ISO/DIS 25222-2

ISO/TC 135/SC 3

Secretariat: DIN

Date: 2025-12-22

Non-destructive testing — Characterization and verification of ultrasonic air-coupled equipment —

Part 2:
Probes

DIS stage

© ISO 2025

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Contents

Foreword

Introduction

Scope

Normative references

Terms and definitions

Symbols

General requirements of conformity

Technical information for probes

General

Probe data sheet

Probe test report

Test equipment

Electronic equipment

Other equipment

Ambient conditions

Performance requirements for probes

Physical aspects

Measurement setup

Pulse properties

Soundfield properties

(informative) Characterization of receiver probes using thermoacoustic excitation

Bibliography

Foreword

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The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of ISO document should be noted. This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

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This document was prepared by Technical Committee ISO /TC 135, Non-destructive testing, Subcommittee SC 3. Ultrasonic testing.

A list of all parts in the ISO 25222 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

Introduction

This document was drafted following and complementing ISO 22232-2:2020 to include air-coupled ultrasonic transducers.

This document is based on the guideline DGZfP US-08E which was originally prepared by members of German Society for Non-Destructive Testing (DGZfP), Committee Ultrasonic Testing, Subcommittee Air-coupled Ultrasonic Testing.

Non-destructive testing — Characterization and verification of ultrasonic air-coupled equipment —

Part 2:
Probes

This document specifies the characteristics of probes used for non-destructive air-coupled ultrasonic testing with centre frequencies from 10 kHz to 3 MHz, with focusing or without focusing means. This document refers to probes based on the piezoelectric effect. Air-coupled probes based on other physical principles can be characterized according to this guideline if it is judged as appropriate, but adaption of tests can be necessary.

This document covers tests under standard environmental conditions. The procedures given in the standard can be applied to extended environmental conditions e.g. higher air pressures, but additional tests can be necessary.

This document excludes periodic tests for probes.

If parameters specified in this document are to be verified during the probe’s lifetime, as agreed upon by the contracting parties, the procedures of verification for these parameters can be selected from those given in this document.

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 5577:2017, Non-destructive testing — Ultrasonic testing — Vocabulary

ISO 9613-2:2024, Acoustics — Attenuation of sound during propagation outdoors — Part 2: Engineering method for the prediction of sound pressure levels outdoors

ISO/IEC 17050-1:2004, Conformity assessment — Supplier's declaration of conformity — Part 1: General requirements

ISO 22232-1:2020, Non-destructive testing — Characterization and verification of ultrasonic test equipment — Part 1: Instruments

ISO 22232-2:2020, Non-destructive testing — Characterization and verification of ultrasonic test equipment — Part 2: Probes

For the purposes of this document, the terms and definitions given in ISO 5577:2017, as well as the following 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

target

reflector used in pulse–echo technique or a microphone or any other type of sensor applied for determination of the exerted sound pressure

peak-to-peak amplitude

difference between the highest positive and the lowest negative amplitude in a pulse

Note 1 to entry: See Figure 1 .

Key

A amplitude

L pulse duration

s time

h peak-to-peak amplitude

Figure 1 — Typical ultrasonic pulse with pulse duration L and peak-to-peak amplitude h

probe data sheet

document giving manufacturer's technical specifications of the same type of probes, i.e. probes manufactured in series

Note 1 to entry: The data sheet does not necessarily need to be a test certificate of performance.

Note 2 to entry: For individually designed or manufactured probes, some parameters might not be accurately known before manufacturing.

