TESTING OF LOUD SPEAKERS 265 



form over the entire audio frequency range considered. The attenuator 

 is adjusted to give the same amphtude of the wave pattern on the oscil- 

 lograph screen as is secured when the switch is thrown to the left and the 

 power is supplied to the loud speaker. With the switch in the latter 

 position the microphone picks up the sound and the wave form is repro- 

 duced upon the fluorescent screen of the oscillograph. The departure from 

 the pure sine wave is indicated readily by the difference in appearance of 

 the pattern from the pure sine wave form secured with the switch thrown 

 to the right. The extent of introduction of harmonics by the loud speaker 

 can be estimated from a slight, moderate or very marked change in the 

 wave form. 



D. Efficiency Frequency Characteristic^''. — The efficiency of a loud 

 speaker at any frequency is the ratio of the total useful acoustical power 

 radiated to the electrical power supplied to the load, the current wave of 

 which exercises a controlling influence on the wave shape of the sound 

 pressure. The plot of efficiency in per cent versus frequency is termed the 

 efficiency characteristic. 



The measurement of efficiency of a loud speaker may be divided into two 

 methods, direct and indirect. One direct method depends on measuring 

 the total energy flow through a spherical surface without reflections. 

 Several indirect methods have been developed. The most common of 

 these consists in measuring the electrical impedance under various con- 

 ditions of diaphragm loading. It has been found in practice that these 

 two methods of determining efficiency are those most widely used at the 

 present time. 



1. Direct Determination of Radiated Power ^''^. —The sound power 

 output from a speaker at a particular frequency may be obtained by 

 measuring the total flow of power through a spherical surface of which the 

 sound source is the center. The surface of the sphere is divided into incre- 

 mental areas and the power transmitted through each area is determined 

 from the sound pressure and the particle velocity as well as the phase 

 displacement between them. To simplify the process, the measurements 

 may be made at a distance sufficiently large so that these quantities are in 

 phase. Then, the radiated power may be determined by measuring the 

 sound pressure or particle velocity over each incremental area (assuming 

 the measuring equipment does not disturb the sound field and no standing 

 wave pattern exists). The total power is equal to the summation of the 



2^ Standards on Electroacoustics, Institute of Radio Engineers, 1938. 

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

 phia. 



