Page 32 - Summer2017

Insect Bioacoustics
Mole crickets (Figure 1E) lack an additional anatomical structure to boost their acoustic output. Rather, they con- struct and sing from burrows that are “designed” (by evolu- tion) to enhance the radiation of the song. The burrows con- sist of two acoustically important components: a bulbous chamber linked by a short constriction to an approximately exponential horn (or in some cases, twin horns) that couples the chamber to the outside. The singing male positions him- self near the constriction with his elevated wings a few mil- limeters into the throat of the horn (Figure 4).
The acoustical properties of this system were probed in mod- el burrows of the species Gryllotalpa australis by replacing the male with a dipole sound source mimicking his vibrating wings (Daws et al., 2012). Both the bulbous chamber and the horn resonate at frequencies similar to the male’s song frequency (2.7 kHz). Measurements of sound pressure at various locations within the burrow showed that the struc- ture supports a standing wave in which sound in the bulb and in the horn are out of phase with a null, where sound pressure is minimal, at the constriction. The opposite phases in bulb and horn reflect the dipole nature of the cricket’s elevated wings, whereby acoustic compression on one side of the vibrating wings is accompanied by rarefaction on the other side. The length of the bulb, about 26 mm, is close to one-quarter the wavelength of the song. As a result, sound that travels from its origin to the rear wall of the bulb and back again (which will thus have traveled half a wavelength) will arrive nearly in phase with sound in the horn, to which it can add constructively. This is not unlike the manner in which the notes produced by brass instruments are deter- mined by their effective lengths (Moore, 2016).
A second acoustically important feature of the burrow is the nearly exponential increase in horn diameter from its nar- row end, where sound is generated, to its opening at the sur- face. As mentioned earlier, the efficiency with which acoustic energy is transferred from its source to the air depends on the relationship between source size and sound wavelength. The increase in effective source area afforded by the horn helps the insect to overcome the impedance mismatch be- tween its wings (ca. 1 cm) and the wavelength of its relatively low-frequency song (12.6 cm). In essence, the male is sing- ing through a megaphone. Singing from a burrow results in a gain in sound pressure of up to 24 dB compared with sing- ing in free air (Bennet-Clark, 1987).
Males construct their burrows incrementally while test- ing the results of their efforts along the way; they dig for a
few minutes, emit a few chirps, dig and shape some more, test again, etc., with the entire process taking up to an hour (Bennet-Clark, 1987). The performance of the burrow, as indicated by the power of the radiated sound, improves throughout this process, although how the male monitors this remains unclear (because he is inside the burrow, near the acoustic null, he cannot hear the radiated sound).
Baffles
Because cricket wings are dipole sound sources, their output is susceptible to acoustic “short-circuiting,” whereby com- pressed air on one side of the vibrating wings flows around the edge of the wing to the rarefied side rather than radiating away from the animal. Short-circuiting occurs when the di- ameter of the source (for crickets, 1 cm or so) is less than half the wavelength of the sound (typically 6-7 cm or greater for crickets); the smaller the source relative to wavelength, the more pronounced the effect (Beranek and Mellow, 2012). In mole crickets, the position of the wings at the constriction between bulb and horn helps to minimize short-circuiting. Field crickets often sing from the narrow entrance to a bur- row, which can help to reduce short-circuiting in a manner analogous to the cabinet in which a loudspeaker might be mounted. Tree crickets (Figure 1F), a subfamily of insects distinct from the more familiar field crickets, use leaves as acoustic baffles to minimize short-circuiting. Males of some species sing from the edge of a leaf, from a notch in the leaf surface, or from the junction between two leaves, orienting their bodies so that their wings are coplanar with the leaf surfaces (Forrest, 1982). The leaves extend the effective size of the male’s wings, making it more difficult for the opposing sound pressures on the two sides to cancel one another. The use of baffles can increase radiated sound pressure by up to 10 dB. Go to https://vimeo.com/120108754 for a video of a male singing from a leaf junction.
Males of some species actually construct baffles by chewing a hole in a leaf (Prozesky-Schulze et al., 1975). When sing- ing, the male positions himself so that his raised wings fill the hole. The sizes of the leaf and hole and the position of the hole within the leaf determine the effectiveness of the baffle. In at least one species of tree cricket, Oecanthus henryi, the nervous system seems to be hardwired for optimizing these parameters. Modeling of the acoustics of the system shows that for maximal effectiveness males should build baffles in large leaves, the size of the hole they excavate should match the area of their wings, and the hole should be positioned centrally within the leaf rather than near its periphery. O.
30 | Acoustics Today | Summer 2017