Symetrix 372 SPL Computer User manual

372 SPL Computer
USERNGUIDE

372 SPL Computer User’s Guide
© January 2001Symetrix, Inc.
All rights reserved.
Printed in the United States of America
Symetrix Part Number 53372-0C00
The information in this guide is subject to
change without notice. Symetrix, Inc. shall
not be liable for technical or editorial
errors or omissions contained herein; nor
is it liable for incidental or consequential
damages resulting from the furnishing,
performance, or use of this material.
Mention of third-party products is for in-
formational purposes only and constitutes
neither an endorsement nor a recommen-
dation. Symetrix assumes no responsibil-
ity with regard to the performance or use
of these products.
Under copyright laws, no part of this user
guide may be reproduced or transmitted
in any form or by any means, electronic or
mechanical, without permission in writing
from Symetrix, Inc. If, however, your only
means of access is electronic, permission
to print one copy is hereby granted.
Permission to copy the Architects and
Engineers Specificiations for written
proposals specifying equipment for sound
reinforcement systems is, also, granted.
Symetrix, Inc.
6408 216th St. SW
Mountlake Terrace WA 98043
USA
Tel:[phone]
Fax:[phone]
Web: www.symetrixaudio.com
Email: [email protected]

1
Contents
Before You Begin 2
What Ships in the Box 2
Getting Help 2
Optional Rackmount Accessories 2
Notational Conventions in this User Guide 2
Operational Safety Summary 3
Equipment Markings 3
Features of the 372 4
Checklist of Tasks for Hardware Connections,
System Setup, and Calibration 5
Making Hardware Connections 6
Overview of Rear Panel Connectors and Connections 6
Sensing Speaker Considerations 8
372 Basic Connection 9
Connecting an External Relay to Drive Amplifiers
Larger than 200W 10
Connecting Up to 20 Slaves 11
Connecting an External Relay to Attach Slaves 12
Connecting a Push Button Switch to Force or
Prevent Sensing 13
System Setup 14
Overview of Front Panel 14
How System Setup Works 15
Description of Menu Features 16
Operating Mode 16
Status 18
History 18
Listen 18
Set MIN Limit 19
Set MAX Limit 19
Averaging Time 19
Gain: Sense Ratio 19
Symetrix 372 SPL Computer User’s Guide
Gap Threshold 20
Force Sense Time 20
AGC 20
Trigger Input 21
Setup Unlocked 21
Calibrating the 372 22
Conditions Under Which the 372
Will Not Sense 23
Signal Flow Diagram 24
Input and Output Connector Wiring 25
Troubleshooting 26
Hardware Specifications 28
Architects and Engineers Specifications 29
Warranty 30
Service 31
Declaration of Conformity 32

2
What Ships in the Box Getting Help
If you have questions beyond the scope of this guide, contact our Customer
Service Department in the following ways:
TUS customers
[phone]
International customers
[phone]
8:00 am to 4:30 pm Monday through Friday, Pacific Time
F
[phone]
[email protected]
www.symetrixaudio.com
Optional Rackmount Accessories
MODEL ITEM DESCRIPTION
RM-3 19" Rackmount Tray 1U shelf
FP-3 Filler Panel Covers unused half of rack tray when
one 372 unit is mounted
PY-3 Y Power Cable Connects a 372 with any other 300
Series product
Contact your dealer or Symetrix for purchasing information.
Notational Conventions in this User Guide
Note Identifies information that needs extra emphasis. Generally
supplies extra information to help you to better use the 372.
CAUTION Identifies information that, if ignored, may cause damage to
the 372 or other equipment in your system.
WARNING Identifies information that, if ignored, may be hazardous to
your health or that of others.
SMALL CAPS Controls, switches or other markings on the chassis of the 372.
Bold Face Indicates menu option in the 372 LCD display.
Before You Begin
| What Ships in the Box | Getting Help | Optional Rackmount Accessories | Notational
Conventions in this User Guide | Operator Safety Summary | Equipment Markings
The 372 unit
One PS-3 or PS-3E (export
version) power supply
This user’s guide
Symetrix 372 SPL Computer User’s Guide

