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Science of Sound<br />
AUDIBLE SIGNALING<br />
Audible Signals designed specifically to alert, warn, communicate<br />
and protect provide a universally understood language<br />
that transcends national borders. Audible signaling devices<br />
typically work by presenting loud, distinctive tones, sequences<br />
of tones or specific voice messages that immediately attract<br />
our attention and focus our thinking. Optimally, audible signal<br />
sound output (in dB) will be greater than 75 dBs and at least<br />
15dBs above ambient noise levels in the surrounding area.<br />
How effective an audible signal is dependent on a variety of<br />
interdependent factors;<br />
• Sound Output Level (in dB)<br />
• Frequency (in Hz)<br />
• Distance from the signaling device<br />
• Ambient Noise Level<br />
• Environmental Influences (i.e. wind speed and direction,<br />
humidity level, precipitation, etc.)<br />
Direction of Sound<br />
How do we know the direction of an approaching train or warning<br />
alarm? To do so, we must examine some characteristics of<br />
the human hearing system. “Binaural localization”, or the ability<br />
of using our two ears to determine from where a sound source<br />
appears, uses three cues:<br />
• Interaural intensity differences - middle and highfrequency<br />
sounds originating from a human subject's left<br />
side will reach the left ear at a higher intensity (volume)<br />
level than the right ear, causing a difference in intensities at<br />
each ear. The head baffles most of the direct sound energy<br />
from reaching the right ear, so that predominantly reflected<br />
sounds arrive at the right ear. Because the reflected sound<br />
has traveled farther and has lost energy in its journey from<br />
source to reflector to ear, the intensity of that sound as perceived<br />
by the right ear is reduced, and the brain tells us that<br />
the sound arrived from the left side.<br />
• Interaural arrive-time differences - while interaural intensity<br />
differences are one clue in determining a sound's pointsource<br />
for mid- to high-frequency sounds, low frequencies,<br />
with their large wavelengths, are not as easily discriminated<br />
using interaural intensity differences. At lower frequencies,<br />
instead, the ear uses time delay-- the short but significant<br />
delay between the left and right ears-- to calculate which<br />
sound arrived first.<br />
• Pinnae of the ears - while interaural intensity and arrivetime<br />
differences gives us lateral cues, telling us left-to-right<br />
information. The pinnae, however, use the shape of the ears<br />
and the strange bumps and ridges to reflect the sound into<br />
the ear. These ridges introduce slight time-delays between<br />
the direct sound and the reflected sound. The time delay<br />
itself is a function of the angle of incidence-- at what angle<br />
the sound bounced off the pinnae.<br />
TYPICAL DB LEVELS WHEN<br />
SELECTING AUDIBLE SIGNALS<br />
In specifying a signaling device, all of the above characteristics<br />
should be considered along with as many factors concerning<br />
the application as can be gathered. Selecting the proper device<br />
for each signaling application need not be difficult if the following<br />
points are observed.<br />
1. Signal Function. Basically, the types of functions to which<br />
signals can be applied are:<br />
- General alarm or emergency<br />
- Start and dismissal<br />
- Paging or coding<br />
- Localized danger<br />
- Indication<br />
The first step in selecting a signal is to carefully define this<br />
function. Is the signal to be a warning, a call, or an instruction?<br />
Will it be used to protect life or property? How much<br />
time will be available to take action? Obviously, the more<br />
critical the application, the more startling the signal generated<br />
should be. A horn is generally the most startling signal.<br />
Its rasping tone commands immediate attention. A single<br />
stroke bell can be used for paging applications where danger<br />
is not at hand. A small buzzer may serve to notify a machine<br />
operator that a particular operation is completed. Audible<br />
signals with pulse and alternating tones are generally more<br />
effective than those with a linear tone.<br />
2. Uniform Sound Distribution: Better signal distribution<br />
can usually be achieved by carefully positioning a number<br />
of smaller signals throughout a given area than by centrally<br />
locating a single large unit.<br />
28<br />
www.edwardssignaling.com
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