The SM5 has the ability to precisely synchronize its internal clock to GPS or GLONASS satellites. This allows you to synchronize multiple SM5 recorders and localize a sound source recorded on multiple microphones.
Acoustic localization, triangulation, or trilateration is a method of determining the precise location of a sound source by recording it with multiple microphones (usually three or more) spread across an area in a microphone array and measuring the time difference between when each microphone records the same sound source.
Under normal circumstances, the clocks in multiple SM5 recorders will drift apart from each other, meaning some clocks will run slightly faster or slower than others. This drift introduces small timing errors that make accurate localization impossible.
By configuring multiple SM5 recorders to continuously synchronize to GPS/GLONASS satellites, you can ensure each recorder's clock is within 30 μs of satellite time for the entirety of the survey, making triangulation possible with a high degree of accuracy.
Required Equipment for Acoustic Localization
- Two or more SM5 recorders
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Each SM5 can only record from two channels at once. Three channels are required for localization, so you must use at least two synchronized SM5 recorders.
Unlike previous Song Meter models, the SM5 has a built-in GPS/GLONASS receiver than can correct for clock drift between multiple recorders. It is not necessary to connect an external GNSS receiver to each SM5.
- Three or more microphones
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To localize a sound source, at least three microphones at different positions must record the sound. Microphones that are separated by some distance provide greater accuracy than microphones that are close together. Using each recorder with one built-in or stub microphone and one external microphone with a long cable is a cost-effective way to separate your microphones.
To survey a larger area, you may need more than three microphones and more than two recorders, distributed such that a sound of interest will be recorded by at least three microphones at a time.
The appropriate distance between the microphones will depend on the size of the area you intend to survey and how far the sound of interest travels.
- Loudspeaker for playback of test sounds
- To ensure your equipment is working correctly and your analysis is correct, you should emit test sounds from a loudspeaker at one or more known positions, then test if your analysis accurately locates the loudspeaker.
- Survey-grade GNSS receiver
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Accurate localization requires precise location data for each microphone. The location data provided by the SM5's internal GPS/GLONASS receiver is only accurate to within several meters, and it cannot measure the location of an external, cabled microphone. You should use a dedicated GNSS receiver with a high degree of accuracy to measure the position of each microphone and your loudspeaker.
At small distance scales, it may be possible to measure the relative positions of the microphones using manual tools like tape measures and compasses, rather than using GNSS position data.
- Temperature and humidity measurement device
- The speed of sound through air varies with temperature and, to a lesser extent, humidity. Accurate localization requires accurately accounting for the speed of sound, so you should record the atmospheric conditions on-site during your survey.
- Editing and analysis software
- You may need multiple software tools to perform the full process of
localization.
- If you wish to combine one-channel or two-channel recordings from multiple recorders into many-channel audio files, you will need software that can do this.
- You will need software to measure the time at which each microphone recorded the same sound. Wildlife Acoustics' Kaleidoscope software can be used for this task on one-channel or two-channel recordings.
- You will need software to process the atmospheric conditions, each microphone's position, and the differences between times of arrival to estimate the location of the sound source. The OpenSoundscape library is one example.