Impedance Tubes

8/18/2019 Impedance Tubes

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IMPEDANCE TUBES

DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN


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TABLE OF CONTENTS

Introduction

Principle of Operation

Measurement Chain

Signal Processing in Impedance Tube Measurements

Market research

An application : Acoustic measurements of a rifle silencer


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• Used for absorbing airborne sound in noise controlapplications

Automotive, Aircraft, Machinery, Building and Appliance Industries

o As single layers of acoustic materialo As multilayer systems (poroelastic, acoustic, and elastic materials)

Examples:• Automotive interior headliners

• Aircraft engine nacelle liners

• To enhance the transmission loss in aircraft fuselage or automotivedashboard applications.

USE OF ACOUSTİC MATERİALS



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According to the end use, materials can be characterised by:

•

Sound absorbing properties; Sound absorption coefficient

Reflection coefficient

Acoustic impedance

• Sound barrier properties;

Sound transmission loss

ACOUSTIC PROPERTİES OF MATERIALS



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1. In-situ testing

2. Free-field methods

3. Reverberation room methods• Using a reverberation room

• Using a small reverberation room adapted to the requirementsof automotive acoustics (alpha cabin)

4. Plane-wave tube methods

• Transfer Function method;*also known as two-microphone method

• The standing wave ratio method;*mostly superseded by the two-microphone method.

DETERMINING THE SOUND ABSORBING PROPERTIES



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1. In-situ testing

2. Two-room method(a reverberation room/anechoic chamber suite)

3. Plane-wave tube methods:

• Transfer matrix method using a four-microphone plane

wave tube

DETERMINING THE SOUND BARRIER PROPERTIES



7/30DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN

PRINCIPLE OF OPERATION


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TRAVELLING AND STANDING WAVE

• A standing wave, also known as a stationary wave, is a wave thatremains in a constant position.

• A travelling wave, propagates in a behaviour which is determined by its wavelength and frequency




This phenomenon occurs due to two propagating travelling waves inopposite direction

STANDING WAVE


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Reflection Coefficient r : Complex ratio of the amplitude of the reflected wave

to that of the incident wave in the reference plane for

a plane wave at normal incidence

Absorption Coefficient α : Ratio of sound power entering the surface of the

test object (without return) to the incident soundpower for a plane wave at normal incidence


ABSORPTION MEASUREMENT


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• ISO 10534 – 1 Absorption Coefficient and Impedance (Standing Wave Ratio)• ISO 10534 – 2 Absorption Coefficient and Impedance (Transfer Function)

• ASTM E1050 Absorption and Impedance (Transfer Function)• ASTM E2611 Normal Incidence Transmission Loss (Transfer Matrix)


CORRESPONDING ISO AND ASTM STANDARTS


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TRANSFER FUNCTION METHOD STANDING WAVE RATIO METHOD

ir

j jH H e H

S

S H

12

11

12

12

102

12

12 x jk

R

I ir

j e H H H H jr r er r r

21 r

21 r

11

s sr

minmax p p s



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DİFFERENCES IN ABSORPTION MEASUREMENT

USING STANDING WAVE RATIO USING TRANSFER FUNCTION



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Surface Impedance Z : Ratio of the complex sound pressure p(0), to the

normal component of the complex sound

particle velocity v(0), at an individual frequency in the reference plane.

Transmission LossTL :

The accumulated decrease in acoustic intensity as anacoustic pressure wave propagates outwards from asource.


