IEC TS 63081:2019

IEC TS 63081 ®
Edition 1.0 2019-12
TECHNICAL
SPECIFICATION
colour
inside
Ultrasonics – Methods for the characterization of the ultrasonic properties
of materials
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IEC TS 63081 ®
Edition 1.0 2019-12
TECHNICAL
SPECIFICATION
colour
inside
Ultrasonics – Methods for the characterization of the ultrasonic properties

of materials
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 17.140.50 ISBN 978-2-8322-7643-3

– 2 – IEC TS 63081:2019 ® IEC 2019
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 List of symbols . 10
5 Overview . 12
5.1 General principles . 12
5.2 Sample preparation . 12
5.2.1 Fluid samples . 12
5.2.2 Solid samples . 13
5.2.3 Sample geometry . 13
5.2.4 Sample stabilization . 13
5.3 Source and receiver transducers . 14
5.4 Transmission versus reflection measurements . 14
5.5 Transducer excitation signal . 15
5.5.1 Frequency dependence of quantities . 15
5.5.2 CW and quasi-CW methods . 15
5.5.3 Frequency modulated pulses and time delay spectrometry . 16
5.5.4 Impulse methods . 18
6 Insertion loss measurement . 19
7 Longitudinal wave speed measurements . 22
7.1 General . 22
7.2 Transducers immersed within fluid material . 22
7.3 Transducers and sample immersed in a coupling fluid . 23
8 Absorption coefficient measurements . 24
8.1 Single sample through transmission method . 24
8.2 Double sample through transmission method . 26
9 Echo reduction (ER) measurement . 27
9.1 Normal incidence . 27
9.2 Oblique incidence . 29
10 Backscatter coefficient measurement. 29
Bibliography . 31

Figure 1 – Schematic showing diffractive spreading between source and receiving
transducers . 14
Figure 2 – Illustration of a typical TDS system . 17
Figure 3 – Development and signal processing for a compensated frequency
modulated signal . 17
Figure 4 – Pulse dispersion in absorbing media . 19
Figure 5 – The additional diffractive spreading encountered in through transmission
measurements . 21
Figure 6 – Source and receiving transducers immersed in a fluid medium to be
characterized . 22
Figure 7 – Source, receiver and sample all immersed in a coupling fluid . 24

Figure 8 – Multiple echoes that are clearly separated in time . 25
Figure 9 – Multiple reflection and transmission phenomena occurring at the surfaces of
a sample . 26
Figure 10 – Schematic presentation of a measurement set-up used to determine the
echo reduction of a test material . 27

– 4 – IEC TS 63081:2019 ® IEC 2019
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ULTRASONICS – METHODS FOR THE CHARACTERIZATION OF THE
ULTRASONIC PROPERTIES OF MATERIALS

FOREWORD
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Technical Specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards
IEC TS 63081, which is a Technical Specification, has been prepared by IEC technical
committee 87: Ultrasonics
The text of this Technical Specification is based on the following documents:
DTS Report on voting
87/718/DTS 87/725/RVDTS
Full information on the voting for the approval of this Technical Specification can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
Words in bold in the text are defined in Clause 3. Symbols and formulae are in Times New Roman
+ Italic.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

– 6 – IEC TS 63081:2019 ® IEC 2019
INTRODUCTION
Many ultrasonic measurement standards contain requirements for the properties of acoustic
materials to be used to construct the measurement equipment relied upon within those
documents. The following are examples of such standards.
• IEC 61161 specifies amplitude reflection factor and acoustic energy absorption for reflecting
targets and absorbing targets and specifies amplitude transmission coefficient for anti-
streaming foils.
• IEC 61391-1 discusses reflection coefficient.
• IEC 61689 defines echo reduction and specifies limits upon its values. The terms reflection
loss and transmission loss are also used, and values specified.
• IEC TS 62306 specifies transmission loss and reflection amplitude reduction.
• IEC 62359 specifies reflection coefficient and absorption.
• IEC 60601-2-37 specifies reflectance and absorption coefficient.
As the list above suggests, a wide range of terms is used to specify the properties of an acoustic
material, and these terms are not used consistently across IEC documents. Furthermore, there
is a degree of duplication with multiple names for the same quantity. This is further confused
since there is no document within the IEC ultrasonics portfolio that defines the methods by
which those properties are measured.
This document seeks to address the shortcomings by providing:
• a clear unambiguous definition of the key quantities of interest during materials
characterization;
• a discussion of similar terms and how they may relate to the key quantities;
• recommended experimental methods for determining the values of key quantities.

