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Home , Diaphragm (acoustics), Equalization (audio), Loudspeaker, Megaphone, Professional audio, Public address system

TC 2

PUBLIC ADDRESS SYSTEM IRISET

Ver 1.0 TC2 1 AGENDA • Application of PA system • • • Loud speakers • • Planning of PA • Installation • Special types of PAIRISET systems • Phasing and matching methods

Ver 1.0 TC2 Public Address System 2 What is a PA system ?

• P.A. System is a type of communication, which can be used to communicate to a limited public over a limited area.

• The basic function of an audio system is to deliver audible and recognizable at comparable level to the listener. IRISET • PA system comprises all the devices and networks that exists between a source of (or its electrical Ver 1.0 TC2 Public Address System 3 equivalent) and its point of final reproduction. Application of P.A. system in Railways IRISET

Ver 1.0 TC2 Public Address System 4 • Passenger amenity • Marshalling Yards • Breakdown Train Emergency Equipment • Special Functions • Railway Workshops • Conferences IRISET

Ver 1.0 TC2 Public Address System 5 Acoustics Acoustics is defined as the "Scientific Study” of Sound, especially of its generation, propagation, reception and interaction with materials and is further described as the "total effect of sound” especially as produced in an enclosed space. IRISET

Ver 1.0 TC2 Public Address System 6 Terms related to the study of Acoustics

IRISET

Ver 1.0 TC2 Public Address System 7 Intensity: The intensity I, is the sound power passing normally through a unit area of space. This relates to of acoustic force. It is expressed in W/m2

Loudness: It is the intensity of the sound stimulus as perceived by the human ear and chiefly a function of . However, it dependsIRISET upon the frequency and the complexity of the waveform.

Ver 1.0 TC2 Public Address System 8 Frequency: Frequency is the number of Cycles per second of a particular sound . It is expressed in Hz. Pitch: It is the frequency of a sound as perceived by human ear. A high frequency gives rise to a high pitch note and a low frequency produces a low pitch note. At Low Freq. : Pitch variesIRISET directly with intensity and At High Freq.: Pitch varies inversely with intensity.

Ver 1.0 TC2 Public Address System 9 : Timbre denotes the harmonic content of a sound wave. Brightness: This refers to the upper harmonic content of a tone. Threshold of Audibility: The lowest acoustic pressure that gives rise to a sensationIRISET of hearing. Threshold of Pain: The highest acoustic pressure to which ear can respond without experiencing pain.

Ver 1.0 TC2 Public Address System 10 Volume Range •It is the difference in amplitude expressed in between the maximum and minimum volumes that can be satisfactorily handled by a system. •In a recording system, the to noise ratio of the system determines the maximum volume of of that system. •For a live pickup, the IRISET ambient noise level of the and the system noise combined limits the volume

range. Ver 1.0 TC2 Public Address System 11 Audibility

• Hearing is the end result of differential pressure changes applied to a membrane within the ear. • If very little or no pressure changes are present, the ear hears nothing. • Conversely, if extreme changes appear, the forced of the membrane may damage the ear. • The ratio of acoustic power at these two limits is the dynamic range of the ear and is approximately 1012 (the threshold of pain representsIRISET 1012 times the power of the threshold of audibility) near the middle of the audio spectrum.

Ver 1.0 TC2 Public Address System 12 Sound pressure and its units

• Sound at a particular point in air is the rapid variation in the air pressure around a steady state value. • This sound pressure is measured in the same units as atmospheric pressure, and since it is an alternating quantity, the term "sound pressure" usually refers to its RMS value. IRISET

Ver 1.0 TC2 Public Address System 13 • At a frequency of 1 KHz, a sound with an RMS pressure of 2 x 10-4 μbar, or about 2 x 10-10 atmosphere, is just below the threshold of hearing for good ears, that is a sound of this magnitude is inaudible, but slightly larger sound pressures can barely be heard. IRISET

Ver 1.0 TC2 Public Address System 14 • This demonstrates the amazing sensitivity of the human ear - it can detect variations in atmospheric pressure as small as a few parts in 1010. One μbar equals one dynes per square centimeter or 0.1 Newton per square meter. A human speaker at a distance of 1meterIRISET generates a sound pressure of 1μ bar.

Ver 1.0 TC2 Public Address System 15 Acoustic impedance

• Acoustic impedance of a sound medium is the complex quotient of the sound pressure and the particle velocity multiplied by the unit of area (square centimeter, square meter, etc.). • The surface of the unit area must lie on the sound wave front, i.e., the surface ofIRISET the area is at all points perpendicular to the direction of sound propagation.

Ver 1.0 TC2 Public Address System 19 • Sound intensity, analogous to electrical power, is the rate at which acoustic energy flows through a unit area normal to the direction of propagation. • It is the product of sound pressure and particle velocity. • Expressed mathematically,IRISET Sound intensity I = PV where P is sound pressure and V is particle velocity.

Ver 1.0 TC2 Public Address System 21 and its units

• In acoustics the accepted standard is a pure 1 KHz tone or narrow-band noise centered at 1 KHz. • The loudness level of any sound is defined as the sound pressure level of a standard sound, which appears to a significant number of observers to be as loud as the unknown. IRISET

Ver 1.0 TC2 Public Address System 22 • Loudness level is measured in "Phon", the loudness level of any sound in phons being equal to the sound pressure level in dB of an equally loud standard sound.

• Thus a sound which is judged to be as loud as 40 dB @ 1 KHz tone has IRISETa loudness level Ls = 40 Phons.

Ver 1.0 TC2 Public Address System 23 LOUDNESS LOUDNESS Decibels SOURCE OF SOUND LEVELS LEVELS (PHONS) (dB) (SONES) Threshold of Pain 140 1024 130 Truck 100 64 90 Orator 80 16 Low conversation 60 4 60 Quite Room 40 1 30 Rustling of Leaves IRISET20 -- 10 Hearing Threshold 4 -- 0

Ver 1.0 TC2 Public Address System 24 MICROPHONES

IRISET

Ver 1.0 TC2 Public Address System 25 • is a , which converts acoustic energy into electrical energy.

