Sound Intensity: Variation of Intensity with Distance

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Sound Intensity Variation of Intensity with
Distance

• Intensity of sound I

r distance between a source and a listener P
Power of the source
Unit W/m2
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Intensity Level Decibels

• Threshold intensity Io 10-12 W/m2
• Sound sensation with the factor by which the
  given sound is more intense than the threshold
  intensity
• Definition

I Sound Intensity Io Threshold Intensity
Unit Decibel or dB
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1001
10-2
10-4
10-6
10-8
10-12
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In 1976, the Who set a record for the loudest
concert. The sound intensity 46 m in front of
the speaker systems was 1W W/m2.
r (m)
I (W/m2)
L (dB)
1
120
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Sound Pressure Level

• Easier than the direct measurement of the
  intensity of a sound wave
• Minimum pressure fluctuation to which the ear can
  respond one billionth (10-9) of atmospheric
  pressure
• The threshold of pain 1/1000 of atmospheric
  pressure
• Definition

, where Po 2 x10-5 N/m2
Unit Decibel, dB
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Sound Level Meter

• Measuring sound pressure levels directly
• Weighing networks
• its response to different frequencies matches
  somewhat the response of the ear
• Average sound level
• At 10 m distance,

pp
ff
45
60
50
60
60
75
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Just Noticeable Difference (jnd or difference
limen)
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Most of musicians ranges less than 15
dB Woodwind worst off Bassoon best with 10 dB
But practice makes it perfect!!! Bassoon 40 dB
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Loudness Level Phons

• Loudness sensation produced by a sound depends on
  the frequency and the behavior of ear.
• Hearing threshold depends on frequency
• Standard frequency 1000 Hz
• Unit of loudness level phons

Determined with a standard frequency 1000 Hz Same
phon Same level of loudness perceived by human
ears
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Loudness Level Phons
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Sound Level Meter

• Three weighting networks
• A network
• Sensitivity varying with frequency so as to
  correspond to 40 phon curve
• Less sensitive to low frequency
• Corresponding well to the subjective impression
  of the listener to the sound measured
• Often in a unit of dBA
• B network
• Corresponding to 70 phon curve
• C network
• Sensitivity independent of frequency

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Loudness Sones

• Comparing mixtures of tones
• Unit of loudness for a subjective scale of
  loudness
• Example
• two sones sound twice as loud as one, 20 sones
  twice as loud as 10
• Subjective scale
• considerable variations among individuals
• different loudness judgements at different time
  by the same person

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Critical Band
Sound of a certain frequency
Formation of a standing wave traveling along the
basilar membrane Maximum position depends on the
frequency of the sound
The mechanical vibration produced by the wave at
the maximum is spread over certain length of the
basilar membrane.
Certain length in the basilar membrane a range
of frequency above and below the frequency of the
incoming sound
Critical band 90 Hz wide below a frequency of
200 Hz 900 Hz t 5000 Hz
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Critical Band Properties
Frequencies within the critical band
Once the basilar membrane is excited, it is hard
to increase the subjective loudness.
Requires more intensity of sound to increase the
loudness
Frequencies outside the critical band
The sensitivity of basilar membrane does not
decrease.
Greater loudness can be achieved than their
frequencies are same.
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Critical Band Properties
Frequencies within the critical band
Individual frequencies cannot be heard separately
by the ear.
Frequencies outside the critical band
Since the frequencies are outside the critical
band, individual frequencies can be heard.
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Masking

• Masking occurs when a sound that is quite audible
  by itself can become completely inaudible if
  another louder sound is present.
• Very important to music
• Threshold of the weaker sound can be raised by
  the presence of the stronger one.

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Masking
What does it occur?
Sounds of widely different frequencies excite
separate portions of the basilar membrane
Loud Masking tones
Will not make below it in frequency, but it will
mask those of higher frequency.
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Hearing Loss
Temporary threshold shift Threshold shift of
hearing due to being deafened momentarily by a
very loud sound such as explosion
? Prolonged exposure to loud sounds can cause
permanent hearing loss
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Sound exposure limits by Walsh-Healy Public
Contracts Act
90 dBA 8-hour working day
95 dBA 4-hour working day
100 dBA 2-hour working day
115 dBA 1/4-hour working day
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Permanent Hearing Loss
Audiometer a device to check hearing loss
Cause
?Long exposure to loud sounds
ex. )Listening loud sound with headphones
? Very loud sounds with a short time ex.) firing
of guns, popgun
? Disease
? Natural aging Presbycusis
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Audiogram
Solid curve Presbycusis Dotted curve hearing
loss produced by exposure to gunfire