The Formula Sound 073 Sentry system, (Sound Limiter or Noise Limiter ), was developed after looking at existing products and addressing their shortcomings. The Sentry Mk2 is the second generation of this product offering more facilities than the original Sentry. The Sentry Mk2 incorporates a measuring microphone inside the unit making it virtually tamperproof. Provision is made to connect an external microphone if required.
A large illuminated bargraph display is incorporated into the unit. The unit should be installed in a position where it can be clearly seen so the artiste has full knowledge of the levels at which he is working. If the noise level drives the display to full scale, which is all the green, amber and red lights lit, after a period of time the power will be turned off. The time period is adjustable between 10 to 70 seconds. Power can only be restored manually by pressing a reset button.

The Sentry is used in conjunction with a contactor or relay usually to control mains power. The connection between the two is at low voltage so the contactor can be located in a fuse cupboard and wired to control all the sockets in the vicinity.

The Sentry has a security loop facility which can be connected to a relay controlled by the fire alarm or switches that detect open fire doors or windows to ensure that the soundproofing of the venue is not impaired. Magnetic reed switches similar to the ones used in intruder alarms can be used as detectors. If the loop circuit is open the Sentry will switch off and it will not be possible to reset the Sentry until the circuit is closed.

The Sentry Mk2 also has provision to operate at two different levels, which may be selected remotely or by switches operated by open doors and windows, or an optional time switch. This allows the unit to cater for changes in noise type (e.g. disco or live music) or changes in surroundings (e.g. a venue situated near offices requiring stricter control during office hours.)

The combination of the security loop to switch off completely and the second level function provides for most situations to be catered for.

A counter which is not resetable is also offered as an option. The counter records the number of times the unit has been reset.

To keep the Sentry system as versatile as possible connectors are fitted which allow the connection of several optional features, including remote reset switches, warning lamps, slave displays, etc.

The Sentry when supplied for entertainment venues can be internally selected to work on a linear or 'C' scale or on Bass. This should be used where Bass, i.e. low frequency noise is the problem. When this option is selected a 300Hz low pass filter with a 12dB per octave slope is inserted in the measuring circuit.

Industrial Applications If units are being used for this application they should be ordered with the 'A' weighting filter fitted. (This will be fitted in place of the Bass option).
One or more units can be used as monitoring devices to give visual displays, which advise the need to wear hearing protection. The unit can also switch on extra warning devices if the noise exceeds a predetermined threshold.

Full installation details and further technical information is provided in the installation manual supplied with each unit.

Latest developments Until recently this was the only way to control mobile or portable systems but we at Formula Sound felt there should be a better way and now there is: The AT-1 controller