Fuzion PLC UK

Symetrix 571 Setup Procedure

The following general points also apply to the new
371 SPL Computer.

Choice of microphones :

The quality of microphone used for sensing is not critical although a full bandwidth unit will obviously give better results than a cheaper, limited bandwidth unit. The pickup pattern of the microphone however is important.

Do NOT use a cardioid type. Cardioid microphones appear initially to be useful as you would think that you could point them in the general direction of the ambient noise. However, cardioid mics are usually employed for vocal use and have frequency response characteristics that are essentially not flat. These characteristics are not desirable for level sensing. Also, cardioid types have differing responses depending on the distance from the sound source. A source that is very close will tend to have more bass response. Ambient noise arrives at the sense position from all directions and to sense it you must use a microphone that has an omni or hemispherical response to pick it up accurately.
PZM types give excellent results.

Before setup, verify the positioning of your sensing microphone(s). The mics must be placed in such a position that they can "hear" both the ambient noise AND the system together. Take care not to place them near any repetitive noises (fruit machines air conditioning ducts etc.) as these will affect overall performance. Also, if the room has high ceilings, the microphones will have to be placed nearer the floor than the ceiling otherwise it is likely that microphone will see the system level as being much higher in level than the ambient space and have more tendency to pick up air movement in ceiling ducts.

If in any doubt, connect a mixer to the microphone and listen to what it hears through headphones. Any problems will become immediately apparent.

Take care to test the system at a very high level before setup to ensure that the acoustic level of the system does not overdrive the microphone input stage. If it does, then the unit will not function correctly. Overdrive of the mic input stage is indicated by the Gain display going blank (unit output level will then be attenuated). If the mic levels are set too low then the Sense Input Error LED will flash.

Setup :

Before setup ensure that you have met all the criteria above.

To set the unit up, the ambient in the room must be at its lowest expected level. If you wish, (if the room is too quiet) you can simulate this ambient level using a (preferably omnidirectional) loudspeaker in the room producing noise such as compressed music (preferred) or pink noise. Take care that the noise does not excite the space so as to introduce standing waves (especially in large, hard walled or hard floored spaces). Music or "real" crowd noise is preferred as it will give better results.

Press the CAL button and play an announcement OR music through the unit. Adjust the MIN level control to the desired level ABOVE the ambient noise you wish to maintain.

Clarification point :

When calibrated, this ratio ABOVE the ambient will be maintained at all times UNTIL you reach the MAX setting - then the system level will STOP rising.

For example if you set the MIN control so that the system level is 6dB above ambient and the ambient level then rises by 10dB, the system level will ALSO rise by 10dB to maintain the output at 6dB ABOVE ambient.

Setup - continued…….

When prompted, (after about 8 seconds in MIN mode after the last time you moved the control) set the MAX level control to the absolute maximum level that the system can endure (or the absolute maximum you wish it to achieve). This will set a maximum that will NOT be exceeded under any circumstances (even if the ambient rises to a point where you would expect it to).

Note !
You cannot calibrate the unit without audio passing through it !

Calibration is now complete.

To check operation :

Measure the average level of the test announcement/music using a sound level meter with the ambient at a low level. Now raise the ambient using a noise source and observe the output level of the 571 rise in proportion to the increase in ambient noise. Best averaging time for the 571 is around 1 minute. Faster times may be used but real world measurements are best achieved when the response rate is slow. Note that the unit will respond slowly, gradually adjusting the system level to raise the system level above the increase in ambient noise.

Run the test for a few minutes to confirm operation.

Useful Tips:

Note that the unit works by comparing the sensing microphone level to the 571’s OUTPUT. This comparison uses the output : microphone level ratio from the setup calibration procedure. If the ambient noise detected at the sense mic rises, the output level is raised (or lowered - if the ambient has reduced) in proportion to what it sees at the device output. The unit averages out the ambient noise over a period of time and then decides to make an adjustment based on the results. Do not expect the unit to go up immediately you raise ambient. It is not designed to do this. It would be very undesirable for the system level to "pump" up and down. You can, if you wish, get the unit to do this, but results will not be "real world" so don’t leave the unit set up in such a way that you can hear it go up and down.

The room must remain at its lowest expected ambient level during the ENTIRE calibration procedure - even when you are setting up the MAX level control !

Any adjustment downstream of the 571 (such as the main amplifier or a 70V/100V line attenuator) will necessitate recalibration of the unit. All adjustments that are required downstream MUST be made BEFORE the unit is calibrated.

The level of signal that is delivered to the unit when setting up MUST be at the level you expect to use in normal use. It’s important to realise that the 571 only makes gain changes in response to changes in ambient. It cannot make accurate changes if the input level feed is changed after calibration.