Automatic volume adjustment
Automatic volume adjustment is a term for volume control that is automatically responsive to ambient (background) sound.
- A louder signal is produced when vehicle and other noise at an intersection is high (as during traffic surge or when a truck is passing).
- A quieter sound is produced when background noise volume is lower (as during night-time hours or lull in traffic).
- A microphone continuously samples the noise levels and varies the volume in response to the existing sound levels. The microphone may be incorporated into the pushbutton housing, or located at the pedestrian signal head.
Automatic volume adjustment is also known as automatic gain control (AGC) or ambient sound adjustment.
Some signals can be pre-set to vary volume within particular ranges.
Most signals with automatic volume control have a minimum limit placed at about 30 dB and a maximum limit at about 90 dB.
Some APS allow the installer to set the range of the locator tone and the WALK indication separately; others have one setting for both.
A signal that is 2–5 dB above ambient sound, as perceived at the departure curb, is loud enough to be heard by pedestrians waiting at that location. If the microphone is installed at the pedestrian signal head that is set back from the curb, the traffic volume sensed by the microphone is not as loud as that perceived by pedestrians waiting at the curb. Therefore, at each installation, the setting may need to be adjusted, depending on the location of the microphone in relation to pedestrians waiting to cross.
Some APS have adjustments for microphone sensitivity as well as WALK indication and pushbutton locator tone volume.
When to use
Automatic volume adjustment is required at all APS.
MUTCD 4E.11, P10 : "Automatic volume adjustment in response to ambient traffic sound level shall be provided up to a maximum volume of 100 dBA. "
How used by pedestrians who are blind or who have low vision
Automatic volume adjustment provides flexibility and allows APS to adjust so they are not disturbing to neighbors at night or times of low traffic volume. This is also helpful to blind or visually impaired pedestrians, as the APS does not drown out essential traffic sounds necessary for crossing.
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