Page 375 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Physical Principles of Sound—Sound 1000 Hz. Sounds above this level induce the
Stimulus and Perception sensation of pain (! B, red curve).
For practical reasons, the decibel (dB) is used
Sound waves are the adequate stimulus for the as a logarithmic measure of the sound pressure
organ of hearing. They arise from a sound level (SPL). Given an arbitrary reference sound
source such as a gong (! A1) and are con- pressure of p o = 2 · 10 – 5 Pa, the sound pressure
ducted in gases, liquids, and solids. The air is level (SPL) can be calculated as follows: [12.2]
the main carrier of sound.
Central Nervous System and Senses ent materials, e.g., at 332 m/s in air of 0 !C. A graphic means that the SPL rises by 20 dB. -2
SPL (dB) = 20 · log (p x/p o)
The air pressure rises and falls rhythmically at the
where p x is the actual sound pressure. A ten-
sound source. These pressure waves (sound waves)
fold increase in the sound pressure therefore
travel at a characteristic sound velocity (c) in differ-
recording of sound waves (! A1) will produce
The sound intensity (I) is the amount of sound
energy passing through a given unit of area per unit
waveform curves. The wavelength (λ) is the dis-
of time (W · m ). The sound intensity is proportional
2
tance between the top of one wave and the identical
to the square of p x. Therefore, dB values cannot be
phase of the succeeding one, and the maximum de-
viation of pressure from baseline is the amplitude
calculated on a simple linear basis. If, for example,
two loudspeakers produce 70 dB each (p x = 6.3 · 10
(a) (! A1). Enlargement (reduction) of wavelength
will lower (raise) the tone, whereas a fall (rise) in
Pa), they do not produce 140 dB together, but a mere
–2
when the intensity level doubles. Thus, !"2 · 6.3 · 10
(! A1). The pitch of a tone is defined by its
Pa has to be inserted for p x into Eq. 12.2.
frequency (f), i.e., the number of sound pressure
oscillations per unit time. Frequency is measured in
Sound waves with different frequencies but
– 1
12 amplitude will produce a quieter (louder) tone 73 dB because p x only increases by a factor of !"2
hertz (Hz = s ). Frequency, wavelength and the
equal sound pressures are not subjectively per-
sound velocity are related: ceived as equally loud. A 63 Hz tone is only per-
–1
f (Hz) ! λ (m) = c (m ! s ). [12.1] ceived to be as loud as a 20 dB/1000 Hz refer-
A pure tone has a simple sinus waveform. The tones ence tone if the sound pressure of the 63 Hz
emanating from most sound sources (e.g., musical tone is 30-fold higher (+ 29 dB). In this case, the
instrument, voice) are mixtures of different frequen-
cies and amplitudes that result in complex periodic sound pressure level of the reference tone
vibrations referred to as sound (! A2). The fun- (20 dB/1000 Hz) gives the loudness level of the
damental (lowest) tone in the complex determines 63 Hz tone in phon (20 phon) as at a frequency
the pitch of the sound, and the higher ones deter- of 1000 Hz, the phon scale is numerically
mine its timbre (overtones). An a (440 Hz) sung by a equals the dB SPL scale (! B). Equal loudness
1
tenor or played on a harp has a different sound than contours or isophones can be obtained by plot-
one produced on an organ or piano. The overlap of ting the subjective values of equal loudness for
two very similar tones produces a distinct effect
characterized by a beat tone of a much lower test frequencies over the whole audible range
frequency (! A3, blue/red). (! B, blue curves). The absolute auditory
Audibility limits. Healthy young persons can threshold is also an isophone (4 phons; ! B,
green curve). Human hearing is most sensitive
hear sounds ranging in frequency from 16 to
20 000 Hz. The upper limit of audibility can in the 2000–5000 Hz range (! B).
drop to 5000 Hz due to aging (presbycusis). At Note: Another unit is used to describe how a tone of
1000 Hz, the absolute auditory threshold or constant frequency is subjectively perceived as
louder or less loud. Sone is the unit of this type of
lowest sound pressure perceived as sound is loudness, where 1 sone = 40 phons at 1000 Hz. 2
3 · 10 – 5 Pa. The threshold of sound is sones equal twice the reference loudness, and 0.5
frequency-dependent (! B, green curve). The sone is /2 the reference loudness.
1
threshold of hearing for a tone rises tre- The auditory area in diagram B is limited by
mendously when other tones are heard simul- the highest and lowest audible frequencies on
taneously. This phenomenon called masking is the one side, and by isophones of the thresh-
the reason why it is so difficult to carry on a olds of hearing and pain on the other. The
conversation against loud background noise. green area in plate B represents the range of
The ear is overwhelmed by sound pressures
362 frequencies and intensities required for com-
6
over 60 Pa, which corresponds to 2 · 10 times prehension of ordinary speech (! B).
the sound pressure of the limit of audibility at
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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