Processing and Control for Professional AV Installations
Legacy Hardware

371 SPL Computer

This simple, yet sophisticated SPL Computer is designed to ensure that foreground music and/or paging announcements are always clearly audible and distinct, but never too loud. Proprietary AmbiSense™ technology monitors ambient noise levels continuously, not just during gaps in the audio program, and responds quickly to sudden changes. Setup is simple: enter the parameters of the “normal” acoustic environment, then tell the 371 how much to change the gain, and how quickly to raise or lower it, whenever measured noise levels deviate from the parameters you have set. The front panel LCD guides you with step-through menus. Install a simple, precise and cost-effective solution: the 371 SPL Computer, backed by over 15 years of SPL processing innovation from Symetrix, the engineering-driven company of signal processing specialists.


Calibration can fail if the Sense Input Gain is set too low (–45 dB) or too high (+20 dB). In the LCD setup menus, adjust Sense Input Gain for a proper gain setting. Check Sense for the current sense input level. The setting should be between -45 dB and +20 dB. You can also check the SENSE INPUT by listening with headphones connected to AMBIENT SENSE MONITOR on the front panel to hear what the 371 is reading.

For most applications, an inexpensive low-impedance, omnidirectional microphone, such as a lavaliere microphone works adequately. Some installers have used boundary microphones with good success (they’re unobtrusive). Other installers have mounted microphone cartridges (available from Mouser Electronics in the U.S.) inside electrical boxes equipped with a single-holed cover. In smaller rooms or acoustically live spaces, a directional microphone is a good choice, because you can use its position to favor the ambient sound and minimize pickup from the sound system loudspeakers. If the sense mic is positioned a relatively long distance from the ambient noise source(s), a directional microphone may improve system performance.

There are two separate AC supplies coming out of the PS-3: 8.5 VAC @ 1A and 34.5 VAC CT @ 250mA. Pinout is as follows: 8.5 VAC on pins 6 and 1, 17 VAC on pins 7 and 3, center tap @ pins 4 and 5 are connected to earth ground. This is when viewed at the end of the PS-3 7-pin DIN plug.

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Try re-calibrating making sure that music/page is playing continuously during the calibration process at its normal or normal to high level. Check levels in Set MIN Limit. Check if the level is set too high in the Gain:Sense Ratio. Check in calibration if the maximum limit in Adjust to Maximum is set too high.

Ensure that “Remote” is enabled in the Output Trim menu.

Ultimately the sensing microphone should be placed so that it’s picking up as much of the ambient noise as possible and as little of the speaker program as possible. The sensing microphone needs to “hear” the ambient sound within the controlled space. It is vital that you place the microphone where it primarily picks up a majority of noise rather than the paging or music that is going through the system. Do not locate the sensing microphone near a localized noise source that is not typical of the ambient noise level of the controlled zone, for example, the noise from a large machine of some sort, or maybe a kids play area, or a video game. If you do this the 371 will think that the zone is noisier than it really is.

During the calibration procedure the 371 uses AmbiSense technology to calculates the percentage of the sense signal that is picked up from the paging and/or music that is going through the sound system. AmbiSense allows music or page pickup without any ill effects on its operation, up to about a 50:50 mix of music/page to ambient. In an extreme case where the sense microphone picks up 100% music/ page (0% ambient noise), AmbiSense will not be able to extract enough information to use for gain control.Basically this is how it works:

During the first stage of calibration, the 371 measures and calculates an averaged value of both the program material, and of the signal picked up by the sense microphone. Then during the second step of calibration, after the installer manually increases the program loudness, another set of readings are taken and averaged. Now, we have four numbers:

1. Average sense signal during quiet room

2. Average sense signal during loud(er) music

3. Average program level during first calibration step

4. Average program level during second calibration step

From these numbers (and other sneaky means) we can then calculate a good estimate of what percentage of the sense signal is due to the pickup of program audio, and therefore what percentage of the sense signal is likely to reflect the actual ambient noise.

Calibrate at a quiet-to-average time in the controlled space. It helps if you have an estimate (sound pressure level) of the maximum desired level. Check the sound system to see that it is capable of delivering that level.

No, the only option would be to use a 12VDC to 117VAC inverter with the supplied PS-3 inline power supply.

Check the ratio setting for Gain:Sense Ratio. At 0.5:1, the gain changes are very subtle. Choose a larger value for more change. Check the bar graph in Sense to see if the sense level is always the same, because then the gain will always be the same. Make sure the unit is not in HISTORY or BYPASS mode.

After a lightning strike, flood, nuclear blast, or other catastrophic disturbance, the 371 may become inoperative if the data stored in its EEPROM memory has been corrupted. The 371 can be returned to its factory default state by performing a master reset. To do this:

1. Unplug AC power
2. Press and hold in the NEXT and EXIT buttons.
3. Reconnect AC power.
4. Let go of the NEXT and EXIT buttons.
5. Press and release EXIT three times.
6. Now the LCD screen will display “EEPROM defaults restored” followed by the software version number. After doing this, all previous setup and calibration data will have been lost and must be re-entered as if it were a brand-new installation.