ULTRASONICS – METHODS FOR THE CHARACTERIZATION OF THE
ULTRASONIC PROPERTIES OF MATERIALS

1 Scope
This document:
• defines key quantities relevant to ultrasonic materials characterization;
• specifies methods for direct measurement of many key ultrasonic materials parameters.
This document is applicable to all measurements of properties of passive acoustic materials
under drive conditions that are not subject to nonlinear acoustic propagation. Whilst there are
materials properties that may be of interest in a nonlinear drive regime, these are currently
outside the scope of this document.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
absorption per unit length
α
component of the attenuation coefficient (IEC 60050-801:1994, 801-23-35) that does not arise
from scattering and is due only to absorption of acoustic energy within the sample
y
αα= f (1)
where
y
α is the absorption constant (dB/(MHz m));
f is the frequency in MHz;
y is the frequency exponent (in general not an integer).
Note 1 to entry: For absorption, y = 2 for water, and in general y is between 1 and 2 for fluids, soft tissues and
tissue mimicking materials.
Note 2 to entry: Absorption per unit length is expressed in units of decibel per metre (dB/m).

– 8 – IEC TS 63081:2019 ® IEC 2019
3.2
amplitude reflection coefficient
R
p
ratio of the pressure amplitude of an acoustic wave reflected from an interface separating two
media to the pressure amplitude of a plane wave incident on that interface
p
r
(2)
R =
p
p
i
where
p is the pressure amplitude of the reflected longitudinal wave, at the reflection angle θ ;
r r
p is the pressure amplitude of the incident longitudinal wave, at the incident angle θ
i i
Note 1 to entry: Care should be taken with the term reflection coefficient as both amplitude and intensity forms
appear in common scientific parlance. This can be particularly problematic when equations involve reflection
coefficient terms, since both are dimensionless, but one varies as the square of the other. Intensity forms of reflection
coefficient are more common in optics.
Note 2 to entry: In general, the equation shown can apply to reflections of different types, each at different reflection
angles but all governed by Snell’s law.
Note 3 to entry: Amplitude reflection coefficient is dimensionless as it is a ratio of quantities. It does not require
the use of a calibrated receiver since measurements are relative in nature.
3.3
amplitude transmission coefficient
T
p
ratio of the pressure amplitude of an acoustic wave transmitted through an interface separating
two media, to the pressure amplitude of a plane wave incident on that interface
p
t
T = (3)
p
p
i
where
p is the pressure amplitude of the transmitted longitudinal wave;
t
p is the pressure amplitude of the incident longitudinal wave
i
Note 1 to entry: Care should be taken with the term transmission coefficient as both amplitude and intensity forms
appear in common scientific parlance. This can be particularly problematic when equations involve transmission
coefficient terms, since both are dimensionless, but one varies as the square of the other. Intensity forms of
transmission coefficient are more common in optics.
Note 2 to entry: In general, the equation shown can apply to transmissions of different types. For example, a
longitudinal wave incident from fluid to solid at an angle will generate two transmitted waves at non-normal incidence,
a shear wave and a longitudinal wave, each at different refraction angles. Each of the transmitted waves is governed
by Snell’s law.
Note 3 to entry: Amplitude transmission coefficient is dimensionless as it is a ratio of quantities. It does not
require the use of a calibrated receiver since measurements are relative in nature.
3.4
backscatter coefficient
η
differential scattering cross-section per unit volume as a function of frequency for a scattering
angle of 180°
−1 −1
Note 1 to entry: Backscatter coefficient is expressed in units of one per second per steradian (s Sr ).

