Francesco Guarato

Features in geometric receiver shapes modelling bat-like directivity patterns

The directional properties of bat ears as receivers is a current area of interest in ultrasound research. This paper presents a new approach to analyse the relationship between morphological features and acoustical properties of the external ear of bat species. The beam pattern of Rousettus leschenaultiis right ear is measured and compared to that of receiver structures whose design is inspired by the bat ear itself and made of appropriate geometric shapes. The regular shape of these receivers makes it possible to control the key reception parameters and thus to understand the effect on the associated beam pattern of the parameters themselves. Measurements show one receiver structure has a beam pattern very similar to that of R. leschenaultiis ear, thus explaining the function of individual parts constituting its ear. As it is applicable to all bat species, this approach can provide a useful tool to investigate acoustics in bats, and possibly other mammals.

Reconstruction of the signal produced by a directional sound source from remote multi-microphone recordings

A mathematical method for reconstructing the signal produced by a directional sound source from knowledge of the same signal in the far field, i.e., microphone recordings, is developed. The key idea is to compute inverse filters that compensate for the directional filtering of the signal by the sound source directivity, using a least-square error optimization strategy. Previous work pointed out how the method strongly depends on arrival times of signal in the microphone recordings. Two strategies are used in this paper for calculating the time shifts that are afterward taken as inputs, together with source directivity, for the reconstruction. The method has been tested in a laboratory environment, where ground truth was available, with a Polaroid transducer as source. The reconstructions are similar with both strategies. The performance of the method also depends on source orientation.

An investigation of acoustic beam patterns for the sonar localization problem using a beam based method.

Target localization can be accomplished through an ultrasonic sonar system equipped with an emitter and two receivers. Time of flight of the sonar echoes allows the calculation of the distance of the target. The orientation can be estimated from knowledge of the beam pattern of the receivers and the ratio, in the frequency domain, between the emitted and the received signals after compensation for distance effects and air absorption. The localization method is described and, as its performance strongly depends on the beam pattern, the search of the most appropriate sonar receiver in order to ensure the highest accuracy of target orientation estimations is developed in this paper. The structure designs considered are inspired by the ear shapes of some bat species. Parameters like flare rate, truncation angle, and tragus are considered in the design of the receiver structures. Simulations of the localization method allow us to state which combination of those parameters could provide the best real world implementation. Simulation results show the estimates of target orientations are, in the worst case, 2° with SNR = 50 dB using the receiver structure chosen for a potential practical implementation of a sonar system.

Airborne broad-beam emitter from a capacitive transducer and a cylindrical structure

Beamwidth broadening of an ultrasonic air-coupled transducer is performed by an emitter constituted of an electrostatic transducer and of a cylinder with an opening at the top covering the surface of the transducer. The acoustic emission is thus forced through a hole smaller than the diameter of the transducers surface. In particular, a cylinder with an upper diameter of 10mm and a height of 5mm ensures the beam pattern of the final emitter is broad across a wide frequency range. Sound attenuation is reduced and lobes in the transducers beam pattern are cancelled. Beam broadening can improve range estimation techniques and ultrasonic sonar as a wider area can be inspected with one emission with no need for scanning.

Sonar localization using receiver beam profile features

A localization method for a sonar system equipped with an emitter and two receivers is presented in this paper. The signal reflected from the target is filtered through the receiver beam patterns whose knowledge is used to estimate the direction of the echo. Indeed, the known emitted signal and measurement of its travel time make it possible to compensate for distance and air absorption effects, and to calculate the ratio between the received echo and the original signal at all frequencies. Simulations with two beam patterns were conducted. A monotonic one over orientations was implemented, to show evidence that linearity reduces ambiguity: in this case, the method returns estimates with an error of less than 1.5° with SNR = 50 dB. If a bats beam pattern is adopted, most of orientations are estimated with an error less than 5°, even when SNR = 15 dB.

A method for estimating the orientation of a directional sound source from source directivity and multi-microphone recordings: principles and application.