• Microphones may be classified into two according to the Mode of Operation: i) Pressure operated,IRISET and ii) Velocity operated

Ver 1.0 TC2 Public Address System 26 Pressure operated • Pressure operated microphones employ a with only one surface exposed to the sound source. • The displacement of the diaphragm is proportional to the instantaneous pressure of the sound wave. • At lower frequencies such microphones generally cause a resonant response, giving rise to a peak that may reach 6 to 8 dB with reference to 1IRISETKHz. • The pressure-operated microphones are carbon, crystal, dynamic and . Ver 1.0 TC2 Public Address System 27 Velocity Operated • A velocity microphone is one in which the electrical output substantially corresponds to the instantaneous particle velocity in the addressed sound wave. • A velocity microphone is also referred to as a gradient microphone. • A gradient microphoneIRISET is a microphone in which the output corresponds to the gradient of the sound pressure. • Ribbon microphones are velocity operated type. Ver 1.0 TC2 Public Address System 28 Specifications of Microphones

IRISET

Ver 1.0 TC2 Public Address System 29 1. Type 2. Sensitivity 3. 4. Max. Sound Pressure level. 5. Impedance 6. Minimum load impedanceIRISET 7. Cable and Connectors 8. Front to Back Ratio

9. Polar Response. Ver 1.0 TC2 Public Address System 30 Type: This specifies the microphone whether it is a Dynamic, Ribbon, Capacitor or Crystal and also specifies whether it is a Pressure gradient or Pressure Operated. Sensitivity: It is the amount of voltage developed or generated by the microphone for an applied sound pressure at a test frequency of 1KHz. It is generally specified as mV/Microbar. ItIRISET is also specified as mV/Pa where Pa is Pascal, which is equal to 10 microbars. The higher the voltage produced the higher the sensitivity and vice versa.

Ver 1.0 TC2 Public Address System 31 Frequency Response: • It is the ability of a microphone to produce a proportionate output to the sound pressure applied for the specified range of frequencies. • The frequency response is distorted when the microphone is kept too close to the mouth. • It generates spherical sound with very high impact pressure when the distance from the mouth increases the spherical sound waves flatten and become plane waves. So the IRISETdiminishes with distance.

Ver 1.0 TC2 Public Address System 32 Maximum Sound Pressure Level

It is the maximum Sound Pressure level that can produce a proportional output with a total harmonic distortion limited to 1%. Impedance It is the impedance offered by the microphone at 1KHz. There are low impedance and high impedance microphones. IRISET Low Impedance means less than 600 ohms

High Impedance meansVer 1.0 more TC2 Public than Address System10K ohms. 33 Minimum Load Impedance:

It is the minimum input impedance of the , which is used to utilize the microphone. The amplifier input impedance should not be less than the minimum load impedance of the microphone specified.

Cables and Connectors: It specifies the type andIRISET length of the cable with a particular connector.

Ver 1.0 TC2 Public Address System 34 Front to Back Ratio:

It is specified in the case of unidirectional microphone, which gives the response of front sound and back sound. Generally it is 20 db.

IRISET

Ver 1.0 TC2 Public Address System 35 Polar response:

It specifies the type of directivity pattern that microphone responds it is a graph of the microphones directional sensitivity. It specifies whether it is an omni- directional, Bi- directional or Uni directional.

IRISET

Ver 1.0 TC2 Public Address System 36 IRISET

Ver 1.0 TC2 Public Address System 37 Microphone Connectors

IRISET

Ver 1.0 TC2 Public Address System 38 Three of the most common connectors used for are 3-pin XLR, 1/4" (6.5mm jacks) and RCA. Both 3-pin XLR and 6.5mm jacks are used with balanced cables whereas RCA connector is used with unbalanced cables. 3-pin XLR connectors are mainly used for . Using a balanced signal reduces the risk of inference. The 3-Pin XLR connector is also the standard connector for digital IRISETconnections (electronically balanced, impedance 110 ohms).

Ver 1.0 TC2 Public Address System 39 Pin Signal 1 Shield 2 Signal + 3 IRISET Signal -

Ver 1.0 TC2 Public Address System 40 Connector pins

3-pin XLR Male

IRISET

3-pin XLR Female

Ver 1.0 TC2 Public Address System 41 Types of microphones

IRISET

Ver 1.0 TC2 Public Address System 42 • • CRYSTAL MICROPHONE • DYNAMIC MICROPHONE • CAPACITOR MICROPHONE • RIBBON MICROPHONE • WIRELESS OR CORDLESSIRISET MICROPHONE

Ver 1.0 TC2 Public Address System 43 CARBON MICROPHONE

IRISET

Ver 1.0 TC2 Public Address System 44 IRISET

Ver 1.0 TC2 Public Address System 45 A carbon microphone, also known as a carbon button microphone (or sometimes just a button microphone), use a capsule or buttonIRISET containing carbon granules pressed between two metal plates.

Ver 1.0 TC2 Public Address System 46 • A voltage is applied across the metal plates, causing a small current to flow through the carbon.

• One of the plates, the diaphragm, vibrates with incident sound waves, applying a varying pressure to the carbon. The changing pressure deforms the granules, causing the contact area between each pair of adjacent granules to change, and this causes the electrical resistance of the mass of granules toIRISET change.

Ver 1.0 TC2 Public Address System 47 • The changes in resistance cause a corresponding change in the current flowing through the microphone, producing the electrical signal. • Carbon microphones were once commonly used in ; they have extremely low-quality sound reproduction and a very limited frequency response range, but are very robust devices. • Disadvantages:- 1. Frequency responseIRISET is limited. 2. Distortion is high.

Ver 1.0 TC2 Public Address System 48 CRYSTAL MICROPHONE

IRISET

Ver 1.0 TC2 Public Address System 49 IRISET

Ver 1.0 TC2 Public Address System 50 Crystals which demonstrate the piezoelectric effect produce voltages whenIRISET they are deformed. The crystal microphone uses a thin strip of piezoelectric material attached to a diaphragm. Ver 1.0 TC2 Public Address System 51 • The two sides of the crystal acquire opposite charges when the crystal is deflected by the diaphragm. • The charges are proportional to the amount of deformation and disappear when the stress on the crystal disappears. • Early crystal microphones used Rochelle salt because of its high output, but it was sensitive to moisture and somewhat fragile. • Later microphones usedIRISET materials such as barium titanate and lead zirconate.

Ver 1.0 TC2 Public Address System 52 • The electric output of crystal microphones is comparatively large, but the frequency response is not comparable to a good dynamic microphone, so they are not serious contenders for the market. IRISET

Ver 1.0 TC2 Public Address System 53 DYNAMIC MICROPHONE

IRISET

Ver 1.0 TC2 Public Address System 54 IRISET

Ver 1.0 TC2 Public Address System 55 IRISET

Ver 1.0 TC2 Public Address System 56 • Dynamic microphones work via electromagnetic induction. • A small movable induction coil, positioned in the of a permanent , is attached to the diaphragm. • When sound enters through the windscreen of the microphone, the sound wave moves the diaphragm. When the diaphragm vibrates,IRISET the coil moves in the magnetic field, producing a varying current in the coil through electromagnetic induction. Ver 1.0 TC2 Public Address System 57 A single dynamic membrane does not respond linearly to all audio frequencies. Advantages: • Relatively cheap and rugged. • Can be easily miniaturized. Disadvantages: The uniformity of responseIRISET to different frequencies does not match that of the condenser microphones.

