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1. INTRODUCTION

In our quest for a quiet living, noise fromvehicular traffic has been drawing attention from

communities worldwide. Various national and

international regulatory authorities have been

continuously reviewing and refining limits for

vehicle noise for many years. Besides, lesser

noise and vibration perceived in the vehicle

lead to better drive comfort and less vibration

fatigue to the passengers. In this context,

automotive NVH has become a very significant

parameter in the overall development of new

vehicles and their sub-systems. Pass-by noiseof the vehicles governed by regulatory bodies

as well as noise in the vehicle interior, power

train, sub-assemblies and different material

treatments need careful investigation for

effective noise refinement activities. Sincenoise and vibration phenomena are dynamic in

nature, its study necessitates use of specialized

transducers making their selection, mounting

and use very critical.

Transducer is a device which converts

one form of energy into another. Its response

mainly depends upon its sensitivity, dynamic

range, frequency response, linearity and

physical parameters like dimensions, mass,

etc. Today we have at our disposal, a varietyof transducers, data acquisition systems

and post processing tools, which are tailor-

Noise &Vibration Measurement Techniques

in Automotive NVH

Manasi P. Joshi, E. Ramachandran and N. V. Karanth

NVH lab, ARAI, Pune

joshi.nvh@araiindia.com

NVH lab, ARAI, Pune

ramachandran.nvh@araiindia.com

NVH lab, ARAI, Pune

karanth.nvh@araiindia.com

Abstract: In todays highly competitive automotive world, Noise, vibration and

harshness (NVH) is one of the thrust areas most of the automotive OEMs are focusing

on for providing better drive and vehicle comfort. For analysis of automotive vehicle

and sub-assemblies for noise and vibration under different vehicle operating conditions

call for specialized transducers and instrumentation. This plays an important role in the

complete process for noise and vibration analysis. Main purpose of this paper is to look

into the various types of transducers and instrumentation systems used for dedicated

noise and vibration assessment involved in automotive NVH. Some of the critical

requirements of the special transducers and instrumentation needed for laboratory

and field measurements are covered. This paper highlights special requirements ofdedicated instrumentation/data acquisition systems for quantitative and qualitative

assessment of critical NVH parameters. Measurement techniques for a few of the

associated NVH parameters required for post-processing and analysis are included

which provide brief review of signal processing and post processing tools. Also this

paper covers guidelines to be followed while using the sensors, data acquisition systems

and important parameters involved in analysis from Instrumentation point of view.

Keywords:Noise, Vibration, Transducer, RPM, Sound Pressure level, Signal Processing

AdMet 2012 Paper No. VN 003

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made for automotive NVH analysis. We will

be discussing various Noise and vibration

measurement sensors, data acquisition

systems, advanced sensors and their uses.

2. VIBRATION MEASUREMENT

TRANSDUCERS

A body is said to vibrate when it describes an

oscillating motion about a reference position.

Components of machines, vehicles and

structures are frequently subjected to cyclic

loads which lead to mechanical vibrations.

In automotive NVH we are mainly dealing

with rotating and reciprocating machinery

vibrations. These structural vibrations occur

as Linear/ lateral vibrations and Torsional

Vibrations. Lateral vibrations are transmittedto body panels by differential mounts and

torsional vibrations are carried to body by the

vehicle structure itself based on the rotational

phenomenon.

Vibration transducers are mainly classified

as Contact type and Non-contact type. For

linear vibration measurements of the vehicle

and engine, accelerometers are used which

are contact type transducers. They are made

from quartz crystals or other piezoelectricmaterials offering excellent long term stability,

good high frequency response and small size.

Laser Doppler vibrometer, is a non-contact

type vibration transducer very useful for high

temperature applications and measurements

on locations difficult to access. Velocity pick-

up possess good mid frequency response

while displacement transducers are preferred

for their good low frequency response.

Torsional vibration measurements are carried

out with non-contact type transducers mostly

in laboratory.

2.1 Accelerometers

Accelerometers are Active sensors.

