Sound Properties and Their Perception
Intensity and the Decibel Scale
Sound waves are introduced into a medium by the vibration
of an object. For example, a vibrating guitar string forces
surrounding air molecules to be compressed and expanded,
creating a pressure disturbance 
consisting
of an alternating pattern of compressions
and rarefactions. The disturbance then travels from
particle to particle through the medium, transporting energy
as it moves. The energy which is carried by the disturbance
was originally imparted to the medium by the vibrating
string. The amount of energy which is transferred to the
medium is dependent upon the amplitude of vibrations of the
guitar string. If more energy is put into the plucking of
the string (that is, more work
is done to displace the string a greater amount from its
rest position), then the string vibrates with a greater
amplitude. The greater amplitude of vibration of the guitar
string thus imparts more energy to the medium, causing air
particles to be displaced a greater distance from their rest
position. Subsequently, the amplitude of vibration of the
particles of the medium is increased, corresponding to an
increased amount of energy being carried by the particles.
This relationship between
energy and amplitude was discussed in more detail in a
previous unit.
The amount of energy which is transported past a given area of the medium per unit of time is known as the intensity of the sound wave. The greater the amplitude of vibrations of the particles of the medium, the greater the rate at which energy is transported through it, and the more intense that the sound wave is. Intensity is the energy/time/area; and since the energy/time ratio is equivalent to the quantity power, intensity is simply the power/area.
Typical units for expressing the intensity of a sound wave are Watts/meter2.
As a sound wave carries its energy through
a two-dimensional or three-dimensional medium, the intensity
of the sound wave decreases with increasing distance from
the source. 
The
decrease in intensity with increasing distance is explained
by the fact that the wave is spreading out over a circular
(2 dimensions) or spherical (3 dimensions) surface and thus
the energy of the sound wave is being distributed over a
greater surface area. The diagram at the right shows that
the sound wave in a 2-dimensional medium is spreading out in
space over a circular pattern. Since energy is conserved and
the area through which this energy is transported is
increasing, the power (being a quantity which is measured on
a per area basis) must decrease. The mathematical
relationship between intensity and distance is sometimes
referred to as an inverse square
relationship. The intensity varies inversely with
the square of the distance from the source. So if the
distance from the source is doubled (increased by a factor
of 2), then the intensity is quartered (decreased by a
factor of 4). Similarly, if the distance from the source is
quadrupled, then the intensity is decreased by a factor of
16. Applied to the diagram at the right, the intensity at
point B is one-fourth the intensity as point A and the
intensity at point C is one-sixteenth the intensity at point
A. Since the intensity-distance relationship is an inverse
relationship, an increase in one quantity corresponds to a
decrease in the other quantity. And since the
intensity-distance relationship is an inverse square
relationship, whatever factor by which the distance is
increased, the intensity is decreased by a factor equal to
the square of the distance change factor. The sample
data in the table below illustrate the inverse square
relationship between power and distance.
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Humans are equipped with very sensitive ears capable of detecting sound waves of extremely low intensity. The faintest sound which the typical human ear can detect has an intensity of 1*10-12 W/m2. This intensity corresponds to a pressure wave in which a compression of the particles of the medium increases the air pressure in that compressional region by a mere 0.3 billionths of an atmosphere. A sound with an intensity of 1*10-12 W/m2 corresponds to a sound which will displace particles of air by a mere one-billionth of a centimeter. The human ear can detect such a sound. WOW! This faintest sound which a human ear can detect is known as the threshold of hearing. The most intense sound which the ear can safely detect without suffering any physical damage is more than one billion times more intense than the threshold of hearing.
Since the range of intensities which the human ear can detect is so large, the scale which is frequently used by physicists to measure intensity is a scale based on multiples of 10. This type of scale is sometimes referred to as a logarithmic scale. The scale for measuring intensity is the decibel scale. The threshold of hearing is assigned a sound level of 0 decibels (abbreviated 0 dB); this sound corresponds to an intensity of 1*10-12 W/m2. A sound which is 10 times more intense ( 1*10-11 W/m2) is assigned a sound level of 10 dB. A sound which is 10*10 or 100 times more intense ( 1*10-10 W/m2) is assigned a sound level of 20 db. A sound which is 10*10*10 or 1000 times more intense ( 1*10-9 W/m2) is assigned a sound level of 30 db. A sound which is 10*10*10*10 or 10000 times more intense ( 1*10-8 W/m2) is assigned a sound level of 40 db. Observe that this scale is based on powers or multiples of 10. If one sound is 10x times more intense than another sound, then it has a sound level which is 10*x more decibels than the less intense sound. The table below lists some common sounds with an estimate of their intensity and decibel level.
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Greater Than TOH |
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While the intensity of a sound is a very objective quantity which can be measured with sensitive instrumentation, the loudness of a sound is more of a subjective response which will vary with a number of factors. The same sound will not be perceived to have the same loudness to all individuals. Age is one factor which effects the human ear's response to a sound. Quite obviously, your grandparents do not hear like they used to. The same intensity sound would not be perceived to have the same loudness to them as it would to you. Furthermore, two sounds with the same intensity but different frequencies will not be perceived to have the same loudness. Because of the human ear's tendency to amplify sounds having frequencies in the range from 1000 Hz to 5000 Hz, sounds with these intensities seem louder to the human ear. Despite the distinction between intensity and loudness, it is safe to state that the more intense sounds will be perceived to be the loudest sounds.
Check
Your Understanding
1. A mosquito's buzz is often rated with a decibel rating of 40 dB. Normal conversation is often rated at 60 dB. How many times more intense is normal conversation compared to a mosquito's buzz?
a. 2
b. 20
c. 100
d. 200
e. 400
2.
The table at the right represents the decibel level for
several sound sources. Use the table to make comparisons of
the intensities of the following sounds.
How many times more intense is the front row of a Smashin' Pumpkins concert than ...
a. ... the 15th row of the same concert?b. ... the average factory?
c. ... normal speech?
d. ... the library after school?
e. ... the sound which most humans can just barely hear?
3. On a good night, the front row of the Twisted Sister concert would surely result in a 120 dB sound level. An IPod produces 100 dB. How many IPods would be needed to produce the same intensity as the front row of the Twisted Sister concert?