Acoustics
When sound enters a new medium, it is reflected, transmitted, or absorbed. Sound can be absorbed by certain materials: rubber, cork, and acoustic tiles, for example. Sound absorbing materials have high absorption coefficients. The absorption coefficient is the fraction of the original sound intensity absorbed by the material at some given frequency. The standard of absorption is an open window with a coefficient of 1.00, since no reflection is possible from an open window. Knowing which materials absorb sound and which reflect it is very important in acoustical engineering projects such as the design of concert halls, which must be built to minimize unwanted effects such as echoes. Echoes are reflected sound, heard enough later than the original sound to be perceived separately, typically when the reflected sound reaches the ear more than a tenth of a second after the original sound was produced. An acoustically corrected room should not have the far wall more than about 56 feet from the speaker in order to eliminate these echoes and the loss of intelligibility they cause.
In the same way that an object between two parallel mirrors will produce multiple images, a source of sound between two parallel sound-reflecting surfaces will produces multiple echoes, or reverberations. Large places, such as auditoriums, have what is known as a reverberation time. This is the time it takes for a certain sound's intensity to decrease to a millionth of its original value in that room. If the time is too great, the original sound lasts too long, and reverberations continue on into the time a speaker wants to move on. These reverberations can mask other sounds, especially those of a higher frequency. If the time is too small, the speaker must raise his voice. A three second maximum for this time is best for a large auditorium.