3
AVIS:
NE PAS OUVRIR
Il ne se trouve a l’interieur aucune piece pourvant entre reparée l’usager.
SEE OWNERS MANUAL. VOIR CAHIER D’INSTRUCTIONS.
S’adresser a un reparateur compétent.
RISQUE DE CHOC ELECTRIQUE
N
o user serviceable parts inside. Refer servicing to qualified service personnel
.
CAUTION
WARNING:
TO REDUCE THE RISK OF FIRE OR
ELECTRIC SHOCK DO NOT EXPOSE
THIS EQUIPMENT TO RAIN OR MOISTURE
DO NOT OPEN
RISK OF ELECTRIC SHOCK
Before You Begin
| What Ships in the Box | Getting Help | Optional Rackmount Accessories | Notational
Conventions in this User Guide | Operator Safety Summary | Equipment Markings
Operator Safety Summary
Follow all warnings and instructions. Install
in accordance with the manufacturer’s
instructions.
Power Source This product is intended
to operate from a Symetrix PS-3 or PS-3E
power supply.
Grounding The chassis of this product is
grounded through the grounding conduc-
tor of the PS-3 or PS-3E power cord. To
avoid electric shock, plug the power cord
into a properly wired receptacle before
making any connections to the product. A
protective ground connection, by way of
the grounding conductor in the power
cord, is essential for safe operation. Do
not defeat the safety purpose of the
grounding plug. The grounding plug has
two blades and a third grounding prong.
The third prong is provided for your
safety. When the provided plug does not
fit your outlet, consult an electrician for
replacement of the obsolete outlet.
Danger from Loss of Ground If the
protective ground connection is lost, all
accessible conductive parts, including
knobs and controls that may appear to be
insulated, can render an electric shock.
Proper Power Cord Use only the power
cord and connector specified for the
product and your operating locale. Use
only a cord that is in good condition.
Protect the power cord from being walked
on or pinched, particularly at the plug,
convenience receptacle, and the point
where the cord exits from the apparatus.
Operating Location Do not operate this
equipment under any of the following
conditions: explosive atmospheres, in wet
locations, in inclement weather, improper
or unknown AC mains voltage, or if
improperly fused. Do not install near any
heat source such as radiators, heat
registers, stoves, or other apparatus
(including amplifiers) that produce heat.
Unplug this apparatus during lightning
storms or when unused for long periods of
time.
Stay Out of the Box To avoid personal
injury (or worse), do not remove the
product covers or panels. Do not operate
the product without the covers and panels
properly installed. Only use accessories
specified by the manufacturer. Clean only
with a damp cloth.
User-serviceable parts There are no
user serviceable parts inside the 372. In
case of failure, refer all servicing to the
factory. Servicing is required when the
372 has been damaged in any way, such
as when a power supply cord or plug is
damaged, liquid has been spilled or
objects have fallen into the apparatus, the
apparatus has been exposed to rain or
moisture, does not operate normally, or
has been dropped.
Equipment Markings
The lightning flash with arrowhead symbol
within an equilateral triangle is intended
to alert the user of the presence of
uninsulated “dangerous voltage” within
the product’s enclosure that may be of
sufficient magnitude to constitute a risk of
electric shock to persons. The exclamation
point within an equilateral triangle is
intended to alert the user of the presence
of important operating and maintenance
(servicing) instructions in the literature
accompanying the product (i.e., this user’s
guide).
CAUTION
To prevent electric shock, do not use the
polarized plug supplied with the unit with
any extension cord, receptacle, or other
outlet unless the blades can be fully
inserted.
Symetrix 372 SPL Computer User’s Guide

