Stabilization of perceived echo amplitudes in echolocating bats. II. The acoustic behavior of the big brown bat, Eptesicus fuscus, when tracking moving prey.

Big brown bats, Eptesicus fuscus, can be trained to use echolocation to track a small microphone with a food reward attached when it is moved rapidly toward them. This situation mimics prey interception in the wild while allowing very precise recording of the sonar pulses emitted during tracking behavior. The results show that E. fuscus intensity compensates, reducing emitted intensity by 6 dB per halving of target range so that the intensity incident upon the target is constant and echo intensity increases by 6 dB per halving of range. This increase in echo intensity is effectively canceled by the reduction in auditory sensitivity due to automatic gain control (AGC) of 6 to 7 dB per halving of range. Intensity compensation behavior and AGC therefore form a dual-component, symmetrical system that stabilizes perceived echo amplitudes during target approach. The same system is present in the fishing bat, Noctilio leporinus, suggesting that it may be widespread in echolocating bats. Correlation analysis shows that, despite large changes in the duration of the pulses emitted by E. fuscus during an approach, the pulse frequency structure is such that the spatial image of the target perceived along the range axis is highly stable. Pulse duration is not reduced in the manner theoretically necessary to eliminate potential echo distortion effects due to AGC, but is reduced in such a way that this distortion is insignificant. During the terminal buzz, a high degree of temporal overlap (relative to pulse duration) occurs between emitted pulse and returning echo.     

Echo-intensity compensation in echolocating bats (Pipistrellus abramus) during flight measured by a telemetry microphone

An onboard microphone (Telemike) was developed to examine changes in the basic characteristics of echolocation sounds of small frequency-modulated echolocating bats, Pipistrellus abramus. Using a dual high-speed video camera system, spatiotemporal observations of echolocation characteristics were conducted on bats during a landing flight task in the laboratory. The Telemike allowed us to observe emitted pulses and returning echoes to which the flying bats listened during flight, and the acoustic parameters could be precisely measured without traditional problems such as the directional properties of the recording microphone and the emitted pulse, or traveling loss of the sound in the air. Pulse intensity in bats intending to land exhibited a marked decrease by 30 dB within 2 m of the target wall, and the reduction rate was approximately 6.5 dB per halving of distance. The intensity of echoes returning from the target wall indicated a nearly constant intensity (-42.6 +/- 5.5 dB weaker than the pulse emitted in search phase) within a target distance of 2 m. These findings provide direct evidence that bats adjust pulse intensity to compensate for changes in echo intensity to maintain a constant intensity of the echo returned from the approaching target at an optimal range.     

Spatial perception and adaptive sonar behavior

Bat echolocation is a dynamic behavior that allows for real-time adaptations in the timing and spectro-temporal design of sonar signals in response to a particular task and environment. To enable detailed, quantitative analyses of adaptive sonar behavior, echolocation call design was investigated in big brown bats, trained to rest on a stationary platform and track a tethered mealworm that approached from a starting distance of about 170 cm in the presence of a stationary sonar distracter. The distracter was presented at different angular offsets and distances from the bat. The results of this study show that the distance and the angular offset of the distracter influence sonar vocalization parameters of the big brown bat, Eptesicus fuscus. Specifically, the bat adjusted its call duration to the closer of two objects, distracter or insect target, and the magnitude of the adjustment depended on the angular offset of the distracter. In contrast, the bat consistently adjusted its call rate to the distance of the insect, even when this target was positioned behind the distracter. The results hold implications for understanding spatial information processing and perception by echolocation.     

Flying big brown bats emit a beam with two lobes in the vertical plane

The sonar beam of an echolocating bat forms a spatial window restricting the echo information returned from the environment. Investigating the shape and orientation of the sonar beam produced by a bat as it flies and performs various behavioral tasks may yield insight into the operation of its sonar system. This paper presents recordings of vertical and horizontal cross sections of the sonar beam produced by Eptesicus fuscus (big brown bats) as they fly and pursue prey in a laboratory flight room. In the horizontal plane the sonar beam consists of one large lobe and in the vertical plane the beam consists of two lobes of comparable size oriented frontally and ventrally. In level flight, the bat directs its beam such that the ventral lobe is pointed forward and down toward the ground ahead of its flight path. The bat may utilize the downward directed lobe to measure altitude without the need for vertical head movements.     

