178 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 32 



be of the order of 16,000 cycles per second for the summation-tone 

 method and 11,000 for the difference-tone method. (Moore and Cur- 

 tis, Bell System Techn. Journ., 1927.) 



Griitzmacher (Elek. Nach. Tech., 1927; Zeit. Tech. Phys., 1929) 

 uses, instead of a mechanical resonator, a low-pass filter arranged 

 to transmit frequencies less than about 30 cycles per second (fig. 

 6). It follows that, in the great majority of cases, both the search 

 tone and all the summation tones are ruled out, and only those differ- 

 ence tones with frequencies less than 30 cycles per second will pass 

 the filter and be recorded by an appropriate amplifier and detector. 

 Thus, as the search frequency is continuously varied, the detector 

 will only respond when the frequency is within 30 cycles per second 

 of that of a constituent tone of the noise. The magnitude of the 

 detector reading can be made to afford a measure of the intensity of 



An.pli? 



MicropVione (exposed l"o 

 i>oise under <iiiali|sis) 



Scurch 

 Frcaucncii 

 Gcncrcil'oi- 



Low Pass 

 "iFill'cr.SO'V 



Aiiipli{ 



Dcl-ecCor^^ 



Figure 6. 



-Griitzmachcr's search-tone metbod of analyzing complex 

 sounds 



the component in question. In the outfit at the National Phj^sical 

 Laboratory the frequency of the search tone is varied from, say, 30 

 to 10,000 cycles per second by the rotation of an air condenser 

 through 180°. 



AURAL MEASUREMENT OF NOISE 



As we have already seen, if we are provided with a standard pure 

 note of medium frequency (above 700 cycles per second), the loud- 

 ness of which is variable at will over a range which has been cali- 

 brated by physical means, then we can evaluate by aural matching 

 or equality the loudness of any other pure note or, in general, of any 

 complex note. Alternatively, measurements may be made of the 

 loudness of the standard note which is just masked or drowned by 

 the sound to be measured. 



Figure 7 shows schematically three types of audiometers wliich all 

 work on the above principle and have been much used for loudness 

 measurements at the National Physical Laboratory and elsewhere. 



In the Siemens Barkhausen audiometer" a standard note of about 

 800 cj^cles and of a high degree of purity is produced bj' an electric 



* Barkhausen, Zeit. Tech. Phjs. vol. 7. p. 599, 1926. 



