Chapter 5(a): Microphones This is where it all begins really – the microphone emulates the human ear and responds to the pressure variations of the sound waves. Microphones range from extremely inexpensive “throw-away” units, to precise studio instruments costing tens of thousands of dollars. They all sound slightly different, and if you ask any three audio professionals what they think of a particular microphone, you will probably get at least 5 different answers. In recent years, the quality of even inexpensive microphones has dramatically improved, and there are some real bargains out there. There are even quality microphones available which plug directly into a computer USB slot. But if you hope to be able to make a wise selection among the plethora of options, you need to know some very basic information. So, once again, we begin with some definitions. Transducer: Any device that transfers the energy of one system to another. These may be the same or different types of systems (i.e. motion to electricity, electric signal to sound, sound pressure to electricity etc.). Microphone: A transducer that converts sound pressure to an electrical signal1. The first broad classification of microphones is as to the manner in which this conversion takes place, and consists of Dynamic and Condenser (or Capacitor) microphone types. Dynamic Microphone: One in which the electrical signal is produced by the movement of a conductor through a magnetic field. There are two major types of dynamic microphones, moving coil, and ribbon. Moving Coil microphone: A coil of wire wrapped around a tube is attached to the rear of the microphone diaphragm (the disc that is moved by the force of the sound pressure), and suspended in a magnetic field. Movement of the diaphragm moves the coil, cutting the lines of magnetic force and inducing a flow of electrons in the wire of the coil. Strength of the resulting signal depends on the strength of the magnet and the number of turns of the wire in the coil, as well as the amount of movement allowed by the diaphragm suspension.
Figure 1: Moving Coil Microphone Elements Ribbon microphone: (also dynamic) Instead of having a moving coil of wire, ribbon mics have a corrugated strip of metal - actually foil as it is extremely thin - that is suspended in a magnetic field. As metal is a conductor, the movement of the foil induces a voltage as it cuts across the magnetic lines of force.
1 So – do you abbreviate it as “mic”, “mike” or “mic.”? Again, 5 soundies in a room, 6 opinions likely. I like to use “mic” since it’s not a “mikerophone”, it’s a “microphone”. I usually skip the period because it keeps the darn word processor from capitalizing the next word and I’m basically lazy.
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Figure 2: Ribbon Mic Elements This produces a voltage that is relatively small and a very small current as the resistance of the foil is very, very small. Ribbon microphones always have a matching transformer that presents appropriate impedance2 to the system (though these are still "low impedance" devices) and provides a voltage more similar to that produced by other microphones. While modern ribbon microphones can be quite robust, it is important to remember that many vintage ribbon mics are extremely delicate, and should be protected against shocks and overloads. Since even modern versions tend to be quite expensive, it probably makes good sense to be in the habit of treating them gently. Condenser (or Capacitor) Microphone: These are microphones in which the diaphragm is one part of a capacitor3 (once called a condenser). Movement of the diaphragm causes the distance between the plates to change which varies the capacitance of the device, causing a corresponding change in voltage if a voltage is being applied.
Figure 3: Condenser Mic Elements
2 More detail later – for now: Impedance is the complex sum of Resistance and Reactance – everything that “impedes”, or gets in the way of the flow of electrons in a circuit. 3 A Capacitor consists of two conductors separated by a non-conductor such that an accumulation of electrons on one conductor drives electrons out of the other via the electrostatic field between them. Electron flow “through” the capacitor is thus dependent on the distance (and material) between the conductors.
Basic Sound Engineering PP271: Chapter 5a 65 This signal is quite small, so is applied to a preamplifier to bring it up to the level of a mic signal. Condenser mics must have both power supplies and preamplifiers. Power supply: can be separate plug-in or battery supply inserted in the mic cable, a simple chamber in the mic for a battery, or Phantom Power - wherein the operating voltage is supplied by the mixer board. More later on phantom power, but briefly the voltage is applied to BOTH signal lines and returned by the shield. Better boards with phantom power allow you to switch it on or off for each line, some offer a range of voltages, as condenser mic supply voltages range from 1.5v to over 50 volts. Electret Capacitor Microphones: are condenser microphones in which the diaphragm is polarized permanently at manufacture eliminating the largest drain on the power supply. This allows smaller batteries to be used, as the pre-amp requires a much smaller EMF4 to operate.
