How loud should the sound be?
Sound should be between 30 dB minimum and 89 dB maximum. At no time should sound be more than 5 dB above ambient sound (except by special actuation for audible beaconing). [MUTCD 4E.06 and 4E.08 Guidance; and PROWAAC 126.96.36.199 G]
Draft Public Rights of Way Accessibility Guidelines specify that the sound level should be between 2 and 5 dBA above ambient sound, measured 36 inches from the pole.
Manufacturers typically set a maximum and minimum output level on APS devices. The maximum should be 89 dBA, as required by OSHA regulations.
The setting of the automatic volume adjustment, also called microphone sensitivity, or automatic gain control (AGC), must be adjusted by the installer to provide output at one of a number of ranges between the maximum and minimum. The number and width of ranges varies by manufacturer.
Automatic volume adjustment
Pre-set automatic volume adjustment or automatic gain controls cannot assure that the volume meets the criterion for distance at which the APS should be audible. Similar automatic volume adjustment settings on APS by different manufacturers may seem to provide quite different loudness, as judged by listeners.
Because of the short duration of pushbutton locator tone and WALK tone pulses, conventional analog or digital sound level meters are not able, in the crosswalk environment, to accurately measure the absolute sound level (dBA) of APS tones, or the sound level of APS tones relative to ambient sound.
At present, setting and evaluation of APS sound level is typically done by ear. The locator tone and WALK indication should be audible within 10 to 12 feet of the device. It is critical for the WALK indication to be audible at the crosswalk waiting location.
Most APS installations evaluated by the authors during 2001-2002 have been set louder than was optimal either for blind pedestrians or APS neighbors.
Measuring sound where audible beaconing is needed
At crosswalks where audible beaconing is needed, sound pressure level (dB) should be evaluated from the middle of the street, when the loud WALK indication has been called, to be sure beaconing will be provided throughout the crossing. However, OSHA limits the maximum output of APS to 89dB, and most manufacturers pre-set this maximum. Therefore, at exceptionally wide crossings, and when and where there is high ambient sound, there may be a distance in the middle of the crosswalk where the beaconing is not readily heard.
Microphone location and effect on perceived loudness
APS devices that respond to ambient sound have microphones to pick up the ambient sound.
An APS microphone should be mounted as close as possible to the position of the pedestrian who is waiting to cross the associated crosswalk, because sound pressure is halved for each doubling of the distance from the sound source in a free field.
The farther from that ideal position the microphone is for a given APS device, the greater will be the following problems.
The best location for the microphone is as close as possible to the position of pedestrians who are waiting to cross the associated street. This results in pedestrians being clearly able to hear APS signals with no need to set the automatic volume adjustment so high that sound levels will be too high at 36 inches from the source, or that APS neighbors will be annoyed.
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