International Journal of Comparative Psychology

Blue

monkeys (Cercopithecus mitis) were trained to detect

acoustic signals

embedded in noise. Masked thresholds were determined for four hu

consonant-vowel speech

sounds (6a, pa, ga, and ka), and four blue monkey

(boom,

pyow, chirp, and trill). The ability of monkey listeners to hear these

was compared with humans. Results showed that monkey and human

was

very similar. The mean difference between species for these eight stimuli in

broad-band

noise environment was 2.3 dB. The signal-to-noise ratio for

ranged from

4.8 dB for the ka to -23.8 dB for the boom. The four monkey calls

audible

at a signal-to-noise level that was 8.1 dB less than that required for the

the speech sounds. However, most of this effect was due to the audibility of

boom.

With the boom excluded, the mean signal-to-noise ratio for detection of the re

7

sounds was -0.5 dB, and the mean difference in the audibility of

and

monkey sounds within this set was 2.6 dB. These results contrast

findings

which used simulated rain forest noise as the masking noise (Brown, 1986).

rain

forest noise, test signals were audible at signal-to-noise ratios approximately

dB

less than those reported here, and the observed difference in the

human and monkey utterances was larger. These findings

variations in

the amplitude and spectrum of the ambient noise may have

influence

on the

audibility of vocal signals in nature.

a strong

suggest that rather small

relative audibility

of

10

In

with previous

the speech

maining

the

detection

of

were

perception

the

hearing

signals in

noise

calls

man

complex