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. Otherwise feedback is created.
A problem with reflecting sound a lot is the fact that it has a decay rate. 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.
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