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At a Glance
Researchers at Colorado State University have developed an open coupler 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 any arbitrarily shaped microphone, such as the relatively popular micro-electro-mechanical system (MEMS) microphones. The device has been tested in collaboration with many groups, including the North Carolina Museum of Natural Sciences, The National Park Service, SciStarter, and several Universities across the nation.
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 result 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 widely used due to their small size, performance, and cost. These and other consumer microphones have been increasingly used in measurement applications and studies, especially those 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 difficult to create for low frequencies. Furthermore, calibration by an electrostatic actuator is only possible for microphones with an accessible metallized diaphragm.
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.
Figure 1. Orthographic drawing of the open coupler showing a center cutaway of the side view (left panel) and front view (right panel)
Figure 2. Depicts the Open Coupler attached to a LD831 (provides bias voltage only), an first audio recorder (to extract mic sensitivity), and a second audio recorder (playback, or signal generator). Also shown here is a temperature and relatively humidity detector and a calibrator to confirm the reference microphone.
- Can calibrate arbitrarily, oddly and/or irregularly shaped microphones
- Accommodates microphones across a wide frequency range
- Large scale spatial recordings can be calibrated and processed to yield sound level data
- Can support studies having large-scale sampling with multiple acoustical recorders
- Accurate, fast, and inexpensive
- High sensitivity
- Calibration of arbitrarily, oddly and/or irregularly shaped microphones
- Acoustical monitoring (e.g., remote areas, natural habitats, etc.)
- High resolution, multichannel audio recordings
- Sound and vibration/acoustical equipment