Wang X. Vehicle Noise and Vibration Refinement

Подождите немного. Документ загружается.

Vehicle noise measurement and analysis 73

Figure 4.3 shows the A, B, C and D weighting sound pressure level (SPL)

curves. As mentioned already, the A weighting network weighs the signal

in a manner which approximates to the human hearing response (as shown

in Fig. 4.2), i.e. the A weighting corresponds to an inverted equal loudness

contour at low SPLs. The B weighting corresponds to a contour at medium

SPLs, and the C weighting to an equal loudness contour at high SPLs. The

D weighting has been the standard for aircraft noise measurement. The A

weighting is well correlated with subjective evaluation and most widely

used; B and C weightings do not correlate well with subjective tests.

Figure 4.4 shows a family of equal loudness contours, i.e the sound pres-

sure level required at any frequency to give the same apparent loudness as

a 1 kHz tone. The amplitude scale for sound work is a logarithmic scale (to

base 10). It ranges from 0 dB (the threshold of hearing) to 140 dB (typical

of the sound level near a jet aircraft engine). An ofﬁ ce area typically mea-

sures around 60 dB. Three decibels is the minimum change in level that

humans can perceive; a change in sound pressure level less than 3 dB will

go unnoticed. Ten decibels of change is required before the sound subjec-

tively appears to become twice as loud or quiet.

To obtain frequency information using a sound level meter, octave analy-

sis is done with ﬁ lters. An octave is a doubling of frequency. It gets its name

SPL (dB)

–10

–20

–30

–40

–50

–60

B + C

Frequency (Hz)

20 1000 2000 20000

4.3 Weighting curves for sound-level meters (courtesy of Brüel & Kjær

Sound & Vibration A/S, Measuring Sound, 1984, p. 11).

Copyrighted Material downloaded from Woodhead Publishing Online

Delivered by http://woodhead.metapress.com

ETH Zuerich (307-97-768)

Sunday, August 28, 2011 12:01:07 AM

IP Address: 129.132.208.2

74 Vehicle noise and vibration reﬁ nement

from the musical scale where one octave covers eight notes of the diatonic

musical scale. For example, the 1 kHz octave band ﬁ lter passes the frequen-

cies from 707 to 1414 Hz (centred on 1 kHz). Remember that ﬁ lters are not

perfect and the frequency limits are not sharp cut-offs – there is some slope

to the sides, and some signals outside this band will sneak in. To get ﬁ ner

resolution of frequency analysis, 1/3-octave bands are available on some

ﬁ lter sets. In 1/3-octave bands the highest frequency of the pass-band is 1.26

times the lowest frequency. The same 1 kHz pass-band in a 1/3-octave ﬁ lter

will have frequency limits of 891 Hz to 1120 Hz. A signiﬁ cant amount of

acoustic data is still measured and reported in octave and 1/3-octave ﬁ ltered

measurements, but these measurements are crude compared to those from

a spectrum analyser that can perform narrowband analysis. A 400-line

analyser set to a frequency span of 2000 Hz will have band-pass ﬁ lters set

at 5 Hz, i.e. 2000/400. The 1000-bin ﬁ lter in this analyser will have lower

and upper cut-off frequencies of 997.5 and 1002.5 Hz. With zooming this

can get much narrower. The superiority of spectrum analysers to obtain

precise frequency information should be evident.

SPL (dB)

130

110

130

110

Frequency (Hz)

20 50 100 200 500 1

k2 k5 k10 k20 k

4.4 Equal loudness contours (courtesy of Brüel & Kjær Sound &

Vibration A/S, Measuring Sound, 1984, p. 8).

Copyrighted Material downloaded from Woodhead Publishing Online

Delivered by http://woodhead.metapress.com

ETH Zuerich (307-97-768)

Sunday, August 28, 2011 12:01:07 AM

IP Address: 129.132.208.2

Vehicle noise measurement and analysis 75

4.2.3 Decibel scales

Sound levels are commonly described in terms of the sound power (W)

output of noise sources and the sound pressure (Pa) amplitude at a given

location. However, decibel scales (dB) are useful due to the wide range of

powers and sound pressure amplitudes that can be encountered.

