Funk & Wagnalls explanation of "Acoustics"

The following is from The Funk & Wagnalls New Encyclopedia, 1994*

ACOUSTICS ( Gr. akouein, "to hear"),

term sometimes used for the science of sound in general, but more commonly for the special branch of that science, architectural acoustics, that deals with the construction of enclosed areas so as to enhance the hearing of speech or music. For the treatment of acoustics as a branch of the pure science of physics, see Sound .

The acoustics of buildings was an undeveloped aspect of the study of sound until comparatively recent times. The Roman architect Marcus Pollio, who lived during the 1st century bc, made some pertinent observations on the subject and some astute guesses concerning reverberation and interference. The scientific aspects of this subject, however, were first thoroughly treated by the American physicist Joseph Henry in 1856 and more fully developed by the American physicist Wallace Sabine (1868-1919) in 1900.

Problems of Design.

Acoustical design must take into consideration 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 best acoustic qualities, rooms are designed to produce sufficient reflections for naturalness, 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 acoustical effect, especially for music; a loud sound should still be barely audible for one to two seconds after the sound has stopped in an auditorium. In a private home a shorter but still discernible reverberation time is desirable.

Materials.

For modifying the reverberations, the architect has two types of materials, sound-absorbent and sound-reflecting, to coat the surfaces of ceilings, walls, and floors. Soft 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 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 at that point in the room. 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 canceled out. Reproduction of sound picked up by microphones also requires the elimination of echoes and interference.

Insulation.

Another aspect of room acoustics is insulation from unwanted sound. This is obtained by sealing even the smallest openings that can leak sound, by using massive walls, and by building unconnected walls separated by dead spaces.

To evaluate the acoustical properties of rooms and materials, the acoustical 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-fiber wedges placed on the surfaces of the walls. A sound-level meter measures sound intensity, the rate of flow of sound energy, which is related to the loudness of a sound, and 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; a jet-airplane noise, 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 judgment of the listener, are measured in sones and phons.

* Infopedia 2.0 © 1992-1996 SoftKey Multimedia, a subsidiary of SoftKey International Inc. SoftKey is a registered trademark and INFOPEDIA is a trademark of SoftKey Multimedia Inc. The Funk & Wagnalls New Encyclopedia are licensed from K-III Reference Corporation. ©1994-96 K-III Reference Corporation. Posted without permission for educational purposes only.