What is passive acoustic monitoring? And why is it used for?

Animals use sound for communication, echolocation, sexual display, and territorial defence, and bioacoustic monitoring involves the recording of those sounds to infer animal distribution, physiological state, abundance, and behaviour.

Passive acoustic monitoring, or just ‘acoustic monitoring’, involves surveying and monitoring wildlife and environments using sound recorders (acoustic sensors). These are deployed in the field, often for hours, days or weeks, recording acoustic data on a specified schedule. After collection, these recordings are processed to extract useful ecological data – such as detecting the calls of animal species of interest – which is then analysed similarly to other types of survey data.

Commercially available bioacoustic sensors usually record either audible range sound (e.g. birds, most mammals, amphibians) or ultrasound (e.g. bats, many toothed whales), and are designed specifically for either terrestrial or marine deployment (hydrophone).

How far away from a microphone can animals be heard?

This depends on the animal species, environment and sensor type. Detection distances are affected by a sound’s amplitude and frequency (how rapidly it attenuates to below a perceptible level):

In general, animals calling at higher amplitudes (more loudly) will be detected at greater distances than those calling at lower amplitudes (more quietly), and higher frequencies also attenuate more quickly than lower frequencies. Site-specific environmental factors also have an impact, such as the medium (air/water), temperature, pressure, humidity, ambient sound levels, and habitat structure such as vegetation and buildings. This means that different species are more readily detectable by acoustic sensors than others, and this can vary between habitat types.

Here is an exemple of biotic and abiotic sound observed in different species with a spectrogramme (a visual representation of the sound):

Signal Processing:

Spectrograms are critical tools in the analysis of acoustic wildlife monitoring data, enabling specific sounds (e.g. animal calls) to be visually recognised and labelled, either manually or using automated classification software.

Audacity, Raven or Pamguard are common software used in marine biology to listen to the recordings.

In summary:

Most current applications of acoustic monitoring endeavour to assess animal population dynamics, behaviour, communities and diversity, or the status of species or populations, often in relation to human activities

• Acoustic monitoring offers advantages over other survey methods, including that it is non-invasive, can survey a broader taxonomic range of species than camera traps, and uses sensors that are relatively easy to deploy and can be left in situ for extended times

• Acoustic monitoring has several disadvantages too, including its inability to detect phenomena that do not emit sound, its dependence on relatively expensive equipment, and high skill level required to analyse what are often massive volumes of data

• In the near future, open-source options for acoustic monitoring hardware and software, sensors integrated with on-board detection and classification capabilities, and networked sensors connected wirelessly will rapidly expand the field of acoustic monitoring

Source: acousticmonitoring guide WWF

To have an idea of a practical sample, have a look at the Vaquita C-POD post