Accelerometers are used widely as their

acceleration output is directly proportional

to the applied force and hence there is no

dependence of the forcing frequency on its

output. They are classified as Piezoelectric

Charge type i.e PE, Piezoelectric IEPE(Integrated Electronic Piezo-Electric) type,

Piezo Resistive (PR) type and Variable

Capacitance (VC) type. For automotive

NVH measurements we use Piezoelectric

IEPE (Integrated Electronic Piezo-Electric)

type accelerometers as they can be used

with long lengths of commonly available

connection cables. Its construction is based

on a combination of seismic mass and

piezoelectric material. They are designed both

as compression and shear types depending

on the direction of force exerted by the mass

on the piezo element. Uniaxial Accelerometers

are used for measurement along single axis

while Triaxial accelerometers are used for

measurement along the three co-ordinate

axes.

Fig. 1: Shear type Uni-axial Accelerometer

Fig. 2 : Tri-axial Accelerometer

Selection Criteria : Accelerometers are

selected mainly based on its sensitivity, mass,

frequency response, dynamic range and

operating temperature.

Important considerations while using

Accelerometer:

1. Mounting: Accelerometer mounting

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can be done using metallic stud, quick-setting

adhesives like Loctite, magnetic base, bees

wax, manually held tips etc. The choice is

mainly governed by surface area available

for mounting, surface temperature, amplitude

of vibration and nature of installation, i.etemporary or permanent. The mounting

becomes very critical in some situations

especially where amplitude and frequency of

vibrations are high.

2. Test Conditions:

2.1. Location of accelerometer and surface

on which it is mounted needs to be checked.

Temperature of surface should be taken

into consideration. For high temperature

surfaces like engine components, hightemperature accelerometers need to be used

(Temperatures in the range of 200-6000C).

2.2. Electrical grounding of the surface

may be required in certain conditions where

electrical ground loops cause interference.

2.3. Size and dimensions of accelerometers

must suit the selected applications. For modal

testing light weight accelerometers must be

used to avoid mass loading effect To carry outthe modal testing of small structure like PCB,

we need small mass accelerometer.

2.4. If long cables are to be used, suitable

conditioner must be selected to minimize

cable noise in signal.

2.5. Possible effect on accelerometer

sensitivity when exposed to high temperature,

humidity, strong magnetic fields and high

shocks should be considered before their use.

2.6 Calibration check must be done for

accelerometer before use.

2.2 Other Vibration Transducers

1. Laser Doppler Vibrometer:It is a non-

contact type sensor. It is good for distant,

difficult to access, high temperature vibration

measurements. It measures vibration velocity

over wide range of frequency. It uses low

frequency vibration wave(0- 6 kHz) modulatedon high frequency (50 MHz) carrier wave.

2. 2D and 3D Laser Scanning Vibrometers

are used for non-contact vibration response

measurement and recording in advanced

modal analysis and engine vibration mapping.

3. Velocity pick up transducer anddisplacement transducer like Eddy current

proximity probe are mainly used when

frequency range of measurement is not very

high.

4. Rotary encoders are used for angular

vibration measurement. In this we connect a

thin disc of equidistant angular slots to the test

object when the beam of light passes through

the slots, voltage pulses will be generated

which will be counted and scaled as measure

of angular velocity. It has very high resolution.

5. Toothed wheel with magnetic sensors

are used in which Twist can be detected across

any two members of the assembly. It is based

on the principle of Ferro-magnetism in which

change in magnetic field gives a sinusoidal

output voltage which is converted to TTL

pulses. Direct measurements on components

like flywheel, gear box etc can be done and is

used for Torsional vibration analysis.