Ver 1.0 TC2 Public Address System 58 RIBBON MICROPHONE

IRISET

Ver 1.0 TC2 Public Address System 59 Principle: The air movement associated with the sound moves the metallic ribbon in the magnetic field, generating an electrical field betweenIRISET the ends of the ribbon which is proportional to the velocity of the ribbon - characterized as a "velocity" microphone. Ver 1.0 TC2 Public Address System 60 Advantages: • Adds "warmth" to the tone by accenting lows when close-miked. • Can be used to discriminate against distant low frequency noise in its most common gradient form. Disadvantages: • Accenting lows sometimes produces "boomy" . • Very susceptible toIRISET wind noise. Not suitable for outside use unless very well shielded.

Ver 1.0 TC2 Public Address System 61 CAPACITOR MICROPHONE

IRISET

Ver 1.0 TC2 Public Address System 62 IRISET

Ver 1.0 TC2 Public Address System 63 IRISET

Ver 1.0 TC2 Public Address System 64 Principle: sound pressure changes the spacing between a thin metallic membrane and the stationary back plate. The plates are charged to a total charge. IRISET

where C is the and V the voltage of the biasing battery. Ver 1.0 TC2 Public Address System 65 Advantages: • Best overall frequency response makes this, the microphone of choice for many recording applications. Disadvantages: • Expensive • May pop and crackIRISET when close miked. • Requires a battery or external power supply to bias

the plates. Ver 1.0 TC2 Public Address System 66 CORDLESS MICROPHONE

IRISET

Ver 1.0 TC2 Public Address System 67 IRISET

HAND HELD CORDLESS MICROPHONE

Ver 1.0 TC2 Public Address System 68 IRISET WIRELESS COLLOR MICROPHONE

Ver 1.0 TC2 Public Address System 69 • A wireless or cordless microphone, as the name implies, is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated. • Also known as a microphone, it has a small, battery- powered radio transmitterIRISET in the microphone body, which transmits the from the microphone by radio waves to a nearby receiver unit, which recovers the audio.

Ver 1.0 TC2 Public Address System 70 • Wireless microphones are widely used in the entertainment industry, broadcasting, and public speaking to allow public speakers, interviewers, performers, and entertainers to move about freely while using a microphone to amplify their voices. IRISET

Ver 1.0 TC2 Public Address System 71 Advantages: • Greater freedom of movement for the artist or speaker. • Avoidance of cabling problems common with wired microphones, caused by constant moving and stressing the cables.IRISET • Reduction of cable "trip hazards" in the performance space.

Ver 1.0 TC2 Public Address System 72 Disadvantages: • Sometimes limited range. • Possible interference with or, more often, from other radio equipment or other radio microphones. • Operation time is limited relative to battery life. • Noise or dead spots (places where it doesn't work, especially in non-diversity systems). • Limited number of operating microphones at the same time and place, dueIRISET to the limited number of radio channels (frequencies).

Ver 1.0 TC2 Public Address System 73 MICROPHONE CONNECTORS

TRS IRISETConnector

Ver 1.0 TC2 Public Address System 74 INSTALLATION PRACTICE

IRISET

Ver 1.0 TC2 Public Address System 75 • All microphones are delicate instruments; they must be carefully handled and never dropped, nor placed where there may be metal dust. • To avoid hum pick-up and especially in case of high impedance microphones, locate as far as possible from electrical apparatus. Do not run microphone leads together with mains IRISETcable.

Ver 1.0 TC2 Public Address System 76

continued….. • For public address locate well away from preferably to rear of the to prevent acoustic 'howl'. • Ribbon microphones should be at least 10” from the speaker. • One should speak intoIRISET the microphone and on no account it should be tested by blowing into the microphone.

Ver 1.0 TC2 Public Address System 77 • When using microphone with long twin core lead (i.e., in low impedance condition) in association with equipment having high impedance input, a step up transformer/matching transformer is required to be interposed in the lead near the equipment input socket. • Microphones shouldIRISET not be mounted on vibrating surface, e.g. , amplifiers, radio, recorders, etc.

Ver 1.0 TC2 Public Address System 78 • Microphones must be protected from strong winds, otherwise 'roaring' noises will result. • It is common practice to provide windscreens in such cases. • A windscreen consists of a framework covered with silk and designed to fit over the outside of a microphone to reduce the effects of wind noise. • A wire frame is clamped over the end of the microphone housing. IRISET • A windscreen is also called a "Wind Gag".

Ver 1.0 TC2 Public Address System 79 LOUDSPEAKERS

IRISET

Ver 1.0 TC2 Public Address System 80 • The function of the is to convert electrical energy into acoustic energy. • Loudspeakers may be classified into two main groups: •Cone Type - i.e., direct radiator, where cone or diaphragm is directly coupled to air.

• Horn Type – i.e.,IRISET indirect radiator, where the diaphragm is coupled to the air by means of horn.

Ver 1.0 TC2 Public Address System 81 Dynamic Loudspeaker

IRISET

Ver 1.0 TC2 Public Address System 82 IRISET

CONE TYPE LOUD SPEAKER

Ver 1.0 TC2 Public Address System 83 • The in moving coil drivers is suspended in a magneticfieldprovidedbytheloudspeaker magnet structure. • As flows through the voice coil (from an amplifier), the magnetic field created by the coil reacts against the magnet's fixed field and moves the voice coil (and so the cone). • Alternating current IRISETwill move the cone back and forth.

Ver 1.0 TC2 Public Address System 84 HORN TYPE LOUDSPEAKERS

IRISET

RE-ENTRANT HORN SPEAKER

Ver 1.0 TC2 Public Address System 85 IRISET

Ver 1.0 TC2 Public Address System 86 (hi-fi) speakers: . These are used to reproduce the generally audible frequency range of 50 Hz to 12 KHz (out of the entire audio range of 20 Hz to 20 KHz). • The frequency response of ordinary speakers is irregular, with a number of resonant peaks and valleys, and has a range of about 60 Hz to 8 KHz only. • By using a fairly large (30cm to 38 cm diameter) and heavy cone, the lowIRISET frequency response of speakers can be extended downward to 45 or even 30 Hz but at the cost of high frequency response.

Ver 1.0 TC2 Public Address System 87 :

• A woofer mainly reproduces lower notes in a musical programme. In some cases it handles all notes below 2 KHz. • For a three-speaker system, it is designed to work up to 500 Hz. A woofer is operated in a closed box (baffle), so that air resistance limits the cone’s movement and avoids damage. IRISET

Ver 1.0 TC2 Public Address System 88 : It is designed to reproduce higher notes in a musical programme and can be used with a woofer in a two- speaker system or with woofer and midrange speakers in a threespeaker system. Tweeter in a two speaker system re-produces frequencies from 1 KHz onwards and in a three speaker system from 5 KHz onwards. covers the range from 8 KHz onwards. A tweeter may beIRISET a small cone permanent magnet speaker or an electrostatic type

Ver 1.0 TC2 Public Address System 89 Crossover network: • The crossover networks are frequency-dividing circuit which ensures that each drive unit is fed only with its correct frequency band. This circuit, in a two-way system directs the high frequencies to a tweeter and low frequencies to the woofer

IRISET

Ver 1.0 TC2 Public Address System 90 The specific purpose of crossover network is: a. To extend the frequency range by the use of two or more speakers of different sizes. b. To avoid distortion which may occur in a single unit. c. To limit the input to the most useful frequency range in a given speaker. d. To protect a delicate HF unit from LF input. e. To facilitate suitable placing of bass and treble speakers for naturalIRISET results.

