lecture 3 jan 31

8/3/2019 Lecture 3 Jan 31

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NVH-SQ Group

University of Windsor Presented by Dr. C. Novak

Automotive Engineering Preparation Programutomotive Engineering Preparation ProgramFundamentals of NVHJanuary 31, 2011

Copyright 2009


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Sound Power

,

The rate per unit time at which airborne sound energy is radiate by a source. Is independent of the acoustic environment in which a source is located. Is an excellent parameter for quantifying the characteristics of a source. , .

) / log(10 ref W W Lw =

Expands to: where Wref = 10 -12

][120)log(10 dBW Lw +=


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Sound Power

Analogy

Pressure p [N/m2

= Pa]Lp [dB]

Temperature t [C]Power P [W]

Power P W

SoundSource

Heater


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Sound Power/SPL/Distance

The relationship between sound power and sound pressure for spherical radiation

where c = 408 RaylWe would like to develop a relationship between SPL and Lw.

Rearranging:

Multi l b 10lo :Gives:

Simplifies to:


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Sound Power/SPL/Distance

,relationship, in terms of Lp and r:

To be further related the source at twodifferent distances r by:


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Sound Intensity

Under free-fieldconditions: P 2

2

, ,describes the amount and

directionof flow of acoustic energy at agiven position

= =

P

r

p

c4 2

2

1

r 1

r 2

Power: P [W]

Intensity: [J/s/m 2] = W/m 2

Pressure: p [Pa = N/m 2]


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Example:

Sound Power= 0.01 Watt c

pr 4

W 22

=

=

r = 1.5 m

Sound Power Sound Pressureound Intensity

Pascal0.532

400000707.0cp

=

==

2000707.05.12

01.02 22

mW

r W

=

==

W = 0.01 Watt

dB01.0

log10

dBWWlog10L

10

010W

=

=

dB1007.7

log10

dBlog10L

4

10

010

=

=

dB532.0

log10

dBpplog10L

2

2

10

20

10p

=

=

dB100LW = dB5.88L =

dB5.88Lp =

LI = L p under free-field conditions


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Characterization of Propagation

.following shows the acoustic propagation for an ideal point source.

Measurements should not be conducted in either the near or reverberant field. Ideally, we should experience a 6 dB reduction per doubling of distance or 20 dB per

decade.

Near field Far field

Lp

2 x distances ance, og r


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Characterization of Sources

Unfortunately most real sources do not,

spherically as in figure (a).

A non-ideal source will have directivitycharacteristics, as illustrated in figure (b),which must be taken into account whenquantifying.


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Characterization of Sources

radiate spherically if directivitycharacteristics are present. e.g. Bearingnoise, gears, hydraulics, etc.

To account for directivity we must determinea directivity index DI for the source.

where Lp ideal is usually taken as Lp avg

Therefore for a real source:


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Characterisation of Sources

Another condition which acousticalengineers must often correct for is thereflectivity from nearby surfaces.

o accoun or e presence o re ec ngsurfaces a directivity factor, Q, is defined interms of the directivity index, where:

Illustrated to the right are the values for Qand DI for common source locations.

It is important to account for this whenusing:


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Characterisation of Sources

r: LLine source

2r: L p 3 dB

r: L p

Plane source

p

r: L p 2r: L p


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Special Room Environments

Anechoic Reverberant


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Basic Design of Reverberation Room


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Questions???

lecture 3 jan 31

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