Doppler-shift compensation in the Taiwanese leaf-nosed bat (Hipposideros terasensis) recorded with a telemetry microphone system during flight

Biosonar behavior was examined in Taiwanese leaf-nosed bats (Hipposideros terasensis; CF-FM bats) during flight. Echolocation sounds were recorded using a telemetry microphone mounted on the bat's head. Flight speed and three-dimensional trajectory of the bat were reconstructed from images taken with a dual high-speed video camera system. Bats were observed to change the intensity and emission rate of pulses depending on the distance from the landing site. Frequencies of the dominant second harmonic constant frequency component (CF2) of calls estimated from the bats' flight speed agreed strongly with observed values. Taiwanese leaf-nosed bats changed CF2 frequencies depending on flight speed, which caused the CF2 frequencies of the Doppler-shifted echoes to remain constant. Pulse frequencies were also estimated using echoes returning directly ahead of the bat and from its sides for two different flight conditions: landing and U-turn. Bats in flight may periodically alter their attended angles from the front to the side when emitting echolocation pulses.     

Stabilization of perceived echo amplitudes in echolocating bats. I. Echo detection and automatic gain control in the big brown bat, Eptesicus fuscus, and the fishing bat, Noctilio leporinus.

Previous research on echo detection in bats has suggested that the effective threshold is a function of the acoustic clutter in the experimental environment, as might be expected given the low ambient noise levels typical of such psychophysical research. This paper demonstrates that theory of signal detectability (TSD) methodology is applicable to bats and uses it to show that an important element of clutter limiting in Eptesicus fuscus and Noctilio leporinus is backward masking of phantom targets by the real echo from the loudspeakers used to generate them. This information suggests that a previous estimate of the magnitude of automatic gain control (AGC) is too high, due to variable backward masking inherent in the experimental method used. A re-examination of gain control using a masking-free method shows that it reduces auditory sensitivity by 6 to 7 dB per halving of target range, rather than 11 dB as previously thought.     

A new microphone system for near whispering

A new microphone system was developed to monitor the human voice near the microphone in a noisy environment. The system is equipped with two special functions in addition to the usual microphone functions: reduction of air-blow effects by the mouth and focused reception to a sound source. A wind filter was developed to reduce the air-blow effects from the mouth during speaking. This filter is a plate perforated by an array of small holes; the method used to design the filter is also presented. To achieve focused reception, four microphones were used in conjunction with a new signal-processing method. The proposed signal-processing method effectively increases the directivity in the desired direction. Additionally, it provides the system with focusing on the source since the source is located adjacent to the system. A prototype of the proposed system was fabricated and subjected to performance tests. The results showed that air-blow effects can be reduced by up to 20 dB and the directional gain is more than 4 dB. The proposed microphone system shows such good performance that it can be used in mobile phones for whispering communication.     

The role of the external ear in vertical sound localization in the free flying bat, Eptesicus fuscus

The role of the external ear in sonar target localization for prey capture was studied by deflecting the tragus of six big brown bats, Eptesicus fuscus. The prey capture performance of the bat dropped significantly in the tragus-deflection condition, compared with baseline, control, and recovery conditions. Target localization error occurred in the tragus-deflected bat, and mainly in elevation. The deflection of the tragus did not abolish the prey capture ability of the bat, which suggests that other cues are available used for prey localization. Adaptive vocal and motor behaviors were also investigated in this study. The bat did not show significant changes in vocal behaviors but modified its flight trajectories in response to the tragus manipulation. The tragus-deflected bat tended to attack the prey item from above and had lower tangential velocity and larger bearing from the side, compared with baseline and recovery conditions. These findings highlight the contribution of the tragus to vertical sound localization in the free-flying big brown bat and demonstrate flight adaptations the bat makes to compensate altered acoustic cues.     