Acoustical Properties: Receiving end: Our brain has an editing facility that allows us to tune in some sounds and tune out others. It is relatively easy to concentrate on one sound out of a mix when we are present in the room. Rooms like a classroom are actually fairly noisy, but we learn to tune out un-wanted sounds such as hum from florescent lights, ventilator noise, noise from the hall and parking lot, etc. Of course, this assumes there is something that we want to hear in the room that the brain can focus on. One of the keys to this facility is a perception of direction of sounds. We perceive horizontal direction (excluding visual cues) largely by evaluating the difference in arrival time of different sounds at each ear. Thus if a sound arrives at one ear earlier than the other, we perceive its direction to be to the side of the "early ear". Microphones in general do not have this ability - a microphone will "hear" all sounds of equal level equally well. The demonstration of this is simple, put a microphone in a "quiet" room and record for a while. On playback, we hear all kinds of noises that are present in the room, and "heard" by the mic, that we didn't hear during the recording session. We hear them on playback because they are coming from a source that we do not automatically tune out - the speaker - so all the cues our mind uses to pick and choose are defeated. This has great import to us as sound engineers, and different types of microphones have been designed to overcome this and other problems related to recording only what we want.5 Microphone Physical Design. The three basic microphone types for directional sensitivity are: Omni-directional, Bi-Directional, and Uni-directional. Directional sensitivity is a function of the physical design of the microphone (except in the case of multiple diaphragm microphones - more later). Omni-directional microphones: also called pressure microphones as they respond to variations in air pressure without regard to direction. They are characterized by only one path for sound to the diaphragm, and only one side of the diaphragm is open to sound. They are essentially a simple sealed container with a diaphragm as one end. If you look at figure 3 on
4 EMF: Electro Motive Force – a sexier term for voltage. It describes what voltage really is in a circuit – it is the “pressure” that causes electrons to flow. The actual flow of electrons is referred to as “current”. 5 It’s also part of why we never think a recording of our voice really sounds like us. The brain filters out and “equalizes” the sound it receives, and in the case of our own voices it also adjusts for the fact that the ears cannot possibly be in the direct path of sounds coming from the mouth, and that there are other paths for the sound of our voice to arrive at our hearing mechanism. It has no accurate reference to do this though, since it has no input from any device which is in the direct path.
Basic Sound Engineering PP271: Chapter 5a 66 page 65 above, you can see that it represents such a pressure microphone. Any pressure variation, regardless of what direction it may come from, will cause movement of the diaphragm. If the microphone body is physically large, the case of the mic may obstruct sounds arriving from off-axis, so they may not be perfectly Omni-directional. Bi-Directional microphones: are equally sensitive to sounds on 0 and 180 degrees (front and back), and insensitive to sounds on 90 and 270 degrees (left and right). Also called pressure gradient mic, as it responds to the relative difference of pressure between front and back.
Figure 4: Bi Directional Microphone Elements (top view)
Physically the bi-directional mic is simply a diaphragm suspended "vertically" so that sound can reach it from any direction. Sounds from the front strike the diaphragm and cause it to move, but by the time that the same "wave" arrives at the rear of the diaphragm it has shifted in phase and helps to "suck" the back side thus reinforcing the movement of the diaphragm.. Sounds from the back have a similar effect. Sound from the side strikes both sides of the diaphragm at the same time and in phase, so the effect is like pushing on a door from both sides at once - nothing happens. As the sound source moves around the mic toward the front, the phase shift of the rear arriving wave gets progressively longer allowing a build in sensitivity. Obviously, we know from our discussion of wavelength that this reinforcement will vary depending on frequency and wavelength. At very low frequencies, there is no real shift in phase because the wavelength is so long; as the wavelength of the signal decreases (frequency increases) the reinforcement provided by the phase shifted rear arrival increases (by 6 dB per octave), until the wavelength is exactly 1/2 the path, at which point maximum phase shift and maximum boost occur. Beyond this point (called the transition frequency), rear effect rapidly drops back off then at wavelengths that are multiples of the path the effect will repeat. - Microphones are physically designed to compensate for the 6 dB/8v (“8v” is a common shorthand for “octave) rising rate by applying a 6dB/8v roll-off up to the transition frequency.
Basic Sound Engineering PP271: Chapter 5a 67 Uni-directional microphones: are sensitive to sounds from one direction or range of directions only. They are essentially a compromise between the omni and the bi - in that the capsule is designed to allow unobstructed arrival at the front of the diaphragm, while a carefully calculated set of obstructions (or labyrinth) are arrayed on the rear. The effect is that sounds coming from the front strike the diaphragm, but also continue on through the phase port and arrive at the back of the diaphragm delayed by the time of travel. So they are out of phase and, depending on wavelength (frequency) they will reinforce the movement of the diaphragm since a zone of rarefaction would hit the back of the diaphragm at the same time as a zone of compression hits the front.
Figure 5: Uni-Directional Mic With Front Arriving Sound Meanwhile sounds from the unwanted directions find essentially an equal length path to the front and rear of the diaphragm, thus arriving in phase and canceling in the same way that the side arriving waves do on the bi-directional microphone.