4
Features of the 372
Uses Speakers for Noise Sensing In less than 1 second, the Symetrix 372 temporarily opens up the amplifier
connection and routes the loudspeakers to a preamplifier for the purpose of
measuring changes in the ambient noise level. Samples during absence of page
and/or music audio, or at preset intervals.
Headphone Monitoring Monitor the sense signal using a separate front panel headphone output.
5 Operating Modes MUSIC Music only or music + page. Use Gap and/or Force Sense Time.
PAGE Only paging and annoucement audio. Use Force Sense Time at
preset intervals.
SLAVE Forces all 372 units in a multi-zone system to execute a sense
sample at the same time.
HISTORY Records and displays the lowest and highest SPL readings from when
the unit was last reset.
BYPASS Bypasses the gain control of the SPL controller and the AGC.
Signal Path Controls mono or stereo signals through Euroblock connectors.
Programmable Sensing Operation Select the auto-sense mode and/or the time interval between sense samples.
Simple Calibration Use step-through menus on the front panel LCD. Perform calibration under
typical installation conditions. No waiting for the quietest or noisiest ambient
environment.
AGC Control Enables or disables auto leveling of input signals.
Ambient Adjustment Choose an adjustment ratio of SPL change vs. program level change.
Sense Signal Monitoring Display numeric reading and relative bargraph of the signal appearing at the
sense terminals.
Listen to sense signal with headphones connected to front panel mono output.
Gain Controls Set minimum and maximum limits for SPL gain range. Gain range is +20 to –30 db.
Averaging Time Choose integration time of the running average SPL.
Symetrix 372 SPL Computer User’s Guide

5
REQUIRED RECOMMENDED OPTIONAL TASK TO DO
HARDWARE Connecting input sources
Connecting output sources (use Symetrix 303 for
unbalanced outputs)
Connecting speaker for sensing and music/paging
Connecting speaker for either music or paging; does not
sense
Connecting an external relay if maximum amplifier
voltage and wattage exceeded
Connecting slave 372s
Connecting a switch or contact closure to manually force
or prevent sensing
SYSTEM SETUP Choose Operating Mode*
View calibration Status
Collect History of highest and lowest SPLs
Set gain level in Listen
Set MIN limit (gain)
Set MAX limit (gain)
Specify number of samples in Averaging Time
Specify Gain: Sense Ratio
Set Gap Threshold
Specify time for Force Sense Time
Enable AGC
Specify type of Trigger Input
Lock or Unlock Setup
CALIBRATION Equating ambient noise level to 0 db (Unity) gain
*Bold face indicates menu name in LCD display
Checklist of Tasks for Hardware Connections,
System Setup, and Calibration
CONNECTIONS
Symetrix 372 SPL Computer User’s Guide

6
Overview of Rear Panel Connectors and Connections
CONNECTION CONNECTOR TYPE WHAT IT DOES
Power Input 7-pin DIN connector Accepts power only from Symetrix
PS-3 or PS-3E power supply.
FROM AMP 2 terminal barrier strip Accepts signal from power amplifier.
This output may be grounded or
bridged. 8A maximum current rating.
If your speaker load exceeds the
following ratings, you will need to use
an external relay (See Connecting an
External Relay to Attach Slaves).
MAX AMPLIFIER OUTPUT RATINGS
16 ohms 400W
70V 300W
8 ohms 200W
4 ohms 100W
TOSPEAKER 2 terminal barrier strip Delivers signal to loudspeaker system.
Do not ground either of these terminals.
SENSE EXT RELAY Euroblock terminals Open-collector drive signal for exter-
nal sense relay. Can power amplifiers
larger than 200W at 8 ohms (See
Connecting an External Relay to
Attach Slaves). Controls as many as
20 slave 372 units directly from the
EXT RELAY output of the master 372.
This output can also drive the TRIGGER
input.
GROUND Euroblock terminals Ground return for external relay and
trigger.
TRIGGER Euroblock terminals Active low input forces sense event.
Can force or prevent a sense event
manually (See Connecting a Push
Button Switch to Force or Prevent
Sensing). Is internally pulled to the 5V
supply by way of a 20k ohm resistor.
From this connection attach slave 372
units to the master 372.
Making Hardware Connections
| Overview of Rear Panel Connectors and Connections | Sensing Speaker Considerations
| 372 Basic Connection | Connecting an External Relay to Drive Amplifiers Larger than 200W
| Connecting Up to 20 Slaves | Connecting an External Relay to Attach Slaves
| Connecting a Push Button Switch to Force or Prevent Sensing
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
Symetrix 372 SPL Computer User’s Guide

