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It has long been recognized that the phenomenon of reverberation may affect the quality of musical tones produced in a room. In the open air, at a distance from any reflecting surface and at short distances from the source of sound it may safely be assumed, I think, that the wave in the air is identical with the wave form produced by the source. That is to say the quality of the tone does not materially change as the wave
advances. But in a room, the resultant vibration at any point is that due to the sound coming directly from the source combined with that which has been reflected from the bounding surfaces of the room.. Now, if these reflecting surfaces reflect all the components of the complex tone in the same degree, the quality of the reflected sound will be identical with that of the direct sound. But as a matter of fact, practically all materials are selective to a greater or less degree, so that, theoretically, at least, the quality of reverberant sound is altered by the room.
Fig. 2 shows the absorption coefficients of hair felt of different thicknesses from 12" to 3" over the frequency range from 128 to 4096 vibrations per second. It will be observed there is a maximum of absorption that shifts to lower frequencies as the thickness of the material is increased. The ratio of the maximum to the minimum absorption is 6.2 for 1" material. This ratio is reduced to 2.5 when we get to a thickness of 3 ". This selective absorption is characteristic of all porous material. One obvious method of ironing out the curve is to increase the thickness. This however is an expensive procedure and as a practical matter is sometimes difficult. It is also possible to reduce the peak absorption relatively to the absorption at other frequencies by surfacing the porous material with a flexible though impervious membrane. Fig. 3 shows the effect of cementing a painted fabric to the surface of 1 " felt. This however, has the disadvantage of producing a decrease in the general efficiency of the material.
The effect of this selective absorption is pronounced in rooms in which the major portion of the total absorbing power in supplied by a selectively absorbing material. The high pitched components of music
Fig. 2
Absorption coefficients of felt of various thicknesses.
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