300 



ARCHITECTURAL ACOUSTICS 



panel loud speaker is employed. At the present time the favored position 

 for the loud speaker appears to be in the instrument panel because in this 

 location the radio receiver, loud speaker, and controls may be combined 

 into a single compact unit. The distribution of sound is excellent in the 

 front seat and good in the back seat. The stiffness presented to the back 

 of the cone is small because the entire radio receiver case volume is used 

 to enclose the back of the loud speaker. Therefore, the response may be 

 maintained in the low frequency range. In order to reduce annoyance 

 from hiss generated in the receiver due to the relatively weak signals 



(r.0005 



..0004 



70 80 90 100 



AT 20" CENTIGRADE 



Fig. 12.13. Curves showing the absorption of a plane sound wave in passing through air, 

 at 20° C. for different frequencies, as a function of the relative humidity. The intensity 

 after a plane wave has travelled a distance x centimeters is /oe~'"^, where h is the intensity 

 at ;(: = and m is the coefficient given by the above graph. (After Knudsen.) 



delivered by the antenna, it is customary to attenuate the response above 

 4500 cycles. The low frequency response in reproduction is usually masked 

 at the higher speeds by wind noise and road rumble. Also see Sec. \\3A1. 

 L. Absorption of Sound in Passing Through Air. — The absorption ^^ of 

 a plane progressive sound wave in passing through air may be several times 

 that predicted by the classical theory. The anomalous absorption is 

 primarily dependent upon the humidity, although it is also affected by im- 

 purities such as H2O, H2, H2S and NH3. This, of course, means that there 

 may be considerable frequency discrimination of the reproduced sound in 

 large theatres where the sound travels a long distance. In addition, the 

 reverberation time will be reduced at the higher frequencies. The co- 



" Knudsen, V. D., Jour. Acous. Soc. Amer., Vol. 6, No. 4, p. 199, 1935. 