7
CONNECTION CONNECTOR TYPE WHAT IT DOES
LINE OUTPUT Euroblock terminals These connectors deliver a differential
balanced output at 200 ohm source
impedance. For unbalanced use, use
the positive line output terminals and
the ground terminals. Ignore (float)
the minus output terminals.
The output stage is a pair of opamps,
with the plus and minus output
terminals at 180°antiphase with each
other. It emulates a grounded center-
tap transformer. The series build-out
resistors are 100 ohms in each leg.
For wiring diagram, see the section
Input and Output Connector Wiring.
LINE INPUT Euroblock terminals Balanced input for the 372. 20k ohm
balanced bridging. For unbalanced
sources, connect the minus input
terminal to the source ground at the
source.
Note
Feeding unbalanced inputs directly from
balanced outputs is not recommended
due to the possibility of ground loops.
You may want to use the Symetrix 303
isolation transformer to break the
ground connection and eliminate the
ground loop.
With unbalanced sources, it is preferable
to carry the low side of the input all the
way back to the ground connection of
the source.
For wiring diagram, see the section
Input and Output Connector Wiring.
Making Hardware Connections
| Overview of Rear Panel Connectors and Connections | Sensing Speaker Considerations
| 372 Basic Connection | Connecting an External Relay to Drive Amplifiers Larger than 200W
| Connecting Up to 20 Slaves | Connecting an External Relay to Attach Slaves
| Connecting a Push Button Switch to Force or Prevent Sensing
Symetrix 372 SPL Computer User’s Guide

8
Sensing Speaker Considerations
Making Hardware Connections
Speaker Location
If a sensing speaker is located near a
localized noise source that is not
typical for the ambient noise level of
the controlled zone, the 372 deter-
mines that the zone is noiser than it
really is. For example, the noise from
a large machine of some sort, or
maybe a kids play area, or a video
game. The solution is to have the
speakers that are near the noise
source driven directly from the
amplifier so that they are not used
for sensing.
| Overview of Rear Panel Connectors and Connections | Sensing Speaker Considerations
| 372 Basic Connection | Connecting an External Relay to Drive Amplifiers Larger than 200W
| Connecting Up to 20 Slaves | Connecting an External Relay to Attach Slaves
| Connecting a Push Button Switch to Force or Prevent Sensing
70V Systems
In a 70V system, the signal level
returned by the speakers when the
372 is sensing is higher, because of
the step-up action of the line trans-
formers. With highly-efficient speak-
ers, this may present the problem of
overloading the sense input of the
372. Use the Listen mode to check
the level of the signal while the 372
senses. You may need to lower the
gain position of the AMBIENT SENSE
GAIN switch. More information about
the gain setting is found in Listen
under the section System Setup.
A common practice in 70V systems is
to provide autotransformers or L-pads
after the line transformer to allow
local volume control. These controls
should not be used with the 372.
Allowing any sort of signal level
control after the 372 invalidates your
calibration settings. If you must
provide a local control, then ensure
that the controlled zone is a minor
part of the entire system. In this
controlled zone, volume level will not
be ideal and sensing should not be
used.
Also, it is imperative that every
speaker and transformer in the
system operate in-phase.
4, 8, and 16 Ohm Systems
In these speaker impedance systems,
the signal levels returned to the 372
are low. Use the Listen mode to
check the level of the signal while the
372 senses. You may need to select
the high gain position of the AMBIENT
SENSE GAIN switch.
Symetrix 372 SPL Computer User’s Guide

9
372 Basic Connection
Making Hardware Connections
Signal I/O Connections
These connectors are designed for
use with bare wire. Do not tin strand
wires before inserting them into the
connectors.
If using unbalanced connections, see
wiring diagram in the section Input
and Output Connector Wiring.
Note
Feeding unbalanced inputs directly from
balanced outputs is not recommended
due to the possibility of ground loops.
An option is to use the Symetrix 303
isolation transformer to break the
ground connection and eliminate the
ground loop.
1Connect your line-level source to
the rear-panel input connectors.
These connectors may be driven
from a balanced or unbalanced
source. When using an unbal-
anced source, connect the output
minus terminal to ground either at
the source or at the 372.
2Connect the outputs as required.
These connectors deliver a
differential balanced output signal.
For unbalanced use, use the
positive line output terminals and
the ground terminals. Ignore
(float) the minus output terminals.
Amplifier and Speaker
Connections
3Connect your amplifier and
speakers to the barrier strip
connectors.
The speaker and amplifier connec-
tions are earth-free (floating).
The 372 treats the speaker
connections after its sensing relay
as if they were a balanced line.
Do not ground either side of the
speaker line. It is OK if the one
side of the amplifier output is
grounded.
| Overview of Rear Panel Connectors and Connections | Sensing Speaker Considerations
| 372 Basic Connection | Connecting an External Relay to Drive Amplifiers Larger than 200W
| Connecting Up to 20 Slaves | Connecting an External Relay to Attach Slaves
| Connecting a Push Button Switch to Force or Prevent Sensing
Symetrix 372 SPL Computer User’s Guide
Input Source
Amplifier
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
1
2
3
3
Connect all speakers used for sensing
and either music or paging to this
output.
These speakers are required.
The relay contacts handle 8A
continuous current.
Connect all speakers that do not sense
to the amplifier output.
Use these speakers for only music or
paging.
These speakers are optional.