Range discrimination by big brown bats (Eptesicus fuscus) using altered model echoes: implications for signal processing

The sonar emissions of two big brown bats (Eptesicus fuscus) were modeled to create a "normal" echolocation signal for each bat which was then used as an artificial echo to synthesize a phantom target. The bat's task was to indicate which of two phantom targets (presented singly) was the "near" target and which the "far" target. Threshold range discrimination at a nominal target distance of 80 cm was about 0.6 cm for both bats. The normal signal was then modified to change the relative energy in each harmonic, the signal duration, the curvature of the frequency sweep, the absolute frequency, the phase of the second and third harmonics relative to the first, or the Doppler shift of the signal. To determine which modifications affected ranging performance, the altered models were used in tests of range discrimination that were interleaved on a day-to-day basis with tests using the normal model. Of the 12 modifications tested, only those changing the curvature of the frequency sweep affected performance. This result appears not to be predicted by current models of echo processing in FM bats. Eptesicus may be able to compensate for certain types of distortions of a returning echo, an ability possibly related to Doppler tolerance or to the characteristics of the natural variation in a bat's emissions.     

Detection of targets colocalized in clutter by big brown bats (Eptesicus fuscus)

Echolocating big brown bats (Eptesicus fuscus) frequently catch insects during aerial pursuits in open spaces, but they also capture prey swarming on vegetation, and from substrates. To evaluate perception of targets on cluttered surfaces, big brown bats were trained in a two-alternative forced-choice task to locate a target, varying in height, that was embedded partway in holes (clutter) cut in a foam surface. The holes were colocalized with the possible positions of the target at distances ranging from 25 to 35 cm. For successful perception of the target, the bat had to detect the echoes contributed by the target in the same time window that contained echoes from the clutter. Performance was assessed in terms of target reflective strength relative to clutter strength in the same time window. The bats detected the target whenever the target strength was greater than 1-2 dB above the clutter.     

Attenuation of vehicle noise over short grass

Field experiments to investigate the attenuation of noise from single vehicles over short grass indicate that for a microphone height of 1·2 m and a source/receiver distance <30 m, the rate of attenuation is 4·8 dB(A) per doubling of distance. Some evidence is presented to suggest that for source/receiver distances >30 m the rate of attenuation increases with distance.     

Echolocation call intensity and directionality in flying short-tailed fruit bats, Carollia perspicillata (Phyllostomidae)

The directionality of bat echolocation calls defines the width of bats' sonar "view," while call intensity directly influences detection range since adequate sound energy must impinge upon objects to return audible echoes. Both are thus crucial parameters for understanding biosonar signal design. Phyllostomid bats have been classified as low intensity or "whispering bats," but recent data indicate that this designation may be inaccurate. Echolocation beam directionality in phyllostomids has only been measured through electrode brain-stimulation of restrained bats, presumably excluding active beam control via the noseleaf. Here, a 12-microphone array was used to measure echolocation call intensity and beam directionality in the frugivorous phyllostomid, Carollia perspicillata, echolocating in flight. The results showed a considerably narrower beam shape (half-amplitude beam angles of approximately 16° horizontally and 14° vertically) and louder echolocation calls [source levels averaging 99 dB sound pressure level (SPL) root mean square] for C. perspicillata than was found for this species when stationary. This suggests that naturally behaving phyllostomids shape their sound beam to achieve a longer and narrower sonar range than previously thought. C. perspicillata orient and forage in the forest interior and the narrow beam might be adaptive in clutter, by reducing the number and intensity of off-axis echoes.     

Sonar inter-ping noise field characterization during cetacean behavioral response studies off Southern California