Table 4.3 shows that sound power outputs of everyday machines lie in

the 0.001 to 1000 W range (a factor of one million times) and the human

voice has a power output in the range of 0.000 000 001 to 0.001 W (also a

factor of one million times).

Table 4.4 shows that everyday noise environments relate to pressure

amplitudes in the range of 0.000 02 to 20 Pa (a factor of one million times).

The human ear can detect sound pressure amplitudes over this whole range

(beyond, in fact: from 20 × 10

−6

to 60 Pa).

Table 4.3 Typical sound power levels for different sources

Source Sound power output (W)

Saturn rocket 25–40 million

Military jet engine with afterburner 100 000

Turbojet engine 1000–10 000

Propeller airliner with four engines 100–1000

75-piece orchestra 10–1000

Small aircraft engine 1–10

Piano 0.1–1

Car alarm 0.1

Hi-ﬁ 0.01

Moving car 0.001–0.01

Fan 0.001

Person shouting 0.0001–0.001

Conversational voice 0.000 01

Whispering voice 0.000 000 001

Table 4.4 Typical sound pressure levels in different environments

Environment Sound pressure

amplitude (Pa)

Description

100 m from Saturn rocket 200 Ear pain threshold

At front of rock concert 20 Potentially damaging

Noisiest factory 2 Harmful

Next to road or railway 0.2 Noisy

Busy restaurant 0.02

Quiet suburban street 0.002 Quiet

Recording studio 0.0002 Very quiet

0.000 02 Threshold of hearing

Copyrighted Material downloaded from Woodhead Publishing Online

Delivered by http://woodhead.metapress.com

ETH Zuerich (307-97-768)

Sunday, August 28, 2011 12:01:07 AM

IP Address: 129.132.208.2

76 Vehicle noise and vibration reﬁ nement

To simplify the situation, sound levels are usually described by using the

decibel scale:

⎡

⎣

⎢

⎤

⎦

⎥

log

ref

(4.1)

where:

ref

(4.2)

The sound pressure level (L

) is expressed as the ratio of the squared sound

pressure amplitude to the threshold of hearing:

ref ref

dB=

⎡

⎣

⎢

⎤

⎦

⎥

⎡

⎣

⎢

⎤

⎦

⎥

10 20

log log

(4.3)

where p

ref

= 20 × 10

−6

Pa.

According to this scale, everyday noise environments fall in the sound

pressure level range of 0–120 dB. By way of comparison, the typical operat-

ing pressures associated with internal combustion engines are:

• Peak cylinder pressure ≈ 60 bar ≈ 230 dB.

• Exhaust pressure wave ≈ 0.8 bar ≈ 192 dB.

• Intake pressure wave ≈ 0.2 bar ≈ 180 dB.

The sound power level (L

) is expressed as the ratio of the sound power

to a reference power of 10

−12

ref

dB=

⎡

⎣

⎢

⎤

⎦

⎥

log

(4.4)

where:

ref

= 10

−12

W (4.5)

As a result, everyday machine power outputs fall in a sound power range

of 70–160 dB and the human voice produces sound power levels in the

30–70 dB range.

4.2.4 Decibel arithmetic

If the sound levels from two or more machines have been measured sepa-

rately and you want to know the total sound pressure level (SPL) made by

the machines when operating together, the sound levels must be added.

However, decibels cannot just be added together directly (because of the

logarithmic scale). Addition of decibels can be done simply using the chart

in Fig. 4.5 where the sound pressures of different machines are uncorrelated

to each other.

Copyrighted Material downloaded from Woodhead Publishing Online

Delivered by http://woodhead.metapress.com

ETH Zuerich (307-97-768)

Sunday, August 28, 2011 12:01:07 AM

IP Address: 129.132.208.2

Vehicle noise measurement and analysis 77

1. Measure the SPL of each machine separately (L

, L

2. Find the difference between these levels (L

− L

3. Enter the bottom of Fig. 4.5 with this difference. Go up until you inter-

sect the curve, then go to the vertical axis on the left.

