Processing and Control for Professional AV Installations
Legacy Hardware

572 SPL Computer

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.


572 Getting Started Control Settings
SENSE GAIN IN if your installation is in a normal noise level area, such as a store.
OUT if your installation is in a high noise level area such as a factory.

OPTIONS Switches All Down


1. Complete all input and output connections.
2. Provide constant music or paging signal to the PAGE or MUSIC Line Inputs, or to the PAGE MIC input during the entire calibration procedure.
3. Press the CALIBRATE button in and then release it.
4. The MIN LED will flash, and the system audio will be at the minimum level for approximately 10 seconds.
5. The MAX LED will flash, and the system audio will be at the maximum level for approximately 10 seconds.
6. The 572 is now calibrated (unless the MUSIC/PAGE ERROR LED flashes, see previous FAQ for help if this happens).

Calibration must be done when ambient noise is at its normal minimum level for the area.

1. Repeat the calibration procedure. Make sure that you calibrate at a time when the ambient noise level is at a minimum, and check that you have set the MIN and MAX controls to give you the desired level adjustment range.

2. Disconnect the speakers from the system and, using a microphone preamplifier connected to the speaker(s), listen in headphones to what the 572 “hears” when it is sensing. Listen for any noises, such as air conditioning noise, that could falsely trigger a gain change. Listen for any hum or buzz. Make the necessary adjustments to correct any problems that you discover (reposition sensing speaker, fix or re-route wiring, etc.).

3. If you are using horn speakers, you may be overloading the sense input of the 572. Try disconnecting the amplifier and horn speakers from the FROM AMP and TO SPEAKER CONNECTIONS of the 572. Now connect the LINE OUTPUT of the 572 to the amplifier input, and connect the amplifier output to the speaker inputs. Then, use a single 8-ohm, paper cone speaker as your “sensing speaker”, connecting it to the FROM SPEAKER connection on the 572.

Quickly turn the SAMPLE INTERVAL control first in one direction, then the other direction, returning it to its original position.

Set OPTIONS switch “B” to the “UP” position. The 572’s GAIN (dB) LED’s will now display the level of ambient noise that is being picked up by the sensing speakers.

NOTE: This interrupts normal operation of the 572. Return OPTIONS switch “B” to the “DOWN” position, to restore normal operation of the 572.

During calibration, the 572 requires continuous paging or music signal at its HOUSE or MUSIC Line Inputs or at the PAGE MIC input. If the MUSIC/PAGE ERROR LED flashes, it means that the calibration was unsuccessful, because there was not enough signal present at the input(s).

1. Make sure that OPTIONS dip switch “A” is in the “DOWN” position.
2. Check that paging or music signals are being generated continuously and that they are at an appropriate level, i.e. line level (0dB) for the HOUSE or MUSIC Line Inputs, and mic level (-40dB) for the PAGE MIC input.
3. Make sure that these signals are actually reaching the desired 572 input (check your cables).
4. If you are applying signal to the PAGE MIC input, check that the PAGE MIC trimpot has been turned clockwise. If this trimpot is turned only part of the way to the full clockwise position, and the MUSIC/PAGE ERROR LED flashes when you attempt a calibration, try turning the PAGE MIC trimpot completely clockwise.