10
Making Hardware Connections
Connecting an External Relay to Drive Amplifiers Larger than 200W
When controlling amplifiers larger
than 200W, you must use an external
DPDT relay. The internal relay of the
372 cannot handle more than 8A of
load current.
RECOMMENDED MAXIMUM
AMPLIFIER WATTAGES
400W 16 ohms
300W 70V
200W 8 ohms
100W 4 ohms
You must supply power for the relay
and the relay must have a back-
biased 1N4002 diode connected
across its coil to absorb the back EMF
from the relay coil. The transistor has
a 60V VCE rating (MPSA 06).
The 24V version of the Potter &
Brumfield PRD11DGO works well. Its
DPDT contacts are rated at 30A.
| Overview of Rear Panel Connectors and Connections | Sensing Speaker Considerations
| 372 Basic Connection | Connecting an External Relay to Drive Amplifiers
Larger than 200W | Connecting Up to 20 Slaves | Connecting an External Relay to Attach Slaves
| Connecting a Push Button Switch to Force or Prevent Sensing
Symetrix 372 SPL Computer User’s Guide
Low (–)
High (+)
Low (–)
High (+)
1N4002 or
Equivalent
Diode
LEFT
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
DC
Power
Supply
(+) (–)
EXT. RELAY
TRIGGER
SENSE
TO
SPEAKER
Amplifier
External DC
Coil Relay

LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
Master 372
Slave 372
Slave Group up to 20 units
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
11
Making Hardware Connections
Connecting Up to 20 Slaves
A master 372 can direct any number
of slave 372 units to take an ambient
sample. You can parallel slave 372
units directly together from the
master unit to enable you to sense at
the same time in adjacent or multiple
installation zones. If the slave 372
units are in remote locations, use an
external relay to connect the slaves
to the master unit (see Connecting an
External Relay to Attach Slaves).
As many as 20 slave 372 units may
be controlled directly from the master
372.
MAXIMUM CABLE LENGTH
Depends upon the difference in
ground potential (if any) between the
individual units.
| Overview of Rear Panel Connectors and Connections | Sensing Speaker Considerations
| 372 Basic Connection | Connecting an External Relay to Drive Amplifiers Larger than 200W
| Connecting Up to 20 Slaves | Connecting an External Relay to Attach Slaves
| Connecting a Push Button Switch to Force or Prevent Sensing
Connect a cable from TRIGGER and
Ground terminals of the master unit.
Connect the other end of the cable to
the TRIGGER input and Ground terminals
of each slave unit in parallel.
From the front panel of the slave 372
LCD menu display:
Press NEXT to gain access to Operating
Mode.
Rotate ADJUST to select Slave mode.
Press NEXT to locate Trigger Input.
Check to see that Force Sense is selected.
(Force Sense is the default setting.)
(See Operating Mode—Slave in the
section System Setup for additional
information)
Symetrix 372 SPL Computer User’s Guide
IMPORTANT!
As of 372 hardware revision C, an
internal jumper (J8) has been added
which must be in correctly config-
ured on the master 372 in order to
properly drive the slaves.
On the master 372, remove the top
and move the jumper (J8) to con-
nect pins 2 and 3. (Pins 1 and 2
are connected at the factory. This is
the default position and this jumper
should only be changed on the mas-
ter 372. J8 should remain in the fac-
tory default position on all slaves).
Moving J8 to connect pins 2 and
3 configures the TRIGGER input
connection on the Master 372 to
become a Slave Drive OUTPUT. This
reconfiguration is necessary to pre-
vent a "false sensing" problem when
multiple zones bleed into each other.
This new way of configuring the
master 372 ensures that all 372's
will sense in sync thus avoiding false
sense readings from zone bleed.

