Tuesday, March 17, 2009

Acoustic Bird Location and Directional Recording

A few weeks ago Victor Obolonkin and I traveled to Pureora Forest to record kōkako calls with my microphone array.

The microphone array is made of 12 microphones mounted on a 5 m beam, as shown mounted on the van.

Using beam-forming techniques, we can turn the microphones into a highly directional microphone which can be electronically "steered" in any direction. This is better than a traditional dish-type of directional mic, because we can steer the beam in any direction after the recording is made, allowing us to listen to two or more birds with overlapping calls. You can listen to an example of the power of the system below. The first file is a recording of a kōkako using a single microphone, which includes background noises from other birds and insects.

The next is a recording using the full array, steered in the direction of the kōkako. Many of the background noises are greatly attenuated, especially the sounds between 6 and 15 seconds.

Here is a photo of the kōkako in its tree, as well as a zoomed in version.

We can also do the opposite: use the array to locate the direction that the sound came from. This array has an angular resolution of approximately 4° for a sound of 1000 Hz, which is much better than a human ear. We intend to use this capability to track endangered birds, such as the kōkako, by using two arrays to triangulate their position.

An example of tracking a sound is shown below, recorded at the Physics Field Site at Ardmore. This shows the sound pressure level over time and angle from the array. You can see a steady source at approximately 90° (straight in front of the array); this is a speaker transmitting a kokako recording. You can also see a source that moves between 120° and 180°, a helicopter flying nearby.

Zoom in for a better view.