The substitution method was adopted from industrial acoustics (Francois and de Montussaint 1972) to “eliminate the influence of the environment'' on measurements of the amplitude of vocalizations given by blue monkeys Cercopithecus mitis and grey-cheeked mangabeys Cercocebus albigena. Measurements were conducted of sound power and sound pressure level of representative utterances. Monkey vocal radiation patterns were also measured. The results showed that vocal amplitude ranged from 62 dB to 100 dB in sound pressure (re l pw). At a distance of 2 m, the loudest calls approached an amplitude of 110 dB SPL, a level about equal to the loudest human yell. The measurements of call amplitude conducted here exceeded those derived from the field by approximately 10 dB. It was shown that the discrepancy in amplitude between these laboratory based measurements and earlier measurements conducted under field conditions (Waser and Waser 1977) was probably due to destructive interference between the direct wave and the “ground wave'', a phase shifted wave reflected from the ground. Measurements of radiation patterns of primate vocalizations showed that, like human speech, directivity was a function of frequency, with high-frequency components being radiated mote directionally than lower-frequency components. However, primate utterances were in general radiated more omnidirectionally than was human speech.
Brown, C.H. (1989). The measurement of vocal amplitude and vocal radiation pattern in blue monkeys and grey-cheeked mangabeys. Bioacoustics 1(4): 253-271