Ver 1.0 TC2 Public Address System 91 Specifications of Loud Speakers

IRISET

Ver 1.0 TC2 Public Address System 92 Impedance: It is the impedance offered by a loud speaker at 400 Hz. The impedance will change with the frequency. Impedance is the electrical characteristic of a speaker that restricts ("impedes") the flow of power from your receiver or amplifier. Impedance is the combination of the resistance of a speakerIRISET plus its reactance. The impedance value of a loudspeaker is expressed in

Ohms. Ver 1.0 TC2 Public Address System 93 Power Handling Capacity (PHC): Loud Speaker manufacturers use this term to indicate the maximum volume of sound that the loud speaker will produce before it runs into distortion. Generally maximum 5% tolerance is allowed. It is also said that the voice coil of the loud speaker can handle the maximum radial power safely. IRISET

Ver 1.0 TC2 Public Address System 94 • For loudspeakers, there is also a thermal and a mechanical aspect to maximum power handling. • Not all energy delivered to a loudspeaker is emitted as sound. • In fact, most is converted to heat, and that heat must not rise too high or damage will follow. • Loudspeaker components have mechanical limits which can be exceeded by even a very brief power peak; an example is the most common sort of loudspeaker driver,IRISET which cannot move in or out more than some limit without mechanical damage.

Ver 1.0 TC2 Public Address System 95 • There are a number of methods used to describe the power of an electrical signal. • These methods are given names such as "instantaneous power", "average power", "RMS Power", "peak power", "Music power", "Program Power" and others. • The most commonly used ratings are “RMS Power” and “PMPO” (Peak Music Power Output). • "RMS power" is practically the same as "Average power". IRISET • Peak power here refers to the maximum amount of power an can possibly handle for an instant without damage. Ver 1.0 TC2 Public Address System 96 “Peak Music Power Output (PMPO)” is of interest more to advertising copy-writers than to consumers. The term PMPO has never been defined in any standard, but it is often taken to be the sum of some sort of peak power for each amplifier in a system. Different manufacturers use different definitions, so that the ratio of PMPO to continuous power output varies widely; it is not possible to convert from one to the other. Most amplifiers can sustain their PMPO for only a very short time, if at all; IRISET loudspeakers are not designed to withstand their stated PMPO for anything but a momentary peak without serious damage.

Ver 1.0 TC2 Public Address System 97 Frequency Response: It indicates the uniform sound pressure throw for the given band of frequencies and it is related with the enclosures that are used. Sound pressure level: Loud speaker manufacturers indicate the accurate of sound pressure in DB SPL at 1meter distance when 1 watt of 1Khz signal feed to the loud speaker. It is also related with the enclosures. Apart from the above specification some manufactures indicate the dimensions,IRISET weight and the size of the magnet used in the loud speaker.

Ver 1.0 TC2 Public Address System 98 PA AMPLIFIERS AND MIXERS

IRISET

Ver 1.0 TC2 Public Address System 99 MIXER A sound mixer is a device which takes two or more audio signals, mixes them together and provides one or more output signals. The diagram on the next slide shows a simple mixer with six inputs and two outputs. As well as combining signals, mixers allow you to adjust levels, enhance sound with and effects, create monitor feeds,IRISET etc. Mixers come in a wide variety of sizes and designs, from small portable units to massive studio consoles. Ver 1.0 TC2 Public Address System 100 IRISET

Ver 1.0 TC2 Public Address System 101 6 Channel Mixer

IRISET

Ver 1.0 TC2 Public Address System 102 Applications

• Some of the most common uses for sound mixers include: • Music studios and live performances: Combining different instruments into a stereo master mix and additional monitoring mixes. • Television studios: Combining sound from microphones, tapeIRISET recorders and other sources. • Field shoots: Combining multiple microphones into 2 or 4 channels for easier recording.

Ver 1.0 TC2 Public Address System 103 • Mixers are frequently described by the number of channels they have. For example, a "12-channel mixer" has 12 input channels, i.e. you can plug in 12 separate input sources. • You might also see a specification such as "24 x 4 x 2" which meansIRISET 24 input channels, 4 subgroup channels and 2 output channels.

Ver 1.0 TC2 Public Address System 104 Features and facilities of an amplifier:

• Accommodating no. of input devices

• Various controls of input devices

• Rated power output

• Output connectivity for loud speakers IRISET • Standby power supply connection

Ver 1.0 TC2 Public Address System 105 IRISET

Ver 1.0 TC2 Public Address System 106 IRISET AMPLIFIER FRONT VEIW

Ver 1.0 TC2 Public Address System 107 AMPLIFIERIRISET REAR VEIW

Ver 1.0 TC2 Public Address System 108 Mic.1 Mic.2 Mic.3 Micic.4 Mic.5 Bass Treble Master

ON/ OFF

Jack 1 Jack 2 Jack 3 Jack4 Jack5

+ __ Pre Amp. out

2A Com. 70V 100V Com. 4Ω 8 Ω 16Ω Line out. IRISET

AMPLIFIER FRONT & BACK VEIW

Ver 1.0 TC2 Public Address System 109 • Technically, any device that applies gain (even negative gain) to a signal is an amplifier. To distinguish it from other kinds of amplifier (such as or line amplifiers), the main amplifiers are often described as "power amplifiers". • A power amplifier takes a line-level signal and reproduces it in a form that will drive a loudspeaker. It converts a low- voltage, high-impedance waveform into a high-voltage, low-impedance waveform.IRISET Its main purpose is to reproduce a low-power signal at high power.

Ver 1.0 TC2 Public Address System 110 • Most modern PA amplifiers use transistors to convert the signal from low to high power. • Typically the work is shared by a number of transistors and these can get very warm when the amplifier is running at full power. For this reason, power amplifiers in PA systems are almost always fitted with cooling fans and ventilation slots. • Connect the line-level signals to the inputs, and connect the outputs to the IRISET speakers. Always power up with the volume controls set to minimum.

Ver 1.0 TC2 Public Address System 111 Performance of amplifiers • Input and output Impedance. • Input voltage to produce full rated output (volume control at maximum position) • Rated output in watts. • Percentage distortion at full output power expressed in relation to harmonics. • Percentage of harmonics present at a quarter, half and three quarters of full rated output. • Frequency response.IRISET • Noise level at full output. • Power Consumption.

