Sound power estimation with an acoustic camera

Abstract: Sound power describes a sound source regardless of its environment and is
useful in noise control applications, but can be cumbersome and time
consuming to measure. Sound power levels can rank different sound sources
and is often restricted in noise control legislation.
An acoustic camera records a sound field with a microphone array. Due to
properties of the array, and by using beamforming algorithms, an acoustic
camera can separate sound from different directions. The acoustic camera
measures sound pressure from a sound source. By assuming directivity
properties the sound power of a sound source can be derived from the sound
pressure.
In this thesis an acoustic camera has been evaluated in order to determine
sound power estimation performance and sound source separation ability.
This is tested by six different measurement set-ups in an anechoic chamber.
Two different sound sources are used in the trials: one reference sound
source and one disturbing sound source. The reference sound source has a
calibrated and documented sound power level to which the measurement results
are compared. Measurements were performed at 1 to 5 m distance from the
acoustic camera with both sound sources. The influence of a disturbing sound
source on the reference sound source sound power level was measured with the
sound sources separated 0.65 m to 2.6 m.
The measurements show that the sound power level could at best be determined
within 1 dB. The acoustic camera can separate different sound sources well.
Influence from a disturbing sound source, 10 dB SPL stronger and distanced 1
m from a reference sound source was 2 to 3 dB for mid-frequency one-third
octave bands at 5 m measurement distance.
Measuring sound power with an acoustic camera is fast and mobile compared to
room interaction methods and sound intensity measurements. The results of
this thesis are useful when measuring sound power levels, especially for
sound sources such as chimney outlets, wind power stations and big objects
that can not be moved or do not fit in a room.