TESTING OF TELEPHONE RECEIVERS 269 



apparatus under test is fed to a cathode ray oscillograph. The deviation 

 from the square wave is shown on the screen of the cathode ray oscillo- 

 graph. Square waves offer a simple and rapid method of including both 

 phase shift and amplitude response in a single test. 



H. Subjective Measurements. — In many cases the apparatus for mak- 

 ing all the objective tests outlined in the preceding sections is not always 

 available. Furthermore, there is always some difficulty in evaluating the 

 objective measurements. For this reason a subjective test of efficiency, 

 frequency response, directional characteristics, nonlinear distortion and 

 transient response in which two or more loud speakers are compared with a 

 reference loud speaker system, is widely used. Or, in other words, this 

 test determines the lumped effects of the following factors: loudness, fre- 

 quency range, tone balance, spacial distribution, quality and hangover. 

 The electrical input is usually broadcasting program material such as 

 speech or music. The inputs should be adjusted until the reference and 

 test loud speakers are judged to have equal loudness. The required atten- 

 uation of the electrical input determines the relative loudness efficiency 

 of the loud speaker under test. The loud speakers should occupy positions 

 which are symmetrical with respect to the room boundaries and the 

 observer. The loud speakers should be sufficiently separated that inter- 

 action is negligible. A number of observers and a variety of program 

 material should be used in order to insure statistical significance. 



11.4. Testing of Telephone Receivers. — ■ The characteristic of a tele- 

 phone receiver should show the performance as normally worn on the ear. 

 The sound intensity produced in the ear by the telephone receiver should 

 be the same as the intensity produced in the ear when the head is immersed 

 in the original sound field. There are two types of measurements upon 

 telephone receivers, namely: subjective and objective. 



A. Subjective Measurements 29.30,304 . — a free progressive sound wave is 

 established by means of a loud speaker driven by an oscillator. The sound 

 pressure at a distance of five feet from the loud speaker is measured by 

 using a calibrated microphone, amplifier and output meter. With the 

 receivers removed, the observer listens to the sound at the point where the 

 sound pressure was measured by the microphone. Next, the observer 

 places the receivers upon his head and the output of the oscillator is trans- 

 ferred from the loud speaker to the receivers by means of a suitable atten- 



ds Olson and Massa, Jour. Acous. Soc. Amer., Vol. 6, No. 4, p. 250, 1935. 

 30 Olson, H. F., Jour. S. M. P. £., Vol. 27, No. 5, p. 537, 1936. 

 3°^ Olson and Massa, "Applied Acoustics," P. Blakiston's Son and Co., Philadel- 

 phia. 