Ver 1.0 TC2 Public Address System 112 Measurements on amplifier • Frequency Response • Amplification (Gain) • Internal Noise Level • Output Power • Harmonic Content • Output Regulation • Power ConsumptionIRISET

Ver 1.0 TC2 Public Address System 113 IRISET

Ver 1.0 TC2 Public Address System 114 Earthing of amplifier units • Earthing of amplifier is necessary to provide greater safety for the person handling the amplifier and protection for the amplifier in case of faults. • The connectivity of Earth to an amplifier is the metal chassis of the amplifier to the Earth or ground. • The chassis does not carry electric current under normal operation. But in the event of fault, when a live wire carrying current comes in contact with the chassis, then this current would flowIRISET to earth.

Ver 1.0 TC2 Public Address System 115 PLANNING OF SOUND REINFORCEMENT SYSTEM

IRISET

Ver 1.0 TC2 Public Address System 116 A sound reinforcement system is governed mainly by the following factors: • Size • Type • Location • Audience to be covered • Type of sound to be IRISETreproduced • Desired psychological reaction on audience.

Ver 1.0 TC2 Public Address System 117 Indoor Installations:

1)Size of auditorium 2)Area to be covered 3)Dimensions 4)Approximate size of audience 5)Actual volume of the auditorium in cubic feet. 6)Reverberation time, if known 7)Seating Capacity 8)Type and distributionIRISET of absorbing materials. 9)Location of source of pick up.

Ver 1.0 TC2 Public Address System 118 10)Desired position of microphone 11)Desired position of loudspeaker 12)Ambient noise level. 13)Type of service. i) Voice or music reinforcement. ii) Remote pick up iii) Orchestra v) Point Source illusion. 14)Frequency characteristics of microphone or pickup. 15)Amplifier IRISET 16)Available . 17)Desired Coverage 18)Permissible Cost

Ver 1.0 TC2 Public Address System 119 Outdoor Installations 1) Area to be covered, in sq. ft. 2) Dimensions 3) Approximate size of audience 4) Desired location of microphone 5) Desired location of loudspeaker. 6) Ambient noise level. 7) Loudest noise whichIRISET the system should over-ride.

Ver 1.0 TC2 Public Address System 120 8) Type of Service. i) Voice or music reinforcement ii) Remote pick up. iii) Orchestra iv) Point Source Illusion. 9) Frequency Characteristics of Microphone 10) Amplifier 11) Audio power available 12) Desired coverage 13) Permissible Cost.IRISET

Ver 1.0 TC2 Public Address System 121 Technical requirement for quality reproduction

Frequency response: The frequency response for the entire system should be with in 3 dB from 100 Hz to 10 KHz. Harmonic distortion: The total harmonic distortion of the entire system shall not exceed 5% at the rated power output of the amplifier.IRISET

Ver 1.0 TC2 Public Address System 122 Signal to noise ratio: The signal to noise ratio under normal operating conditions of the amplifying systems as a whole with flat operation of the tone control shall not be worse than 50 dB. Note: The normal operating conditions are those where sound pressure level of 70 to 80 dB is maintained. Sensitivity: System should be capable of direct operation from input voltage ratingIRISET 0.5mV to 1.5V.

Ver 1.0 TC2 Public Address System 123 ACOUSTIC SURVEY

The object of an acoustic survey is to determine the acoustic defects in relation to the location of the sound sources so that necessary acoustic correction can be done to ensure an optimum, acceptable sound distribution.

ACOUSTIC DEFECTS: • Echo IRISET • Flutter • Reverberation Ver 1.0 TC2 Public Address System 124 Echo: The sound reflection reaching a listener’s ear at least 1/15th of a second after the original sound is termed as echo. Flutter: Where parallel surfaces like side walls exist, there is a tendency for the sound energy to decay in series of steps, rather like a series of echoes of diminishing intensity, where the interval between successive step is the time of sound to be reflected from one IRISETsurfaces to the opposite surface. The effect is flutter. Continued……. Ver 1.0 TC2 Public Address System 125 Reverberation: It is an accumulation of echoes, one interfering with and masking the other, so that the individual echoes cannot be distinguished. It is the persistence of sound by reflection from surround surfaces after the source sound has ceased. IRISET

Ver 1.0 TC2 Public Address System 126 Reverberation • Reverberation is the collection of reflected sounds from the surfaces in an enclosure like an auditorium. • It is a desirable property of auditoriums to the extent that it helps to overcome the inverse square law drop- off of sound intensity in the enclosure. • However, if it is excessive, it makes the sound becomes muddy, garbled. • To quantitatively characterize the reverberation, the parameter called theIRISET reverberation time is used.

Ver 1.0 TC2 Public Address System 127 Reverberation Time:

• In order to provide a reproducible parameter, a standard reverberation time has been defined as the time for the sound to die away to a level 60 decibels below its original level. • Around 2 seconds is desirable for a medium-sized, general purpose auditorium that is to be used for both speech and music. • A classroom should be much shorter, i.e. less than a second. And a recordingIRISET studio should minimize reverberation time in most cases for clarity of recording.

Ver 1.0 TC2 Public Address System 128 • The reverberation time is strongly influenced by the coefficients of the surfaces and volume of the room. • The absorption coefficient of a surface typically changes with frequency, so the reverberation time is likewise frequency dependent. • A table of absorptionIRISET coefficients can be used in calculations of reverberation time.

Ver 1.0 TC2 Public Address System 129 Acoustic corrections • Requisite for acoustic corrections: The type and extent of acoustic defects are to be known, so that the acoustic correction can be applied. • Effect of parallel walls: Parallel walls must be avoided as far as possible. • Effect of side walls: Random reflections from the side walls will reduce the flutter and high efficiency absorbent material may be used on the side walls to reduce the flutter. • Areas opposite to sound source: Curvilinear surfaces and large areas of reflectionIRISET opposite to the sound normally give rise to echo and they must be avoided.

Ver 1.0 TC2 Public Address System 130 PA System for VIP functions

IRISET

Ver 1.0 TC2 Public Address System 131 IRISET

132 Selection of devices: Microphones: Generally dynamic microphones are used for such occasions, as these are having unidirectional characteristics, which help in reducing feedback/howling, and also having low impedance permits long microphone leads.

Connecting microphones: Use of more number of microphone generally essential in large stages. In such cases, output from several microphones should be connected to a mixing system and the common output will be connected to Aux. of main amplifier. Microphone IRISET cables carry low-level signal currents and are therefore, susceptible for electrical interference.