Figure 6: Uni Directional Mic With Rear Arriving Sound
Basic Sound Engineering PP271: Chapter 5a 68 One consequence of the design of both bi-directional and uni-directional microphones is something called proximity effect. Remember our old friend the inverse square law as it applies to the roll-off in sound pressure level with distance. Every doubling of distance results in a 6dB drop in level, at least within our free field. If our sound source is, say, 2 feet away from the microphone, then it will take another two feet of travel for the wave to drop by that 6dB. The path from the front to the back of the microphone, either around the body for the bi-directional, or through the rear phase port for the uni-directional isn’t likely to be more than about an inch or so. We can figure out how much the sound pressure will have dropped over that path length by comparing the two distances – let’s say that our initial distance is two feet, or 24”, and our second distance, including the path through the port or around the mic is 25”. Remember the formula from Chapter 2 for sound level drop over distance:
Dr LP = LPR + 20log Dm Where LP = Sound Pressure Level @ Measurement point
LPR = Sound Pressure Level at a Reference Point
€ Dr = reference distance
Dm = measured distance Since we aren’t really worried about the actual sound pressure at the point, only the difference, we can use a variation of this that simply shows us the dB difference between the two points:
Dr LDif = 20log Dm
LDif = the difference in dB in sound level due to distance
Dr = reference distance
€ Dm = measured distance We can plug in our distances, using the 24” as our reference, and 25” as the measured distance and we’ll see: 24inches L = 20log Dif 25inches Or
LDif = -0.354 dB SPL So, less than€ a dB difference between the two paths. But, what if we have a rock’n roll singer swallowing the microphone? Let’s recalculate using the closer distances. Now, the initial distance is ½”, and the second distance is 1-1/2”. 0.5inches L = 20log Dif 1.5inches Or
LDif = -9.54 dB SPL Remember that€ a difference of 10dB SPL is quite large, most people perceive it as a factor of 2 difference. What impact does this have? For the sounds arriving from a distance, the portion of the wave arriving at the rear of the mic diaphragm has about equal level to the
Basic Sound Engineering PP271: Chapter 5a 69 portion arriving at the front, so the mic works as expected. But, when the sound source is very close, the rear-arriving waves are proportionally much lower in level than the front arriving waves, so the direct sound from immediately in front of the mic is the predominant component of the resulting signal, or put differently, the rear arriving wave is so much lower than the direct wave, that it has little impact on the movement of the diaphragm. However, recall that these microphones are designed to compensate for the rear wave induced rising response with a 6dB/8v roll-off in response. So, the end result of this is that the microphones tend to exhibit a boost in low frequencies when worked very close – this is called the proximity effect, and is most pronounced in uni- directional microphones, though also present in bi-directional mics. It is not present in uni- directional microphones as there is no path to the rear of the diaphragm, and no need for the physical compensation. Put simply, proximity effect is the tendency for directional microphones to become “boomy” or “bassy” when worked closely. There are some uni- directional microphones which are specifically designed to overcome this effect, notably the Electro Voice “Variable-D” microphones, such as the RE-16, and RE-20.
Figure 7:Electro Voice RE-16 Variable D Microphone
Figure 8: Electro Voice RE20 Variable D Microphone On the RE-20 in Figure 8, the grille-work that extends down the body of the microphone covers the additional rear ports that reduce the proximity effect by providing multiple paths for the rear wave to arrive. On the body of the RE-20, you can also see a small switch with settings marked “—“ and “/”. The position marked “/” engages an electronic bass roll-off circuit to reduce the remaining proximity effect. Such switches can be found on most uni- directional microphones. Note that if the vocalist holds the microphone so that their hand covers up the ports, the variable-d feature will be defeated! Microphone Polar Patterns: are simply methods of graphically expressing the behavior of a microphone. How would we show the way a microphone responds to sound from different directions? One way would be to measure pick-up at specific angles for each mic, but this could be tedious, and what if we wanted to know the performance in between points? The method that has been developed is very simple. A microphone is set up on a stand with a speaker some specific distance away, then as the speaker plays a continuous signal, the
Basic Sound Engineering PP271: Chapter 5a 70 microphone is slowly rotated through 360 Degrees, and its output recorded on a circular piece of paper. This gives us an easy to read picture of what the response of the mic is at any point. Figure 9 is the polar pattern for an omni-directional microphone, in this case a theoretical “perfect” omni, which has equal pickup in 360 degrees.
