Vacuum‐isolation vessel for measurement of thermal noise in microphones

Abstract

In the past, the inherent electronic and thermal noise in a condenser microphone and the associated preamplifier were measured in facilities with poor acoustic isolation properties, especially at low frequencies where 1/fn (n = 1,2) noise sources are dominant. These background noise levels constitute one of the most important specifications of a microphone, for they determine the lowest sound pressure that can be detected. Therefore, a new method for measuring the thermal agitation noise within a bandwidth 1–25 000 Hz has been developed. The presentation will discuss the theoretical and experimental measurements of thermal agitation noise in a condenser microphone by using a high‐vacuum isolation apparatus to achieve an acoustically quiet and vibration‐free environment. The 1‐in. condenser microphone is placed inside the isolation vessel where the pressure inside is atmospheric. The isolation vessel is suspended inside a larger vessel, in which the intervening air is evacuated. This method gives an accura...