The potential negative effects of sound, particularly active sonar, on marine mammals has received considerable attention in the past decade. Numerous behavioral response studies are ongoing around the world to examine such direct exposures. However, detailed aspects of the acoustic field (beyond simply exposure level) in the vicinity of sonar operations both during real operations and experimental exposures have not been regularly measured. For instance, while exposures are typically repeated and intermittent, there is likely a gradual decay of the intense sonar ping due to reverberation that has not been well described. However, it is expected that the sound field between successive sonar pings would exceed natural ambient noise within the sonar frequency band if there were no sonar activity. Such elevated sound field between the pings may provide cues to nearby marine mammals on source distances, thus influencing potential behavioral response. Therefore, a good understanding of the noise field in these contexts is important to address marine mammal behavioral response to MFAS exposure. Here we investigate characteristics of the sound field during a behavioral response study off California using drifting acoustic recording buoys. Acoustic data were collected before, during, and after playbacks of simulated mid-frequency active sonar (MFAS). An incremental computational method was developed to quantify the inter-ping sound field during MFAS transmissions. Additionally, comparisons were made between inter-ping sound field and natural background in three distinctive frequency bands: low-frequency (<3 kHz), MFA-frequency (3–4.5 kHz), and high-frequency (>4.5 kHz) bands. Results indicate significantly elevated sound pressure levels (SPLs) in the inter-ping interval of the MFA-frequency band compared to natural background levels before and after playbacks. No difference was observed between inter-ping SPLs and natural background levels in the low- and high-frequency bands. In addition, the duration of elevated inter-ping sound field depends on the MFAS source distance. At a distance of 900–1300 m from the source, inter-ping sound field at the exposure frequency is observed to remain 5 dB above natural background levels for approximately 15 s, or 65%, of the entire inter-ping interval. However, at a distance of 2000 m, the 5 dB elevation of the inter-ping SPLs lasted for just 7 s, or 30% of the inter-ping interval. The prolonged elevation of sound field beyond the brief sonar ping at such large distances is most likely due to volume reverberation of the marine environment, although multipath propagation may also contribute to this.     

Evaluation of an auditory model for echo delay accuracy in wideband biosonar

In a psychophysical task with echoes that jitter in delay, big brown bats can detect changes as small as 10-20 ns at an echo signal-to-noise ratio of approximately 49 dB and 40 ns at approximately 36 dB. This performance is possible to achieve with ideal coherent processing of the wideband echoes, but it is widely assumed that the bat's peripheral auditory system is incapable of encoding signal waveforms to represent delay with the requisite precision or phase at ultrasonic frequencies. This assumption was examined by modeling inner-ear transduction with a bank of parallel bandpass filters followed by low-pass smoothing. Several versions of the filterbank model were tested to learn how the smoothing filters, which are the most critical parameter for controlling the coherence of the representation, affect replication of the bat's performance. When tested at a signal-to-noise ratio of 36 dB, the model achieved a delay acuity of 83 ns using a second-order smoothing filter with a cutoff frequency of 8 kHz. The same model achieved a delay acuity of 17 ns when tested with a signal-to-noise ratio of 50 dB. Jitter detection thresholds were an order of magnitude worse than the bat for fifth-order smoothing or for lower cutoff frequencies. Most surprising is that effectively coherent reception is possible with filter cutoff frequencies well below any of the ultrasonic frequencies contained in the bat's sonar sounds. The results suggest that only a modest rise in the frequency response of smoothing in the bat's inner ear can confer full phase sensitivity on subsequent processing and account for the bat's fine acuity or delay.     

Hybrid active and passive control of fan noise

This paper deals with the global reduction of axial flow fan noise in ducts in a building using a hybrid passive-active noise control method. The effectiveness of using an infra-red device as a reference signal source is also investigated. It is shown that using such a hybrid noise control system over an axial-flow fan reduces the overall sound pressure level by 5 dB(A) in the surrounding environment and global control of the blade passing frequency can also be achieved. This paper also shows that using an infra-red device as a reference signal source produces marginally better control as compared with using a microphone reference sensor. Moreover, long term stability is guaranteed and the possibility of acoustic feedback is eliminated.     

Hearing thresholds of a harbor porpoise (Phocoena phocoena) for helicopter dipping sonar signals (1.43-1.33 kHz) (L)

Helicopter long range active sonar (HELRAS), a "dipping" sonar system used by lowering transducer and receiver arrays into water from helicopters, produces signals within the functional hearing range of many marine animals, including the harbor porpoise. The distance at which the signals can be heard is unknown, and depends, among other factors, on the hearing sensitivity of the species to these particular signals. Therefore, the hearing thresholds of a harbor porpoise for HELRAS signals were quantified by means of a psychophysical technique. Detection thresholds were obtained for five 1.25 s simulated HELRAS signals, varying in their harmonic content and amplitude envelopes. The 50% hearing thresholds for the different signals were similar: 76 dB re 1 μPa (broadband sound pressure level, averaged over the signal duration). The detection thresholds were similar to those found in the same porpoise for tonal signals in the 1-2 kHz range measured in a previous study. Harmonic distortion, which occurred in three of the five signals, had little influence on their audibility. The results of this study, combined with information on the source level of the signal, the propagation conditions and ambient noise levels, allow the calculation of accurate estimates of the distances at which porpoises can detect HELRAS signals.     