Figure 9: Omni Directional Mic Polar Pattern To read the polar graph, note that the degrees indicate direction of the sound source, so 0 degrees would be a source directly in front of the microphone, 90 degrees would be one from the right side, 270 degrees would be one from the left, and 180 degrees one from directly behind. The outer ring of the graph is the 0dB or highest level. In this graph, there is a 5dB difference between rings, so the second ring in from the outside would correspond with a level that is 5dB down from the outer ring, the third ring in would be 10dB down, and so on. The little microphone symbol in the middle is just to orient the direction – often it will not be shown. Figure 10 shows a polar response graph for a bi-directional microphone. As you can see, it shows that the response of the microphone is strong in front and back, then begins to drop off around 45 degrees off axis, with the drop off increasing as you move around to the side of the mic, and it shows the two lobes of pick-up, one in front and one in the rear.
Figure 10: Bi-Directional Microphone Polar Graph
Basic Sound Engineering PP271: Chapter 5a 71 As different frequencies have different wavelengths, they will behave differently in a given microphone. For that reason it is important to have more than just one pattern on the graph for each mic. A Uni-directional mic that is very directional at higher frequencies may well behave almost like an omni-directional at very low frequencies. This could be a problem if we set up such a mic to record vocals with its supposedly "dead" side facing a bass amp for example.
Figure 11: Polar Pattern With Multiple Traces In the polar pattern for a uni-directional microphone shown in Figure 11, each trace represents a different frequency. The innermost trace is the pick-up pattern for the microphone at 1k Hz, the solid line of the next trace is the pattern at 500 Hz, and the outermost dashed line is the trace at 100 Hz. So, looking at this one graph, we can predict the way the microphone will work, and we are alerted to the fairly large off-axis response at low frequencies. This much variation is quite common, it all comes down to physics. Looking at the inner two patterns for the uni-directional mic, you can see the origin of the common name for them, “cardioid” (as you can see in the illustration, frequently misspelled by the author as “cardiod”) – this comes from the fact that the pickup pattern of the most common uni-directional mic is roughly heart-shaped. The term cardioid is applied to the class of uni-directional mics that have the most pronounced and clear heart-shaped response. There are several variations on this response; subcardioid, supercardioid and hypercardioid mics being the most common. Subcardioid Microphones fall somewhere between a cardioid and an omni-directional mic. Subcardioids are rare, one example is Schoeps MK-21, and they may sometimes be called Forward-biased omni-directional mics. They have a gentler drop in response to the rear and wider range in the front, with less frequency irregularities than "straight" cardioids. Supercardioid - A cardioid with a more pronounced directionality. The physical tricks used to increase the forward bias of the mic also lead to a small increase in the rear lobe of the polar response graph, that is more sound from "off-axis", as well as a more pronounced tendency to behave differently with varying frequency.
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Figure 12: Supercardioid Microphone Polar Graph Hypercardioid – A more extreme version of the Supercardioid, with much larger rear lobe, and more inconsistency in frequency response.
Figure 13: Hypercardioid Mic Polar Graph Anechoic Chamber: The process of measuring microphone polar graphs must be done in an environment that is free from sounds other than the intended source, and in fact one that is acoustically "dead", or offers no reflective surfaces for the sound from that source. In other words we must be sure that the measured sound is in fact coming only from the direction of the speaker in order for the graph to mean anything. The Anechoic Chamber is a room built to these specifications. The large wedge shapes that you see in the photo of an anechoic chamber in Figure 14 are designed to eliminate reflections of sound waves. Notice that the floor of the chamber is a wire mesh grid, and that the wedges extend down below the floor.
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Figure 14: Measuring Microphone Patterns in an Anechoic Chamber Anechoic chambers are eerie environments – when the door is closed, you can actually experience a loss of balance and disorientation because the constant bombardment of external noise goes away. If you stand quietly in such a room, you will begin to become aware of a soft noise of fluid flowing – it is your own circulation system, you actually can hear the blood flowing in your body. (And, before anyone asks, no, the dapper looking gentleman adjusting the microphone is not your instructor in his erstwhile youth.) Off-Axis Coloration - Directional characteristics of cardioid mics are dependent on physical path length differences to front of diaphragm and rear of diaphragm, so the longer the wavelength, the less relative difference, and the less directional the mic becomes. Put differently, the lower the frequency, the less directional the microphone becomes. This can be very significant in a studio situation where, for example, the acoustic guitar microphone, though faced away from the Bass, may still pick up significant sound energy from the bass. Given the path length difference between the mic intended for the Bass, and the acoustic Guitar mic, some phase distortion is inevitable when the final mix down occurs.