12
Making Hardware Connections
Connecting an External Relay to Attach Slaves
Use an External Relay:
To connect more than 20 slave 372
units together from the master unit to
sense at the same time in adjacent or
multiple installation zones.
—or—
To connect slave 372 units in remote
locations from the master unit.
If some of the slave units are a long
distance away from the master, a
direct connection might give rise to
ground loop problems. To avoid this,
use an external relay to isolate the
connection between the master and
the distant slaves.
Connect a cable from TRIGGER and
Ground terminals of the master unit to
the External Relay control and Ground
terminals on the rear panel of the
external relay.
Connect cables from the external relay
outputs to the TRIGGER and Ground
terminals of each slave unit in parallel.
From the front panel of the slave 372
LCD menu display:
Press NEXT to gain access to Operating
Mode.
Rotate ADJUST to select Slave mode.
Press NEXT to locate Trigger Input.
Check to see that Force Sense is selected.
(Force Sense is the default setting.)
(See Description of Menu Features,
Operating Mode—Slave in the section
System Setup for additional information)
| Overview of Rear Panel Connectors and Connections | Sensing Speaker Considerations
| 372 Basic Connection | Connecting an External Relay to Drive Amplifiers Larger than 200W
| Connecting Up to 20 Slaves | Connecting an External Relay to Attach Slaves
| Connecting a Push Button Switch to Force or Prevent Sensing
Symetrix 372 SPL Computer User’s Guide
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
Ext
Relay
Control
1234
Slaved Groups
Sen Sen Sen Sen
External Relay Pack
Master 372
Slave 372
Slave Group 1 Slave Group 2
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
IMPORTANT!
Please see notes on previous page
regarding internal jumper configura-
tion of the master 372.

13
Making Hardware Connections
Connecting a Push Button Switch to Force or Prevent Sensing
You can manually prevent sensing or
manually control sensing by using a
push button switch.
In conjunction with the TRIGGER
connection on the rear panel, you
must also choose Trigger Input in
the LCD menu display to enable either
a force sense or prevent sense event.
Prevent Sense is useful in the Music
mode during voice announcements, in
conjunction with an external contact
closure. In Music mode the 372 has
no other way to know if an announce-
ment is happening.
An example of an external contact
closure might be a spare set of
contacts on a push-to-talk micro-
phone. The 372 is prevented from
sensing the ambient room level
whenever these contacts are pressed
closed. Otherwise, a timed sense
event might “cut off” the announcer
when he/she is speaking.
LEFT
372
LINE OUTPUT
RIGHTLEFTRIGHT
LINE INPUTFROM AMP
INPUT
POWER
CONNECT TO
SYMETRIX
PS-3 OR PS-3E
POWER
SUPPLY ONLY
EXT. RELAY
SENSE
TRIGGER
TO SPEAKER
Push Button Switch
Connect a momentary contact closure
between the TRIGGER and Ground
terminals.
From the front panel of the slave 372
LCD menu display:
Press NEXT to gain access to Trigger
Input.
Check to see that Force Sense is selected.
(Force Sense is the default setting.)
(For additional information, see in the
section System Setup, Description of
Menu Features—Trigger Input)
Connect a normally-open set of switch
or relay contacts between the TRIGGER
and Ground terminals.
From the front panel of the slave 372
LCD menu display:
Press NEXT to gain access to Trigger
Input.
Rotate ADJUST to select Prevent Sense.
(For additional information, see in the
section System Setup, Description of
Menu Features—Trigger Input)
| Overview of Rear Panel Connectors and Connections | Sensing Speaker Considerations
| 372 Basic Connection | Connecting an External Relay to Drive Amplifiers Larger than 200W
| Connecting Up to 20 Slaves | Connecting an External Relay to Attach Slaves
| Connecting a Push Button Switch to Force or Prevent Sensing
Symetrix 372 SPL Computer User’s Guide
To Force a Sense Event
To Prevent a Sense Event

