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J. Perelman
"Entertaining physics". Book 1.
Chapter 10. Sound and hearing


Who many times visited various theatres and concert halls, that is well-known that the sense of hearing rooms are with good acoustics and poor acoustics; in some areas the voices of artists and sounds of musical instruments distinctly heard in the far distance, in the other - sounds even near perceived indistinctly. The reason of this phenomenon is very well described in the book American physicist wood: “Sound waves and their applications”.

“Any sound made in the building for quite a long time dealt at the end of the sound source; due to multiple reflections several times he walks around the building, and meanwhile, there are other sounds, and the listener is often unable to catch them in proper order and to sort them out. For example, if the sound lasts 3 seconds and he said with a speed of three syllables per second, then the sound waves corresponding to 9 syllables, will move around the room all together and creates a total mess and noise, due to which the listener will not be able to understand the speaker.

Caught in such circumstances, the speaker remains to speak very clearly and not too loud. But usually the speakers, just the opposite, trying to speak loud and it only amplify the noise.

Not so long ago the construction of a theater with good acoustics regarded as a happy accident. Currently found techniques of successful struggle with unwanted sound duration (called “reverb”), which spoils the hearing. This book is not the place to enter into details, interesting only for architects. I note only that struggle with poor acoustics is to create surfaces that absorb unwanted sounds. The best absorber of sound is an open window (as best absorber of light is a hole; square meter open window even adopted for the unit, which measures the absorption of sound. Very good - although twice as worse than an open window, - absorb the sounds themselves by theatre: everyone ravenshaw in this regard, about half a square meter open window. And if it is right a comment by one of physics, that “the audience absorbs the speaker in the truest sense of the word, it is not less true that the empty hall unpleasant for the speaker in the immediate sense.

If the sound absorption is too high, it also creates a poor audibility. First, excessive absorption mutes the sounds, and secondly, it reduces the reverb to such an extent that the sounds are heard as if dangling and impress some dryness. So, if you want to avoid too long reverb, and too short reverb is also undesirable. The best amount of reverb for the various facilities are not the same and must be installed when the design of each room.

In the theatre there is another matter of interest from the point of view of physics: prompt booth. Have you ever taken any notice that in all the theatres she has the same form? This is because the prompt-box - sort of physical device. The ceiling of the booth is a concave acoustic mirror, which has a twofold purpose: to delay the sound waves coming from the mouth breather towards the audience, but also to reflect these waves towards the stage.

Entertaining physics J. Perelman


System Orphus


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