UNIT 3 STUDIO ACOUSTICS AND RECORDING TECHNIOUES 1 Structure 3.0 Introduction . - 3.1 Objectives 3.2 Studio Acoustics 3.2.1 Nature of Sound 3.2.2 Hearing Characteristics 3.2.3 Acoustic Quality of Enclosed Spaces 3.3 Recording Techniques 3.3.1 Sound Pick-Up 3.3.2 Signal Processing 3.3.3 Recording 3.3.4 Level Control 3.4 Recording Medium 3.5 LetUsSumUp 3.6 Check Your Progress: Possible Answers 3.0 INTRODUCTION In the previous two units we have discussed the different components of the sound broadcasting chain in general and microphones, loudspeakers and sound mixers in particular. You are already conversant with various facilities and technical features of sound mixers and microphones. As has been mentioned earlier, quality recording demands a good knowledge of studio acoustics and optimum use of these elements. This unit has been designed to give you an Insight into various concepts relating to studio acoustics from the view point of a Programme Producer, and to provide an overview of sound recurding techniques in general including practical tips for making quality sound recordings. 3.1 OBJECTIVES After studying this unit, you should be able to: explain the fundamentals of sound signal and hearing characteristics of the human ear; identify the different acoustical effects related to enclosed spaces and make use of this information while recording; select the suitable microphone for different applications and use it appropriately for efficient sound pick-up; and describe the important characteristics of different recording media from a user's point of view and be able to carry out signal processing effectively. 3.2 STUDIO ACOUSTICS Acoustics refers to the science of sound. Environment plays a very important role in creating sound recordings. In enclosed spaces like studios, auditoria etc., the quality of sound produced depends on the design of studio and the materials used for sound proofing. Good acoustics conditions for studios mean the following requirements: the studio should have suitable reverberation and sound quality in accordance with studio use. Requirements of reverberation are different for different types of audio programmes; displeasing sounds should not be heard; there should be no intrusive noise from the surrounding areas; and sound from the studio should not leak to surrounding areas. The above requirements are generally taken care of at the time of acoustic design of the studio. However, as a Producer, you should understand the basics of sound and acoustics for producing quality recordings. Studio Acoustics And 3.2.1 Nature of Sound Recording Techniques Sound is a pressure wave which is generated naturally by the movement of surface, such as strings or skins (like in string and percussion instruments), variation of air flow in a tube past an obstruction (as in wood wind, brass or vocals) etc. Apart from these natural sources of sound, there is ever growing family of synthesizers or electronic keyboards which generate sound electrically and have no audible existence until the loud-speaker converts them into sound. The sound waves propagate in air as very small pressure variations of the static atmospheric pressure. A sound wave comprises of succession of compression (pressure higher than atmospheric) and rarefactions (pressure less than atmospheric). One set of compression and rarefaction comprise one cycle of frequency. There is no actual movement of air particles from sound source to the listener. Sound propagates only by transfer of pressure variations. MOVING PISTON ALTERNATINW I ,7-COMPRESSIONS /\ AND RAREFACTIONS PRESSURE -ATMOSPHERIC ALONE '- PRESSURE WAVE - Propagation of plain sound waves Sound wave propagation works exactly like propagation of ripples in a water pond when disturbed by throwing a stone. The speed of sound wave propagation is about 332 rnlsec. Have you ever wondered why thunder is heard later than the lightening flash, though both originate at the same time? The reason is that light travels with a speed of 300,000 Krnl sec. Thus lightening is seen immediately even if the clouds are a few kilometers away but sound takes a few seconds to travel the same distance. Frequency range of audible sound varies from 20 cycles Isec or Hertz (written shodly as Hz ) to 20,000 Hz. Most musical sounds have their origin in simple harmonic motion having a single frequency with multiple overtones. The particular overtones and their relative intensities are the governing factors of the quality of sound. Faithful reproduction of overtones brings naturalness to the reproduced sound. Sound generally propagates in all directions. The intensity of sound reduces in proportion to square of the distance from the source. When the sound waves strike boundaries of the studio, a part of its energy is absorbed, a part is transmitted through the boundary and the remaining part is reflected back. The quantum of reflection depends upon nature of the surface. Reflections IS are strong from hard surfaces like stonebrick wall, glass surface, doors, tiled floorslwall etc. whereas they are weak from sqft surfaces like curtains, carpeed floor, soft furniture, fibrous material faced by perforated hard surfaces etc. The reflected sound may undergo multiple reflections before becoming inaudible. Control of these reflections to a large extent decides 1 the acoustic quality of the studio. DIRECT~EARLY~REFLECTION~ - - - - - - - .