Airborne Insects As Potential Sources of Acoustic Reflectivity
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538 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 14 Airborne Insects as Potential Sources of Acoustic Re¯ectivity J. R. RILEY AND A. S. EDWARDS Natural Resources Institute Radar Unit, Great Malvern, United Kingdom 21 July 1995 and 1 April 1996 ABSTRACT Laboratory measurements of the acoustic backscattering cross sections of a variety of insects at 3.4 kHz are reported. The authors argue from these data, and from what is known (from radar studies) about insect con- centrations in the lower atmosphere, that acoustic returns from insects may seriously contaminate sodar data. They suggest that this possibility should be carefully considered when interpreting backscatter coef®cient data acquired in acoustic sounding experiments. 1. Introduction that the diurnal and seasonal variation of the phenom- enon indicated that the observed dot echoes were more a. Radar ``angels'' probably produced by insects, and the acoustic echoes In the early days of radar, operators occasionally re- certainly appeared to be similar to those previously at- ported anomalous echoes from clear air for which there tributed to ¯ying grasshoppers by Cronenwett et al. was no obvious cause (see references in Crawford (1972). More recently, De et al. (1994) have also sug- 1949). These mysterious echoes that sometimes ap- gested that the dot echoes detected by their sodar were peared as point targets and sometimes as extended struc- of atmospheric origin. The purpose of this paper is to tures and in expanding rings, came to be known as radar demonstrate that airborne insects may on occasion be ``angels'' and formed the subject of much ingenious sources of both acoustic dot echoes and of distributed speculation (Gordon 1949). In particular, it was hy- acoustic re¯ectivity, and to suggest that this possibility pothesized that angels indicated the presence of micro- should be carefully considered when sounder echoes are refractive structures in the atmosphere, associated with being interpreted. The paper follows an earlier note on turbulent layers and with convective bubbles (Plank the same topic (Riley 1994). 1958). It eventually became clear, however, that as Crawford (1949) had originally suggested, most angels seen on ordinary radars could be con®dently attributed 2. Insects as acoustic re¯ectors to either airborne insects (Campistron 1975) or to birds (Pollen 1972; Eastwood 1967). Specialist, highly sen- a. Bat studies sitive radars do indeed produce spectacular displays of atmospheric refractive-index gradients (e.g., Browning It is well known that many species of insectivorous and Watkins 1970; Jordan and McLaughlin 1992), but bat locate their prey by acoustic sounding, and that air- these do not appear as point targets. borne insects must therefore be effective re¯ectors of sound, at least at the frequencies used by bats (typically 30±80 kHz). Laboratory backscattering studies designed b. Sodar angels to investigate the sonar mechanisms of hunting bats During the sixth ISARS meeting in Athens in 1992, have focused primarily on the temporal variation of Pahwa et al. (1992, unpublished manuscript) described 80-kHz echoes from ¯ying insects (Roeder 1963; Kober some sodar observations of dot echo structures and pos- and Schnitzler 1990), and there appear to be no mea- tulated that the dots might indicate the presence of mi- surements reported in the literature of insect acoustic crostructures in the lower atmosphere. It seemed to us backscattering cross sections at sodar frequencies (1.5± 4 kHz). We have therefore made some measurements in this frequency range of the acoustic cross sections of several species of moth and of a locust. Cross sections Corresponding author address: Dr. J. R. Riley, NRI Radar Unit, were measured with the insects presenting their under- North Site, Leigh Sinton Rd., Great Malvern, Worcestershire WR14 ILL, United Kingdom. side aspect, that is, the attitude from which they would E-mail: [email protected] be viewed when over¯ying a sodar. q1997 American Meteorological Society Unauthenticated | Downloaded 09/25/21 06:23 PM UTC JUNE 1997 NOTES AND CORRESPONDENCE 539 ®ed and bandpass ®ltered, and the returns from cali- bration targets and from insects were displayed on an oscilloscope and recorded. The insects were either alive during the experiments and made comatose by cooling, or were freshly killed (by freezing). Signal amplitude was calibrated in terms of cross section by using the expression 25 s5 K42pr, b 129 (where K 5 2p/l, r is the sphere radius, and l the acoustic wavelength) to estimate the acoustic backscat- tering cross section sb of the calibration spheres (Riley 1994). This expression is accurate only for spheres that are small enough to lie in the Rayleigh scattering region, that