ACOUSTIC DESIGN [Film Technology]

acoustic design 
[Film Technology]

     Acoustic design must take into consideration the fact that, in addition to physiological peculiarities of the ear, hearing is complicated by psychological peculiarities. For example, sounds that are unfamiliar seem unnatural. Sound produced in an ordinary room is somewhat modified by reverberations due to reflections from walls and furniture; for this reason, a broadcasting studio should have a normal degree of reverberation to ensure natural reproduction of sound. For the best acoustic qualities, rooms are designed to reflect sound sufficiently strongly to give a natural quality, without introducing excessive reverberation at any frequency, without echoing certain frequencies unnaturally, and without producing undesirable interference effects or distortion. The time required for a sound to diminish to one-millionth of its original intensity is called reverberation time. An appreciable reverberation time improves the acoustic effect, especially for music; in an auditorium a loud sound should still be barely audible for one to two seconds after its source has stopped. In a private home a shorter but still discernible reverberation time is desirable. Accoustic design Materials - To modify the reverberations the architect has two types of materials, sound-absorbent and sound-reflecting, to coat the surfaces of ceilings, walls, and floors. Soft Accoustic design materials such as cork and felt absorb most of the sound that strikes them, although they may reflect some of the low-frequency sounds. Hard Accoustic design materials such as stone and metals reflect most of the sound that strikes them. The acoustics of a large auditorium may be very different when it is full from when it is empty; empty seats reflect sound, whereas an audience absorbs sound. In most cases, the acoustics of a room will be satisfactory if a proper balance between sound-absorbing and sound-reflecting materials is created. Troublesome echoes may frequently occur in a room that otherwise has a proper overall reverberation time if the ceiling or a wall is concave in shape and is highly reflecting; in such cases, sound may be focused at a particular point, making the acoustics bad there. Similarly, a narrow corridor between parallel reflecting walls may trap sound by repeated reflection and cause troublesome echoes, even though the overall absorption is sufficient. Attention must also be given to the elimination of interference. Such interference arises from the difference in the distances traversed by the direct and the reflected sound and produces so-called dead spots, in which certain ranges of frequency are cancelled out. Reproduction of sound[Sound Reproduction] picked up by microphones also requires the elimination of echoes and interference. Accoustic design Insulation - Another important aspect of room acoustics is insulation from unwanted sound. This is obtained by carefully sealing even the smallest openings that can leak sound, by using massive walls, and by building several unconnected walls separated by dead spaces. To evaluate the acoustic properties of rooms and materials, the scientist uses tools such as anechoic chambers and sound-level meters. The anechoic chamber is a room free from echoes and reverberations in which all sound is absorbed by glass-fibre wedges placed on the surfaces of the walls. A sound-level meter measures sound intensity, which is the rate of flow of sound energy and is related to the loudness of a sound. The meter expresses the result in decibels (dB), a logarithmic unit. In a quiet residence the sound-level meter would read about 38 dB. An ordinary conversation would increase the sound-level reading to about 70 dB. The sound intensity of an air-raid siren could reach about 150 dB; that of a jet plane, around 120 dB. When perceived sound intensity is doubled, its power level increases by 10 times, or 10 dB. Loudness levels, which depend upon the judgement of the listener, are measured in units called sones and phons. 

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