3.5
density
mass density
ρ
at a given point within a three-dimensional domain of quasi-infinitesimal volume dV, scalar
quantity equal to the mass dm within the domain divided by the volume dV
ρ = dm/dV
Note 1 to entry: Mass density is an intensive quantity describing a local property of a substance.
Note 2 to entry: The concept of mass density may also be applied to the mass m in a domain D with a volume V,
m 1
leading to the average density .
ρ ρvd
av
∫
V V
D
Note 3 to entry: Mass density is expressed in units of kilogram per metre cubed (kg/m ).
[SOURCE: IEC 60050-113:2011, 113-03-07]
3.6
echo reduction
ER
reduction in pressure amplitude of an ultrasonic plane wave resulting from its reflection from an
interface between two media
p
r
ER=−20 log dB

p
i (4)
=−20 log R dB
( )
10 p
where
p is the pressure amplitude of the reflected longitudinal wave;
r
p is the pressure amplitude of the incident longitudinal wave
i
Note 1 to entry: In general, the reflected waves are governed by elastic Snell’s laws and the reduction in pressure
amplitude is a function of the angle of the incidence of the plane-wave on the surface.
Note 2 to entry: Echo reduction is expressed in decibels (dB).
3.7
group velocity
v
g
velocity in the direction of propagation of a characteristic feature of the envelope of a pulse
dω
Note 1 to entry: Group velocity is commonly defined in terms of angular frequency ω and wavenumber k as
v =
g
dk
and differs from phase velocity only in a dispersive medium.
Note 2 to entry: Group velocity is ordinarily the velocity of propagation of the energy associated with the
disturbance.
Note 3 to entry: Group velocity is expressed in units of metre per second (m/s).
[SOURCE: IEC 60050-801:1994, 801-23-21, modified – In the definition, "non-sinusoidal
disturbance" has been replaced by "pulse".]
==
– 10 – IEC TS 63081:2019 ® IEC 2019
3.8
insertion loss
IL
reduction in pressure amplitude of an ultrasonic plane wave resulting from the insertion of a
sample in the acoustic path

p
s
(5)
IL=−20 log dB

p
ns
where
p is the amplitude of the received pressure wave with the sample in the path (with sample);
s
p is the amplitude of the received pressure wave without the sample in the path (no sample)
ns
Note 1 to entry: Care should be taken as insertion loss is sometimes incorrectly labelled transmission loss.
However, transmission loss is a more general term describing loss of signal between a source and a receiver.
IEC 60050-801:1994, 801-23-39 defines transmission loss as "reduction in sound pressure level between two
designated locations in a sound transmission system, one location often being at a reference distance from the
source". As such it can include contributions from the directivity functions of both source and receiver as well as
acoustic spreading. These are functions of the experimental configuration and not the material under investigation.
Note 2 to entry: Insertion loss is expressed in decibels (dB).
3.9
longitudinal wave speed
c
L
magnitude of the velocity of a free progressive longitudinal wave
Note 1 to entry: Longitudinal wave speed is expressed in units of metre per second (m/s).
3.10
phase velocity
v
p
velocity in the direction of propagation of a surface of constant phase
ω
Note 1 to entry: Phase velocity is commonly defined as where ω is the angular frequency and k is the
v =
p
k
wave number.
Note 2 to entry: Phase velocity is expressed in units of metre per second (m/s).
[SOURCE: IEC 60050-801:1994, 801-23-20]
4 List of symbols
A toneburst amplitude in single sample absorption coefficient measurements
A , A amplitude of nth and mth echoes in single sample absorption coefficient
n m
measurements
A area of a transducer aperture
B bandwidth of time delay spectrometry (TDS) tracking filter
c longitudinal wave speed in the coupling fluid
CF
c longitudinal wave speed
L
e excitation signal used with compensated frequency modulation (CFM)
CFM
E spectral modulus of CFM signal
CFM
ER echo reduction
f frequency
F focal length of a transducer
H frequency spectrum of transducer used in calculation of CFM signal
s
IL insertion loss
k wave number
p(x,t) pressure signal as a function of time, measured at position x
P(x,f ) Fourier transform of p(x,t)
p pressure amplitude of the incident longitudinal wave
i
p amplitude of the received pressure wave without the sample in the path
ns
pressure amplitude of the reflected longitudinal wave
p
r
ˆ
p pressure amplitude of the reflected longitudinal wave, corrected for imperfect
r
reflector
p amplitude of the received pressure wave with the sample in the path
s
p pressure amplitude of the transmitted longitudinal wave