4. Add the value indicated (ΔL) on the vertical axis to the level of the

noisier machine (L

); this gives the sum of the SPLs of the two machines

1+2

5. If three machines are present, repeat steps 1 to 4 using the sum obtained

for the ﬁ rst two machines and the SPL of the third machine.

Example:

1. Machine 1 has L

= 82 dB; machine 2 has L

= 85 dB.

2. Difference L

− L

= 3 dB.

3. Correction (from chart) ΔL = 1.7 dB.

4. Total noise = 85 + 1.7 = 86.7 dB.

Mathematically, decibel addition has this structure:

LLL X

12 10

10 10

10 10 10 10

[]

−log log

ref

(4.6)

and decibel subtraction has this structure:

LLL X

12 10

10 10

10 10 10 10

−

=−

[]

=−

[]

−log log

ref

(4.7)

where the sound pressures of different sources L

and L

are uncorrelated

to each other.

ΔL (dB)

– L

(dB)

1. 7

510 150

4.5 Adding sound levels (courtesy of Brüel & Kjær Sound & Vibration

A/S, Measuring Sound, 1988, p. 30).

Copyrighted Material downloaded from Woodhead Publishing Online

Delivered by http://woodhead.metapress.com

ETH Zuerich (307-97-768)

Sunday, August 28, 2011 12:01:07 AM

IP Address: 129.132.208.2

78 Vehicle noise and vibration reﬁ nement

The combination of two identical sound levels produces a sum which is

3 dB greater than the individual levels. Combining a sound level with

another which is 10 dB less in magnitude, it produces a sum that is negli-

gibly greater than the highest sound level.

4.3 Vehicle noise

Whenever vehicle power is generated, transmitted or used, a fraction of

this energy is converted into sound power and radiated into the air.

Generally the fraction of available vehicle power that is converted into

acoustical power is very small. However, very little acoustical power is

needed to make a sound source audible.

A vehicle generates two types of noise: noise that travels in air is called

airborne noise, while noise that travels in vehicle structures is called

structure-borne noise. There are two types of noise – broadband and nar-

rowband, or pure tones. The broadband noise can be desirable or objection-

able depending on the frequency content, amplitude and structure-borne

versus airborne content. The pure tones are almost always objectionable.

Resonances, which also cause pure tones, may be of two kinds: structural

or acoustical. Structural resonance is the easier of the two to identify and

correct. Simply ﬁ nd the ‘singing’ part and stiffen it. An acoustical resonance

is found by matching a length of ductwork with the wavelength of the

objectionable tone.

4.3.1 Direct sound-generation mechanics: airborne sound

An arbitrary noise is generated by a physical mechanism and then radiated.

The physical mechanism could be:

• Hot displacement of ﬂ uid mass or volume (loudspeaker, exhaust

tailpipe)

• Unsteady combustion (engine, furnace, boiler)

• Unsteady ﬂ uid transport (ﬂ ow noise)

• Fluctuating force on a ﬂ uid (fans, wires in a ﬂ ow)

• Fluctuating shear force (shear layer noise in a jet).

When controlling the sound power output from a direct sound generation

mechanism at source, the following strategies can be employed:

• Reduce the source strength by alternating the parameters within the

physical process itself, such as the rate of combustion, ﬂ ow rate, the

force imparted to the ﬂ uid, etc.

• Reduce the physical size of the source as a means of reducing the source

strength.

Copyrighted Material downloaded from Woodhead Publishing Online

Delivered by http://woodhead.metapress.com

ETH Zuerich (307-97-768)

Sunday, August 28, 2011 12:01:07 AM

IP Address: 129.132.208.2

Vehicle noise measurement and analysis 79

4.3.2 Indirect sound-generating mechanism:

structure-borne sound

An arbitrary noise is generated from a ﬂ uctuating force which excites struc-

tural responses and sound radiation from the structure. The common indi-

rect sound sources on vehicles are:

• Casing vibration

• Bearing noise

• Electric motor or generator noise

• Belt, chain or gear drives

• Internal combustion engine structural noise.

Controlling noise at the indirect sound sources is done by one or more of

the following means:

• Reducing the amplitude of the force or, where a number of independent

cyclic forces are present, arranging their phasing so as to obtain

cancellation.

