Power Output and Efficiency of Receivers and Loudspeakers

Of the total effective power input to the diaphragm I²R_d, little is radiated in the form of sound waves. For some telephone receivers.

the average ratio of the acoustic power output to the electrical power input is below 1 per cent.⁴² Thus, if the input current of the receiver tested is 0.0025 ampere (a large input), the power input will be I²R = 0.0025² X 210 = 0.00131 watt, or 1310 microwatts. The value 210 ohms was obtained from Fig. 32 for a frequency of 820 cycles. Using an efficiency of 1 per cent, the power output in speech sounds would be 13.1 microwatts. This is a much larger output than under actual operating conditions. (Only about 10 microwatts of power is given out by the average voice in talking, page 30.)

A dynamic (moving-coil) loudspeaker will handle a continuous power input of about 5 watts. Auditorium dynamic loudspeakers will handle about 20 watts. Technical literature^4,3,44 contains widely differing figures for the efficiency of dynamic loudspeakers; the variation is not surprising because of the many types available, the various uses, and the difficulty in measuring efficiencies. Based on references 43 and 44, and other sources of information, it is estimated that a dynamic loudspeaker in a baffle or cabinet has an efficiency of 2.5 to 10 percent, that a dynamic loudspeaker in a directional baffle has an efficiency of 5 to 15 per cent, and that a dynamic loudspeaker in a short exponential horn has an efficiency of about 15 to 25 per cent. For the moving-coil dynamic unit driving a long exponential horn, the efficiency is estimated to be about 20 to 40 per cent. These units can handle a continuous power input of about 20 watts. It is emphasized that these are average estimated figures and that many exceptions exist. Standard methods have been devised² for determining the efficiency of loudspeakers.

In the design of sound systems for a theater⁴³ or an auditorium,⁴⁴ the acoustic power required must be known. The acoustic power requirements of auditoriums are given⁴⁴ as follows: 50,000 cubic feet, 1.2 watts; 100,000 cubic feet, 2,0 watts; 250,000 cubic feet, 4.0 watts; and 600,000 cubic feet, 8.0 watts. This same reference gives the equation

where V is the volume of the auditorium in cubic feet, 7 is the sound intensity in watts per square centimeter, and T is the reverberation time in seconds. Also, this reference states that for orchestral reproduction the level should be +100 decibels (10^-6 watt per square centimeter), based on the zero level of page 39, and that for speech the level should be +80 decibels (10^-8 watt per square centimeter). These data assume average noise conditions.