Numerical design and testing of a sound source for secondary calibration of microphones using the Boundary Element Method

Secondary calibration of microphones in free field is performed by placing the microphone under calibration in an
anechoic chamber with a sound source, and exposing it to a controlled sound field. A calibrated microphone is also
measured as a reference. While the two measurements are usually made consecutively, a variation of this procedure,
where the microphones are measured simultaneously, is considered more advantageous from the metrological point of
view. However, it must be guaranteed that the two microphones receive the same excitation from the source, although
their positions are some distance apart to avoid acoustic interaction.
As a part of the project Euromet-792, aiming to investigate and improve methods for secondary free-field calibration
of microphones, a sound source suitable for simultaneous secondary free-field calibration has been designed using
the Boundary Element Method. The source has a central plug with the eect of reducing the spatial variations of
the sound pressure at the microphone positions within the frequency range of the calibration. After some preliminary
measurements, the source has been tested thoroughly in a calibration setup at the acoustic laboratory of the Danish
Fundamental Metrology Institute (DFM). The design and verification of the source are presented in this communication.