Page 343 - NS-2 Textbook

338 NAUTICAL SCIENCES

sound in terms of its power per unit area, as described
above, there is another 'widely used meaSUl'e of sound in-
tensity relative to the quietest sound the ear can hear.
This measure, called relative illtellsity or lIoise level, is cal-
culated in units called decibels. A sound having 0 decibels
is equal in intensity to the lowest that can be heard, 10- 12
walts per square meter. On the decibel scale a sound of
10
100 decibels would be 10 times as intense as a sound of
o decibels. A sDtmd of 120 decibels is the loudest sound
that the ear can stand without pain as the eardrum be-
gins to tear. Sound decibel levels that are negative indi-
cate a sDtmd that is too faint to be heard without ampli-
fication, as for example, distant fish sounds in the ocean.

THE PHYSIOLOGY OF SOUND An illustration of the Doppler effect As the sound source 5 moves
to the right, observer 0 1 hears a higher frequency than observer Oz
Without a human ear to hear a sound wave there would because of the compression of the sound's wavelength (h,) as the
be no sOlmd, only noise. The sound waves are gathered source approaches. The observer Oz hears a lower frequency be-
cause of the stretching of the wavelength (hz) resulting from the
and funneled by the outer ear into an opening through source's motion away. The difference between frequencies heard by
the skull called the ear calla I. At the inner end of the ear the two observers and the source frequency is called the Doppler
canal is a very thin sensitive membrane called the shift.
eardrum. Its extreme sensitivity is indicated by the fact
that it can detect sOlmd intensities of 10- 12 watts per increase in frequency caused by compression of the dis-
square metel; equivalent to a pressure of only 2 x 10- 5 tance benveen waves. When the SOUl'Ce is opposite you,
newtons (the metric emit of force) per square meter! It is you hear the same frequency as the whistle or horn puts
obvious that you should be very careful to protect yom out. When the train or auto moves away, the effect is to
eardrums from loud or highly focused sOlmd, such as increase the distance betvveen waves, thus causing a de-
that produced by highly amplified music or earphones. crease in the frequency reaching your ear. This phenom-
Beyond the eardrum is the middle ear. Here three del- enon is known as the Doppler effect, named for the Aus-
icate bones called the hanunel~ anvil, and stilTUP trans- trian physicist Christian Doppler (1803-1853). The change
mit the sound from the eardrum to the illller ear, where a between the highest and lowest frequencies heard and
liquid-filled structure called the cochlea is located. Sound the source frequency is called the Doppler shift.
vibrations in the liquid are sensed by special cells that The Doppler shift can be used to determine the speed
translate the mechanical vibrations to electromagnetic and direction of motion of a sound's source, such as a
nerve impulses. These impulses travel through the audi- submarine in the ocean. It also occurs with electromag-
tory nerve to the brain, where the person interprets them netic waves such as radio and light. By analyzing the
as sound. Doppler shift in light from a distant star, for instance, as-
Some animals such as bats and dogs have ears that tronomers can determine the speed at which it is moving
are sensitive to sowlds above the 20,000-Hz upper fre- away from us. Radar detectors use the Doppler shift to
quency limit that hlilllans can hear. SDtmds in this region determine the speed of baseballs and automobiles.
are called ultrasoulld. Bats use these high intensities to
navigate by means of echoes returned from objects
around them, and dog owners may use ultrasonic dog SOUND IN THE SEA
whistles to call their pets.
Since Navy ships and submarines operate in the sea, the
characteristics of sound in seawater are of special interest
THE DOPPLER SHIFT to Navy people. The speed of sound waves traveling
through seawater is affected by three factors: (1) its tem-
You may have noticed the apparent change in frequency perature; (2) its pressme, a function of depth; and (3) its
or pitch of a train whistle or automobile horn as the train salinity, or salt content.
or auto approaches, passes, and departs. Actually, there Temperature is by far the most important of these
is no change in the frequency emitted by the source. factors. The speed of sound changes from 4 to 8 feet per
There is, ho,vever, a change in the frequency reaching the second for every degree of temperature change. The tem-
ear, because of the relative lllotion between the source perahlre of the sea varies from freezing in the polar seas
and you. As the train or auto approaches, the effect is an to more than 85 degrees F in the tropics. It may decrease