3. NOISE MEASURMENT TRANSDUCERS

Sound is a propagating type of energy

which consists of longitudinal/spherical/

cylindrical waves with compressions and

rarefactions. Any unwanted sound is known

as Noise. In automotive NVH, the noise

investigation is mainly carried out as vehicle

interior and exterior noise. Interior noise

is due to radiation from the engine, intake

system, and ancillary systems and from

structural vibrations. Road and tyre noise,

wind noise, exhaust system, brake system are

the contributors for exterior noise. We usually

express noise in terms of Sound pressure

level as sound pressure is what human ear

actually senses. To measure this noise we

use measurement microphones which are

different from commercial audio system

microphones. Measurement microphones are

passive sensors having very high sensitivity,

upper frequency response and dynamic range

which make them specialized. Acoustic fieldis generally classified into Free field, pressure

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field and diffuse field from measurement

point of view. Based on this, microphones are

mainly classified as below:

3.1. Free field Microphone

This type of microphones is generally used

for single source by pointing them directly at

the source at a specified distance, usually

1meter. It measures the sound pressure at

the point of measurement, with minimum

influence of the microphone itself. This is

useful while measuring noise in open areas,

anechoic chambers where fairly accurate

estimation of the noise generated by the

source is needed, eg. Vehicle Pass by noise,

tyre noise measurement.

3.2. Pressure field Microphone

This type of microphone measures

the sound pressure that exists in front of

the diaphragm with same magnitude and

phase at any position in the field. It is used

in closed areas, cavities or pipes which are

smaller in dimension than the wavelength. It

measures the sound coming from the source

in a direction pointing at the microphone. It is

used for measurement of pressure in cavities,enclosures and for microphone calibration.

3.3. Random Incidence / Diffuse field

Microphone

This type of microphone measures the

sound in the diffuse field. It is used for sound

pressure measurements of church, big halls

with hard walls or reverberation rooms. It usually

has good Omni-directional Characteristics.

Based on construction, microphones are

classified as carbon, condenser microphone,

piezoelectric and surface microphones. For

NVH measurements Condenser microphones

are widely used. It is having disk-shaped

back-plate, isolated from the cylindrical metal

housing at the end of which a metal diaphragm

is stretched. This diaphragm and the front of

the back-plate form the plates of the active

capacitor which generates the output signal of

the condenser microphone. It is having high

and stable sensitivity hence preferred.

Selection Criteria

1. Microphones are mainly chosen

based on the type of sound field, sensitivity,

frequency response, dynamic range and other

physical parameters.

2. Microphone size in general governs its

frequency response and sensitivity. Standard

sizes available for selection include 1 inch,

1/2 inch, 1/4 inch and 1/8 inch microphones.

Smaller diameter microphones usually have

lower sensitivity and higher upper frequency

limit.

Applications

Condenser microphones are used for soundpressure level measurement. It can be used in

Sound Intensity probes as a pair of 1/2 inch or

1/4 inch microphones along with preamplifier.

In advanced technique like beam forming and

near field acoustic holography microphones

are used in array. Surface microphones are

used for acoustic-fatigue testing of aero

planes, wind-tunnel measurements, medium-

to high-level measurements, measurement in

confined spaces etc.

Fig. 3: Different dimension (Diameter)microphones and Condenser Microphone

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Important considerations while using

Microphones:

1. Mounting of microphone should be done

very carefully. It should be placed at the

recommended distance as specified in

the standard from source and use withprotective cover like windscreen.

2. Cable length must be checked to avoid

noise problem in signal.

3. Microphones must be carefully handled

as a short fall or mechanical shock can

damage its thin diaphragm

4. Correct type of microphone must be

used as per the acoustic field conditions

5. Check whether the microphone is

externally polarized which requires

external polarization voltage or pre-

polarized.

6. Avoid use of the microphone at high

temperature, humidity, vibrations, and

magnetic fields.

7. Calibration check must be done before

using the microphone.

