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

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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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