Ver 1.0 TC2 Public Address System 133 Continued… • The microphone cables shall be isolated from power, loudspeaker and cables. • Joints in the cables should be avoided as far as possible. • The plugs and sockets used for microphone cables should have strong self-cleaning contacts so as to eliminate noise and they shall be non-reversible. • Microphone cables should be laid without sharp bends as far as possible. Inside buildings, they may be laid on the floor along the walls or under the carpet to avoid any obstruction for the participantsIRISET on the dais.

Ver 1.0 TC2 Public Address System 134 • In case more no. of microphones are being used, numbering on microphone stands is necessary for easy identification. • Twisted pairs of conductors with sufficient insulation, screened continuously with close mesh of tinned braid shall beIRISET used.

Ver 1.0 TC2 Public Address System 135 Amplifying systems

• The output power of the amplifying system should be so chosen as to be capable of establishing clarity sound at any point amongst the audience. • The gain controls of the amplifying system should be so set that the signal reach each member of audience at comfortable listening level, i.e. during weak passage the signals are distinctly audibleIRISET at each point, while loud passage these do not cause annoyance.

Ver 1.0 TC2 Public Address System 136 The amplifying system should have a gain sufficient to deliver the required output power. However the level should not be less than 60 dB. The amplifiers should preferably be in multiples of 100W capacity, one for each group instead of using high power sets for entire installation.

IRISET

Ver 1.0 TC2 Public Address System 137 • 100% standby amplifiers should be provided so that speech is not being held up due to defects in the working of amplifiers. • Easy changeover arrangement for switching from defective amplifiers to the standby amplifiers should be done by providing changeover switches. • Earthing should be IRISET done for all the amplifier systems properly.

Ver 1.0 TC2 Public Address System 138 Loud speakers

• The number of loud speakers required, their location, direction and the power input to the loud speakers installed will have to be decided with the object of maintaining the intensity of reproduced sound above the local noise level. • The loud speakers used should have adequate power handling capacity and should normally of high efficiency type. • For better reproduction directional type of loud speakers (column) should be used.IRISET

Ver 1.0 TC2 Public Address System 139 • The vertical directivity pattern of the system should be such as to feed the audience at uniform level. • Column speakers are ideal for obtaining the vertical directivity pattern. • The directivity of such speakers should be such as to provide sufficient intelligibility at all points of the seated area and avoid feedback to microphone. • The spacing betweenIRISET two columns in a row should be approximately 8 meters apart.

Ver 1.0 TC2 Public Address System 140 Connecting loudspeakers a)All the loud speakers in each group should be connected in parallel and in phase across the output line. b) The pair of from each group should be terminated on the announcer’s panel at the amplifiers end, so that the line could be isolated from the output of the amplifier in case of any line fault or changeover to a standby amplifier. IRISET

Ver 1.0 TC2 Public Address System 141 c) When a number of loudspeakers are connected to the same output circuit, matching transformers shall be used with each loud speaker and connect to the 100V output line, so that it consumes the rated power.

IRISET

Ver 1.0 TC2 Public Address System 142

Power supply: The installation should be normally operated from 230V single-phase 50Hz AC mains supply with voltage regulating device. The system should also connect to 12/24V battery, which provides continuous operation without any interruption. Changeover to battery operation is automatic when AC main supply fails. No battery current is consumed when the amplifier is working on AC mains. IRISET

Ver 1.0 TC2 Public Address System 143 Installation

1. All equipment should be robustly made and designed for continuous operation. Equipment should be securely installed in such a manner as to have convenient access to all sides of it. 2. When the number of equipment is not large, they may be placed on a table IRISET and wired. The positioning of the equipment is such that the lengths of the inter connecting cable are kept minimum for convenience.

Ver 1.0 TC2 Public Address System 144 3. In case the number of equipment is large, it is desirable to mount them in racks of suitable dimensions. The height of the rack will depend upon the number of equipment to be mounted and accommodation available, ensuring that all manual controls are with in easy reach. 4. The patch cords if used should be tested and neatly arranged to avoidIRISET obstruction and should be easily identified.

Ver 1.0 TC2 Public Address System 145 5. The items should be installed at appropriate time after other like decoration, seating etc. are completed. This will minimize the risk of damage or loss. The wiring for the loud speakers and microphones may be laid just sufficiently in advance of the appropriate time for completing the installation so that preliminary tests that may be IRISET necessary to decide on the type and position of loud speakers could be made.

Ver 1.0 TC2 Public Address System 146 MODEL SSA – 160 EM

Power Output 220 W RMS Max. Output Regulation 2dB no load to full load at 1 KHz 5 x Mic. 0.65mV / 4.7kΩ Input channels 2 x Aux. 100mV / 470kΩ Line input 1 V / 50kΩ Frequency response +/IRISET- 3dB @ 50 to 15,000Hz

Ver 1.0 TC2 Public Address System 147 MODEL SSA – 160 EM

Signal to Noise 60dB Ratio Tone controls Box speaker position Bass: 10dB @ 100Hz Treble: Driver unit position 10dB @ 10KHz Bass: 10dB @ 100Hz Treble: IRISET10dB @ 10KHz Pre Amp. Out 200mV / 600Ω Line output 1V / 1kΩ

Ver 1.0 TC2 Public Address System 148 MODEL SSA – 160 EM Output taps for 4Ω & 8Ω (for direct connections) speakers 70V & 100V (for use with LMT) AC: 220-240V / 50Hz Power Supply DC: 24V (2 x 12V car batteries) AC power consumption 300VA

DC power consumption 4A

Dimensions IRISETW 420 x H 155 x D 320 mm

Weight 13.6 kg approx.

Ver 1.0 TC2 Public Address System 149 Special types of sound reinforcement systems IRISET

Ver 1.0 TC2 Public Address System 150 i) Power ii) Paging System iii) Centralized Sound redistribution system iv) Intercommunications System v) Integrated Sound system vi) Train entertainmentIRISET system vii) Portable P.A. System.

Ver 1.0 TC2 Public Address System 151 Power Megaphone

IRISET

Ver 1.0 TC2 Public Address System 152 Power Megaphone • It is used where human voice has to be reinforced, and portability is required. • Typical applications are in ARTs, fire fighting, marine services and in crowd control.

• It consists of a sensitive carbon microphone, an efficient horn type speaker, aIRISET battery made up of flash light cells, the entire unit combined in a trigger operated assembly resembling a megaphone.

Ver 1.0 TC2 Public Address System 153 PAGING AND TALKBACK

• The paging and talk back system utilizes the horn type loud speaker as a loud speaker as well as a microphone.

• Yardmaster is provided with an amplifier, one microphone and a loud speaker. Audiovisual indications are provided on the Yard Master’s control console. All the speakers are connected in voltageIRISET matching system.

Ver 1.0 TC2 Public Address System 154 • Paging and talk back loudspeaker equipment for use in Railway Marshalling yards broadly consist of microphones, amplifiers, loudspeakers with associated line matching transformers, switching equipment and control panels, the loudspeakers being connected to the amplifiers and switching equipment throughIRISET appropriate aerial wires/cables and wiring, protectors and terminals.

