impedance tubes
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IMPEDANCE TUBES
DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN
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DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN
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
DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN
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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• The standing wave ratio method;*mostly superseded by the two-microphone method.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
DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN
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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
DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN
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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
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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
DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN
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)
DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN
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.
DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN
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
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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)
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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
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MEASUREMENT CHAIN
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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
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SYSTEM COMPONENTS
The measurement chain includes;
Tubes
Microphones Sound source
Amplifiers
Analyzer
Data acquisition and signal processing software
Computer
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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
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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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DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN
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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DATA ACQUISITION AND SIGNAL PROCESSING ÖZGÜR PALAZ – ONUR AKAYDIN
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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