Fundamentals of Noise
• Sound - Defined as any pressure variation in a medium (Air,
Water or other mediums) that the human ear can detect
• Sounds unpleasant and unwanted are called “Noise
• “Audible dynamic range of human ear
20 x 10-6 Pa to 200 Pa
20 x 10-6 Pa Threshold of Hearing of 0 dB
200 Pa - Threshold of Pain - 140 dB
160 dB - Threshold of permanent deafness
• Sounds of different frequency, at a constant sound pressure level
do not evoke equal loudness sensations
• Loudness level expressed as Phon
0 Phon is 0 dB SPL at 1000 Kz
40 Phon is 40 dB SPL at 1000 Hz
The Ear’s Most Important Function : To Receive Speech
Important Ranges of Human Bearing :
Range of Hearing
20 Hz – 20 kHz
All sounds preceived by normal subjects
Range of Speech
100 Hz – 10 kHz
All sound produced in human speech
Range of Articulation 200 Hz – 6 Hz
Range needed to hear every syllable of English speech
Range of Intelligibility
500 Hz – 2500 Hz
Range needed to understand English speech (even though every
syllable may not be properly heard)
Minimum Audible Field (MAF)
Level at which each tonal signal is barely received by people with
500 – 2500 Hz
20 – 100 Hz, 15,000 –
Why measure sound?
• Provide definite quantities which describe and rate
• Permit precise,scientific analysis of annoying sound
• Engineering aspects of noise control
• Ascertain the probable damage to ears
• Improvement in building design
• Diagnostic tool
1. Sound Pressure
2. Sound Power
3. Sound Intensity
Sound pressure level (SPL)
P = RMS Sound Pressure
Po = Reference sound pressure 20 x 10-6 Pa
Equivalent sound pressure level
Leq = 10 log
∫ P o 2 dt
= Sound Pressure
Leq has the same energy content as the varying sound level
SPL Depends on
Orientation of Receiver relative to machine
Environment of measurement room
Why Use A Logarithmic Measure Of Sound ?
Enables coverage of entire hearing range on single meter.
Corresponds roughly to ear’s behavior:
Twice as loud.
Three times as loud.
Four times as loud.
Eight times as loud.
Corresponda to ear’s perception of change:
1 db change…….. Hardly noticeable.
3 db “
……... Noticeable, not significant.
5 db “
10 db “ ……... Double loudness
0.5 db “ ……... Noticeable only in “A-B” tests.
Sound power level (SWL)
W = Acoustic power of source
W0 = Reference sound power = 10-12
• SWL is a function of source only and is independent of
the acoustic environment
• Widely used for rating and comparing equipment and
also in noise control.
Sound Intensity level
SIL = 10 LOG
Io = Reference Intensity = 10-12 W/M2
• Average rate of flow of energy per unit time through a
• Uses special intensity probes – Two microphones for
• Used for identification of noise sources
• Provides information on direction of acoustical energy
1. Intensity is a vector quantity as it has both magnitude
2. Intensity is a measure of the concentration of acoustic
power across a unit area.
3. Intensity is dependent on the source’s properties and
distance from the source.
4. Intensity is difficult to measure directly, but can be
A – Weighted
Reciprocating pumps(>1600 rpm)
115 - 138
Pile driving equipment (upto to 6 ton drop hammer 103 - 131
96 - 126
Generators ( 1.25 – 250 kVA)
99 - 119
Industrial vibrating screens
100 - 107
95 - 120
Room air-conditioners (up to 2 hp)
55 - 85
Tractors and trucks
110 - 130
Sound power level data are useful
• To calculate the approximate SPL at a given distance from a
machine operating in a specified environment
• To compare the noise radiated by machines of same type and
• To compare the noise radiated by machines of different types
• To determine whether a machine complies with a specified
upper limit of sound emission
• To plan in order to determine the amount of transmission loss
or noise control required under certain circumstances
• To assist in developing quiet machinery and equipment.
LP = Lw + 10 log 10 [
+ ] dB
R = Room constant =
S1α1 + S 2α 2 + ...........
S1 + S 2
Q = Directivity factor depends on the shape and the
complexity of source
Free space, spherical radiation
Centre of flat surface, hemispherical radiation
Centre of edge formed by junction of two
adjacent flat surfaces
Corner formed by junction of three
For a typical air conditioned room at a distance
LP = Lw – 8 dB
Meter response control
– Meter responds quickly to step changes in
continous sound level, approximately 0.2
–Meter responds slowly to step changes in
continuous sound level. Approximately 1.0
IMPULSE – Meter responds to maximum RMS value of
repetitive impulsive sounds.
-Meter responds to maximum peak value of
impulsive sound, even of single impulses
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