is, where 2pr/l K 1. For a frequency of 3.4 kHz FIG. 1. Experimental layout. The transmitting antenna was a 0.6- this corresponds to diameters much less than 3.6 cm. m-diameter paraboloid, ®tted with a 3.4-kHz 0.5-W transducer at the focus and with a 1.2-m-long cylindrical shroud. The receiver was an We used calibration spheres with diameters in the range unshrouded 0.6-m paraboloid with a miniature microphone (sensitiv- 1.91±1.26 cm, which barely meet this criterion, but are ityÐ64 dB at 1 kHz, 0 dB 5 1Vmb21) at the focus. Both antennas close enough to it for the equation to produce reasonable pointed horizontally over the smooth, concrete ¯oor to the measure- estimates of cross section. ment point 6 m in front of the them, where calibration spheres and insect targets were placed. c. Results The results of the measurements are summarized in b. Experiments Table 1, together with estimates of backscattering cross sections at the commonly used sodar frequency of 1.5 The experiments were carried out in a large (9.5 m kHz, derived by applying the scaling factor (1.5/3.4)4 3 11.3 m 3 4.6 m) anechoic enclosure with a ¯at and to the cross-sectional data measured at 3.4 kHz. very smooth concrete ¯oor. The equipment was ar- Radar backscattering cross sections of insects have ranged so that the ¯oor acted as the mirror in an acoustic been found to be rather similar to those of water spheres analog of the Lloyd's mirror arrangement (Longhurst of equivalent mass (Riley 1985), and so it was of interest 1963). This arrangement had two advantages. First, the to see if there were an analogous relationship for acous- insects and calibration spheres could be simply placed tic cross sections. Our estimates of insect scattering on the ¯oor in front of the acoustic source and receiver cross sections at 1.5 kHz have therefore been plotted as (Fig. 1), obviating the need for elaborate anechoic sup- a function of their mass in Fig. 2, together with the port structures. All that was needed to hold the insects acoustic backscattering cross section calculated for wa- in place was a single, vertical pin, held upright by a ter spheres, also plotted as a function of mass (Riley small piece of Plasticine ®xed to the ¯oor. Second, un- 1994). It can be seen that the insect underside aspect like the electromagnetic case, acoustic power density is cross sections do indeed increase with mass in a similar a maximum at the mirror surface (approximately four way to those of water spheres, although the insect values times the free space value), so targets on it are optimally are some 10 dB higher. It is to be emphasized that insect placed to act as re¯ectors. Calculations of power density acoustic cross sections, like their radar analogs, will be as a function of height above the ¯oor showed that the aspect sensitive, and that the ``water sphere plus 10 dB'' ®rst interference minimum was at 30 cm, and that over relationship is presented here only as a convenient the body length of the largest insects (5 cm) illuminating means of estimating the effect that airborne insects may power varied by less than 10%. Because the acoustic have on vertically looking acoustic sounders. We found that, unlike the case with radar (Riley 1973), the wings echoes from the insects were expected to be very weak, of the insects made a major contribution to their cross we used a close working range (6 m), and this dictated section, a result similar to that found in the 80-kHz the use of separate (bistatic) transducers for transmission backscatter experiments (Roeder 1963; Kober and and reception, so our results do not correspond exactly Schnitzler 1990). Re¯ections from moth wings were to monostatic backscattering cross sections. However, higher than those from locust wings. the angle subtended between the transducers at the target position was small (68), so the difference is unlikely to be signi®cant. Because the transmitter transducer avail- 3. Sodar returns from insects ¯ying in the lower able for the experiments operated most ef®ciently at 3.4 atmosphere kHz, our measurements were made at this frequency (l a. Dot echoes from individual insects ø 10 cm). The pulse length was set at 3 ms. Cronenwett et al. (1972) deduced that the dot echoes The signal from the receiver microphone was ampli- detected at an altitude of 100±250 m above their 1.5- Unauthenticated | Downloaded 09/25/21 06:23 PM UTC 540 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 14 TABLE 1. Measurements of the acoustic backscattering cross sections of various insects at 3.4 kHz. The measurements were made using the apparatus shown in Fig. 1 and by comparing the amplitude of the signals returned from the insects with those returned from a series of solid spheres. We also list in the table estimates of insect backscattering cross sections at the frequency of 1.5 kHz commonly used in sodars.