6. SOUND PRODUCTION AND DETECTION
173
system undoubtedly produced a complex of acoustic energy involving
both pressure waves and displacement. The Enger air-loudspeaker system
was even more complex acoustically, since the speaker was close enough
to the water surface to induce near field, and the energy from the back
of the speaker was not damped. The Jacobs-Tavolga system was based
essentially upon the suggestions of Parvulescu (1964, 1967) and van
Bergeijk (1967a), and was an attempt to attain a pure pressure field,
although its “purity” could not be measured. The Weiss system produced
a push-pull effect in the water and probably approached a pure near-field
condition.
Weiss (1967, 1969) described his apparatus as producing a “uniform
sound field,” but he presented no data to validate this point. If by uniformity he meant equal sound pressure at all points, this would be a
remarkable achievement, especially in a small tank. Although he made no
measurements of displacement, he claimed to be able to separate inner
ear from lateral line reception. By contrast, Cahn et al. (1969) obtained
separate pressure and velocity (displacement) measures in a tank similar
to Weiss’s, and they found variations in the pressure levels at digerent
part of the tank over a range of about 10 dB.
At lo00 Hz, Enger’s and Jacobs and Tavolga’s data were roughly in
agreement, but Weiss’ thresholds were about 20 dB higher. All the audiograms were essentially flat over the 20&1000-Hz range, except Enger’s
air-loudspeaker data, which showed a significant rise in threshold at 200
Hz. Above 1000 Hz, Enger obtained thresholds of about -10 dB pb at
5000 Hz, while the other two audiograms rose abruptly to about f 3 0
dB pb at 3000 Hz.
A factor in threshold studies that must be considered is the level and
spectrum of background noise in the experimental tank or chamber.
Tavolga (1967b) demonstrated that masking noise effects in fishes are
essentially similar to those in human hearing studies (Fletcher and
Munson, 1937). In general, if the thresholds are 10 dB or more above the
noise level in the region of the test frequency, the probability is high that
the threshold values obtained are not masked and are unaffected by the
ambient noise. The threshold data obtained by Jacobs and Tavolga
(1967) gave values that were at least 10 dB higher than the noise level
in the appropriate band, and at least 30 dB higher than the spectrum level
(in noise per cycle). Unfortunately, Enger (196713) and Weiss (1967)
gave no data on ambient noise levels.
With so many differences in conditioning techniques, acoustic circumstances, and, above all, the lack of comparable noise level data, it is not
possible at this time to bring all the above results into consonance. However, Wodinsky (1969) found that the high thresholds obtained by Weiss
173
system undoubtedly produced a complex of acoustic energy involving
both pressure waves and displacement. The Enger air-loudspeaker system
was even more complex acoustically, since the speaker was close enough
to the water surface to induce near field, and the energy from the back
of the speaker was not damped. The Jacobs-Tavolga system was based
essentially upon the suggestions of Parvulescu (1964, 1967) and van
Bergeijk (1967a), and was an attempt to attain a pure pressure field,
although its “purity” could not be measured. The Weiss system produced
a push-pull effect in the water and probably approached a pure near-field
condition.
Weiss (1967, 1969) described his apparatus as producing a “uniform
sound field,” but he presented no data to validate this point. If by uniformity he meant equal sound pressure at all points, this would be a
remarkable achievement, especially in a small tank. Although he made no
measurements of displacement, he claimed to be able to separate inner
ear from lateral line reception. By contrast, Cahn et al. (1969) obtained
separate pressure and velocity (displacement) measures in a tank similar
to Weiss’s, and they found variations in the pressure levels at digerent
part of the tank over a range of about 10 dB.
At lo00 Hz, Enger’s and Jacobs and Tavolga’s data were roughly in
agreement, but Weiss’ thresholds were about 20 dB higher. All the audiograms were essentially flat over the 20&1000-Hz range, except Enger’s
air-loudspeaker data, which showed a significant rise in threshold at 200
Hz. Above 1000 Hz, Enger obtained thresholds of about -10 dB pb at
5000 Hz, while the other two audiograms rose abruptly to about f 3 0
dB pb at 3000 Hz.
A factor in threshold studies that must be considered is the level and
spectrum of background noise in the experimental tank or chamber.
Tavolga (1967b) demonstrated that masking noise effects in fishes are
essentially similar to those in human hearing studies (Fletcher and
Munson, 1937). In general, if the thresholds are 10 dB or more above the
noise level in the region of the test frequency, the probability is high that
the threshold values obtained are not masked and are unaffected by the
ambient noise. The threshold data obtained by Jacobs and Tavolga
(1967) gave values that were at least 10 dB higher than the noise level
in the appropriate band, and at least 30 dB higher than the spectrum level
(in noise per cycle). Unfortunately, Enger (196713) and Weiss (1967)
gave no data on ambient noise levels.
With so many differences in conditioning techniques, acoustic circumstances, and, above all, the lack of comparable noise level data, it is not
possible at this time to bring all the above results into consonance. However, Wodinsky (1969) found that the high thresholds obtained by Weiss