t
q(x,t) pressure signal as a function of time after delay applied, measured at position x
Q(x,f ) Fourier transform of q(x,t)
R amplitude reflection coefficient
p
t time
S sweep rate of TDS source signal
T amplitude transmission coefficient
p
x distance/position
y frequency exponent of absorption per unit length
Z acoustic impedance
v group velocity
g
v phase velocity
p
α absorption per unit length
Δx thickness (either of sample or vessel wall)
Δt change in time
Δφ change in phase
η backscatter coefficient
φ phase
ρ density
τ time delay
angular frequency
ω
– 12 – IEC TS 63081:2019 ® IEC 2019
5 Overview
5.1 General principles
It is important when measuring materials characteristics that the equipment set-up environment
and method have as little effect on the results as possible. Clause 5 discusses some of the set-
up issues and items to be noted when performing these measurements.
The application of a consistent and well understood protocol is the key to deriving meaningful
measurements that are reproducible and transportable. Controlling as much of the test
environment as possible, from temperature to sample preparation and conditioning, has an
important impact on the measurement quality and confidence.
Having water-filled tanks where the temperature can be controlled and measured to the desired
accuracy is important. Similarly, other aspects of the water quality employed for the
measurements (dissolved gas content and conductivity) may be important subject to the applied
technique.
In addition, measurements should be repeated at different conditions (distance, orientation
driving signal) depending on the property to be measured, in order to understand and minimize
set-up and instrumentation effects. To this end, once the type of measurement has been
selected for the particular parameter, sources of uncertainty should be investigated and
quantified [1]. Type A uncertainties can be quantified by repeating measurements and building
up an appropriate uncertainty budget. Type B sources of uncertainty will require modelling or
parallel/confirmatory validation studies.
5.2 Sample preparation
5.2.1 Fluid samples
When working with fluids, immersing the test transducers in the fluid is ideal. However,
practically fluids often need to be housed within a container which is immersed within a test
tank. Such containers are typically either rigid walled vessels (such as a parallel walled cell
culture flasks) or have an acoustically thin membrane at either end of the acoustic path through
the material.
Rigid-walled vessels are likely to have the benefit that they have parallel walls, and therefore
maintain a uniform thickness of sample. However, the material used to construct the walls of
the flask is likely to be acoustically mismatched to water, leading to significant reflections at the
interfaces. Additionally, they can exhibit acoustic absorption and dispersion.
Whilst membranes may be thin relative to the acoustic wavelength, their influence on the
determination of ultrasonic power as part of the radiation force balance measurement is well
documented [2],[3], and such effects need to be avoided. Care should also be taken to avoid
expansion of the membrane during the process of filling the measurement vessel with the fluid
under test. Particularly if the vessel is surrounded by air during filling, membranes can expand
to a convex shape and thus the vessel might become an acoustic lens which further perturbs
the measurement. It is best practice to use flat restraining plates in contact with the membrane
to minimize expansion during filling.
Given the possible artefacts introduced by a measurement vessel, measurements should be
conducted in a manner that separates the properties of the fluid under test from those of the
vessel containing it. A description of methods to accomplish this is given in Clause 6.
When testing fluids, the sample ideally needs to be degassed to a constant level. During sample
preparation, it is easy to introduce air into the sample when pouring the liquid. Care should be
taken to minimize this; however, it is likely that gas will still be introduced particularly for viscous
liquids. Therefore, once the sample is prepared, it may be useful to degas the sample with
agitation in a vacuum chamber jar to allow any entrapped gasses to be liberated.