• Reducing the vibration response of the structure to a given force

input.

• Improving the acoustic radiation efﬁ ciency of the structure at a

given frequency by altering the critical frequency of the radiating

component.

The wavelength of sound is calculated by using:

344m s

(4.8)

where:

λ = wavelength (m)

f = frequency (Hz)

344 m/s is the speed of sound in air (at 1 atm pressure and 300 K).

It is useful at this point to look at some numbers:

• The lowest frequency audible to the typical human subject is 20 Hz,

wavelength in air 17.15 m.

• The highest frequency audible to the typical human subject is 20 kHz,

wavelength in air 17.15 mm.

• The lowest amplitude audible to the typical human subject is 20 ×

−6

Pa, particle velocity in air 0.000 05 mm/s.

• The highest amplitude audible to the typical human subject without

experiencing ear pain is 200 Pa, particle velocity in air 481 mm/s.

Copyrighted Material downloaded from Woodhead Publishing Online

Delivered by http://woodhead.metapress.com

ETH Zuerich (307-97-768)

Sunday, August 28, 2011 12:01:07 AM

IP Address: 129.132.208.2

80 Vehicle noise and vibration reﬁ nement

4.4 Measuring microphones

Measurements provide deﬁ nite quantities which describe and rate sounds.

Sound measurements also permit precise, scientiﬁ c analysis of annoying

sounds. The measurements give us an objective means of comparing annoy-

ing sounds under different conditions.

Sound levels are measured using a microphone attached to some form

of electrical signal conditioning and analysis equipment. There are many

types of microphone with different operational means of converting ﬂ uctu-

ating pressure (or pressure difference) into an electrical signal. Two basic

types of microphone are condenser and electro-dynamic. The condenser

microphone is favoured for vehicle noise investigations because of its fre-

quency response. Electro-dynamic microphones start to lose sensitivity

around 80 Hz and are down considerably at 50 Hz. Condenser micro-

phones, on the other hand, remain linear below 50 Hz and some are linear

even down to 20 Hz. Condenser microphones are also more sensitive than

electro-dynamic types because of their use of pre-ampliﬁ ers.

The pre-ampliﬁ er converts the microphone’s high output impedance to

low impedance suitable for feeding into the input of accessory equipment.

This impedance conversion next to the microphone serves to minimize the

pickup of noise in the signal cable to the accessory equipment.

4.4.1 Condenser microphones

The operating principle of a condenser microphone is to use a diaphragm as

the moving electrode of a parallel plate air capacitor. It features a tensioned

metal (nickel) diaphragm supported close to a rigid metal back-plate. The

microphone’s output voltage signal appears on a gold-plated terminal

mounted on the back-plate which is isolated from the microphone casing (or

cartridge) by an insulator. The cartridge’s internal cavity is exposed to atmo-

spheric pressure by a small vent and the construction of the microphone is

completed with the addition of the distinctive diaphragm protective grid.

The diaphragm and back-plate form the parallel plates of a simple air

capacitor which is polarized by a charge on the back-plate. When the dia-

phragm vibrates in a sound ﬁ eld the capacitance varies and an output

voltage is generated. The voltage signal replicates the sound-ﬁ eld pressure

variations as long as the charge on the microphone back-plate is kept ﬁ xed.

The sensitivity of the condenser microphone is discussed below.

Sensitivity

As might be expected, the larger the electrodes within the microphone, the

greater is the voltage produced by a given deﬂ ection of the diaphragm, and

the greater is the sensitivity of the microphone.

Copyrighted Material downloaded from Woodhead Publishing Online

Delivered by http://woodhead.metapress.com

ETH Zuerich (307-97-768)

Sunday, August 28, 2011 12:01:07 AM

IP Address: 129.132.208.2

Vehicle noise measurement and analysis 81

Precision condenser microphones usually come in four sizes, denoted by

their external diameters:

• One inch (25.4 mm)

• Half inch (12.7 mm)

• Quarter inch (6.35 mm)

• Eighth inch (3.175 mm).

