Sound Propagation And Temperature

A final issue that needs to be discussed is the influence temperature has on the propagation of acoustic signals. Temperature gradients across upper forest canopies at dawn and dusk reduce the amount of excess attenuation in forests by trapping sound energy under the canopy (Waser and Waser, 1977; Wiley and Richards, 1978; van Staaden and Romer, 1997; Romer, 1998). The broadcast area of acoustic signals has been shown to be greatest during these times (Figure 7.8)

Distance (m)

FIGURE 7.8 Decrease in sound attenuation due to thermal inversion. The differences in temperature above and below the canopy cause sound to be reflected back toward the ground increasing the range of acoustic signals. Excess attenuation and upward-refracting sound conditions during the day (open circles) limits the transmission distance of the call while calling at night increases the transmission distance more than 10-fold due to the downward-refracting temperature inversions. (From van Staaden, M. J. and Romer, H., J. Exp. Biol., 200, 2597-2608, 1997. With permission from The Company of Biologists, Limited.)

Distance (m)

FIGURE 7.8 Decrease in sound attenuation due to thermal inversion. The differences in temperature above and below the canopy cause sound to be reflected back toward the ground increasing the range of acoustic signals. Excess attenuation and upward-refracting sound conditions during the day (open circles) limits the transmission distance of the call while calling at night increases the transmission distance more than 10-fold due to the downward-refracting temperature inversions. (From van Staaden, M. J. and Romer, H., J. Exp. Biol., 200, 2597-2608, 1997. With permission from The Company of Biologists, Limited.)

(Henwood and Fabrick, 1979; van Staaden and Romer, 1997). Young (1981) hypothesised that dawn-dusk chorusing in cicadas is an adaptation for communicatory optimisation. Thus the influence of temperature on sound transmission may have been selected for the dawn-dusk chorusing activity seen in many acoustic animals. The increased sound transmission at dawn and dusk has also been hypothesised to be a selective advantage for endotherms (Sanborn et al., 1995a, 1995b, 2003a, 2003b, 2004; Sanborn, 2000; Villet, et al., 2003).

The propagation of vibratory signals is also influenced by temperature. Sound propagation and communicatory distance of the vibratory signal of the mole cricket, Gryllotalpa major Suassure, was shown to vary with temperature (Hill and Shadley, 2001).

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