Microphone Electrical Properties: Impedance (Z): The opposition of a circuit to the flow of AC, the complex sum of resistance and reactance. Reactance: Opposition to AC current exhibited by inductors and capacitors. Low Impedance: Generally considered as impedance lower than about 600 Ω. Note that impedance is dependent on frequency to a large extent. High Impedance: Generally considered as impedance in the range of several thousand ohms, frequently in the neighborhood of 10 KΩ or higher. NOTE: There is a gap between the commonly accepted ranges of low and high impedance. Equipment that falls into this range may be considered either high or low impedance depending on the other equipment it is used with. If impedance is the opposition to signal, which is actually caused by the physical structure of wires and properties of electronic parts of the mic itself, then it is reasonable to expect that the high impedance microphone will "soak up" more of the signal than will the low impedance mic, so correspondingly less signal will be available to make the trip to the mixer. In addition there is a phenomenon related to
Basic Sound Engineering PP271: Chapter 5a 74 the cable length caused by the fact that the wires of the cable can be thought of as a long capacitor. The cables impedance will actually change as the frequency increases due to the way a capacitor behaves, and this will have the effect of lowering the impedance of the mixer/cable combination. This will reduce the voltage "dropped" across these loads. The voltage between two points is determined by the resistance or impedance between those points, i.e. two points separated by a wire in a circuit would develop no voltage, but put a resistor between the points and a voltage will be developed. For this reason we always will find that the most successful arrangement in electronic equipment is for the output impedance to be lower than the input impedance of the next stage. Modern practice indicates that there should be a 1:10 ratio of impedance between output and the next input. That is, the input of the mixer should have at least 10 times the impedance of the output of the microphone. High impedance mics are limited to cable runs of about 20 to 25 feet at a maximum, while the low impedance mic can often have cable lengths of several hundred feet without severe losses. In fact the lower the impedance the longer the cable run, and mics with impedances in the range of 75 to 100 Ω can have cables in the 500 to 900 foot length range. Sensitivity: Mic sensitivity ratings give standardized references to allow us to compare one mic to another in selecting the one for the job. There are two ratings that you may encounter, the current international standard Open Circuit Voltage Rating and the older Maximum Power Rating. Open Circuit Voltage Rating is a measure of the output voltage of the microphone into air (not hooked to any load) with a standard sound pressure field of 1Pascal (which would be 94dB SPL) at 1k Hz, 1 volt is used as the reference for the electrical measurement if expressed in dB (dBV). Usually, it will be expressed as mV/Pa – for example, from several manufacturer’s data sheets: Beyerdynamic Opus 83: “Open circuit voltage at 1k Hz (0dB=1V/Pa) 3.2mV/Pa = -50dBV ElectroVoice RE20: “Open circuit voltage at 1k Hz: 1.5mV/Pa” ElectroVoice ND67a: “Open circuit voltage at 1k Hz: 3.1mV/Pa” Schoeps CMH 64U: “Sensitivity measured at 1k Hz: 13mV/Pa” Maximum Power Rating is a measure of the power dissipated by a load-impedance equal to the impedance of the microphone, when a sound pressure field of 10 microbars is introduced. The 0 dB reference here is 1 milliWatt. This rating is no longer widely used. Microphone Overload: There are two basic types of overload to be concerned with, acoustical and electrical. Acoustical Overload is a case wherein the sound pressure field is simply too strong for the microphone's diaphragm to move smoothly, or one which forces the diaphragm to move
Basic Sound Engineering PP271: Chapter 5a 75 beyond its design limits. This is rare with modern mics, but can still be encountered with older ribbon mics and inexpensive microphones. Electrical Overload would be the situation wherein the output from the mic is simply too strong for the input stage of the console. This is the reason most consoles today have attenuators (sometimes called “pads”) controlled by switches in their input stages. Barring the built-in attenuator, there are plug in versions available to be put in line with the microphone. Another type of electrical overload may occur in condenser microphones when the output of the condenser diaphragm is too large for the input stage of the preamplifier. In this case it is sometimes possible to physically insert a pad in line between the capsule and the preamp, or there may be a switch on the microphone body for this purpose. Microphone Technique John Woram's first law: Never use more than two microphones! Well, OK that’s really not a practical rule for reasons we will discuss, but there are some reasons behind the stricture. Perceptions of direction are related to the difference in time between the arrivals of sound signals at our two ears. Differences are very small and are frequency-dependent; that is longer wavelengths tend not to seem as directional as shorter wavelengths. A large part of the directional information is related to phase shift between arrivals of the sound signal at the two ears - longer wavelengths don't go through reversal in such a short distance. Using two microphones some distance from the sound allows us to pick up the phase clues in a similar fashion to the way the ears do, while a multi mic method totally blows these clues away by locating the mics close to the individual sections of the orchestra. Even with the engineer using the pan controls on the mixer to "locate" the individual sounds, the time clues are gone, as most sounds arrive at the mic, and then at the tape simultaneously. Often the facts of real life leave us no alternative to multi-micing, but there are some techniques that retain at least a semblance of the natural sound reception and allow us to reproduce these effects on tape and in the playback system, to the listener. Stereo Microphones are microphones with two diaphragms built into one case for convenient stereo recording; usually the diaphragms are rotatable through some angles with respect to each other to allow different pick-up arrangements. M-S recording (Middle Side)(Mono-Stereo) is a method using one cardioid and one Bi- Directional mic and a matrix system to combine the sound in a way that produces a high degree of left/right discrimination. Since the Bi-Directional mic produces sound that is in phase with the cardioid on one side and out of phase with the cardioid on the other, by using the "sum and difference" signals we get two signals that are equivalent to having two cardioid mics aimed at an angle to each other. Advantages of MS over using two cardioids are: 1. You can adjust the apparent sensitivity and pattern of the two "ghost" cardioids by adjusting the relative signal of the sum and difference signals. 2. You can simply record the output of the two mics on tape and perform the matrixing later in the studio at leisure. 3. The figure 8 mic does a better job of picking up reverberant signals from the sides, ultimately giving a more realistic sound. 4. The cardioid signal may be used as a mono signal if both a mono and stereo signal is required.
