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Field Adjustments

Setting and evaluating sound levels

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 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.
  • Automatic volume adjustment technology used by different manufacturers varies in the rate of sampling of ambient sound and in the speed with which output adjusts to changes in ambient sound.
  • Some APS, and some installations will be more subject to responding to their own noise than others. For example, as the WALK signal continues throughout the Walk interval, the signal may get louder and louder in response to its own noise.
  • Different tones or speech will seem louder or quieter depending on their frequency content, although they may measure the same on the dBA scale.
Measuring the sound level
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.
  • Microphones for pedhead type devices are typically in or on the pedhead, incorporated into the APS.
  • Pushbutton integrated devices may have microphones at the pedhead or the microphones may be incorporated into the pushbutton housing.
Microphone location
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 ambient traffic sound when a microphone is located on a signal pole 10 feet from the curb line will be quieter at the microphone than at the street; therefore the resulting output will be quieter than it would be if the ambient sound were measured near the street.
  • The WALK indicator, if it comes from the same pole location, may already be too quiet because the ambient sound level has been measured too far from waiting pedestrians, and it will be quieter still when its sound reaches the ears of pedestrians waiting at the crosswalk because the sound has traveled farther to reach them.
  • If the microphone is closer to the intersecting street than to the street the pedestrian is waiting to cross, or the microphone is oriented toward the intersecting street, it will respond to the traffic sound on that street instead of the street the pedestrian is waiting to cross. This may result in WALK signals that are too loud or too quiet as perceived by pedestrians waiting to cross.
Ideal microphone location
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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