RESULTS OF THE WORLD'S FAIR HEARING TESTS 555 



telephone receivers in parallel on the output.^ Vertical cut phono- 

 graph records contained instructions for the test and the tones used 

 in the test. To insure a favorable ratio of signal to record and am- 

 plifier noise throughout the test, the test tones were recorded at con- 

 stant level, and the desired level changes were obtained by changes 

 in the attenuation network made in synchronism with the turntable. 



Output of each phonograph reproducer and amplifier was checked 

 daily, and held within limits which varied from ± 0.5 db at the lowest 

 frequency to ± 2 db at the highest frequency. Performance of the 

 attenuation networks was determined twice during the season by 

 careful measurements of voltage at each test level and each frequency. 

 They were found to give the expected values of attenuation within 1 db 

 at all levels. The efficiency of each receiver was measured on a rigid 

 closed coupler. The standard deviation of all the receivers used was 

 about 1 db at the lower frequencies and 3 db at 7040 cycles. Check 

 measurements were made at intervals of about one month on each 

 receiver. The mean response of all the receivers varied by less than 

 1 db during the season. The ten machines were alike in output 

 (at the test level nearest the reference level) within ± 1.0 db at the 

 three lower frequencies and within ±1.5 db at the two higher fre- 

 quencies. 



In addition to the above measurements, listening tests were made 

 daily by one of the engineers in charge of the equipment, and the girls 

 who conducted the test listened frequently throughout the day by 

 means of monitoring receivers. 



Hearing Test Results in Terms of Pressure 

 AND Intensity Level 



In order to compare the results of the Fair tests with other data on 

 hearing, calibrations have been made of the receivers that were used 

 in the tests. This was done by measuring the pressure levels developed 

 by the receiver at the opening of the ear canal for a small group of 

 people, using a special search tube transmitter so designed that the 

 tube could be inserted under the receiver cap into the opening of the ear 

 canal. Such a calibration gives an ear canal pressure level in terms 

 of receiver voltage levels. The authors are indebted to Mr. W. A. 

 Munson of these Laboratories for the calibrations. They are pre- 

 liminary in character and may need modification in the light of 

 subsequent studies. 



With the aid of these calibrations, ear canal pressure levels may be 



* F. A. Coles, "Hearing-Test Machines at the World's Fairs," Bell Laboratories 

 Record, 18: 290, June 1940. 