TRANSMISSION LOSS MEASUREMENT


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Tranmission loss measurement is based on determining the “TransferMatrix” that includes the information of acoustic pressure and particle velocity

on both sides of the specimen




d xa

a

xa

a

u

p

T T

T T

u

p

2221

1211

0

TRANSMISSION LOSS CALCULATION

d xb

b

xb

b

u

p

T T

T T

u

p

2221

1211

0

22211211 )(

2

T cT cT T

et

jkd

t TLn 1log20 10

(Transmission Coefficient)

(Normal Incident Transmission Loss)



SEVERAL PROPERTIES AND ADVANTAGES OF

IMPEDANCE TUBE MEASUREMENT

• Strongly dependent on geometrical properties of the system

• Useful frequency range depends on the tube diameter and microphone spacing

•

Easy-to-handle, cheaper and faster when compared to traditional methods (e.g.reverberation room method)

• Measurement can be achieved with small amounts of test materials



MEASUREMENT CHAIN



Digital signalgeneration

MEASUREMENT CHAIN

Impedance Tube with Testing Specimen

Data Logging andSignal Processing

Digital-to-AnalogConversation

Computer andThe Software

Analyzer

Power Amplifier

Sound Source Speaker Transducers Microphones

Pre-Amplifier

Input Filter Anti-aliasing etc.

Analog-to-digitalConversation



SYSTEM COMPONENTS

The measurement chain includes;

Tubes

Microphones Sound source

Amplifiers

Analyzer

Data acquisition and signal processing software

Computer



MEASUREMENT STEPS

The input signal to be generated can be in forms of;

• A periodic signal with certain frequency• Band-limited white noise• Random noise with uniform spectral density over interested range

• Impact

For a single tone input signal, only a frequency component (FRA) can be investigated

For other multi-tone signals, spectrum analysis (FRF) can be determined



MEASUREMENT STEPS

Amplification should provide following features;

• A flat spectrum in the interested frequency range• High levels of signal-to-noise ratio

***The best performing amplifiers are the ones having vacuum tubes.

The sound source, speaker, should be capable of;

• Providing the input signal spectrum in its bandwidth• Including properties of low impedance, high QMS and low QTS, long strokeof the voice coil inside


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MEASUREMENT STEPS

The main structure, impedance tubes must have;

• Certain values of geometric dimensions• Stainless material, enough wall thickness for insulation, specially treated

internal structure

• No internal cracks, faults or holes (except for relief openings)

Microphone properties;

• Each microphone must be identical with each other• Proper bandwidth,• Very flat frequency response• Providing geometric demands



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MEASUREMENT STEPS

The signal processing software,a critical part of the process,

• Must have the capability of making frequency analysis• Should be communicating with the analyzer ,as fast as possible• Should have easy-to-use interface for most users

• Must include signal processing options like windowing, averaging, filtering ...• Must have Export and Import options• Have the capability of generating signals including random, pseudo-random,

periodic signals etc.



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SIGNAL PROCESSING IN IMPEDANCE TUBE MEASUREMENTS



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SIGNAL PROCESSING

In measurement following analog signal processes are done;

Analog filtering (anti-aliasing...)

Amplification (power amplifiers and conditioning amplifiers)

DAC and ADC convertion


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SIGNAL PROCESSING

Most of the signal processing is done digitally;

Filtering (if necessary !)

Averaging (Linear, Exponential, etc.)

Windowing

Spectrum Analysis

Cross Spectrum

Autospectrum

Other (Transfer function calibration,calculations of acoustic properties etc.)


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MARKET RESEARCH



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OVERVIEW ON MARKET

Company Tube

Analyzer

Hardware Software

Transmission

Loss

Frequency

Range

Brüel & Kjær4206, 4206A,

4206TPULSE 7758 Yes 50Hz - 6.4kHz

BIAS Engineering XN-1105 NI 4431 LabVIEW Yes 300Hz - 9kHz

01dB-Metravib SCS9020B PRO-117 dBAlphaTest Optional 50Hz - 6.4kHz

BSWA SW230, SW260SW433, SW463

MC3022MC 3044

No Yes125Hz -6.3kHz

LMS No PimentoTest-

procedureNo ?

Ono Sokki SR-4100A SR-4100B

DS-2000 DS-0257 No 50Hz - 6.4kHz

SCIENSP-2001,SP2002

Yes Yes Yes100Hz -6.3kHz

SCS SCS 9020 Yes SCS 902A Yes 50Hz - 6.4kHz

Sinus AFD 1000 Soundbook AFD 1001 No100Hz – 4

kHz

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