Wednesday, 12 March 2008

Bats, bombers and acoustic mirrors

One reason why I first started working with bats was that bat work combines my love of wildlife with my longstanding interest in electronics. Fortunately people who do surveys with me tend to tolerate my tendency to come up with strange new ideas or pieces of equipment!

Several years ago I found myself wondering whether parabolic dishes might have a place in bat work. These are used in the recording of bird calls and other wildlife sounds and look similar to TV satellite dishes. In essence, the dish reflects sounds into a focal point, where a microphone is placed. This has two effects:
1. Sounds are concentrated by the dish, making them louder.
2. Sounds are picked up from a narrow direction, filtering out background sounds from either side.

Incidentally, not just any dish will work: a parabola is a specific curve, calculated using a mathematical formula.
I experimented with an old Sky TV dish and found that when I put a bat detector at the focal point of the dish, bat calls became louder and so could be heard from further away. However, it was clear that this would have limited practical uses, as it only worked when the dish was pointed directly at a distant bat. People recording bird calls can see a bird and point a dish at it. Bats don't allow that luxury!

More recently I became aware of something called a parabolic plane. This is sometimes used in microwave radio applications. It is like a dish, but only curved in one plane, a bit like Rolf Harris's wobble board. Between the wars gigantic parabolic planes up to 200 feet long by 26 feet high were built at strategic sites around Britain's coast. These "acoustic mirrors" were used to listen for approaching bombers: operators could hear an aircraft from 20 miles away. Unfortunately they were never used for their intended purpose: by the time World War II broke out, radar had taken over.



Image (c) English Heritage



The beauty of a parabolic plane is that it only concentrates sound from the horizontal plane, so that a bat flying in front of one at any height should be heard. It could be possible to create a kind of listening curtain: in theory any bat passing through it would be detected. To test this I made up a prototype reflector, using foam board, card and sticky-backed plastic (eat your heart out, Blue Peter) and tried it, using a low-level ultrasound source as an artificial bat.



The prototype is only 60cm by 20cm, so I wasn't expecting miracles, but tests at varying ultrasound levels showed that, using the reflector a Duet bat detector could pick up the artificial bat between 30% and 70% further away than it could on it's own. Positioning of the detector microphone is critical, as it needs to be at the precise focal point of the reflector for best results. The half-brick will need to be replaced by something with a bit more finesse!

There is some interesting potential for parabolic planes, but there are some potential draw-backs too. In theory, the bigger the reflector, the further away you can hear bats. But a bigger reflector also means the bat would be audible for a shorter time, passing through a narrower curtain. (The acoustic mirrors could hear aircraft from 20 miles away and place them to within 1.5 degrees.) Ideally a bat needs to be audible for 1-2 seconds, to be sure enough of it's call is
recorded, to allow for identification, so this may limit possible size..

Another problem is the practicality of lugging a big reflector around. Even using it in a fixed position, more than 2-3 metres in width would probably be impractical, unless as a permanent fixture. Any fixed-location survey involves predicting where the bats will fly, and they don't alway read the same books as us!

More about acoustic mirrors: http://www.ajg41.clara.co.uk/mirrors/

My website: Plecotus.co.uk

No comments: