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Science of Sound<br />
Physical Principles of Sound<br />
The sources of sound are many and nearly infinitely varied.<br />
Sound can be described in terms of pitch– from the low rumble<br />
of distant thunder to the high-pitched buzzing of a mosquito–<br />
and loudness. However, it is important to understand that these<br />
are subjective qualities that depend in part on the hearer's<br />
sense of hearing. Objective, measurable qualities of sound<br />
include frequency and intensity, which are related to pitch and<br />
loudness. These terms, as well as others used in discussing<br />
sound, are best understood through an examination of the<br />
physical principles of sound.<br />
To understand the basics of sound, it is important to first<br />
understand some basic related scientific principles.<br />
At its most fundamental level, sound is the mechanical<br />
disturbance of a medium, either gas, liquid or solid. All sounds<br />
are created by causing a medium to vibrate, be it a bell, horn,<br />
siren, whistle or the wings of a cicada. Sound is propagated<br />
through mediums by causing adjacent particles to vibrate in<br />
a similar fashion– a bell vibrates at a given frequency and<br />
thereby displaces adjacent air molecules. This process continues,<br />
and eventually air particles in our ears bump into tiny hairs<br />
in our inner ear; these hairs send electrical impulses to our<br />
brain, which tells us that we are hearing a particular tone.<br />
Put more simply, sound can be described as the transmission<br />
of pressure, from an initial source to a listener through the air<br />
(or other medium).<br />
The most popular analogy to sound wave propagation is the<br />
example of a pebble that is dropped into a pond. The pebble,<br />
on its initial collision with the water’s surface, produces ripples<br />
originating from the “point source” of the rock’s entry, spreading<br />
in all directions. Due to the mass of the water molecules, energy<br />
is expended in making the water ripple. So as the ripples<br />
travel further and further away from the rock's point of entry,<br />
the ripples lose intensity. Sound behaves in the same manner.<br />
As sound travels, it loses energy, sounding “softer”. A law<br />
known as the inverse-square law dictates the amount of energy<br />
lost per unit distance— in a free-field, doubling the distance<br />
quarters the sound energy, given a point-source.<br />
Speed of sound in various mediums<br />
Medium<br />
Speed<br />
in feet<br />
per second<br />
Speed<br />
in meters<br />
per second<br />
Air at 59°F (15°C) 1,116 340<br />
Aluminum 16,000 5,000<br />
Brick 11,980 3,650<br />
Distilled water at 77°F (25°C) 4,908 1,496<br />
Glass 14,900 4,540<br />
Seawater at 77°F (25°C) 5,023 1,531<br />
Steel 17,100 5,200<br />
Wood (maple) 13,480 4,110<br />
The rate at which variations in air pressure occur is referred<br />
to as frequency and is expressed in cycles per second (cps)<br />
or Hertz (Hz). The human ear is typically capable of hearing<br />
sounds produced in the 20 to 20,000 Hertz range.<br />
The range of sound pressure to which the ear will respond<br />
is extremely wide (on the order of several million to one).<br />
Because of this, a linear scale to compare different sound<br />
pressures becomes as impractical as the use of a yardstick to<br />
measure miles. A logarithmic scale, is a far more suitable scale<br />
for this type of measurement. Therefore, the decibel, which is<br />
a logarithmic unit, is used to measure sound pressure levels.<br />
A very important concept to note when using decibels, is that<br />
for each additional 3 dBs, the sound pressure doubles! (e.g. a<br />
signaling device rated 83 dB at 10 ft. is TWICE as powerful as<br />
one rated at 80 dB at 10 ft.).<br />
Loudness is determined by the magnitude of these variations.<br />
Greater variations in pressure produce louder sounds. The<br />
louder the sound, the more our ear drum moves. The volume<br />
(or magnitude) of any particular sound is referred to as its<br />
“sound pressure.” Under normal atmospheric conditions, the<br />
maximum sound pressure level attainable is 194 dB. Sound<br />
pressure levels in excess of 120 dB may be painful. Above 150<br />
dB they can result in ear damage. A distance must generally<br />
be specified along with the dB rating to fully describe a sound.<br />
The sound pressure level changes 6 dB for each halving or<br />
doubling of distance. For a change in distance of ten times, the<br />
sound pressure level changes 20 dB.<br />
Here are some common sound levels to give you a framework<br />
for understanding the different sound levels:<br />
Source in dB Activity<br />
170-180 dB stun grenades (hearing tissue death occurs)<br />
140-150 dB firearms, jet engine, rock concert<br />
120-130 dB<br />
jackhammers, heavy construction tools,<br />
loud car stereo<br />
100-110 dB motorcycles, chainsaws, loud nightclub<br />
90 dB a hair dryer<br />
85 dB city traffic<br />
80 dB alarm clock<br />
60 dB normal conversation<br />
50 dB<br />
30-40 dB<br />
moderate rainfall, the average ambient sound level in<br />
one’s home.<br />
a quiet room or library, a whisper, the bedroom while<br />
sleeping.<br />
TM<br />
www.edwardssignaling.com 27