The Importance of Measurement Microphone Venting
Introduction: Venting holes
A measurement microphone’s venting holes are used for static pressure equalization.This means that the microphone can equalize the internal static pressure behind the diaphragm to the same static pressure as outside the microphone. So, the venting hole is basically a small air path that allows the inner volume's pressure to follow the ambient pressure outside the microphone.
Measurement Microphone capsule with all its parts
Proper static pressure equalization is extremely important if we want to make a reliable acoustic measurement. Suppose the static pressure inside the microphone is different than the static pressure outside. In that case, the microphone will not only measure acoustical pressure variation but also a static pressure differential between inside and outside the capsule, leading to erroneous results.
Venting holes are also partly responsible for controlling the low-frequency cutoff (the frequency where the microphone's frequency response is down 3 dB) of the measurement microphone, so design changes to the vent will be used to control the low-frequency response of the unit. The venting must be carefully designed so it allows static pressure equalization without affecting the desired low-frequency response of the microphone. In general, the low-frequency cut-off of the microphone set depends on the microphone's capacitance, the preamplifier's input impedance, venting design, and positioning of the venting in the sound field (directly exposed to or outside the sound field).
Below 10 – 20 Hz, the frequency response of the microphone is also influenced by its time constant. The time constant for a measurement microphone is
around 0.1 s. This is good enough to get a flat frequency response down to 3 – 5 Hz while getting a microphone that is fast enough to equalize for static pressure changes. When the microphone is exposed to a sudden static pressure change (like when mounted in a sound calibrator/pistonphone for
calibration or in a pressure chamber) it is always good practice to wait a couple of seconds to let the microphone equalize its internal static pressure and let the diaphragm return to its normal position.
Why are there different microphone venting locations?
There are two main ways of measurement microphone venting: Front venting (also called side venting) and rear venting. Most GRAS microphones are rear vented but many are available in a front-vented configuration. For example, the GRAS 40AO ½″ Prepolarized Pressure Microphone capsule is rear vented but can be found in the 40AO-FV version, which is front vented. In most cases, front or rear venting will not make any difference in the measurement, but there are some specific applications that require the use of a front-vented microphone.