276 MEASUREMENTS 



be modified by a factor in computing the reverberation time of a room. 

 It may be said, however, that this data indicates the relative efficiency of 

 the various materials. 



11.10. Measurement of Transmission Coefficient. — The transmission 

 coefficient of a partition or wall is defined as the ratio of the transmitted 

 sound energy to the rate of the incident flow of sound energy. The sound 

 insulating properties of a partition consist of a determination of its trans- 

 mission coefficient. In a general way, the noise reduction caused by a 

 particular structure may be obtained by measuring the difference in level 

 of a sound source with and without the intervening partition. For defi- 

 nition and equations relating to transmission loss and reduction factor See 

 Sec. 12.2M. The sound source should be fed with a warbled frequency or 

 rotated in a circle to average out reflection errors. The noise meter, Sec. 

 11.7, or, as a matter of fact, any of the sound measuring system, Sec. 11.3yf2, 

 may be used to measure the sound reduction. In some cases it may be 

 desirable to use two or more samples of diff"erent shapes and areas. When- 

 ever possible the measurements should be made under operating conditions. 

 For the transmission characteristics of various structures see Table 12.3, 

 Sec. 12.2M. 



11.11. Audiometry*^' ^^. — The acuity of hearing is measured by an 

 audiometer. The audiometer consists of an audio oscillator for gener- 

 ating pure tones, an attenuator calibrated in decibels and a telephone 

 receiver, Fig. 11.19. The usual range of test tones are 128, 256, 512, 



AUCMO 

 OSCILLATOR 



ATTENUATOR 



0] 



TELEPHONE 

 RECEIVER 



Fig. 11.19. Schematic arrangement of the components of an audiometer. 



1024, 2048, 4096, 8192 cycles per second. The tone generated in the ear 

 phone should be reasonably free from harmonics. The telephone receiver 

 is calibrated as outlined in Sec. 11.4. The reference level is the normal 

 threshold of audibility, Fig. 13.1. This level is the zero level of the 

 audiometer. The person to be tested wears the earphone in the normal 

 manner and the level at which the sound is no longer audible is noted on the 

 attenuator. A person with normal hearing will show no hearing loss, while 

 the person who is hard of hearing will show a hearing loss. These results 



^^ Fletcher, H., " Speech and Hearing," D. Van Nostrand Co., New York. 

 •"^ Proposed Specifications for Audiometers for General Diagnostic Purposes, 

 Jour. Acous. Soc. Amer., Vol 9, No. 1, p. 72, 1937. 



