Latest research results for 2008

• Acoustic tracking [PDF]

Research results for 2007

• Acoustic tomography [PDF]
• Robat [PDF]

Biomimetics and Theory Group

At the University of Leeds, we are providing the biological support for the BIAS group. We provide biological expertise in bat echolocation through access to a literature database and existing research expertise. We also provide recorded echolocation calls to the consortium, as well as training courses in bat biology and produce original research to feed into the BIAS group. The interaction is two-way, with us receiving new ways of acquiring and analysing bat echolocation calls. Our work is split into a number of project areas. These revolve around the theme of establishing exactly what the call structures are that bats use for echolocation, how these structures are linked to the prey and foraging environment of the bat and how they convey information on target structure back to the bat.

A Noctule bat

Acoustic Tracking of Bats

Echolocation calls are very directional, with the higher frequencies more directional than the lower frequencies. The higher frequencies are also absorbed more by the atmosphere. This means that a recording made of the echolocation call may not be a true representation of the emitted call. In order to know what the call really is, we need to know the distance and angle of the bat to the recording microphone. In order to do this, we have developed an acoustic tracking system. Eight microphones record the time of arrival of each echolocation call as the free flying bat flies through the array. We can then use an optimization procedure to reconstruct the flight path of the bat. Two high quality recording microphones on a 4m pole then record the call structure, and can also give the intensity of the emitted call. The whole system runs from one laptop computer so is fully field-portable. The recordings are made at a sampling rate of 1 MHz and 16 bit, giving a huge increase in resolution from previous recordings.

The acoustic tracking array deployed in the field.

One of the eight tracking microphone stations.

The multi-harmonic echolocation call of Tadarida aegyptiaca

Echoes from insect prey

Bats are limited to echolocating in the range 10 kHz to 200 kHz. At the lower end, the wavelength of the sound is larger than their prey, so most of the sound energy diffracts around the target rather than echoes back. At the high end, the wavelength is smaller than the object, so it reflects well, but is absorbed by the atmosphere. Within this range, we assume that the echolocation call structure is matched to the prey size and shape and the foraging environment. Predicting how sound reflects from even simple shapes like spheres and disks is not easy since the size of the object and the wavelength of sound for bats fall into the region where creeping waves affect the target strength, the so-called Mie region. Most insect prey is also acoustically ‘soft’, with the potential to set up vibrational modes within the prey, further blurring simple assumptions. To investigate reflections from simple shapes and real prey items we have developed a rig to broadcast pulsed ultrasound, simulated and real echolocation calls at a variety of targets and record the echoes. This rig can also move along a linear track at up to 7 ms-1 so that we can also look at Doppler effects of the sweep rate of the echolocation call.

The bat back-pack

In collaboration with Strathclyde University, we have been developing a back-pack which can be mounted on a bat. A microphone is mounted between the bat’s ears, which receives the returning echo from targets as the bat flies and echolocates. The information is relayed via a radio-link to a base station, which synchronises with a high speed video camera. This gives us the actual echo structure the bat receives in free flight, as well as the visual information as to what the bat is doing at that moment.