- MULTIPLE REFLECTIONS Process of reflections from different surface -. Recording You have studied in the previous unit that the intensity of sound is measured in terms of sound pressure variations and is expressed in decibels (as).Typical sound pressure levels of different sound sources encountered in day-to-day life are: Forest 20 dl3 Quiet living room 40 dB Business office 65 dB Street traffic 80 dB Pop Music 100 dl3 Jet take-off (100 m distance) 125 dl3 3.2.2 Hearing Characteristics ' The human ear is a remarkable sensor. It can perceive sound levels right from rustling of Ii leaves (level of about 30 dB) to the roar of an aircraft engine (level of about 120 dB). Thus the human ear has a very high dynamic range i.e. level difference between the loudest and quietest sounds. Such high dynamic range cannot be recorded on any recordmg medium. You have to reduce the dynamic range by boosting the level of low level passages and reducing the level of high level passages. The ear has a non-linear response to both level and frequency. The ear is most sensitive at frequencies around 2 KHz.At low sound levels, the ear is much less sensitive to low frequency sounds than the mid frequency sounds. However at high sound levels, the sensitivity is more or less the same as at low and mid-frequencies. Loudness pattern of ear is given in equal loudness contours in figure below: Equal ToiZdness contours of human ears (Phon is a unit of Equal loudness contours of human ears- reference 1 Khz.) intelligibility of sound reduces significantly in the presence of noise. We have all experienced the phenoaenon that we have to raise our voices to make ourselves heard on a crowded railway platform. A difference of at least 20 dB between desirable sounds and noise is essential for intelligibility. This is a very important criteria that decides the dynamic range I of recording medium. The ear also has some very special characteristics. If a sound is received within about 30 milli-seconds of the direct sound, it is perceived as part of the original sound and adds to its loudness. However if delay is 50 m seconds or more, it is perceived as a separate sound. This effect is known as "Hass Effect". During acoustic design, care is taken to avoid reflections with delay &weeding50 m seconds in the recording and listening area. The two ears cleverly decide the direction of sound. We would not have been able to perceive Studio Acoustics And the direction of sound if we did not have two ears. Direction is decided by combination of Recording Techniques intensity and time difference of sound signal arriving at the two ears. The hearing mechanism is so intelligent that it can precisely decide the direction of sound even when there are strong reflections from the surroundings. The ears latch on to the first arriving sounds. These aspects of hearing help in deciding stereo sound pick-up methods. 3.2.3 Acoustic Quality of Enclosed Spaces We have mentioned earlier that studio or enclosed spaces like auditoria must have suitable reverberation time depending on the use to which the studio is put to. Let us now discuss the meaning of reverberation time and the various parameters connected with this. Reverberation Time As already mentioned, a sound signal urrlergoes multiple reflections from walls, floor and It ceiling of any enclosed space b~f~rebecoming inaudible. At every reflection, the level of reflected sound is less than the incident sound. Initially, the reflections are spaced in time but after the onset of multiple reflections, the reflections are very closely spaced as shown in 1 figure below. BANG Process of reverberation . Thus the sound once created does not die immediately. The time taken for a sound signal to reduce in level by 60 dB is defined as Reverberation Time (RT). RT is the most widely used acoustic quality parameter of an enclosure. 1 RT of studios varies fiom 0.3 seconds to more than 1 second depending upon use and volume. Desirable RT is low for spoken-word studios and listening environment. The recommended value is about 0.35 seconds At higher RT values, clarity of speech suffers. However, for music stud~os,higher RT values are recommended. Desirable RT is related to volume of the studio. The h~gherthe volume, the higher is the recommended RT. You can get a broad idea about the RT of a studio by clapping once and listening to the response.