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19 August 2022
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FORMULA SOUND SENTRY Mk2FORMULA SOUND AT-1 Automatic volume controller, rackmountFORMULA SOUND AT-1 Automatic volume controller, wall mountFORMULA SOUND CONTACTOR for Sentry

The Sentry Mk2 can be helpful with regard to legislation concerning noise in the workplace. One or more units can be used as monitoring devices to give visual displays which advise the need to wear hearing protectors. The unit can also switch on extra warning devices if the noise level exceeds a predetermined threshold.

Under the Environmental Health act, entertainment licences can be at risk if a Local Authority decides excessive noise from entertainment venues is a nuisance. But what happens in an entertainment venue when a live band or mobile disco is using their own equipment? It is still the responsibility of the venue manager to ensure that the regulations are adhered to and this is now possible with the Sentry.

The permitted level of noise in any venue is determined by the local environmental health officer. The sensitivity of the Sentry can be set up in accordance with any of these regulations. If the permitted levels are exceeded in any venue where a Sentry is installed and the warnings ignored, the offending equipment will be disconnected from mains power. Re-connection is only possible by operating the reset button. The Sentry has the advantage over other units in that it features a large bar-graph VU meter with 23dB range, within an 85dB to 120dB SPL operating environment, to give visual indication of the noise level in a venue. Anyone can see what the volume level is and how their actions are contributing to this level.

The Sentry works in conjunction with a 220-240V coil mains contactor to control mains power outlets. Alternatively an auxiliary connector is provided to utilise 12V contactors (this includes the EMO mains switcher). The unit operates by supplying a continuous voltage to the contactor which is connected to designated power outlets. If the unit is tripped or should anyone disconnect the Sentry, the power outlets will be disconnected. Alternatively the Sentry can make available 220/240V AC for mains controlled switches.

The bar-graph meter is colour coded from green to red, the red section increasing in size to aid viewing, and indicates the volume level in the venue. While the meter is operating in the green section, with even an occasional peak into the red, there is no need for concern. If the two red “WARNING” segments are lit this indicates that the volume level in the venue is nearing the permitted limit. A connector is provided so that a mains powered warning lamp or beacon can be connected which operates at the same point as these two red sections of the meter. This lamp or beacon may be situated in a remote position. If the noise level is high enough to light the “over limit” section (3 red segments) of the meter, the noise has exceeded the permitted limit.

If this is allowed to continue the unit will trip and remove the power to the contactor. Visual indication is provided by the lamp adjacent to the re-set button labelled “Power off”. The unit must then be re-set manually in order to restore power. The delay between “over limit” and “power off” is adjustable. A remote reset switch (not supplied) can be connected to the unit. The unit can also operate on 2 preset thresholds (e.g. for different night and day settings), with the presets selected manually by use of a key switch or similar, or automatically by a timer switch (not supplied). Provision to connect a security loop is provided, to monitor the opening of windows or doors which may increase the sound level output from a building.

Other indicators show if the microphone circuit has been tampered with (“mic tamper”), if a security loop has been broken (“security loop”) or if the level 2 threshold settings have been selected (“level 2”).

The improved case design incorporates cable entry knock-outs and screw terminal connections for all wiring. A removable cover provide access to all connectors, with anti-tamper seals provided for integrity checking.

The unit has its own internal microphone and can easily be adjusted to operate at different levels. The facility to use an external microphone is incorporated into the unit (microphone not supplied) to allow remote area monitoring from the main display. Alternatively, additional slave units can be used to display levels from the main display unit, within adjacent areas.

Technical Specification:
Connectors: All connections are on screw terminals.
Display: Large bar graph VU meter with 13 segments each 20mm x 12mm.
Reset indicator, Mic tamper, Security loop, Level 2.
Microphone: The unit is fitted with a built-in electret microphone. Sensitivity range - 85dB-120dB SPL continuously available. Provision is mode for an external microphone (not supplied). Microphone response may be internally selected for ‘A’, ‘C’ or Linear weighting.
Delay: The delay between “over limit” and “power off” is internally selectable - 10 seconds and 70 seconds.
Power input: 220/240V AC 50Hz or 110/120V AC internal select.
Dimensions: 320(W) x 261(H) x 120(D)mm.
Finish: High impact polycarbonate. Flame resistance - UL 94-V2. Colour Grey (RAL7035). Clear polycarbonate cover. The unit should only be adjusted by a competent operator using a precision sound level meter.

AT-1 Attenuation Controller

The Sentry Mk2 can be accompanied with an AT-1 controller which effectively turns the system into a complex device for monitoring and controlling the level at which audio equipment can operate, as opposed to the sentry’s familiar, total audio power-out! Connected only by a 4 core control cable to the Sentry Mk2, the AT-1 acts as a variable attenuator to a stereo line-level feed (e.g. between mixer and power amplifier) as the Sentry reaches it’s threshold and also gives a visual LED indication as to the amount of attenuation taking place.
Inputs and outputs are on balanced or unbalanced XLR or 6.35mm jack, with power source at 220V AC .25A, IEC Inlet, with no lead supplied. Two versions are available - a package housed in a robust 1U high 19” rack unit for installed sound systems and also a wall mount version that is ideal for visiting sound systems.

Technical Specification:
Input connectors: 2 x XLR 3 pin balanced or unbalanced line level.
2 x 6.35mm 3 pole jack bal or unbal line level.
Output connectors: 2 x XLR 3 pin balanced or unbalanced line level.
2 x 6.35mm 3 pole jack bal or unbal line level.
Max Input Level: +20dBu.
Max Output Level: +20dBu into a 600ohm load at < 0.01% distortion.
Input Impedance: 20Kohms balanced, 10Kohms unbalanced.
Noise: EIN < -90dBu.
Attenuator range: Unity gain to -90dB.
LED indicators: -3dB, -6dB, -9dB, -12dB, -15dB, -18dB, attenuation. I/P CLIP
Power source: 220V AC .25A, IEC Inlet, with no lead supplied.
Dimensions: Rack version - 482(w) x 44(h) x 125(d)mm
Wall version - 380(w) x 50(h) x 125(d)mm.
Panel finish: Black anodised aluminium with etched silver notation.
Case finish: Black plastic coated steel.


A fully fused 32 amp contactor in a black-enamelled adaptable box with cable entry knockouts on all sides is also available. A 12 volt interface allows the use of low voltage cable between the contactor and Sentry. Dimensions 254 x 159 x 76mm.

  Description Price £
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FORMULA SOUND SENTRY Mk2 Stock code: 42-664
Ordered on demand, available in 1‑2 working days. Need it sooner? Call us
FORMULA SOUND AT-1 Automatic volume controller, rackmount Stock code: 42-658
Ordered on demand, cancellations may incur charges, available in 2‑3 working days. Need it sooner? Call us
FORMULA SOUND AT-1 Automatic volume controller, wall mount Stock code: 42-659
Ordered on demand, cancellations may incur charges, available in 2‑3 working days. Need it sooner? Call us
FORMULA SOUND CONTACTOR for Sentry Stock code: 42-660
Ordered on demand, cancellations may incur charges, available in 1‑2 working days. Need it sooner? Call us