14
System Setup
| Overview of Front Panel | How System Setup Works | Description of Menu Features
Overview of Front Panel
CONTROL CONTROL TYPE WHAT IT DOES
SETUP LCD menu display Two line display shows levels and
settings of selected menu feature.
NEXT From normal operation, the first press
enters the setup mode; subsequent
presses step through the setup process.
EXIT Exits the setup mode. The unit reverts
to normal operation.
ADJUST Rotary control used in setup mode for
parameter setting.
CAL Press to have the 372 read the
current ambient level and equate this
reading to 0 db (Unity) gain. Displays
the relative ambient reading which it
considers as normal. CAL is only
active in normal operating mode and
when setup is unlocked.
AMBIENT SENSE Headphone jack 1/4 inch TRS jack suitable for head-
phone impedances of 60 ohms or
higher, stereo or mono. Enables
listening to the output of the sensing
system when 372 is in Listen mode.
Helpful for troubleshooting or for
figuring out what the 372 is listening
to. For more information, see
Description of Menu Features, Oper-
ating Mode—Listen.
MONITOR Adjusts the volume level at the
headphone jack.
GAIN 3-position switch adjusts the sensitiv-
ity of the sensing circuitry to accom-
modate different conditions. Set the
gain level while in the Listen mode.
For more information, see Description
of Menu Features, Operating Mode—
Listen.
Note
Once the 372 has been set up, do not
change the AMBIENT SENSE GAIN position.
372
COMPUTER
SPL
HIGH
MID
LOW
CALADJUST
EXITNEXT
MENU
SETUP
GAINMONITOR
AMBIENT SENSE
Symetrix 372 SPL Computer User’s Guide

15
System Setup
How System Setup Works
REQUIRED RECOMMENDED OPTIONAL MENU FEATURE IN LCD DISPLAY
Choose Operating Mode*
View calibration Status
Collect History of highest and lowest
SPLs
Set gain level in Listen
Set MIN limit (gain)
Set MAX limit (gain)
Specify Averaging Time
Specify Gain: Sense Ratio
Set Gap Threshold
Specify time for Force Sense Time
Enable AGC
Specify type of Trigger Input
Lock or Unlock Setup
*Bold face indicates menu name in LCD display
| Overview of Front Panel | How System Setup Works | Description of Menu Features
Use the two push buttons, NEXT and
EXIT, with the parameter adjustment
knob, ADJUST, to gain access to menu
setup options in the LCD display.
1Press NEXT to enter the setup
mode.
2Rotate ADJUST to select a specific
value or choice.
3Press NEXT again to gain access
to another menu option. Subse-
quent presses of Next change
menu options.
4Press EXIT to end the setup
process, save any changes made,
and return the unit to normal
operation.
—or—
Allow 30 seconds of inactivity,
after which the unit returns to
normal operation. (Does not
apply while using the Listen
feature.)
5After setup is complete, calibrate
the unit (See the section Calibrat-
ing the 372).
Symetrix 372 SPL Computer User’s Guide

16
Description of Menu Features
ROTATE ADJUST TO
SELECT VALUE OR
PRESS NEXT TO SELECT MENU FEATURE CHOICE DESCRIPTION
Operating Mode Music This mode considers everything as if it were music,
even though it may contain a mix of music and
announcement audio.
Varies the gain of this program channel to match the
ambient noise level.
How Sampling Works —Takes SPL measurements during
silent portions in the program material. If the 372 doesn’t
find gaps, then it forces a sense period (samples for about
0.75 seconds) after which, it restores the program channel
signal.
How You Can Control Sampling —You can force a sense
period manually and select the time of the forced event.
See Trigger Input in this section to enable this feature.
Can also perform simple AGC on the input signal to
improve the consistency of the output level. See AGC in
this section to enable this feature.
Page Select this mode if you are doing paging or
announcement audio only.
Varies the gain of the program channel to match the
ambient noise level.
How Sampling Works —The presence of audio in the
program channel prevents sampling from taking place.
Sampling stays “locked out” until the program channel has
remained silent for at least 20 seconds. In the rare event
that a page or announcement begins while sampling is
taking place (samples for about 0.75 seconds), the current
sense cycle/period will complete and then further sampling
is prevented.
How You Can Control Sampling —If your application cannot
tolerate paging interuptions, you can force sensing to
occur after a page announcement. See Force Sense Time
in this section to enable this feature.
Can perform simple AGC on over-threshold compression
on the paging signal at 3:1 ratio on signals above 0 dBu
(defeatable). See AGC in this section to enable this
feature.
System Setup
| Overview of Front Panel | How System Setup Works | Description of Menu Features
REQUIRED RECOMMENDED OPTIONAL
 
