Acoustic Source Localization under Variable Speed of Sound Conditions

What is it about?

A microphone can record the sound waves of a human speaker or of a musical instrument. Multiple microphones at different positions serve also to determine the location of the speaker or musician. When the distances from each microphone to the sound source are slightly different then also the times of arrival of the sound waves differ. These differences can be determined by signal processing methods. When the speed of sound is known then the time differences can be converted to range differences. They allow finally to determine the location of the acoustic source. This paper discusses solutions for the more involved case, that the exact value of the speed of sound is not known. Several possibilities are considered for a joint estimation of speed of sound and source location .

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Why is it important?

The speed of sound depends on the air temperature and to a lesser extent also on the humidity of the air. These values are fairly constant for indoor applications. However, as digital devices become smarter, cheaper, and smaller, they support also outdoor applications. Then the speed of sound may vary between day and night and between summer and winter. The simple assumption of a constant speed of sound might lead to localization errors. In these cases, the speed of sound must be treated as an additional unkown value.

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