acoustics



acoustics defined in 1939 year

acoustics - Acoustics (Gr. akouein, to hear);
acoustics - The normal interpretation of the word is the behaviour of sound in rooms and halls, as it affects the hearing of speech and music. The Greeks and later the Romans designed large amphitheatres with acoustic effects in mind. In modern building great attention is given to the subject.

The following are the main requirements which have to be met:

(1) Loudness should be adequate. The distance that the human voice can carry in the open air is usually 50 ft., which means restricting the number of persons in an outdoor audience to 2,000.

(2) Reverberation should be controlled, particularly inside halls. Each sound must die away rapidly enough so that it is not confounded with subsequent sounds. At onetime it was thought that the stretching of wires across a hall reduced reverberation, but this has been disproved. The rate of decay of reverberation, in terms of the time that a note takes to die away to inaudibility, can be calculated and checked by test. About one second is found to be satisfactory for small halls and up to two seconds for large halls.

Reverberation can be controlled by the use of sound-absorbent materials. Porosity of the surface is an essential, and special plasters and fibre boards are available for this purpose, the sound absorption qualities being as much as ten times those of normal interior surfaces. The sound absorption of a person is about 30 times that of a wooden seat, and approximately 2¼ times that of an upholstered seat. Since often as much as half the sound absorption in a theatre may be due to the audience, the additional advantage of upholstered seats is clear.

(3)Echoes require to be controlled. The time interval between direct and reflected sound should not exceed l/50th of a second.

(4)External, main, and partition walls must be sufficiently soundproof to exclude outside noise.

Noise may be transmitted to a room by windows, ventilators, or cracks; by vibration of the walls, ceiling, or floor; and by direct transmission of waves from the walls, ceiling, or floor.

Resistance to airborne sound is related to the weight of the intervening structure and is not much affected by cavities unless there is a discontinuity, i.e. a hollow wall, in the structure. The nature of the material is' of importance, e.g. increasing the thickness of a brick wall ten times will only double the sound insulation, whereas with hard felt the insulation is increased directly in proportion to the thickness.

The joint work of the National Physical Laboratory and the Building Research Station has made available much valuable information, and experiments have shown that discontinuity can be of great value, not only for airborne sound but especially where vibration is set up through impact. In a block of flats, for instance, where a false ceiling is provided, the provision of partitions separate from the main structure is of great value. In another example a ½-in. thick quilting of glass silk was laid on the structural concrete floor; on this building paper was laid before placing a 2-in. reinforced concrete floor, resting on the glass silk. By Q. B. Palkiner Nuttall.

near acoustics in Knolik


acoustichome
letter "A"
start from "AC"
acquaviva, claudius

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