5.2.2 Solid samples
Many solid samples will require machining to yield a sample of the appropriate dimension. In
these cases, the material finish should be smooth relative to the smallest wavelength in the
ultrasonic signal being used to interrogate the material. Where this involves polishing, sanding
or similar techniques, any debris/residue from the machining process should be removed prior
to characterization.
Some solid samples, such as resin systems, start in a fluid form. In these cases, the comments
raised in 5.2.1 with regard to the avoidance of trapped air are equally valid. If the resin system
contains high- and/or low-density fillers, these can have the tendency to slump or float during
cure. This should be avoided by rotating the sample during cure to ensure a uniform distribution
of filler.
5.2.3 Sample geometry
A sample the thickness of which is such that the transit time of the acoustic signal is larger than
the duration of the signal of interrogation is preferable for some measurements such as
attenuation characterization. However, there may be cases (e.g. very high attenuation
materials) where this is not practical. Thin samples should be mounted in a fixture to prevent
possible lens effects from loose materials or effects from drum skin fixtures. Very thick samples
may be needed for some material characterization measurements including:
• measurement of absorption coefficient in very low loss materials;
• measurement of the velocity or speed of sound of a sample within a water bath, whose
longitudinal velocity is similar to that of water.
In both these cases, a propagation distance of many wavelengths inside the material under test
is needed to ensure a measurable change in the quantity being evaluated.
The sample should have lateral extents such that the wave diffracted around the edge of the
sample can easily be time resolved from the direct wave passing directly through the sample.
Careful consideration of the lateral dimensions of the container should be employed as this may
dictate the lowest frequencies which can be used for characterization and also influence the
positioning of the container within the acoustic field.
It is recommended that samples are prepared with parallel sides. A constant thickness greatly
facilitates measurement of those quantities involving a distance dependence (such as
absorption coefficient). It also helps to minimize experimental error due to mis-alignment since
refraction effects are equal magnitude and opposite direction at the front and rear surfaces of
a parallel-sided sample. Consideration of parallel surfaces is particularly important when
measuring fluid samples within a measurement cell (container) that has a flexible membrane at
one end or at both ends. Overfilling of the measurement vessel can result in a convex surface
on the end membrane and this can cause lens effects. When parallel-sided samples are not
used, testing in several orientations should be performed to gain an average measurement.
5.2.4 Sample stabilization
The measurement of samples immersed in a fluid medium is common. When a sample is
immersed in fluid, there is the possibility of:
• absorption of the fluid medium by the sample;
• micro air-bubbles being trapped on the surface of the sample;
• a temperature differential between the sample and the immersing fluid.

