Approaching the Capability of Bats and Dolphins

Bats and dolphins have developed very sophisticated means of object detection, location and characterisation. Their capabilities in the generation, reception and processing of acoustic signals go way beyond man’s current understanding. Current technology drivers in most scientific, military and industrial fields include increasing resolutions, lowering power consumption and improving material assessment and characterisation. Existing solutions often result in current technologies merely being driven harder, but new, novel technologies can be developed from a fresh view of the way bio-acoustic systems solve similar problems.

How bats and dolphins achieve such high levels of object detection, location and characterisation has been the focus of much research. One aim being to develop similar, ‘bio-mimetic’ systems, but it is still unclear the effort that is required to approach the capability of bio-acoustic systems. Recent experiences suggest that new research should be ‘bio-inspired’ and investigate the way in which energy is delivered to and returned from the source of investigation.

New Research: The BIAS Project

Researchers from the NERC British Geological Survey, the Universities of Southampton, Edinburgh, Leeds, Strathclyde, Leicester and Fortkey Ltd have formed the BIAS consortium to undertake research into the tools / techniques used routinely by nature that hitherto have not been embraced by man. Key challenges for a new approach include breaking the quarter wavelength barrier that currently limits spatial resolution, and the development of new signals for improved power efficiency and material property characterisation. The project will establish an experimental programme to be undertaken at three ultrasonic laboratory facilities:

• firstly, a waterborne, high frequency ultrasound facility focusing on medical physics applications
• secondly, a waterborne, low frequency ultrasound facility focusing on the physical characterisation of materials for geological applications,
• and thirdly, an airborne, low frequency ultrasound facility focusing on the characterising the effect of aspect angle on echo patterns.

Figure 1: Test tank at the Ultrasonics Research Laboratory at the British Geological Survey

Novel Ultrasonic Systems for New Coded Waveform Generation and Acquisition

A new study of the phase and magnitude of the echo-reflected wave is key to understanding the achievements of bats and dolphins. The Ultrasonics Research Laboratory at the British Geological Survey has commissioned Alba Ultrasound Ltd., Glasgow, UK to provide very wideband piezo-composite transducers to generate a new family of coded waveform signals to be used in this study. The bandwidths and efficiencies will be further improved when combined with new transducer matching networks commissioned by Blacknor Technologies, Portland, UK. The generation and detection of these new coded waveforms requires systems with very long memory lengths and high sampling frequencies at high dynamic ranges offered by ZTEC instrumentation.

Figure 2: ZTEC ZT530PXI-01 Arbitrary Waveform Generator providing drive for Alba Ultrasound transducers

ZTEC’s ZT530PXI-01 Arbitrary Waveform Generator has the voltage output capability to directly drive Alba Ultrasound’s wideband transducers. The ZT530PXI-01 offers sampling rates of up to 400MSs-1 at voltage levels of 20V peak to peak unlike many other competing products, which only offer 3V peak to peak.

Figure 3: 40, 000 samples for a 200kHz to 800kHz linear upsweep with tail of trailing zeros; Lower Left: Start frequency 200kHz; Lower Right: End Frequency 800kHz

ZTEC’s ZT530PXI-01 Arbitrary Waveform Generator can provide up to 4MSamples and the ZT410PXI-51 Digital Oscilloscope can acquire up to 16MSamples. Long memory lengths are vital for the generation and detection of signals such as wideband linear chirps. The linear upsweep above comprises 10, 000 samples at 10MSs-1 with a start frequency of 200kHz and an end frequency of 800kHz within 1ms, and is succeeded by a tail of 30, 000 points. The 200kHz to 800kHz upsweep was acquired with the ZT410PXI-51 with 125, 000 samples at 100MSs-1.

Figure 4: Step quantisation on a 200kHz signal sampled at 100MSs-1:
Upper: x1 interpolation on the ZT530PXI-01; Lower: x8 interpolation on the ZT530PXI-01

Step quantisation is a phenomenon that occurs when digitising analogue signals and can be seen as the staircase structure on the signals in the above figure. Step quantisation can introduce phase distortion to the signals, which can add unwanted phases for example when driving power amplifiers. Step quantisation can be reduced in the signal emitted from the ZT530PXI-01 using the interpolation feature to provide a smother signal input into the power amplifier.

Signal discrimination techniques can be much improved even if the transmitting transducer technology is bandwidth limited. For example, Barker code sequences can be sent to a transmitting transducer, each of which initiates a wave packet mainly containing damped oscillation at the resonant frequency of the transducer. The multiple reflections when returned to this transducer produce a composite signal that is the result of the convolution of all the echo returns from the various reflectors. Not only is the transmitted signal of long duration, but also the detection of pulse echoes requires a very long acquisition window. The ZT410PXI-51 with its combined 16MSample memory and very high sampling rate offers the capability of very high sampling rates of long duration events. This allows time delay measurements to very high resolution during pulse compression. The example below shows the deconvolution of an 11-point Barker code emitted from a piezoelectric transducer from a complicated pulse train comprising multiple echoes.

Figure 5: Transducer response to a series of impulses in an 11-point Barker Code
Deconvolution of the upper source signal from the lower signal containing multiple packets (13) of the source signal. Time resolution of the deconvolution dependent upon sampling interval

Bat and Dolphin Signals: Generation and Detection

Many bat species emit frequency modulated (FM) or combined FM and constant frequency (CF) signals, in pulse lengths ranging from 0.3 to 300 ms, with frequencies ranging from around 10 to 200 kHz. The figure below shows a complex chirp signal emitted from bat, and the time-frequency spectrogram shows that two FM signals are emitted, and, while one is delayed from the other, there is still some pulse overlap between the two signals.

Figure 6: Chirp emitted from a bat comprising pulse overlapped FM signals

The majority of dolphins emit clicks comprising a few cycles that don’t tend to change much in duration or shape, but where the frequency band appears to be related to signal intensity. High intensity signals tend to have centre frequency of 100 kHz or above and low intensity signals tend to have centre frequencies around 30 to 60 kHz, as shown below.

Figure 7: Simulated dolphins clicks: High intensity 100 – 150 kHz succeeded by a low intensity 60 kHz signal

Recent research has shown that using this range of frequencies dolphins can detect differences in the wall thickness of metal cylinders of around 250 microns, or 1/50th of the wavelength. We cannot achieve anything like this at present but we hope to redress this balance with our new research programme.

Contributed by
David Gunn and Peter Jackson, British Geological Survey, Keyworth, Nottingham, UK
Said Assous and Clare Hopper, Leicester University, Leicester, UK

This work is being under taken as part of the Biologically Inspired Acoustic Systems (BIAS) project, which is funded by Research Councils UK via the Basic Technology Programme.

For more information, please visit http://www.biasweb.co.uk.