Description:
Adaptive Coupler for Calibration of Arbitrarily Shaped Microphones
At a Glance:
The open coupler is an acoustical calibration device that can accommodate a microphone of arbitrarily cross-sectional shape and operates over a wide frequency range
The coupler can be used for arbitrarily shaped microphones, compared to what is currently being used
The main market is the sound and vibration/acoustical equipment
Background:
Couplers and adapters are designed to be used with laboratory and working standard microphones which are generally cylindrical in shape and have a diameter of approximately 23.77 mm, 12.7 mm, or 6.35 mm. However, there are many situations in which a microphone does not fit a standard coupler. The use of a ruggedized environmental housing or housing with attractive acoustical properties can results in a shape that disturbs the fit to a coupler. Furthermore, there are numerous microphones with dimensions other than that of standard microphones. For example, micro-electro-mechanical (MEMS) microphones are very small and widely used due to their size, performance, and cost. These and other consumer microphones have been increasingly used in measurement applications and the cost of such equipment is attractive, especially for studies entailing large-scale spatial sampling. Relative to standard microphones, consumer microphones are typically less robust to changing environmental conditions and require more frequent calibration. Alternative systems to accommodate the physical dimensions of a microphone are not uncommon and custom adapters can be machined for a particular coupler and microphone geometry with considerable effort. Alternatively, the free-field response of a microphone can be assessed with an anechoic chamber. However, anechoic chambers are expensive and the lower the frequency, the more difficult it is to create a sufficiently anechoic space. Calibration by an electrostatic actuator is only possible for microphones with an accessible metallized diaphragm.
Technology Overview :
The open coupler is an acoustical calibration device that can accommodate a microphone of arbitrary cross-sectional shape. The microphone port of the open coupler is a flexible membrane of open cell foam which adapts to the shape of the microphone. Typical acoustical couplers constitute a closed system and variation in the quality of the seal leads to variation in pressure, especially at low frequencies. A reliable seal requires that the shape of the microphone matches that of the coupler orifice, and custom adapters can be machined for a particular coupler and microphone geometry with considerable effort. In contrast, an open system freely exchanges matter with its surroundings. The permeability of open cell foam creates an open system which is less sensitive to the seal around the microphone and relaxes the sensitivity to radiation loading. Instead of relying on cavity and microphone parameters, the interior space is excited with a sound source and the pressure at the face of the membrane is measured with a reference microphone for use with a direct comparison calibration method. The interior cavity of the coupler is also lined with foam to dissipate energy at high frequencies. Damping increases the density of cavity modes, lowering the frequency at which acoustic field becomes diffuse. The small size and damping create uniform conditions at low and high frequencies respectively, allowing for the sensitivity of a microphone to be assessed over a wide frequency range.
Benefits:
Can calibrate arbitrarily, oddly and/or irregularly shaped microphones
Can accommodate a microphone of any cross-sectional shaped with the ability over a wide frequency range
