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FUNDAMENTALS
OF

SOUND / NOISE

INTRODUCTION

Sound is a sensation of acoustic waves (disturbance /pressure
fluctuations setup in a medium)

Unpleasant, unwanted, disturbing sound is generally treated as
Noise and is a highly subjective feeling
...

Sound can also be viewed as an excitation of the hearing
mechanism that results in the perception of sound
...


Sound is characterized by a number of basic phenomena
...

•Wavelength
• Pitch

PARTICLE MOTION

The maximum particle
velocity is very small, less
than 0
...

To lower the level of a
sound, we must reduce
the particle velocity
...

The maximum velocity is called the velocity amplitude, and the maximum
displacement is called the displacement amplitude
...


SPEED OF SOUND
The rate at which the disturbance (sound wave) travels

The speed of sound in air is about 1,130 ft/sec at normal temperature and
pressure
...

You can gauge the distance of a thunderstorm by counting the time
between the sight of the lightning strike and the sound of its thunder
...


Sound Intensity

SOUND LEVELS AND THE DECIBEL
THE DECIBEL IS UNIT OF MEASURE IN THE AUDIO FIELD
...

A LEVEL IS BASICALLY A FRACTION
EXPRESS THE INTENSITIES AS LOGARITHMS OF THE RATIOS AS THE SOUND
FOLLOWS LOG PATH
...

THUS, THE INTENSITY RATIO IN DECIBELS BECOMES:

THIS VALUE IS CALLED THE SOUND INTENSITY LEVEL (IL IN DECIBELS) AND
DIFFERS FROM INTENSITY (I in watts/m2)
...
3*10-3 W/m2

98 dB

Walkman at Maximum Level

1*10-2 W/m2

100 dB

Front Rows of Rock Concert

1*10-1 W/m2

110 dB

Threshold of Pain

1*101 W/m2

130 dB

Military Jet Takeoff

1*102 W/m2

140 dB

Instant Perforation of Eardrum

1*104 W/m2

160 dB

NOTE : The large range of associated intensity is one of the reasons we need alternate scale

SOUND POWER
Intensity : Average Rate of energy transfer per unit area

W
I=
4π r 2

W/m

p2
W = 4π r I = 4π r
Watt
ρ 0c

2

2

Sound Power Level:

2

W
SWL = 10 log10
dB
Wref
Reference Power Wref =10-12 Watt

Peak Power output
output:
Female Voice – 0
...
004W,

A Soft whisper – 10-9W,

An average shout – 0
...

rate of energy flow per unit area

Sound Power:
for noise rating of machines
...
00002 N/m2 (threshold of hearing)
to
20 N/m2

(sensation of pain)

1000000 times larger

Atmospheric Pressure is 105N/m 2 , at normal temp
so the peak pressure associated with loudest sound is 5000 times smaller
than atm
...

Some small sources do not radiate sound equally in all
directions
Directivity of the source must be taken into account to calculate
level from the source power

EFFECT OF HARD REFLECTING GROUND
Radiated Sound Power of the source can be affected by a rigid,
reflecting planes
Strength and vibrational velocity of the source does not change but the
hard reflecting plane produces double the pressure and four-fold
increase in sound intensity compared to monopole (point spherical
source)
If source is sufficiently above the ground this effect is reduced

Free Field Condition

Diffuse Field

ENVIRONMENTAL EFFECTS
...

(Remember 6dB change represents doubling of sound pressure!!)

Ear is not equally sensitive at all frequencies:
highly sensitive at frequencies between 2kHz to 5kHz
less at other freq
...


COMBINATION OF SEVERAL SOURCES
Total Intensity produced by several sources
IT=I1+ I2+ I3+…
Usually, intensity levels are known (L1, L2,…)

 IT 
LT = 10 Log  −12 
 10 

 I1 
L1 = 10 Log  −12 
 10 

L1 
 L3 
 L2 
  10 

 10 

 10 
 


 
LT = 10 Log 10
+ 10
+ 10
+
...

ISO(1999): Increase in SPL from
90 to 93dB, must reduce time of
exposure from 8 to 4 hours

OSHA: with every 5dB increase,
reduce exposure by half

OSHA: OCCUPATIONAL SAFETY AND HEALTH ADMINISTRATION

ERRORS OF THE ORDER OF 6DB AROUND 400HZ DUE TO REFLECTIONS

Thanks

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