Ver 1.0 TC2 Public Address System 155 Platform announcement systems

The location may be broadly divided into: i) Platforms ii) Circulating area iii) Waiting Rooms iv) Restaurants.

The distribution lines areIRISET divided into groups according to the above .

Ver 1.0 TC2 Public Address System 156 Train entertainment system

These consist of two basic systems: i) Low Level System. ii) High Level System. In type (i), each coach has its own amplifier, and in (ii) the main amplifier is on the dining car and high voltage distribution line (100V) is taken to each coach. For this arrangement, the entireIRISET rake of carriages have to be wired internally and fitted with external coupling units.

Ver 1.0 TC2 Public Address System 157 i) ‘Live’ voice announcements by the guard i.e., manual system. ii) Pre-recorded announcements operated by the guard i.e., semi automatic system. iii) Pre-recorded announcements operated automatically during the progress of the train i.e., automatic system. • In the automatic system, pre-recorded announcements are initiated at appropriate points in the journey by special devices, which measuresIRISET accurately the distance traveled.

Ver 1.0 TC2 Public Address System 158 Conference system

IRISET

Ver 1.0 TC2 Public Address System 159 Conference system mainly consists of one Chairman unit, one secretary unit, delegate units as desired and central amplifier with connecting cable and loudspeaker system.

Chairman unit: This unit specially designed for chairperson, who is chairing the meeting. It consists of built-in loudspeaker and highlyIRISET sensitive electret condenser microphone mounted on flexible gooseneck arrangement.

Ver 1.0 TC2 Public Address System 160 IRISET

ELECTRET CONDENSER MICROPHONES

Ver 1.0 TC2 Public Address System 161 IRISET Chairman unit

Ver 1.0 TC2 Public Address System 162

• The microphone of the unit is equipped with a lockable switch for speaking and a ring LED indicator for visual identification of the speaker.

• It will also have a non-lockable PRIORITY switch, which interrupts and mutes the delegates unit microphones. The built-in speaker hasIRISET volume control for setting desired volume levels.

Ver 1.0 TC2 Public Address System 163 IRISET Delegate units: These units are similar to chairman unit with the exception of the priority switch not being provided.

Ver 1.0 TC2 Public Address System 164 Secretary unit: This unit enables proceedings to be recorded through a cassette recorder,IRISET for a stenographer present to take notes and to relay pre-recorded messages if any to delegates. Ver 1.0 TC2 Public Address System 165 Central Amplifier: It isIRISET provided for connecting conference units consisting of chairman, secretary and delegate units.

Ver 1.0 TC2 Public Address System 166 AMPLIFIERIRISET REAR VEIW

Ver 1.0 TC2 Public Address System 167 PHASING AND MATCHING METHODS IN SOUND REINFORCEMENT SYSTEMS IRISET

Ver 1.0 TC2 Public Address System 168 Phasing of loud speakers

• In a multiple loudspeaker system, when two or more driver units/loud speakers are facing in the same direction and are installed in same area, it is essential that their diaphragms/cones act in unison. For this action, the loud speakers must be connected in phase with each other is called as phasing of loudIRISET speakers.

Ver 1.0 TC2 Public Address System 169 • Loud speaker manufacturers are providing some markings on the winding part of the speaker for the purpose of perfect phasing. • It may be in the form of a “red dot (.)” or + sign at one terminal and “black dot (.)” or - sign on the other. • The connections to the voice coil whether in series or in parallel, must be madeIRISET in such a manner that in any one instant all diaphragms must be moving outward or inward in unison.

Ver 1.0 TC2 Public Address System 170 Matching methods in sound distribution system Any system of connecting a number of speakers to an amplifier has to serve two purposes:- a) Deliver the full available output of the amplifier, in correct proportions, to the individual units of the system. b) Do so efficiently. The "correct proportions"IRISET is a matter of distribution, the realization of full available output, efficiently, is a matter of matching by impedance.

Ver 1.0 TC2 Public Address System 171 For correct transfer of power from the amplifier to the speaker group, we require correct impedance matching.

The effective impedance of the load should be matched with the output impedance of the amplifier. For this, the distribution of number of speakers connected to amplifier may be either in series, parallel or combination of series parallel can be done. For connecting the speakersIRISET in impedance matching method to an amplifier the following procedure must be followed.

Ver 1.0 TC2 Public Address System 172 Com 8 Ω 4 Ω 8 Ω 16 Ω 8 Ω

When speakers are connected in series, the total impedance is the sum of the individual impedance of speakers.

As shown in figure, each speaker will have an impedance of 8Ω; the resultant impedanceIRISET will be of 16 Ω. So for the perfect impedance matching we will select the 16 Ω taping of the amplifier. Ver 1.0 TC2 Public Address System 173 Group A Group B

Com. 16 Ω 16 Ω 4 Ω

8 Ω 16 Ω 16 Ω 16 Ω

Impedance of group A = R1+ R2 = 16+16 = 32 Ω.

Impedance of groupIRISET B = R3+ R4 = 16+16 = 32 Ω.

Ver 1.0 TC2 Public Address System 174 Voltage matching • Connecting of loud speakers to the amplifier with line matching transformers to either 70V or 100V tapping. • This method is based on "constant voltage" system, which simplifies to a great degree the computation of the total impedance of the system. • It also permits the addition of speaker to an existing system, without theIRISET recalculation of the load and source impedances so long as the total power consumed by the loud speaker is less than or equal to the amplifier rating. Ver 1.0 TC2 Public Address System 175 Voltage matching

• To generate this high voltage, the amplifier is equipped with a step-up transformer,IRISET which transforms the regular output voltage, in the range of 15 to 30 Volts, up to the necessary 70V (or 100V respectively).

Ver 1.0 TC2 Public Address System 176 • These systems work by including transformers at the input to each speaker and directly after the amplifier output (see fig.). The transformers are used to convert the impedance of each speaker to a higher value, and to convert the amplifier output impedance to a correspondingly high value. • In a 100 volt line system, speaker impedances (with transformers) may range from below 20 ohms to as high as 10,000 ohms or more. • But you won’t needIRISET to calculate the load impedance in ohms, because of how the high impedance approach works.

Ver 1.0 TC2 Public Address System 177 Connectivity in 70V constant voltage system

IRISET

Ver 1.0 TC2 Public Address System 178 High impedance (100 volt or 70 volt line) systems have three major advantages over low-impedance systems: 1) System impedance-matching is made much easier — it is simply a matter of adding up speaker power taps and selecting an amplifier rated for at least that much power plus an allowance for headroom. 2) Line loss is greatly reduced, especially over long cable runs, resulting in better performance and reduced cost compared to long low impedance lines. 3) The amplifier outputIRISET is electrically isolated from the speaker line by the output transformer, protecting the output stage against a grounded line.

