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Frequency in Cycles per Second Figure 4.
sound level of 77 phonsf the deviations were nowhere more than 3 phons. The final arrangement of the loudspeaker units and their adjustment resulted in a considerable improvement of the sound level through the frequency range. This result, based on theoretical calculation, is produced by the effect of concentration due to the adjustment of the loudspeaker units.
A comparison of the loudspeaker arrangement described above with that of an arrangement located at onequarter of the screen height without special adjustment of the loudspeaker direction may be of interest. Figure 4 shows the frequency characteristics taken at a distance of 262 ft from the loudspeaker units. Curve II refers to the loudspeaker arrangement at one-quarter the screen height while Curve I refers to the final arrangement. The data for both curves were obtained with the loudspeakers equally powered (50 v), corresponding to an amplification power
of about 40 w. Comparison of the two curves shows an increase in the sound level of about 8 phons within the range of 100-8000 cycles/sec. Concentration of the sound waves produced considerable improvement in acoustic efficiency and eliminated overloading at high sound levels, i.e. distortion.
In addition, it is evident from the two curves that a notable enlargement of the frequency band in both high and low ranges has occurred. If Ji and Jn be the amount of energy of sound at loudspeaker units I and II, the following equations apply:
101ogJr; -a
10 log ^ = a + 8 Jo
where Jo is the sound energy at threshold. Subtracting the first equation from the* second :
f The loudness level, in phons, of a sound is numerically equal to the sound pressure level in decibels, relative to 0.0002 microbar, of a simple tone of frequency 1000 cycles /sec which is judged by the listeners to be equivalent in loudness.
10 log ^ 10 log i
Jo J0
or 10 log ^
Jjl Ji
8 8
.8 6.3
Hans Simon: Open-Air Sound Theater
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