Sources of background noise in low-frequency condenser microphones

The background noise of a condenser microphone originates from external sources (electromagnetic interference, vibration) and internal sources (self-noise). The leading internal sources include the membrane damping resistance, the polarization voltage resistance (air condenser microphone only), the input resistance to the preamplifier, the capillary vent resistance, and a source of 1/f noise. These sources are modeled in a noise-equivalent circuit, from which an expression for the noise power spectral density is derived. This expression is compared to measurements on a low-frequency electret condenser microphone taken in an acoustic isolation vessel. The capillary vent and 1/f noises vary inversely with frequency and, thus, impact the background noise levels in the infrasonic region.