Basic Sound Engineering PP271: Chapter 5a 76 Note: the matrix can be done away with if you can use phase reversal "Y" plugs and several channels of a mixer (or a mixer with polarity reversal capability). Blumlein Pair: is a method using two Bi-Directional elements, either stereo mic or two separate mics set at a 90° angle and located about as far from the ensemble as half the width of the ensemble. Stereosonic Recording is a method using two Bi-Directional elements, either stereo mic or two separate mics set at a 45° angle and located about as far from the ensemble as half the width of the ensemble. X-Y Recording is similar to the Stereosonic, except two cardioid elements are used and the angle between them may be varied to allow for greater distance from the ensemble. This method is perhaps better when there is an audience as the pickup of the audience is reduced, however the reverberation of the room acoustics are somewhat muted with this technique. There is a variation of the technique with the mics spread apart a bit – but this can introduce phasing problems problems if the spread is too far. Binaural Recording is one of the most interesting and misunderstood methods. In Binaural, the microphones are placed so as to closely resemble the placement of the human eardrum, often even located in a "dummy Head" to enhance the effect. The catch is that the listener must use headphones in order to hear the effect; binaural recordings played back over stereo speakers are simply strange, as the sounds seem to abruptly change directions. ORTF System: A pair of Cardioid microphones are placed 17 cm. apart at an included angle of 110°, the name comes from the use of this method by the French National Broadcasting Organization (Office de Radiodiffusion Télévision Française) Spaced Pair: Often a pair of Omni-Directional microphones spaced a short distance apart and parallel to each other, facing the source. When spaced much farther apart than the average distance between the ears, a "hole" in the middle of the response often opens, leaving the stereo image without a well-defined "center". This is frequently addressed with a third center microphone, judiciously mixed in to fill the center image, though this can reduce the "spaciousness" of the image. These are frequently called an A-B Pair, or sometimes a Near- Coincident Pair. Devotees argue that the omni-directional mics used are much more faithful to the frequency spectrum than any system using cardioid or Bi-Directional mics can possibly be. Additional Mics can be used with various pair systems to "warm up" various sections, but the more you use them the more "blurred” the clean stereo image will become. One promising system, developed by Dennon and Soundstream for digital recording of orchestra (Dennon release of Mahler Symphonies on CD) involves the use of computer controlled delay systems on mic lines that allows the engineer to reestablish the correct time and phase clues into the sound before it is put on tape. Multi Mic Technique: Why would we use more than two microphones? 1. Ensemble may not be self-balancing - there may be a need for the control of multiple microphones. 2. A room with the proper acoustics may not be available, or it may be too noisy for recording with stereo techniques. 3. Some instruments may need more "punch" for the type of music.
Basic Sound Engineering PP271: Chapter 5a 77 4. One instrument or voice may require special effects or modification. 5. All members of the ensemble may not be able to get together at one time, or in one place.
Generalities For Microphone Placement: 1. LISTEN TO THE INSTRUMENT!! a. Spend some time listening to the instrument in the room b. Move around the instrument to find where the sound is best c. Put on headphones, hook up the mic, and move it around 2. Pay attention to characteristics of Microphones a. directional response b. off-axis response c. proximity effect 3. Listen for Balance problems a. placement in mix (no 20' wide drums) b. phase reversals c. room reflections 4. Use the minimum mics practical for the job Specifics: 3:1 rule - If a mic is 1' from performer, no other mic should be within 3' of the first to allow enough separation between microphone signals to avoid phase cancellations. Note the origin of sound in the instrument - i.e. on a horn such as a sax, a significant portion of the sound is developed after the bell, so a mic stuffed into the bell looses this. The same is especially true with a piano - best sound may be 3' away from the piano for full effect of harmonics and reverberation of body Despite the point above, the closer to the instrument you get, the less background noise and off axis noise you need worry about. This becomes a balancing act (pun intended). Off-Axis Response - a mic with very directional response at 1k HZ might well be nearly omni-directional at 20 HZ or 30 HZ. Several mics such as this used to mic other instruments would give a "muddy" sound, as much low frequency info from bass and drums (and even piano, or room noise) may be present on all tracks. This is one place where bi-directional mics are often used, as they are very dead to the sides over a broad frequency spectrum. Use of Gobos6 & Booths: These can be used to good effect, but we need to watch out for the problem of unequal diffraction and reverberation effect. A booth has a very different characteristic to an open studio, so the sound on that track may be noticeably different especially during solos. Also a Gobo may absorb some of the natural sound from the
6 No lighting peeps – that doesn’t refer to a small piece of metal with a pattern! In the studio, a gobo is a movable partition that often has a sound absorbing surface on one side and a reflecting surface on the other.