基于传声器阵列过顶测量结果的飞机起落架噪声研究

应用由111个传声器组成的平面传声器阵列对当前流行的民用客机进场着陆过程中的机体噪声源进行了实验测量,本对七架窄体客机和七架宽体客机的起落架噪声进行了分析,得到了起落架噪声的频谱特性、指向特性和声级变化。研究发现,起落架噪声的频谱是由宽频随机噪声与一些较为明显的单噪声源组成,起落架噪声的指向性类似于一个水平放置的偶极子。不同飞机起落架噪声的声级相差较大,这说明可以通过重新结构设计降低起落架噪声。     

数控传声器阵列反卷积法对飞机噪音的应用与实验研究

本文主要讨论传声器阵列反卷积法对于飞机发动机噪音测量的实际应用及反卷积法对于飞机过顶测噪的数值模拟。传统"波束成型法"的成像是点源与点分布函数卷积的结果,只有在满足瑞利限制的前提下可以分辨多个相距较近的声源。本文介绍了一种简称反卷积法的传声器阵列新方法,它是将原有的Beamforming结果作为已知条件,从中提取有用的声源信息,去除掉旁瓣的干扰,使得处理出来的声源幅值与位置更加准确。观察运动声源会产生多普勒效应,特别是在频域中处理声信号时会使发声源产生频率偏移,因此在反卷积法使用前引入平均多普勒频移因子,它使得运用反卷积法在频域中预测飞机进场噪音成为可能。     

Threshold received sound pressure levels of single 1-2 kHz and 6-7 kHz up-sweeps and down-sweeps causing startle responses in a harbor porpoise (Phocoena phocoena)

Mid-frequency and low-frequency sonar systems produce frequency-modulated sweeps which may affect harbor porpoises. To study the effect of sweeps on behavioral responses (specifically "startle" responses, which we define as sudden changes in swimming speed and/or direction), a harbor porpoise in a large pool was exposed to three pairs of sweeps: a 1-2 kHz up-sweep was compared with a 2-1 kHz down-sweep, both with and without harmonics, and a 6-7 kHz up-sweep was compared with a 7-6 kHz down-sweep without harmonics. Sweeps were presented at five spatially averaged received levels (mRLs; 6 dB steps; identical for the up-sweep and down-sweep of each pair). During sweep presentation, startle responses were recorded. There was no difference in the mRLs causing startle responses for up-sweeps and down-sweeps within frequency pairs. For 1-2 kHz sweeps without harmonics, a 50% startle response rate occurred at mRLs of 133 dB re 1 μPa; for 1-2 kHz sweeps with strong harmonics at 99 dB re 1 μPa; for 6-7 kHz sweeps without harmonics at 101 dB re 1 μPa. Low-frequency (1-2 kHz) active naval sonar systems without harmonics can therefore operate at higher source levels than mid-frequency (6-7 kHz) active sonar systems without harmonics, with similar startle effects on porpoises.     

Active noise control in finite length ducts

A simple technique for the active control of sound in ducts, initially suggested by Olson and May [1], is investigated in detail. A simple, “virtual earth” principle, feedback loop is used to drive the sound pressure to a minimum at a microphone placed close to a loudspeaker in the duct wall. This produces a reflection of downstream travelling plane waves. A detailed investigation of the loudspeaker near field has enabled the optimum position of the microphone to be identified. The system is shown to be especially effective at the frequencies of the longitudinal duct resonances, where the acoustic response of the duct produces a high loop gain. Results are presented which show a reduction of up to 20 dB in the amplitude of low frequency broadband noise at a position downstream of the cancelling source.