Taking into account directivity of real sound sources makes it possible to try solving an interesting and biologically relevant problem: estimating the orientation in three-dimensional space of a directional sound source. The source, of known directivity, produces a broadband signal (in the ultrasonic range, in this application) that is recorded by microphones whose position with respect to source is known. An analytical method to process the recorded signals and estimate source orientation is developed in this paper. Experiments testing method performance in estimating source orientation were performed both in a laboratory environment with a Polaroid transducer as source and in a flight room with a Myotis daubentonii bat. In the first case, results showed the estimation method to be accurate and pointed out its limitations. The latter case is significant as an example biological application of the method for extracting behavioral features from bats; results are compared with alternative calculations based on microphone root-mean-square (rms)-pressure values.

The anti-bat strategy of ultrasound absorption: the wings of nocturnal moths (Bombycoidea: Saturniidae) absorb more ultrasound than the wings of diurnal moths (Chalcosiinae: Zygaenoidea: Zygaenidae)

ABSTRACT The selection pressure from echolocating bats has driven the development of a diverse range of anti-bat strategies in insects. For instance, several studies have proposed that the wings of some moths absorb a large portion of the sound energy contained in a bats ultrasonic cry; as a result, the bat receives a dampened echo, and the moth becomes invisible to the bat. To test the hypothesis that greater exposure to bat predation drives the development of higher ultrasound absorbance, we used a small reverberation chamber to measure the ultrasound absorbance of the wings of nocturnal (Bombycoidea: Saturniidae) and diurnal moths (Chalcosiinae: Zygaenoidea: Zygaenidae). The absorption factor of the nocturnal saturniids peaks significantly higher than the absorption factor of the diurnal chalcosiines. However, the wings of the chalcosiines absorb more ultrasound than the wings of some diurnal butterflies. Following a phylogenetic analysis on the character state of diurnality/ nocturnality in the Zygaenidae, we propose that diurnality in the Chalcosiinae is plesiomorphic (retained); hence, the absorbance of their wings is probably not a vestigial trait from an ancestral, nocturnal form but an adaptation to bat activity that overlaps their own. On a within-species level, females of the saturniids Argema mittrei and Samia cynthia ricini have significantly higher absorption factors than the males. In the female S. c. ricini, the higher absorption factor corresponds to a detection distance by bats that is at best 20-30% shorter than that of the male. Summary: Moth wings partly absorb the ultrasonic calls of bats to reduce predation. Different moths fly at night or day, and this work compares their absorption of ultrasound.

Directional properties of bat ears for target localization

Among several bat acoustic capabilities, it is reported that the error they show in discriminating the direction of a target in the elevation plane is within a few degrees. Many researchers have investigated bat echolocation and attempted to explain which features their accuracy relies on. Some works have pointed out the importance of beam patterns associated to the ears of the bats: indeed, ear shapes are very complex and different across species. This allows bats to significantly discriminate among the directions the echoes of their calls come from. Also, binaural cues for sound localization have been found to be determinant for some bat species. A method to estimate target orientation based on the beam patterns of the receivers and their binaural geometric relationship is developed for a sonar system emulating bats echolocation: a broadband signal is produced in the ultrasonic range and the echoes are collected by two receivers. The location of the target with respect to the bat is given by spherical coordinates including the distance from the bat and the orientation (defined by azimuth and elevation angles). While distance of the target from the sonar system is recovered through the time from the emission to the reception of the signal, the orientation is estimated from knowledge of the beam profiles of the receivers. The ratios between received and original signals’ spectra are calculated for the left and the right receiver across frequencies after compensating for distance effects. Orientations ensuring attenuation values in the beam patterns closest to the ratios are selected. Among these, one orientation for the left receiver and one for the right, fulfilling appropriate mutual geometric relations given by the geometry of the situation, are selected, these two defining the direction of the signal echo. The beam patterns associated to the left and the right ear of the Phyllostomus discolor bat are used within a limited range of orientations. A model of the emission of the signal and its reception is simulated for a set of target orientations spanning the beam pattern domain. Additionally, several noise levels are considered. Estimates of directions of the echoes are performed and the error for each of them with respect to the true orientation is calculated. Most error values are comparable to those reported in the literature and associated to some bat species. The beam patterns have very irregular shapes over the set of orientations that change with the frequency range: indeed, the accuracy of the method in estimating the true orientation depends on the choice of the frequency range and on the area of orientations over which the beam pattern is defined. This approach makes it possible to investigate which acoustic features ensure good orientation estimates in order to explain why bats have such complex beam patterns filtering the echoes. As this method relies only on knowledge of the receivers’ beam patterns and geometric relations based on the binaural configuration, it appears feasible that bats utilise this effect.