[Source: ISO 22232-2:2020, 3.3]

probe test report

document showing compliance with this document giving the measured values of the required parameters of one specific probe, including test equipment and conditions

[Source: ISO 22232-2:2020, 3.4]

dead time after transmitter pulse

time interval following the start of the transmitter pulse during which the amplifier is unable to respond to incoming signals, when using the pulse-echo technique, because of saturation by the transmitter pulse

[Source:ISO 22232-1:2020, 3.3]

Table 1 — Symbols

An ultrasonic probe complies with this document if it fulfils all of the following requirements:

1. the probe shall comply with Clause 8;
2. a declaration of conformity according to ISO/IEC 17050-1:2004 shall be available;
3. the ultrasonic probe shall be clearly marked to identify the manufacturer, and carry a unique serial number or show a permanent reference number from which information can be traced to the datasheet and probe test report;
4. a probe data sheet corresponding to the ultrasonic probe shall be available, which defines the performance criteria for the items given in Clause 8 ;
5. a probe test report shall be delivered together with the probe, which includes at least the test results given in Clause 8 .

Table 2 summarises the tests to be performed on air-coupled ultrasonic probes.

Table 2 — List of tests for air-coupled ultrasonic probes

The test conditions and the equipment used for the evaluation of the probe parameters shall be listed (see Table 3).

For individually designed or manufactured probes some parameters may not be accurately known prior to manufacturing. In that case the measured values shall be used as reference values.

The probe data sheet gives the list of information to be reported for all probes within the scope of this document (see Table 3).

The probe test report gives the measured values of the required parameters of one specific probe and other information from the probe data sheet (see Table 3). The probe test report shall include the unique serial number or the permanent reference number to provide a unique assignment between the specific probe and the probe test report.

Table 3 — List of information to be given in a probe data sheet and a probe test report

The ultrasonic instrument (or laboratory pulser/receiver) used for the tests specified in Clause 8 shall be of the type designated on the probe data sheet and shall comply with ISO 22232-1:2020 as applicable. The following modifications apply to ISO 22232-1:2020:

1. The frequency range of the instrument shall cover the bandwidth of the air-coupled ultrasonic probe under test.
2. A unipolar or bipolar rectangular tone burst pulser or a bipolar sine wave pulser or a spike pulser may be used.
3. The pulser shall provide sufficient pulse power to ensure that each rectangular pulse of a tone burst meets the nominal voltage. Output impedance of the pulser shall be documented.
4. The length of the unipolar rectangular pulse shall be adjustable to the nominal frequency of the probe.
5. Preamplifiers shall be used as required by the equipment. The bandwidth, the input impedance and the preamplifier gain shall be documented.

Where more than one type of ultrasonic instrument is designated, the tests shall be repeated with each of the additional designated types.

Testing shall be carried out with the probe cables and electrical matching devices specified on the probe data sheet for use with the particular type of ultrasonic instrument. The specifications of the probe cables and electrical matching devices shall be documented.

In addition to the ultrasonic instrument or laboratory pulser/receiver, the items of equipment essential to assess probes in accordance with this document are as follows:

1. an oscilloscope or digitizer with a minimum sampling frequency equal to 10 times the expected upper cut-off frequency f_u of the transmitter or receiver probe, whichever is greater;
2. a frequency spectrum analyser with a minimum sampling frequency of 10 times the expected upper cut-off frequency f_u, or an oscilloscope/digitiser or computer capable of performing discrete Fourier transforms (DFT).
3. a thermoacoustic broadband transmitter providing transient single pulses without ringdown. The pulse bandwidth shall cover the expected bandwidth of the tested probe (this may depend on the distance and position of the measurement).
4. a broadband microphone such that the bandwidth of the microphone and the amplifier covers the bandwidth of the probe under test.
5. a laser Doppler vibrometer with a minimum sampling frequency of 2,5 times probe center

frequency if a Fast Fourier transform (FFT) measurement is done or a minimum sampling

frequency of 10 times probe center frequency, if a time measurement (measuring real displacement or velocity) is used. The bandwidth of the laser Doppler vibrometer shall cover the bandwidth of the probe under test.

The specifications of applied items shall be recorded.

For measuring distances, a ruler or a measurement tool with a similar accuracy shall be used.