Most of them are omnidirectional, i.e. sensitive to sound arriving from

all directions. The two smallest microphones have the best omni-

directional characteristics at audio frequencies. They respond equally to

all frequencies arriving from all directions because their physical presence

in the sound ﬁ eld does not have a big inﬂ uence on incoming sound

waves. The larger (one-inch) microphones, as a direct result of their

size, are not sensitive to frequencies above 5 kHz which approach from the

sides and rear of the microphone. The omnidirectionality of the one-inch

microphone can be improved by ﬁ tting a nose cone or the special

windscreen.

The open-circuit sensitivity is usually quoted for direct comparison

between microphones. This is the voltage (mV) produced per pascal of

pressure at 250 Hz with 200 V polarization (except for a few microphones

requiring 28 V or 0 V polarization). The open-circuit sensitivity is the sen-

sitivity of the capsule on its own before it is electrically loaded by being

attached to a pre-ampliﬁ er. The sensitivity of the microphone varies with

temperature and atmospheric pressure.

Frequency response

There are three basic types of microphone:

• Free-ﬁ eld-response

• Pressure-response

• Random-response.

Free-ﬁ eld-response microphones are used for measuring sound coming

mainly from one direction. Their frequency response curve is designed to

compensate for the pressure build-up at the diaphragm caused by interfer-

ence and diffraction effects. Measured sound-pressure levels are assumed

equal to those that would exist in the sound ﬁ eld if the microphone were

not present. This means that the physical size of the microphone is small

compared with the wavelength of the impinging sound, its presence does

not affect the local sound ﬁ eld and it measures the true ﬂ uctuating pressure.

The response of the microphone in these conditions is known as the free-

ﬁ eld response. At higher frequencies – near the resonant frequency of the

microphone – the impinging sound is reﬂ ected and diffracted by the pres-

ence of the microphone and, as a result, the pressure at the diaphragm is

Copyrighted Material downloaded from Woodhead Publishing Online

Delivered by http://woodhead.metapress.com

ETH Zuerich (307-97-768)

Sunday, August 28, 2011 12:01:07 AM

IP Address: 129.132.208.2

82 Vehicle noise and vibration reﬁ nement

increased from its free-ﬁ eld value. The response of the microphone to this

artiﬁ cially higher pressure is the pressure response.

It is also possible to construct microphones that respond linearly with

frequency (up to a limiting frequency) to the actual pressure at the dia-

phragm. These are known as pressure-response microphones. Pressure-

response microphones do not compensate for the pressure build-up at the

microphone diaphragm – they measure the actual sound-pressure level at

the diaphragm. Uses include measuring sound pressure levels at a surface

or in a closed cavity. Pressure-response microphones can be used as free-

ﬁ eld microphones if they are oriented at right angles to the direction of

sound propagation, but their effective frequency range is then reduced.

When making measurements within small cavities or couplers or when the

microphone diaphragm is mounted ﬂ ush with a hard surface, it is the pres-

sure response that is of interest. When making measurements in a diffuse

ﬁ eld it is the random incidence response that is of interest and a random

incidence microphone should be used.

The frequency response of any device that operates over a range of fre-

quencies is that range of outputs that are within 3 dB of a 0 dB reference

line as shown in Fig. 4.6. This −3 dB point usually occurs at between 1 and

3 Hz at low frequencies, above which the open-circuit sensitivity of a preci-

sion condenser microphone remains constant with changing frequency.

However, the sensitivity does become frequency-dependent at higher fre-

quencies. The change in sensitivity with frequency is known as the fre-

quency response of the microphone. At higher frequencies the microphone’s

–5

Free-field response

Pressure response

Random incidence response

50 200 1000 5000

4.6 Frequency responses of free-ﬁ eld, pressure- and random-response

microphones (courtesy of Brüel & Kjær Sound & Vibration A/S,

Measuring Sound, 1988, p. 22).

Copyrighted Material downloaded from Woodhead Publishing Online

Delivered by http://woodhead.metapress.com

ETH Zuerich (307-97-768)

Sunday, August 28, 2011 12:01:07 AM

IP Address: 129.132.208.2