A monopole sound source is used for reciprocal measurements in vehicles.
It is an omni-directional volume velocity sound source.
The source has a particle velocity reference sensor so the output can be measured.

The sound power output of a monopole source is proportional with the frequency squared.
The sound pressure at one meter therefore low at low frequencies.
To be able to reach relative high sound pressure at low frequencies with an acceptable source velocity
one needs to increase the source diameter.
For an omni directional behaviour however, the diameter should be small compared to the wavelength
Because of that contradiction two monopole sources are designed.
The low frequency monopole with a source diameter of 12cm and a
high frequency monopole with a source diameter of 15mm.  

A head shaped sound source is developed to be able to calculate what a Human would hear.

Datasheets

• The low frequency monopole 30Hz-300Hz
• The high frequency monopole 100Hz-6kHz
• Head shaped source (100Hz-6kHz)

Key papers

• Microflown based monopole sound sources for reciprocal measurements, H.E. de Bree and Tom Basten, SAE, 2008