4. OTHER TRANSDUCERS AND

ACCESSORIES4.1 Tachometers

Tacho sensors are used to acquire the

speed of rotating machines; in automotive

NVH, this is usually done by measurement

of engine speed in rpm at different driving

conditions. Here simultaneous acquisition

of RPM is performed along with acoustic

and vibration data at different engine

speeds. Based on the types and adaptability

of mounting arrangement and surface

temperature, tacho sensors are selected. A

few of these are listed below:

1. Optical sensor: The Optical Sensor is

capable of detecting a reflected pulse from

a target pasted with Reflective Tape or high

contrast color differences (such as black and

white) at specified distances. Mounting of

sensor is a critical parameter.

2. Laser tacho probe: Some specially

designed Laser tacho probes are alsoused for contact-free speed measurements

requiring better accuracy and for operation at

longer distances on rotating or reciprocating

machine parts.

3. Contact type Tachoprobes based on

vibration measurement and capacitance

variations are also used commonly for engine

speed measurements.

4. Other sensors like V-Box or Radar are

also used for vehicle speed acquisition.

5. Magnetic pick up sensors are also used

for RPM acquisition.

The sensors are usually supplied with

dedicated conditioning unit with provision of

generating TTL pulse output which can be fedto data acquisition system.

4.2. Data Acquisition Systems

A typical data acquisition system consists

of transducers / sensors with necessary

signal conditioning, multiplexing and data

handling data conversion, data processing,

data transmission, and storage and display

systems. To optimize the characteristics of

systems, sub-systems are added or combinedtogether. Generally analog signals are

acquired and converted into digital form for

processing and displaying the desired result.

Generalized data acquisition systems have

the following functional blocks:

Fig. 4: Functional Block diagram of Data

acquisition system

For NVH applications we use high end

data acquisition systems having following

capabilities:

1. Multichannel data acquisition systems

where large no. of sensors can be

connected and data can be acquired

in real time. Normally 4-100 channel

modular data acquisition systems

comprising of multiple hardware frames

which can be synchronized.

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4.4. Electromagnetic Shaker:

1. It works on the principle of electromagnetic

induction.

2. By using different types of electrical

inputs to the driving coil like sine, sweptsine, random, pseudo random, burst

random etc, corresponding motion can

be generated.

3. Mainly electromagnetic, hydraulic and

pneumatic shakers are used depending

on the frequency range and force

requirement in modal analysis and in

transfer path analysis.

4.5. Sound Level Meter

1. Sound level meters (SLM) are used for

sound pressure level measurement.

2. It is a handheld instrument having one or

two measurement channels.

3. For onsite noise measurement requiring

overall sound levels to be recorded,

sound level meters are widely used

because of portability and user friendly

operation.

4. Some high-end SLMs have capabilityto measure vibration as well as sound

intensity.

4.6. Calibrators

1. Portable field Calibrators are quite often

used to check the sensor operational

condition as recommended by many

standards. For microphones, calibrators

use very accurate sound sources having

94dB or 114 dB SPL @ 1000 Hz or

piston-phones 124 dB @ 250 Hz.

For accelerometer calibration, vibration

exciters producing 1g acceleration

@ 160 Hz are very popular for onsite

calibration check as it is quicker.

2. For laboratory calibration of microphones,

precision sound sources with specially

designed acoustic couplers driven by

variable frequency signal generators are

used for calibration of the sensitivity and

frequency response of the microphones.

Laboratory calibration of the

accelerometers use an electromagnetic

shaker (more often two are used

depending on the frequency range of

calibration) driven by a power amplifier

and precision signal generator. The

amplitude of vibration is kept constant by

incorporating a closed loop feedback.

3. Impact hammers are calibrated with

the help of known reference mass and

calibrated vibration sensor.

4. For different DAQs automated hardware

checks are implemented for calibration.

5. Apart from periodic calibration of

transducers and hardware, field

calibrators are very frequently used

nowadays during measurements

(especially for large channel noise

and vibration data acquisition at

site) for performance check of the

complete measurement chain including

transducers, cables and DAQ. Frequency

of actual calibration for sensor should

be predefined and usually sensors are

calibrated annually and in few cases

once in two years.

