automotive nvh - noise...3. hearing impairment. temporary, permanent 3000 –6000 hz, with the...
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Automotive NVH - Noise
Mr. K.P.Wani,
Manager, ARAI Academy
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Outline What is NVH ?
Effects of Noise
Introduction to Sound
Sound waves
Decibel scale – dB
Measure of Sound
Octave Bands
Need for Automotive NVH
Interior noise of vehicle
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What is NVH ?
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Hearing Impairment
Temporary, Permanent
3000 – 6000 Hz, with the largest effect at 4000 Hz
Hearing impairment is not expected to occur at LAeq,8h levels of 75 dB(A) orbelow, even for prolonged occupational noise exposure.
In case of environmental and leisure time noise, LAeq,8h of 70 dB(A) or belowwill not cause impairment, even after a life time exposure.
For adults exposed to impulse noise at the workplace, the noise limit is set to apeak sound pressure level of 140 dB.
In case of children, sound pressure level should never exceed 120 dB
EFFECTS OF NOISE
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What is sound? Frequency & wavelength• Sound is a disturbance that propagates through a medium having
properties of inertia ( mass ) and elasticity. The medium by which the audible waves are transmitted is air. Basically sound propagation is simply the molecular transfer of motional energy. Hence it cannot pass through vacuum.
Frequency: Number of pressure cycles / time
also called pitch of sound (in Hz)
Guess how much is particle displacement??
8e-3nm to 0.1mm
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• Speed of sound in air is 343 meters per second
• Length of one pressure variation = Wavelength(λ) [m]
• λ = speed of soundf (frequency)
• f = speed of soundλ (wavelength)
What is sound? Frequency & wavelength
Pressure
Pmax Pmax
Pmin
Distance[m]
Wavelength, λ [m]
What is sound?
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THE PROPAGATION OF SOUND
Sound is a wave front and can not propagate without a medium.
No sound would be heard in a vacuum
Air is a standard medium by presence of which we hear and communicate
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What do we hear? Audible frequency range
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The reflection of sound follows the law "angle of incidence equals angle ofreflection", sometimes called the law of reflection.
The reflected waves can interfere with incident waves, producing patterns ofconstructive and destructive interference. This can lead to resonances calledstanding waves in rooms.
It also means that the sound intensity near a hard surface is enhanced because thereflected wave adds to the incident wave, giving a higher pressure amplitude thangenerated.
THE REFLECTION OF SOUND
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THE DIFFRACTION OF SOUND
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THE ABSOPRTION OF SOUND
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If you strike a tuning fork and rotate it next to your ear, you will note that the sound alternates between loud and soft as you rotate through the angles where the interference is constructive and destructive.
THE INTERFERENCE OF SOUND
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SOUND WAVES
LONGITUDINAL
TRANSVERSE
SPHERICAL
SOUND WAVES
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LONGITUDINAL WAVES
In longitudinal waves the displacement of the medium is parallel to thepropagation of the wave. A wave in a "slinky" is a good visualization. Soundwaves in air are longitudinal waves.
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TRANSVERSE WAVES
For transverse waves the displacement of the medium is perpendicular to the direction of propagation of the wave. A ripple on a pond and a wave on a string are easily visualized transverse waves.
Transverse waves cannot propagate in a gas or a liquid because there is no mechanism for driving motion perpendicular to the propagation of the wave.
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THE SPHERICAL SOURCE – IDEAL RADIATION
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FREQUENCY
The number of vibrations, or complete cycles, that take place in one second is the frequency (f).
Frequency is measured in units of Hertz (Hz).
One Hz = One cycle per second
The frequency range of the human ear varies considerably among individuals.
A young person with normal hearing will be able to perceive frequencies between approximately 20 and 20,000 Hz. With increasing age, the upper frequency limit tends to decrease.
Frequencies around 2,000 Hz are the most important for understanding speech, while frequencies between 3,000 Hz and 4,000 Hz are the earliest to be affected by noise.
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THE WAVELENGTH, l• The distance traveled by a sound wave during one sound
pressure cycle is called the wavelength (l).
• A wavelength is usually measured in meter or feet
l
m
A
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THE SPEED, c
• The speed (c) at which sound travels is determinedprimarily by the density and the compressibility of themedium through which it is traveling.
• Speed increases as the density of the medium increasesand its compressibility decreases.
• Speed is typically measured in meters or feet persecond.
• In air, the speed of sound is approximately 344 metreper second (1130 feet per second).
• In liquids and solids, the speed of sound is much higher.
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In air - 344 m/s (1240 km/h).
