Last time I showed some results from test data aimed at finding out how well directions can be determined using the cross-correlation technique. I’m now going to look at some real data and see how well it does, and for that I’m using a couple of observations made back in January. The first is of a wattle bird and the second is a bat.
So firstly, here’s the bird: a little wattlebird (Anthochaera chrysoptera). It’s a pretty noisy one and had a broad audio spectrum, so is a nice test case. At the time of recording, the bird was just sitting in a tree and not moving. Here’s a plot that summarises the data:
The direction measurements all look good and consistent across the frequency range. This is pretty surprising given that the baseline is only 15mm which is 13% of a wavelength at 2.9 kHz (118 mm). The ability to measure position is related to signal strength, so here’s a plot that shows that behaviour for the bird data:
The above plot shows that the scatter in direction measurements decreases with increasing signal strength as expected. Even at such a short baseline compared to the wavelength, it’s possible to get angles to about 20 degrees at 3-sigma for strong signals.
Next is a bat observation which was a little more challenging to process given the higher frequency and the issue of resolving phase turn ambiguities. Fortunately this wasn’t a problem in this case. Here’s what the data look like after processing:
This is pretty exciting I reckon! The phases clearly show the bat moving from left to right with the angle changing by about 60 degrees over 7 seconds. It looks as though the systematics kick in in delay at about the 10 us peak-to-peak level which at this frequency and a 15mm baseline corresponds to 10 degrees in direction: