The VSA series is a self-contained sound detection system listed by Underwriters Laboratories, Inc. for the primary protection of reverberant and non-reverberant vaults, such as bank and mercantile vaults. The VSA system contains a microprocessor driven DC differential balance control which is adjusted so that the impedance of the microphone circuits are balanced with the internal impedance of the differential circuit. The VSA will alarm through it’s SPDT alarm relay contacts if the microphone circuit impedance increases or decreases.