4.7. Low Noise Cables

For connecting microphone and

accelerometer to the data acquisition system,

shielded low noise cables are used having

BNC / Lemo type connectors. This helps in

better noise immunity even when longer cables

are used. Cross LAN cable, RS 232, and USB

cables are generally used for communication

of data acquisition system with PC.

5. SIGNAL CONDITIONING AND DATA

PROCESSING TECHNIQUES

Signal conditioning provides electrical

signal proportional to transducer output which

can be used for display and further analysis.

It provides scaling of signal, special filters for

parameter conversion and remove unwanted

noise from signal. Basically Time (Overall

level), Frequency analysis (FFT, CPB) and

Order analysis are the three commonly used

data analysis techniques in NVH.

1. Overall analysis: This is the most

fundamental and commonly usedanalyzer for investigation of noise

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and vibration. It makes use of an on-

line integrator to compute the overall

energy content of the time signal (root

mean square value).It employs linear or

exponential averaging of the sound or

vibration signal.

2. FFT (Fast Fourier Transform)

Analysis: It is used for narrow band

frequency analysis. In this the algorithm

is usually implemented using PC based

software. It provides high frequency

resolution and fast processing of signal

with the currently available computational

resources. It is mainly used for vibration

signal analysis. It processes the data

with the help of anti-aliasing filters and

various windowing techniques

3. CPB (Constant Percentage Bandwidth)

Analysis: It is used for 1/1 octave,

1/3rdOctave, 1/12th Octave analysis. Its

main application is in analysis of noise

signals, the filter bandwidth so chosen to

represent frequency selectivity of human

ear.. It implements filtering using a bank

of parallel digital filters using dedicated

hardware or a combination of hardware

and software. It is used mainly for real

time data analysis.

4. Order Analysis:For rotating machinery

most of the physical events are related to

the rotational speed and its harmonics.

This rotational speed harmonic is called

as an Order. It looks at events in terms

of the rotational speed and not time or

frequency. Vehicle and Power train noise

and vibration data analysis for varying

speed conditions is carried out using

order analysis.

5. Frequency response function(Transfer Function): It is the transfer

function in frequency domain which

determines relations ship between

input and output independent system

parameters. FRF is mainly used in

frequency domain measurements and

dynamic testing.

6 Impulse response function (Transfer

Function): It is the transfer function

in time domain which determines

relationship between input and output fora given system.

7 Noise Mapping:This technique is used

mainly for noise source identification

which is crucial for development

activities for noise reduction. To locate

and quantify the noise sources, sound

intensity mapping, near field acoustic

holography, beam forming techniques

are used. They help in identifying noise

sources and quantify contribution of

various sub-systems.

Fig. 6: Different types of plots in Signal

Processing

6. ADVANCED ANALYSIS TECHNIQUES

6.1. Acoustic Holography Technique:

Near field acoustic holography (NAH)

issued for visualization of acoustic radiation

from multiple sources as a color map. In

this technique instead of measuring the data

at all planes of propagation, the acoustic

field is reconstructed above and below the

hologram plane. Transient and Non stationary

noise mapping can be done with holography

which cannot be done using sound intensity

technique. It characterizes all sound field

descriptors like pressure and intensity as a

function of time or RPM. It is useful for real

time data analysis. It outputs a time sequence

of snapshots of a selected acousticalparameter in a calculation plane parallel to

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the measurement plane, combining time and

space resolution principles. Measurement

is done with help of microphone array which

performs simultaneous acquisition. It is very

useful technique as it can do the mapping for

large sources and interprets the data faster

than sound intensity technique.

6.2. Beam forming

Beam forming is very similar to a camera,

in the sense that the array of microphones

behaves as a lens. By varying the distance

between the array and the object, sound fields

can be determined for objects of many sizes

with a wider frequency range and bandwidth.

Beam forming is widely used in applications

for exterior vehicle noise (outdoor, indoor)or wind-tunnel applications (exterior and

interior). It provides a directional source map

from the position of the array with a useful

opening angle of 60.

