Passive acoustic monitoring (PAM) refers to methods for monitoring animals by using the sounds they produce; in particular we are interested in methods that allows us to estimate their abundance. Typically this refers to automated collection of sounds using microphones or hydrophones. Arguably some historical bird surveys were also PAM studies, since human observers rarely see birds in dense canopy forests but detect them by the sounds they produce.
Recently we have started looking into tracking individual animals using PAM, and investigating whether we can detect changes in behaviour in response to disturbance.
When a sound is recorded on a microphone or hydrophone (underwater microphone), the signal to noise ratio (SNR) can be measured. SNR compares the loudness of the recorded sound, known as the received level (RL), with background noise level (NL). The received level of a sound can be further broken down into (1) the source level (SL) i.e., how loud the sound was when first produced, (2) the transmission loss (TL) i.e., how much energy the sound loses as it travels through air or water, and (3) array and processing gain (AG and PG). Array and processing gain are ways in which the signal strength can be increased either through features of the recording equipment (AG) or the way in which the recording is analysed (PG). We normally think of calculating a ratio by using division. However, the above values are measured in decibels, which are on the logarithmic scale. Therefore, the various values are added and subtracted from each other to give the signal to noise ratio. The passive sonar equation is a very useful model for helping us to understand how sounds propagate over space. It has been used in our work to model the spread of human-caused noise as well as to estimate how far we can detect vocalising animals.
Küsel, E.T., D.K. Mellinger, L. Thomas, T.A. Marques, D.J. Moretti, and J. Ward. 2011. Cetacean population density from single fixed sensors using passive acoustics. Journal of the Acoustical Society of America 129: 3610-3622.
Marques, T. A.; Thomas, L.; Ward, J.; DiMarzio, N. & Tyack, P. L. 2009. Estimating cetacean population density using fixed passive acoustic sensors: an example with Blainville's beaked whales The Journal of the Acoustical Society of America, 125: 1982-1994
Marques, T. A.; Thomas, L.; Martin, S. W.; Mellinger, D. K.; Ward, J. A.; Moretti, D. J.; Harris, D. & Tyack, P. L. 2013. Estimating animal population density using passive acoustics. Biological Reviews, 88: 287-309