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