Basic Sound Engineering PP271: Chapter 5a 78 instrument you are trying to record, or it may have different reflective effect at different frequencies, leading to more potential muddiness. Contact Mic: These can be used to good effect as they respond to mechanical vibrations as opposed to sound in the air, but also they often reduce the "musicality" of an instrument. Potential also exists for phase reversals depending on the wiring of the pickup. Lavaliere Mic: A good Lavaliere (i.e. Sony ECM50) is often used with acoustic instruments such as Bass Viol. Lavalieres have a rising high end response so that they will overcome the bassiness of the position they are used, and this compensates somewhat for the problem caused by close micing the instrument. Electric Guitar: Often we use a Direct Box - essentially a matching transformer that steps down the output to mic line level. Some direct boxes will also work with a speaker level output from an amp, and these are useful for older amps that only apply their effects to the amplified side. If direct boxes are unavailable or nixed by the performer, then a cardioid mic can be used in a "mouse" type position or a PZM mic might be used. Acoustic Guitar: Depends on the circumstance - if solo instrument, a good cardioid (or omni in perfect recording situation w/ no off axis noise) can be used, being careful to avoid proximity effect, and maintaining consistent spacing from the mic. A PZM may be used in addition to improve the "spaciousness" of the image. Drums - Whole separate course could be taught on recording drums! Basic approach is to match mic to portion of kit and big rule is KEEP MICS AWAY FROM STICKS7. • Bass - mic must be able to take high levels, & while high frequencies are not as important, still want to be able to hear the attack sound. Good choice is Sennheiser 421, or any good durable cardioid. Bad choice would be ribbon mic, or condenser w/o a physical pad. frequent placement is to remove front skin and place inside drum with blanket or pillows to reduce reverb. If situation allows (rare) leave skin on and locate mic about 1' to 18" from drum near floor. If there are two Bass drums, use two mics, don't try to cheat with one. Specialty “microphones” like the Yamaha SubKick bass mic can be used to enhance the pickup.
Figure 15: Yamaha Sub Bass Mic • Snare - Mic is often basic cardioid i.e. SM58 or Audix OM5/3 placed about 3" above & in front of drum. Other options include specialty mics that clip to the stand and have a capsule on a wand. Underside of drum is usually unsuitable due to bass and/or
7 Because just asking the drummer to keep the sticks away from mics isn’t going to get you anyplace! (Nor should you expect it to.)
Basic Sound Engineering PP271: Chapter 5a 79 chains. Take special precautions to avoid sticks, and find out where the drummer puts spares. • Toms - may be many or only a couple - similar choices and placement to snare drum - two adjacent toms may share a mic if location allows. Larger floor toms need more distance from mic to develop a full sound, smaller toms may require mics with better high frequency response. • Cymbals, High-Hat - Number varies wildly - worst case you can just allow other drum mics to pick these up, but in best of all situations, use a good condenser cardioid on a boom about 2' above each group. These will also help to pick up the higher attack sounds from the drums and add presence and fullness to the drum image. The high-hat may deserve its own cardioid in addition located below, though often there will be a tom mic in the vicinity that will cover this. Sometimes a spaced or coincident pair of cardioid condenser microphones on a boom above and behind the drummer may pick up the entire top end of the kit and also contribute to the attack sound of the entire kit. These are frequently called “overheads”. • Misc. percussion toys - Often additional toys like bells, triangles, slides etc are in use. Discuss with the percussionist how and where they are to be used and provide an appropriate mic/mics for them. In a pinch the overhead mics or one of the toms may be used if the percussionist knows which one you expect him/her to be using. • Vocals - This is the bear with the drummer. Best bet is a high quality "headset" mic worn by the drummer. Barring this, the only other option is some kind of boom stand located so as to position the mic correctly. Note that you must keep both the mic and its stand and cords away from the sticks, but you must also locate the mic so that the drummer can still drum while singing (or would that be sing while drumming?) Piano - Another deceptively difficult instrument to mic correctly. During a quiet studio recording of a solo, a stereo mic technique could be used with the mics about 8 to 10 feet from the piano. More common situations require compromise of some sort. PZM or PCC mics can be mounted on a music stand about 4' away, or they can be placed on the underside of the lid, and/or under the piano. A cardioid may be used about 4 feet from the open lid aimed at the lid. At the cost of sacrificing the reverberant field, a punchy, "honky-tonk" sound may be achieved by placing two cardioid mics in the sound holes in the soundboard, aimed down in the second and fifth holes. For very close proximity situations the lid can