6.3. Transfer Path Analysis:

Transfer Path Analysis or TPA system is

used to identify and evaluate structure borne

and airborne energy transfer routes / paths

from excitation source to receiver. It quantifiesthe various sources and their paths and figure

out which ones are important, which are

contributing to the noise issues and which

one are cancelling others. It mainly consists of

Data Acquisition System having more number

of channels, Noise Measurement sensors

(Microphones) and Vibration measurement

sensors (Accelerometers) for measuring

different transfer functions including tacho

sensors for speed measurement.

6.4. Sound Quality Analysis

Sound quality and Psycho acoustic

evaluation with the help of jury testing

in listening rooms provide correlation for

measured and human perceived sound from

different noise sources. This technology has

become important in interior cabin noise

assessment, sound quality of engines etc. It

plays vital role in introducing engine noise to

electric vehicles which is inherent and quiet. To

quantify the phenomenon called Harshness,sound quality is useful. It helps in analysis of

different parameters required to overcome the

harshness.

6.5. 3D Laser Vibrometer

It is a latest technology used for distant,

difficult to access, high temperature locations

vibration measurements. It consists of

scanning mirrors. It measures the velocity

signal. It does very fast measurement and

interpretation of data.

8. Wavelet Analysis (Multi Resonance

Analysis)

In MRA different frequencies are analyzed

with different time windows. In this analysis

bank of high pass and low pass filters isprovided. Wavelet transformation is a form

of MRA. Wavelet analysis is of Continuous

and discrete type wavelet analysis (CWT

and DWT). Both CWT and DWT operate

on digitized time signals. CWT provides

continuous range of scales. DWT provides

discontinuous frequencies, the level of

discontinuities determined by the number of

filters applied.

7. GOOD MEASUREMENT PRACTICES

1. Thorough knowledge of the noise and

vibration generation mechanism will

make it easy to choose the correct

transducer and appropriate data

acquisition system.

2. Selection of transducer, transducer

mounting and handling should be

done with proper care for accurate

measurement and results.

3. Before using any transducer ormeasuring device, calibration check

must be done.

4. Routing of cables is critical to minimize

noise in the signal.

5. Important considerations while using the

transducers are mentioned above which

must be followed for better use of it.

6. As these transducers are very costly

because of their precision and

manufacturing process, differentparameters like ageing of transducer,

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frequency of calibration should be

followed as per data sheet from supplier.

Ultimately proper usage and storage of

transducer will increase its functional life.

Hence check points before measurement

and while doing the measurements must be

followed rigorously as practice.

8. FUTURE TRENDS

8.1 Particle velocity (p-u) probes

This is the advanced technique for

sound intensity measurement. It measures

particle velocity directly. The size of the p-u

probe is small which is good for small cavity

measurements. It is having wide range

frequency response (20 Hz 20 KHz).

8.2 Multi field Microphones

Multi-field Microphones are ideal for any

situation in which the nature of the sound

field is unpredictable, or the direction of the

dominant noise source is difficult to pinpoint

or shifts over time. These measurement

microphones can guarantee accurate

measurements in free or diffuse sound fields

at any angle of incidence.

8.3 MEMS (Micro Electromechanical

Systems) Accelerometer

Although these type of accelerometers

have been developed in the mid 90s for

general purpose applications research is

ongoing for development of these sensors for

NVH applications. These sensors are currently

used in cameras, laptops, smart phones and

vehicle air bags etc.

9. CONCLUSION

An attempt was made in this paper

to review various transducers and signal

conditioning used in automotive noise and

vibration measurements. Also few of the

more frequently used signal analysis tools

and techniques for NVH applications along

with advance techniques and future trends

were also discussed. It covers some of good

measurement practices to be followed while

using sensors and data acquisition systems.

10. REFERENCES

1. Li Tan, Digital Signal Processing

Fundamentals & Applications

2. Bruel and KjaerPrimers for Measuring

Sound and Measuring Vibrations.

3. Bruel and Kjaer Handbook on

Microphones and Accelerometers