Ver 1.0 TC2 Public Address System 179 • In a large installation requires a large number of speakers to be connected at a distance. As the distance increases the length of the cable increases so the strength of the sound signal decreases. • Hence in such installations where the length of the cable is substantial, line matching transformers (LMT) are being used in voltage matchingIRISET method, between the output of the amplifier and each individual speaker to be connected.

Ver 1.0 TC2 Public Address System 180 • LMT is having multiple high impedance tapings on primary side and standard impedance tapings of COM., 4 Ω, 8 Ω and 16 Ω on secondary side. • The primary side will be connected to output of amplifier to 100V line and the secondary will be connected to loudspeaker.IRISET LMT will act as a step-down transformer to the speaker.

Ver 1.0 TC2 Public Address System 181 Required impedance (Z) = E2/P

= (Output Voltage) 2 ______Desired power So that, an amplifier employing a 100-volt output tap, the formula is reduced to:

Required impedanceIRISET = 10,000

Desired power Ver 1.0 TC2 Public Address System 182

• Suppose you're connecting hundreds of speakers to a single 8-ohm amplifier output. It can be difficult to wire the speakers in a series-parallel combination having a total impedance of 8 ohms. • Also it is bad practice to run speakers in series because if one speaker fails, all the speakers in series are lost. This changes the loadIRISET impedance seen by the power amplifier.

Ver 1.0 TC2 Public Address System 183 • With a 70V/100V system you can hang hundreds of speakers in parallel on a single amplifier output if you provide a matching load. • In addition, a 100V distributed system is relatively easy to design, and allows flexibility in power settings. • A low-impedance system might employ 30 speaker clusters spaced 100 feet apart, each cluster powered by a 1000W amplifier for extra headroom. • A high-impedance versionIRISET of that system might use only one amplifier providing 70V/100V. The cost savings is obvious.

Ver 1.0 TC2 Public Address System 184 DISADVANTAGES OF 100V OPERATION 1. One disadvantage of a 100V system is that the transformers add expense. 2. Another disadvantage is that transformers can degrade the frequency response and add distortion. IRISET

Ver 1.0 TC2 Public Address System 185 INSTALLATION Here's a basic procedure that neglects transformer insertion loss: 1.Do NOT connect the 70V speaker line to the power amplifier first. 2.Install a transformer at each speaker location, or use speakers with built-in transformers. 3.Connect each speakerIRISET to its transformer secondary tap. The tap impedance should equal the speaker impedance. Ver 1.0 TC2 Public Address System 186 4. Connect each transformer primary to the 100V line from the power amplifier. Choose the tap that will deliver the desired wattage to that speaker. 5. Add the wattage ratings of all the primary taps. This sum must not exceed the amplifier's wattage rating. If it does, change to a lower-wattage primary tap of one or more transformers,IRISET or use a higher-power amplifier.

Ver 1.0 TC2 Public Address System 187

6. Now connect the 100V speaker line to the 100V output of the amplifier.

As an example, suppose you are setting up a 100V system with 8-ohm speakers and a 60W power amp. Connect the 8- ohm secondary taps to each speaker. Suppose the total speaker wattage is 55 IRISET watts. This is acceptable because it does not exceed the amplifier of 60 watts.

Ver 1.0 TC2 Public Address System 188 Here's a more detailed procedure that emphasizes impedance matching: 1. Compute the minimum safe load. The minimum safe load impedance that can be connected to the amplifier is given by Z = E² / P Where: Z = minimum safe load impedance, in ohms. E = speaker line voltage (70V or 100V) P = maximum continuous average power rating of power amplifier, in watts. IRISET

Ver 1.0 TC2 Public Address System 189 An example: For an amplifier rated at 100 Watts continuous average power, the minimum load impedance that may be connected safely to the 70.7V output.

Choose transformer taps. Tap the primary at the desired power level for the speaker, and tap the secondary at the impedance of the speaker.IRISET The sum of all the power taps for all the speakers should not exceed the power output of the amplifier. Ver 1.0 TC2 Public Address System 190 Note: Changing the power tap also changes the load impedance seen by the amplifier. Raising the power tap lowers the load impedance, and vice versa. Also, changing the power tap changes the SPL of the loudspeaker. Reducing the power tap by half reduces the SPL by 3 dB, which is a just-noticeable difference in speech sound level. If a particular speaker is too loud or too quiet, you can change its power tap. Just be careful thatIRISET the total power drain does not exceed the power output of the amplifier.

Ver 1.0 TC2 Public Address System 191 3. Connect the speakers together. Connect all the speaker-transformer primaries in parallel. Run a single cable, or redundant cables, back to the power-amplifier transformer secondary. But DON'T CONNECT IT NOW. 4. Measure the load impedance. Before connecting the load, first measure its impedance with an impedance bridge (a simple low-costIRISET unit is adequate).

Ver 1.0 TC2 Public Address System 192 • Here's why you must do this: If the load impedance is too low, the power amplifier will be loaded down and may overheat or distort. It's a myth that you can connect an unlimited number of speakers to a 70V line. • If the load impedance measures too low, re-tap all the speakers at the next-lower power tap. This raises the load impedance. Measure again. • Usually, it's no problem if the load impedance measures higher than the matching value (the calculated minimum safe load impedance).IRISET The system will work, but at reduced efficiency.

Ver 1.0 TC2 Public Address System 193 • Typically there is more than enough power available, so efficiency is not a problem. If for some reason, the power is limited, then the system should be wired for maximum power transfer. • This occurs when the measured load impedance matches the calculated minimum safe load impedance. If the load impedance measures above this value, you can re-tap all the speakers at the next-higherIRISET power tap and measure again. This tap change lowers the load impedance.

Ver 1.0 TC2 Public Address System 194 Recommended sizes of cables and their maximum distances:

0.75mm² 1.0mm² 1.5mm²

30 watt load 800m 1066m 1600m

60 watt load 400m 533m 800m

120 watt load 200mIRISET 266m 400m

Ver 1.0 TC2 Public Address System 195 Effects of using 100v, 70v or 50v taps on amplifiers with 100v line speaker circuits: Amplifier Tapping Speaker output 100v 100% - Full Power 70v 50% - Half Power (-3dB) 50v 25% - Quarter power (-6dB) Loss & Distance 1m 2m 4m Effects SPL (dB) @ 1w 97 91 85 SPL (dB) @ 2w 100 94 88 SPL (dB) @ 4w 103 97 91 SPL (dB) @ 8w 106 100 94 SPL (dB) @ 16w 109 103 97

IRISET Add 3dB when Deduct 6dB when power is doubled distance is doubled

Ver 1.0 TC2 Public Address System 196 IRISET

Ver 1.0 TC2 Public Address System 197