How much ultrasound does a quad-copter produce and does it interfere with recording bat? We used a bat detector and high quality sound recorder to record ultrasound from a quad-copter at different distances with the detector facing towards and away at each point. Here are the results, plotted as volume against frequency. On this scale most bats are going to be between 2.5 and 6.0.
As you can see from the reference horizontal line (which is set to the same value in both graphs). Interference was a lot lower when the detector was facing away. Additionally the further away we got from the quad-copter the less interference we had.
When we attach the detector to the drone it is going to be facing away, so the next question is how far away does it need to be. I re-plotted the 'facing away' plot on a log scale so we can see the low volume levels better.
The pink line is our reference since this is the reading we got when the quad-copter was off. So it looks like we could get away with the detector being 3 meters away, facing away. It is also interesting that the loudest frequency changes as we moved away. Closest to the quad-copter the loudest frequency was at the same frequency as many bat but by 3m out bat frequencies were some of the quietest.