Symetrix 572 SPL Computer Used

The Symetrix 572 SPL Computer is an automatic level controller that maximizes intelligibility by changing gain in proportion to environmental noise level changes; in essence, controlling the volume of the background/paging system by measuring the volume of the ambient noise and then adjusting the system gain accordingly. Unique to the 572 is its ability to utilize the sound system’s  loudspeakers as noise measurement transducers, in place of the usual microphones. The 572 switches the speaker line from the amplifier’s output to its own sensing input. In less than one second it reads the ambient noise level and switches the speaker line back to the amplifier. Special impedance matching, frequency shaping, and level shifting circuits allow the 572 to acquire precise relative noise measurements from virtually any speaker line, with any number of speakers of any impedance, transformer coupled or direct coupled, 25V or 70V.