be closed in the last method with some padding required for the stands and cords. Also the lid may be closed with a PZM mic attached to its underside. Another method would be to use an electrostatic or contact type piano pickup unit, though these will, of course, eliminate any of the sound that develops with distance. Recently, some very creative systems have been developed to suspend microphones inside the piano. Special Purpose Microphones There are a number of microphone designs that are intended for special purposes. These range from tiny concealable microphones that can be connected to wireless transmitters and worn by performers in stage plays to microphones designed to mount on the floor or on other surfaces, to multiple diaphragm microphones which attempt to record a 3-dimensional soundscape. We won’t talk about all of these here, but we do need to look at some of them. Boundary Layer Microphones: In this sense, a boundary is a surface, most often a floor but sometimes a wall or even a large piece of Plexiglas or wood. The Boundary Layer then, is the
Basic Sound Engineering PP271: Chapter 5a 80 area very close to that boundary, where direct sound and sound reflected from the boundary essentially arrive at the same time. Consider the case in the illustration below.
Figure 16: Multiple Paths to Microphone Because there is both a direct path and a significantly longer reflected path from the floor, we will have at least two signals arriving at the microphone with one delayed in time. This will result in phase cancellation at the microphone, resulting in comb filtering, where sound at some frequencies will cancel out because a pressure zone arrives at the same time as a reflected rarefaction zone. Because we are still working fairly close to the microphone, the reflected energy has not had enough time to significantly reduce in level, so the cancellation may be severe. Remember from chapter two that we call this kind of cancellation “comb filtering” because a graph of the frequencies have regular dips that resemble the tines of a comb. Figure 16 shows an example of comb filtering in a room response curve.
Figure 17: Comb Filtering in Room Frequency Response If the microphone can be located closer to the floor, the potential reflection path from the floor is greatly reduced.
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Figure 18: Reduced Reflecting Path In the 1970’s Crown International introduced a line of microphones called PZM mics, for Pressure Zone Microphone. The PZM was built with a very small omni-directional microphone capsule suspended roughly one millimeter above an aluminum plate and facing down. The plate could be placed on the floor, or on other surfaces, and the tiny spacing between the mic and the plate assured that there would be virtually no path length difference between the direct sound and any reflection that could get to the mic from the boundary surface. In addition, the close placement of the capsule to the boundary resulted in a 6dB increase in sensitivity due to pressure doubling caused by the adjacent surface. The pick-up pattern of the PZM mic is hemispherical – so while it is very good for eliminating reflections and phase cancellations, it also is very good at picking up anything nearby – for example, the orchestra in a pit on the backside of the mic, or the sound of punters in the first row unwrapping candy. Today, many manufacturers make boundary layer microphones of varying design. Some use the same arrangement as the Crown PZM, some simply place the diaphragm facing up in a disc, some actually are made to be inserted into holes drilled in the floor or other surface.
Figure 19: Crown PZM Microphones
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Figure 20: Bayer Boundary Mic Figure 21: Schoeps Boundary Mic Not long after the PZM microphone, Crown introduced the Phase Coherent Cardioid, or PCC microphone. The PCC takes advantage of the boundary layer as well, but it uses a cardioid capsule aimed forward rather than an omni capsule aimed down. The PCC mics work well for picking up vocals across a small stage, and the cardioid pattern helps to reduce the feedback problem that often makes a PZM unusable for live events. They are not magic though, there is still really no substitute for getting in close to a microphone.
Figure 22: Crown PCC 160
Other microphone manufacturers also make PCC type microphones, though as with the PZM, they can’t use the three magic letters because Crown owns the trademark. (Bruce Bartlett, who developed the PCC160 for Crown now manufactures his own variation of the microphone with a slightly more directional pattern)
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Figure 23: Shure EZB EasyFlex Boundary Microphone
Usually, as with the Shure product shown in Figure 22, they will just call them Boundary Microphones. In the case of the Shure EZB, and several other microphones, they are available with either cardioid or omni capsules. Schoeps takes a different approach by providing a special boundary clip that can mount several of their very small omni and cardioid microphone modules.
Figure 24: Schoeps BLC Mounting Plate
In the second part of chapter 5, we will investigate the subject of wireless microphones.
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