– 14 – IEC TS 63081:2019 ® IEC 2019
All of these phenomena can result in an inaccuracy when quantifying the properties of the
sample. The sample should be immersed in the fluid medium prior to measurement to ensure it
has opportunity to normalize to its surroundings. An immersion period of 1 h is often sufficient
but this may vary from sample to sample and thus the investigator should determine an
appropriate stabilization period.
NOTE Speed of sound or phase velocities in materials are temperature dependent and arrival time of a pulse or
toneburst can be measured very accurately, for example, using an oscilloscope. By immersing the test container in
a fluid of different temperature, the arrival time of the ultrasound excitation at the receiver can be monitored and will
shift in time as the specimen gradually becomes thermally equilibrated. This provides the investigator with information
on when thermal equilibrium has been achieved.
5.3 Source and receiver transducers
When conducting materials characterization experiments there are often advantages to be
gained by using large area receivers, both in terms of maximizing signal-to-noise ratio and by
reducing the effects of diffractive loss. This contrasts with normal practice for acoustic output
measurements where smaller receivers are preferred due to the minimization of directivity and
spatial averaging effects. A general expression for the diffraction loss between a circular source
and circular receiver is available [4] and the geometry is as shown in Figure 1. If both
transducers have the same diameter, a simpler single-integral solution for the diffraction loss
exists [5]. It has also been shown that use of very large receivers in materials characterizations
measurements results in a procedure that is free from diffractive loss artefacts [6]. Correction
for diffractive artefacts should be applied to all materials characterization measurements,
particularly for measurements of attenuation of low loss media where there is a large mismatch
in speed of sound between the test specimen and the water used as a reference medium.

Figure 1 – Schematic showing diffractive spreading between source and receiving
transducers
Diffractive spreading as a function of distance or at a specific distance can be determined by
measuring the spectrum of the signal received from a transmitter and receiver at the selected
distance(s) and dividing it by the spectrum of the signal measured when the transmitter and
receiver are in close proximity in a medium with negligible absorption (water). If there is non-
negligible absorption in the medium, the measurement should be corrected for it.
5.4 Transmission versus reflection measurements
Materials characterization has been conducted in a variety of different ways. Some methods
employ reflected signals only [6],[7],[8], whilst others are based solely on the transmission of
ultrasound through a sample [9],[10], [11]. There are yet further methods that incorporate both
reflected and transmitted signals [12],[13],[14]. Choice of characterization technique may be
influenced by the properties of the material being measured and/or the limitations of the sample
geometry and orientation. The following factors should be considered.
• Any measurement method that relies upon the rear surface reflection requires the acoustic
sample to have travelled across the thickness of the sample twice. For very high attenuation
materials, this may introduce signal amplitude losses that exceed the dynamic range of the
system. Therefore, transmission methods (with only a single transit across the sample) may
be preferable.
• Reflection methods of determining absorption often rely upon theoretical values of
amplitude transmission coefficient. These are frequently derived from calculations of
characteristic acoustic impedances of the media involved, which in turn are based upon
experimentally determined values for density and longitudinal wave speed. This extended
chain of calculations can lead to accumulation of uncertainty at each stage.
• Methods combining reflected and transmitted signals offer the possibility to measure the
thickness of the sample ultrasonically. This eliminates the need to independently measure
the thickness of the sample (which can be difficult with compliant, gel or fluid samples).
5.5 Transducer excitation signal
5.5.1 Frequency dependence of quantities
Many of the quantities defined in this document may vary with frequency, although some
ceramic and metallic media may have properties that are constant over a broad range of
frequencies. Other materials (e.g. visco-elastic polymers) may exhibit significant variation in
their properties as a function of both temperature and frequency. Typically, the modulus of
elasticity of these materials will be complex-valued and frequency dependent. Complex-valued
elastic moduli introduce to the description loss mechanisms which leads to ultrasonic
absorption. From causality considerations it follows that there is also dispersion in the material’s
phase velocity [15], [16], [17], [18], [19]. Therefore, the user should consider whether
measurements of material properties at a single frequency are adequate or whether
measurements are needed at multiple frequencies.
No single type of transducer excitation signal will be optimum for every measurement.
Consequently, it will be necessary to establish a compromise between multiple considerations,
some of which may be mutually incompatible. When selecting a transducer signal, the user
should consider the following:
• whether measurements are needed at just one frequency or over a range of frequencies;
• how much attenuation the signal is likely to experience;
• how much dispersion the signal is likely to experience;
• whether the data analysis technique is predicated upon achievable pre-conditions (e.g. the
ability to temporally resolve incident and reflected signals);
• how rapidly the measurement can be conducted;
• The availability of necessary equipment (e.g. specialist source transducers, receivers or
instrumentation).
5.5.2 CW and quasi-CW methods
When a transducer is driven with a continuous wave (CW) sinusoidal excitation, its output will
be mono-harmonic provided that:
• the transmission system does not introduce any harmonic distortion;
• the source pressure radiated by the transducer is below the threshold of nonlinearity of the
medium.
This permits high energy levels to be input to the material sample and is thus a useful technique
for high attenuation materials as it offers the potential for high signal-to-noise ratio.
Furthermore, even if the material is dispersive, because the source signal contains only one
frequency, the variation of sound speed as a function of frequency cannot normally be
determined without changing transducers. However, care should be taken to ensure that energy
levels are not so high as to generate any heating of the specimens and therefore a change in
its temperature or that of the water bath.

