Physical Security

Noise sensor

by Mark Rowe

The latest Public Address and Voice Alarm (PA/VA) system in the Praesensa range from Bosch is the Praesensa Ambient noise sensor. Bosch the manufacturer says it is particularly for applications such as campuses, shopping malls, airports, railway terminals and factories.

The buzz of a crowd, the passing roar of vehicles or the hum of equipment can affect the intelligibility of voice messages. Similarly, music or messages can cause discomfort when they are played too loudly in a quieter building. Automatic Volume Control (AVC) helps to deal with these ambient noise changes. AVC – also known as Noise Dependent Volume Control (NDVC) – changes the loudspeaker volume according to the ambient noise; and automatically solves the dilemma of a system being too loud in a quiet place versus too soft in a noisy one.

The new noise sensor monitors the ambient noise level and alerts the system controller about changes. Announcements are then automatically adjusted to a comfortable level above the ambient noise, and background music is adjusted to maintain the right atmosphere.

Up to four sensors can operate together to cover a large area. In the event of a failure or disconnection of the device, the announcement volume is automatically set to its maximum within the applicable control range. This fail-safe ensures that the system will not interpret a missing sensor as a silent ambient level and decrease the announcement level mistakenly.

For AVC actions to be discreet yet effective, its adaptation threshold, range, slope and speed can be configured for a specific application and location. Its behaviour is different for short announcements compared to ongoing background music. Sample-and-hold mode is used for live speech calls and playback of prerecorded messages. The noise level is sampled and the latest level information is held and used during the call; this is not affected by the sound of the call itself or its associated reverberation and echoes. Tracking mode is used for background music; the ambient noise level is tracked, allowing the continuous adjustment of the background music’s volume to suit the environment.

The sensor is connected to the IP network – one CAT5e cable can accommodate data exchange and power over Ethernet (PoE). The device communicates ambient noise level data to the Praesensa system controller, which adjusts the output level of the involved amplifier channels accordingly. A back box is included for mounting on solid ceilings and walls. Without the back box, the sensor can be flush mounted in hollow walls or suspended ceilings. The device is water resistant (IP54) with or without the back box and can be used indoors and outdoors. It comes with a black and white front cover.

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