After our initial field testing with the first bat plane (have a look at the blog post here) we knew that we needed to reduce the noise seen in the sonograms. This has been the driving force behind making the Mk2, Mk3 and purchasing the Talon.
The noise is a product of the propeller spinning and as such we've tried to reduce the noise heard by the ultrasound detector by separating the propeller and detector by as much distance as possible.
For the Talon we tested both the Peersonic and Soundtrap (which has changed name to Open Acoustic Devices due to a conflict of names - the actual detector is called the AudioMoth) detectors in a variety of positions to compare noise levels. The positions were:
* This would be achieved by attaching an extension to the front of the plane. For fun we've called this the Nimrod option. It'll look something like the re-fueling pipe extending from the front of the plane:
Let's take a look at the data we produced from the two detectors in the various positions. For reference, the throttle was set at the minimum value required for level flight (circa 35%) in order to minimise noise which mimics what we would do in field testing.
Here are the sonograms:
The first noticable difference between the two detectors is that the Soundtrap picks up a lot more noise. There are a number of possible reasons for this. One we know will be a factor is that the Peersonic has a microphone which actively suppresses noise below 20kHz.
Looking at the difference between the various positions, it's apparent that the noisiest position is under the belly (also closest to the propeller) and the quietest is 5cm from the nose. The quietest is somewhat of a surprise, we expected the 15cm from the nose position to be quieter as it is further from the propeller. Perhaps this is due to the airframe blocking the sound at the 5cm position.
In conclusion, the current thinking is to use the 5cm position for field testing. As there is still some time before the bats come out of hibernation we think it might be worth testing a further extended-Nimrod option, with the microphone circa 60cm from the nose and we'd also like to compare the signal to noise ratio for the Peersonic and Soundtrap (AudioMoth) in the preferred position using the ultrasound source (see the blog post here about using this previously).
Keep checking out the blog for more!
UPDATE: Soundtrap has now changed name (due to a conflict of names) to Open Acoustic Devices. The detector we're using is now called the AudioMoth. For this blog post we'll keep the references as 'Soundtrap' as it was at the time.
Here's the original post:
A short time ago we were contacted by a small group of academics who have designed and produced a small ultrasound detector. The detector is called a Soundtrap and their detector is designed as a low-cost open-source acoustic logger for biodiversity and environmental monitoring. For more information on Soundtrap, please visit their website: http://soundtrap.io/
The group is made up of researchers at the University of Oxford and University of Southampton and they have been kind enough to send us one of their detectors to do some testing with. The detector has already been deployed for a number of experiments around the world and if you're interested in it then we'd highly recommend checking out their website.
One of the major benefits of this detector is the size - it's tiny! This is a big help as the less weight we carry when flying the better; although, when using a detector on the boat this is less important. Have a look at the photo below to see just how small it is next to a normal size business card:
During our weekend testing of the new plane (the Talon, see the blog post here) we were actually able to do some side-by-side comparison testing with the Soundtrap and the Peersonic and we'll be putting up another post about this shortly.
Our plan at the moment is to run both the Soundtrap and the Peersonic concurrently to give us the best chance of capturing some good data.
We'd also like to take this opportunity to say thank you to Alex, Peter and Andrew at Soundtrap who have been fantastic!