Symetrix 372 SPL Computer User’s Guide

17
Description of Menu Features continues
ROTATE ADJUST TO
SELECT VALUE OR
PRESS NEXT TO SELECT MENU FEATURE CHOICE DESCRIPTION
Operating Mode continues Slave Select this mode if you want to designate the 372
unit as a slave. Slave units connected to a master
372 unit enable you to sense at the same time in
adjacent or multiple installation zones.
This mode senses only in response to an external sense
command received by way of the TRIGGER input connector
on the rear panel of the slave.
As many as 20 slave 372 units may be controlled directly
from the EXT RELAY output of the master 372. Connect a
cable from EXT RELAY and Ground terminals of the master
unit to the TRIGGER input and Ground terminals of each
slave unit in parallel (See Connecting Up to 20 Slaves).
More than 20 slave 372 units may be controlled using an
external relay (See Connecting an External Relay to Attach
Slaves).
If some of the slave units are a long distance away from
the master, a direct connection might give rise to ground
loop problems. To avoid this, use an external relay to
isolate the connection between the master and the distant
slaves (See Connecting an External Relay to Attach Slaves).
On the front panel of the slave units, press NEXT to locate
Trigger Input in the LCD menu display. Check to see that
Force Sense is selected. Force Sense is the default setting.
History Select this mode if you only want the 372 to collect
data about the highest and lowest ambient SPL in
the installation zone(s).
The 372 does not perform any gain control in this mode.
You can determine the start of the history period or collec-
tion of data. See History later in this section to reset or
start the collection of the relative SPL history.
To view the current SPL readings, see Listen.
Bypass Select this mode when you want to leave the 372
physically connected in the signal path, only pass
the signal unaltered, and never take SPL samples.
This feature is useful if you encounter setup problems in an
installation. You can temporarily disable the 372 until you
have the time to correct the problem. Or when the action
of the 372 is temporarily unwanted, for example, when an
unusual event is taking place.
You can view your control settings in this mode. Any
changes you make to your control settings in this mode,
will not take effect.
The gain is always held at unity (0 dB) in Bypass mode.
Bypass mode is not a hard-wire bypass.
System Setup
REQUIRED RECOMMENDED OPTIONAL
 
Symetrix 372 SPL Computer User’s Guide
| Overview of Front Panel | How System Setup Works | Description of Menu Features

18
Description of Menu Features continues
ROTATE ADJUST TO
SELECT VALUE OR
PRESS NEXT TO SELECT MENU FEATURE CHOICE DESCRIPTION
Status Displays the current calibration status of the 372.
Cal Level Ambient signal level measured at the last sense period.
Min Level Minimum gain that the 372 is permitted to have.
Max Level Maximum gain that the 372 is permited to have.
History Reset to Zero Displays numeric readings of the highest and lowest
SPLs from whenever the 372 unit was calibrated or
from when the setting was last reset.
Resetting the 372 to Zero starts a new history period,
where the 372 begins to collect the highest and lowest
relative SPL readings.
Listen Select Listen in order to set the gain level.
There is a 2 second delay prior to entering this function.
The 372 switches the system into Listen mode, silencing
the speakers and using them as microphones to read the
ambient sound level.
The relative SPL reading then displays in numeric form
and, also, as a bargraph.
Adjust the AMBIENT SENSE GAIN switch on the front panel
until the indicated level is higher than at least –40 dB, but
no more than 0 dB. This setting range allows the 372 to
have enough valid information to control the system gain.
Note
Once the 372 has been set up, the AMBIENT SENSE GAIN switch
position should never be changed.
In this mode, you can plug in headphones to listen to the
ambient sound the 372 reads. Adjust the volume of the
headphones by rotating MONITOR.
The headphone is a helpful troubleshooting tool for identi-
fying noise sources that may be misdirecting the 372 or
hum and noise picked up in the speaker wiring that may be
causing sense errors.
System Setup
REQUIRED RECOMMENDED OPTIONAL
 
Symetrix 372 SPL Computer User’s Guide
| Overview of Front Panel | How System Setup Works | Description of Menu Features