– 16 – IEC TS 63081:2019 ® IEC 2019
In practice, CW excitation leads to standing wave fields within the measurement tank and there
is no ability to temporally separate directly incident signal from those reflected from the
structures of the measurement tank. Whilst applying anechoic coatings (e.g. absorbing tiles) to
reflective surfaces can reduce this effect, it is preferable to use a transducer excitation that is
intrinsically time limited. A toneburst signal provides the high energy benefits of a CW signal,
whilst also having limited time duration to permit identification and rejection of unwanted
reflection artefacts.
Care should be taken to ensure that the transducer has reached a steady-state, and thus a
quasi-CW, condition. Many transducers take a few cycles to ring-up and ring-down (see for
example the tonebursts in Figure 8). The varying amplitude within the first-two and last-two
cycles may introduce measurement inaccuracy. Therefore, measurements should only be made
during the constant envelope portion of the signal (cycles 3 to 6 in Figure 8). Extending the
length of the toneburst may be used to increase the usable, constant amplitude portion of the
signal. However, this can only be done if the dimensions of the sample are such that there is
adequate temporal separation of multiple echoes within the sample and between the sample
and the transducer.
Quasi-CW signals can also be used to determine the frequency dependence of a material’s
characteristics if the centre frequency of the toneburst can be altered. This can be undertaken
by manually adjusting the operating frequency of the source signal generator, but this is very
time consuming and automated adjustment (via computer control) is recommended. Care
should also be taken to ensure that the acquisition record length is also adjusted as a function
of frequency. As frequency increases, the duration of a fixed cycle count toneburst will
decrease. It is therefore possible that the transducer ring-down section will start to appear within
the acquired signal window and this may lead to measurement errors.
5.5.3 Frequency modulated pulses and time delay spectrometry
5.5.3.1 General
When measurements are required over a broad spectral range, frequency modulated pulses
(sometimes called swept sinusoids or chirps) or time delay spectrometry may be suitable.
Frequency modulation in its simplest form involves a signal the instantaneous frequency of
which varies linearly from a lower to an upper frequency. This is a linearly frequency modulated
(LFM) signal. The sweep rate may be nonlinear and exponentially swept sinusoids are common.
The amplitude of the instantaneous frequency component need not be constant. This can be
used to provide a broader bandwidth acoustic signal [20],[21] by driving a transducer with a
signal that is an inverse of its frequency response. These are often referred to as compensated
frequency modulated (CFM) signals.
5.5.3.2 Time delay spectrometry (TDS)
Time delay spectrometry (TDS) measures the frequency response of a system by applying a
swept frequency acoustic source and using a tracking receiver to select only those through-
transmission signals that have the desired time delay corresponding to the ultrasound transit
time directly from the transmitter to the receiver (Figure 2). This system can be used to provide
broadband measurements of complex hydrophone sensitivity as well as frequency-dependent
acoustic characterization of materials (attenuation, group velocity and phase velocity).
In this technique, a transducer is driven with an LFM signal. As a res
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