predictions. The fundamental frequency of midshipman calls hovers near
100 Hz (Fig. 2.10), which is close to the predicted cutoff frequency for calls
in this environment (Bass unpublished).
Crawford et al. (1997) report a transmission loss close to 16 dB per distance doubling (about 50 log 10 (r)) for the naturally generated sounds of
weakly electric mormyrids (Pollimyrus isidori) in 2–3 m water of a freshwater flood plain with a dense clay-like bottom in Mali, West Africa (water
temperature approximately 28°C). This value, like that for the calls of
toadfish, far exceeded the predicted values for either spherical or cylindrical spreading, in both cases likely resulting from the combined influence of
very shallow depths and substrate composition. Crawford et al. (1997) estimated the cutoff frequency as 335 Hz at 2 m for mormyrids in this habitat.
As the authors note, the fundamental frequency of many of the mormyrid
sounds is close to 300 Hz and so is not expected to propagate very far except
when the water depth exceeds 2 m.
Mann and Lobel (1997) studied the propagation of naturally generated
courtship sounds of another marine species, the damselfish (Dascyllus
albisella), at water depths of 7 m; the substrate was a mix of sand, shells, live
coral, and coral rubble. As expected, transmission loss for the two highest
frequency components (398 and 501 Hz) of this species’ pulse-like calls was
the most within the first 4 m from the reference point and showed little
attenuation at distances up to 11–12 m. The two lowest frequency components (251 and 316 Hz) continued to attenuate beyond 4 m. Signal attenuation approximated theoretical losses that were intermediate between
cylindrical and spherical spreading.
A recent study of sound propagation in bullfrogs in a freshwater pond
also provides some basic information on underwater sound transmission
(Boatright-Horowitz et al. 1999). Sound propagation was studied in three
different ponds with depths of 1.25 m (mud bottom covered with detritus),
2.0 m (sand bottom), and 8.0 m (sand and stone bottom). For pure tones, the
cutoff frequency was 1.8–2 kHz at a depth of 1.25 m and 1.6–1.8 kHz at
depths of either 2 m or 8 m. Transmission loss in the 2-m and 8-m ponds was
consistent with theoretical values for loss due to cylindrical spreading,
whereas it was much greater than predicted in the 1.25-m pond. As the
authors point out, the latter was likely due in part to the “softer” bottom of
the 1.25-m pond. Bullfrog advertisement calls underwent losses that
approached values predicted for spherical rather than cylindrical spreading.
Together, the variance in the results across the range of studies discussed
above is consistent with the predicted influence of depth and bottomsubstrate composition on signal transmission.
12.3. Increasing Signal Detection
Wiley (1983) points out a number of traits that can lead to the increased
“reliability of detection” of signals, including small repertoires and redundancy. Both of these traits apply to teleosts such as the plainfin midship2. Physical Acoustics of Underwater Sound Communication
51
100 Hz (Fig. 2.10), which is close to the predicted cutoff frequency for calls
in this environment (Bass unpublished).
Crawford et al. (1997) report a transmission loss close to 16 dB per distance doubling (about 50 log 10 (r)) for the naturally generated sounds of
weakly electric mormyrids (Pollimyrus isidori) in 2–3 m water of a freshwater flood plain with a dense clay-like bottom in Mali, West Africa (water
temperature approximately 28°C). This value, like that for the calls of
toadfish, far exceeded the predicted values for either spherical or cylindrical spreading, in both cases likely resulting from the combined influence of
very shallow depths and substrate composition. Crawford et al. (1997) estimated the cutoff frequency as 335 Hz at 2 m for mormyrids in this habitat.
As the authors note, the fundamental frequency of many of the mormyrid
sounds is close to 300 Hz and so is not expected to propagate very far except
when the water depth exceeds 2 m.
Mann and Lobel (1997) studied the propagation of naturally generated
courtship sounds of another marine species, the damselfish (Dascyllus
albisella), at water depths of 7 m; the substrate was a mix of sand, shells, live
coral, and coral rubble. As expected, transmission loss for the two highest
frequency components (398 and 501 Hz) of this species’ pulse-like calls was
the most within the first 4 m from the reference point and showed little
attenuation at distances up to 11–12 m. The two lowest frequency components (251 and 316 Hz) continued to attenuate beyond 4 m. Signal attenuation approximated theoretical losses that were intermediate between
cylindrical and spherical spreading.
A recent study of sound propagation in bullfrogs in a freshwater pond
also provides some basic information on underwater sound transmission
(Boatright-Horowitz et al. 1999). Sound propagation was studied in three
different ponds with depths of 1.25 m (mud bottom covered with detritus),
2.0 m (sand bottom), and 8.0 m (sand and stone bottom). For pure tones, the
cutoff frequency was 1.8–2 kHz at a depth of 1.25 m and 1.6–1.8 kHz at
depths of either 2 m or 8 m. Transmission loss in the 2-m and 8-m ponds was
consistent with theoretical values for loss due to cylindrical spreading,
whereas it was much greater than predicted in the 1.25-m pond. As the
authors point out, the latter was likely due in part to the “softer” bottom of
the 1.25-m pond. Bullfrog advertisement calls underwent losses that
approached values predicted for spherical rather than cylindrical spreading.
Together, the variance in the results across the range of studies discussed
above is consistent with the predicted influence of depth and bottomsubstrate composition on signal transmission.
12.3. Increasing Signal Detection
Wiley (1983) points out a number of traits that can lead to the increased
“reliability of detection” of signals, including small repertoires and redundancy. Both of these traits apply to teleosts such as the plainfin midship2. Physical Acoustics of Underwater Sound Communication
51