Open Coupler: An Acoustic Calibration Device

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.

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.