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What
is sound intensity level? Explain clearly the differences between sound
intensity level and loudness. |
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Sound intensity level h is defined for
a sound compared to the minimum detectable intensity as |
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where I is the intensity of sound and
Io = 10-12 Wm-2 is the minimum
detectable intensity. |
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Sound intensity level is a physical property
that measures the power of sound per unit area. |
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Loudness is the physiological perception of
sound intensity. It is a sensation of the human ear and may vary from one
person to another for the same sound intensity. |
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Loudness depends on frequency and duration. |
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We are most sensitive to sound frequency of
1 kHz to 5 kHz. A long burst also seems to be louder than a short burst
of the same sound intensity. |
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Loudness of sound is expressed in phon
or dB(A). |
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If the loudness is 30 dB(A), it is as loud
as a 1 kHz sound at 30 dB. |
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| b. |
Explain
how the sound intensity level changes when, separately,
i)
the power of the source is doubled |
4
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Sound waves are usually emitted from a point source. Suppose
the power of the source is Po. Assume that sound travels
adiabatically without energy loss. The intensity of sound at a distance
r from the source is |
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If the power of the source is doubled, the new intensity
is I' = 2 I. |
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The change in intensity level is |
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ii)
the distance from the source is doubled |
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If the distance from the source is doubled, the new intensity
is |
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The change in intensity level is |
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| c. |
Roughly
sketch three curves of equal loudness on a graph of sound intensity level
against sound frequency for
i)
30 dB(A) |
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ii)
60 dB(A) |
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iii)
90 dB(A) |
3
marks |
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| d. |
Mark
on your graph in (c) the approximate region corresponding to the sound
produced by
i)
a human being |
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ii)
a bat.
Explain your answer
briefly. |
2
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Sound produced by a human being cannot be too
loud. Also, the frequency is limited, approximately from one hundred to
several thousand hertz. |
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Sound produced by bat are ultrasonic. i.e.
well above our audible frequency range of 20 kHz. |
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| e. |
In
a Hi-Fi system, there is a switch labelled “loudness” with on-off positions.
It is known that the switch is useful only at quiet listen level. Explain
the function of the switch using your curves in (c). |
2
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The equal loudness curve does not have the
same shape for all loudness. |
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They are flattened for loud sound and more
curved for soft sound. |
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If the overall power of a hi-fi system is reduced,
sound at very high frequency and very low frequency are more difficult
to hear, while sound at ~1 kHz are more easily heard (thus emphasized unintentionally). |
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The loudness switch is to compensate for the
poor sensitivity of our ear to low pitch and high pitch by automatically
increasing the power at these frequencies. With this switch, when the overall
power is reduced, sounds of all frequencies seem to decrease in loudness
altogether. |
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