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.

Driving Times in NZ

After our trip around the South Island, I got to thinking about how long it takes to get everywhere in this small country. So I made this map of the driving time from our house in Auckland to everywhere in the country. The driving times come from a Matlab script I wrote that queries Google maps, and the mapping functions are from M_Map for Matlab, using shoreline data from GSHHS. Click it for higher resolution.

MLS and Kasami Set Generators for Matlab

Linear feedback shift registers (LFSR) are a simple method of generating sequences, including pseudorandom number sequences. Some of these LFSR sequences have special properties; a maximal length sequence (MLS or m-sequence) has a large autocorrelation at zero lag, with near zero autocorrelation elsewhere. This 'impulsive' autocorrelation function allows one to quickly determine the impulse response of a linear time invariant (LTI) system.

Maximal length sequences are also the base for sets of sequences with good correlation properties. One such set is the small set of Kasami sequences. These sequences have small off-peak autocorrelations and also small cross correlations between sequences. This property allows for code-division multiplexing as well as accurately determining the arrival time of a transmitted sequence, even in the presence of other interfering transmissions.

I've uploaded some Matlab code for generating LFSR sequences, m-sequences, and sets of Kasami sequences, as well as examples. As alwasy, find it here. Please review, rate, or leave feedback so that I know people are finding these uploads interesting.

MLS Generator for ATMEGA8

Maximal length sequences (MLS or m-sequences) are useful for system identification and digital communication. For example if a MLS is used to excite the input to a linear time-invariant (LTI) system, the system's impulse response can be determined from the cross correlation of the input and output. The impulse response can be useful in its own right or can be windowed to obtain a quasi-anechoic frequency response.

This code is written for an AVR ATMEGA8, which is an easy-to-use, low-cost microcontroller, but should work with others. It outputs the MLS on pin 18 (PB4). A clock signal is output on pin 19 (PB5), transitioning high on each value. Pin 17 (PB3) is a synchronizing signal, going high for one sample at the start of each loop of the sequence.

The MLS is generated by a 16-bit linear feedback shift register. The "feedback" variable can be changed to other values to generate other sequences. See http://www.ece.cmu.edu/~koopman/lfsr/index.html for other values.

The code is written for gcc with -O3 optimization and the loop runs in a minimum 19 clock cycles if the delay function is removed. The "NOP" (No OPeration) lines must be tuned if you use a different compiler, to ensure that each branch of each "if" statement takes the same number of cycles.

"N_delay" can be used to change the loop's speed. It is currently set for a loop of 73 cycles or approx 219 kHz for a 16 MHz clock.

Download from here. You have to sign up (free), but if you're interested in AVR microcontrollers you should be a member anyway.

Acoustic Tomography

One of the projects that I'm currently working on is a method of measuring temperature and velocity fields using sound, called acoustic tomography. This method relies on the two properties of sound: it's speed is temperature dependent, and the sonic speed is relative to the motion of the fluid (in our case, air). The principle is similar to that used for sonic anemometers, the time taken for a sound to cross a distance is measured in two directions and the mean wind speed and temperature can be determined. If we set up a large number of speakers and microphones around a measurement area, it is possible to reconstruct the temperature and/or velocity fields within the area. This can be used to generate an image or video of the temperature and wind distribution.

This has been done before, however we are looking at much smaller scales in order to measure the flow in street canyons.

For more information, see my paper at the New Zealand Acoustic Society conference. Example code is also available from my section of the Matlab File Exchange.

The image above shows the temperature (colours) and wind velocity (arrows) field for votex shedding behind a cylinder. The top image shows the simulated flow, and the bottom one shows the reconstruction. The video below shows multiple frames of similar data. Unfortunately the resolution is low, if you would like to see the full video, please contact me.

New Slide Background

In preparation for next week's presentation, I got Amy Templeman to design some new Powerpoint slide backgrounds. I really like them, so I thought I'd share. The top one is for Nutaksas; the other is for the University of Auckland.

Upcoming Conferences

Next week (Nov 24-28), Travis will be speaking at two conferences. He will be presenting some recent developments in acoustic tomography at ENZCON 2008, "New Zealand’s leading national conference in electronics", as well as the New Zealand Acoustic Society Conference 2008. Acoustic tomography is a non-invasive method of measuring temperature and flow fields by measuring the time taken for sound to cross an area or volume.

Colleagues from the University of Auckland will also be presenting at both conferences.

Watch this space for details.

PhD Thesis Available

My PhD Thesis, "Online Learning of a Neural Fuel Control System for Gaseous Fueled SI Engines," is now available at lulu.com. This is a beautifully bound 153-page hardcover version of the dissertation including all the papers and a listing of the computer code used. If the price is too expensive for you, please feel free to email me for a free electronic copy. I'm not posting direct links for the first couple years so I can get some feedback as to who is interested in the work.

The cover was designed by Amy Templeman.

Internoise 2008

Travis will be presenting two papers at the Internoise 2008 conference next week. The papers are entitled "Experimental Characterization of Sound Propagation in a Dense New Zealand Forest" and "Long Range Identification of Wildlife Using Phased Arrays of Microphones: A Feasibility Study".

The first is a study which measures the sound attenuation of the very dense forests on New Zealand; that is, how quickly a sound decreases in sound pressure level over distance. We found that sound is attenuated more in the native forests of the Hunua Ranges than in any forest in the literature, at approximately 0.5 dB/m.

The second paper investigates the feasibility of locating birds by their calls, with the particular application of finding the location of kokako in the Hunua Ranges. Due to the high attenuation of sound through the forest, we investigated a scheme which would place arrays of microphones on towers above the canopy. This was compared to a more traditional method of installing a grid of individual microphones near the forest floor. This work used the forest reverb model seen earlier on this blog.

Details to follow.

Thesis Defense Update

I am happy to announce that, as of Wednesday, I am now officially Dr. Travis Wiens. My thesis defense went well and had a lot of good questions. My dissertation, entitled "ONLINE LEARNING OF A NEURAL FUEL CONTROL SYSTEM FOR GASEOUS FUELED SI ENGINES" will be available shortly. If you'd like a copy, please feel free to contact me.

I'd also like to announce that Nutaksas will now be adding minor surgery to our list of services available.