In wood - 3,962 m/s (11 times of air)
In steel - 5,029 m/s (15 times of air)
THE SPEED OF SOUND IN DIFFERENT MEDIA
Speed, C = ( Y/ρ)1/2 for solids
= ( K/ρ)1/2 for fluids
Where, Y = Young’s Modulus
K = Bulk Modulus
ρ = density dependent on temperature TC
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Gas Temperature (°C) Speed in m/s
Air 0 331.5
Air 20 344
Hydrogen 0 1270
Carbon dioxide 0 258
Helium 20 927
Water vapor 35 402
THE SPEED OF SOUND IN DIFFERENT GASES
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THE dB - Decibel SCALE
• Any quantity evaluated as a 10 times the logarithmic ratio of the quantity with respect to a particular reference value is a dB value
velocity in dB = 10log10 (v/vref)2 , vref = 1e-8 m/s
pressure in dB = 10log10 (p/pref) 2 , pref = 20 uPa
• It is generally used when the variance of amplitude of a quantity is very high. e.g. sound pressure level varies from 0 dB ( 20 uPa) to 134 dB (100 Pa )
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2 dB + 2 dB 4 dB
Arithmetic operations only be done in linear value mode and NOT in dB
THE dB SCALE
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20 Hz = 17 meters
Frequency & wavelength
Frequency, f [Hz]
20 50 100 200 500 1k 2k 5k 10k 20kLow High
Wavelength, λ [m]
20 10 5 2 1 0.5 0.2 0.1 0.05 0.02
Long Short
Low = LongHigh = Short
20 kHz = 0.017 m = 1.7 cm
λ λ λ
Time [s]
Pressure
Time [s]
Pressure
Time [s]
Dist. [m] Dist. [m] Dist. [m]
Pressure
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Audible range
Ordinary piano scale
Violin
Double Bass
Cello
Soprano
Bass
Female speech
Male speech
Frequency range necessaryfor understanding speech
55
Hz
11
0 H
z
22
0 H
z
44
0 H
z
88
0 H
z
17
60
Hz
35
20
Hz
70
40
Hz
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.5 H
z
14
08
0 H
z
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Sound pressure level
0
140
120
100
80
60
40
20
dB
Sound pressure
μPa
20
200 000 000
Threshold of hearing
Threshold of pain
20 000 000
2 000 000
20 000
2 000
200
200 000
Quiet room / Library
Lawn mower
Phone ringing
What is sound? Level, the dB-scale
Quiet countryside / calm human breathing
Concert
Normal conversation
TV
Jet take-off
Car(moving)
Traffic on major road
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Sound pressure level
0
140
120
100
80
60
40
20
dB
Threshold of hearing
Threshold of pain
Quiet countryside / calm human breathing
Quiet room / Library
Normal conversation
Lawn mower
ConcertJet take-off
Phone ringing
Car(moving)
What is sound? Level, the dB-scale
TV
Traffic on major road
Perception of sound pressure levels
Veryquiet
Quiet
Noisy
Verynoisy
Intolerable
85 dB
120 dB
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Threshold of hearing varies with frequenciesHuman ear most sensitive in the range of 2 kHz - 5 kHzLess sensitive in lower frequencies
What do we hear? Level vs. frequency
Threshold of hearing
140[dB]
120
100
80
60
40
20
0
20 50 100 200 500 1k 2k 5k 10k 20k
Frequency [Hz]
Sound p
ress
ure
level Risk of damage
Threshold of pain
Music
Speech
Veryquiet
Quiet
Noisy
Verynoisy
Intolerable
SOUND FIELDS
Near Field Far Field
Free Field Reverberant Field
- 6 dB / doubling distance
SOURCE Log10 (distance)
SPL,
dB
Sensitive to Low frequency
For correct measurement of sound at particular frequency, one should be at least ½the wavelength away from the boundaries.
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The sound pressure in Pa varies from 10 uPa ( in complete absence of audible sound) to 108 uPa ( the threshold of pain) which would impossible to put in scale
Sound Pressure Level (SPL) is used as the fundamental Measure of sound (amplitude) .
Lp = 10 Log10 (P/Pr)2
Where Lp = sound pressure level, dB
P = root mean square sound pressure, uN/m2
Pr = reference sound pressure of 20 uN/m2
Log10 = Logarithm to the base 10
Due to this logarithmic nature the sound varies from 0 to 140 dB
THE MEASURE OF SOUND
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THE INVERSE SQUARE LAW
For a free field
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DIFFERENT SOUND WEIGHTINGS
dB
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DIFFERENT SOUND WEIGHTINGS
The most common weighting networks are designated A, B, and C. They were designed to approximate the equal-loudness contours at
•Low sound pressure levels for the A -network
•Medium sound pressure levels for the B-network, and
•High levels for the C-network.
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THE OCTAVE BANDS
The Octave bands are the representation of noise spectra over a wide frequency band and are embedded in basic sound measuring instruments
fc
f2 f1
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THE OCTAVE BAND FILTER
f2= Upper cut-off frequency
f1 = lower cutoff frequency
fc = Center frequency
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The typical relationship between upper and lower cut-off frequencies is
f2 = 21/n(f1)
fc = 21/2n(f1)
n = 1 # 1/1 Octave band
n = 3 # 1/3 Octave band
n = 12 # 1/12 Octave band
The 1/3rd Octave bands with center frequencies from 63 Hz to 8000 Hz are used in most applications and are internationally standardized.
The Octave bands are basically used to have a overall distribution of noise as low /and or high frequency sound
THE OCTAVE BANDS
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Narrow band
1/ 1 Octave band 1/ 12 Octave band
1/ 3 Octave band
THE OCTAVE BANDS
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THANK YOU!!
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