The following reflectors shall be used:

1. A steel ball or rod with a hemispheric ended smooth reflective surface. For each frequency range the diameter of the ball or the rod to be used is given in Table 4.
2. A large planar and smooth steel reflector. The reflector’s lateral size shall be at least ten times wider than the beam width of the probe under test measured at the end of the focal zone, as defined in 8.4.5.

Table 4 — Steel ball or rod diameters for different frequencies

A setup with a manual or automated scanning mechanism or a robot with at least these five free axes shall be used:

• three linear axes x, y, z;
• two angular axes.

The scanning mechanism used should be able to maintain alignment between the reflector and the probe in the x- and y-directions within ±0,1 mm at 100 mm distance in the z-direction, where z is the direction of the acoustical axis.

If the amplitudes of ultrasonic signals are recorded automatically, the system shall have sufficient accuracy. In particular, consideration shall be given to the effects of the system bandwidth, spatial resolution, data processing and data storage on the accuracy of the results.

Typical setups to measure the pulse properties and the soundfield properties of air-coupled probes are specified in 8.2.

Where values for wave propagation in air are specified in this document, they are based on a sound velocity of 343,21 m/s and a characteristic acoustic impedance of 413,3 Pa s/m, which are valid for standard temperature 20 °C and standard pressure 101 325 Pa. For deviating temperature T (in °C) the approximation given in Formula (1) can be used for the sound velocity:

(1)

The dependence of the sound velocity on frequency and pressure are normally insignificant. However, the acoustic impedance, which is the product of the speed of sound and the density, depends on atmospheric pressure, because the air density depends on pressure. Air density will decrease by about 1 % for a decrease of 10 hPa in pressure or 3 °C increase in temperature.

Variations in temperature and pressure as well as air draft shall be prevented. The air temperature shall not deviate by more than ±2 °C during the characterization of air-coupled probes. The room temperature and air pressure shall be measured and reported in the probe test report.

The room surrounding the test setup should be large compared to the measurement distance. It shall be ensured that the air-coupled ultrasound is not reflected by any walls, obstacles or parts of the test setup.

Care should be taken about the influence of sound attenuation in air, which, at high frequencies, causes a downshift of the measured center frequency when using broadband probes (see Table 5). The sound attenuation in air and its dependence on frequency are described in ISO 9613-2:2024.

For example, if a 250 kHz probe with a relative bandwidth of 15 % is characterized, at the distance of 200 mm the resulting spectrum would have a maximum at 249 kHz. However, if the relative bandwidth is 0,38, the maximum would be at 243 kHz.

Table 5 — Downshift of the measured center frequency due to attenuation in air

The outside of the probe shall be visually inspected for correct identification, correct assembly and for physical damage which can influence its current or future reliability.

No visible damage of the probe surface used for transmitting and receiving ultrasound is allowed.

This and the following clauses describe the measurement setups to find pulse properties described in8.3, and beam properties described in 8.4. Pulse properties are all measured at a fixed distance and position (stationary measurements), except for the measurements of the dead zone. Beam properties are measured with the help of a positioning system described in 7.2 (scanning measurements).

Five different measurement setups are possible, depending on the type of probe:

1. Characterization using pulse-echo technique,
2. characterization of pairs of probes using through-transmission technique,
3. characterization of transmitters using a microphone,
4. characterization of transmitters using a laser Doppler vibrometer,
5. characterization of receivers using thermoacoustic or some other broadband excitation.

These five setups are described in 8.2.2, 8.2.3, 8.2.4, 8.2.5 and Annex A, respectively.

Many air-coupled probes are designed to be used both as transmitter and receiver. Such probes can be characterized using pulse-echo technique in setup a), but also using setups b) to e). Air-coupled probes designed as either transmitter or receiver cannot be characterized using pulse-echo technique. For these probes, one of the measurement setups b) to e) shall be applied.

There are several options to drive a transmitter probe:

1. with unipolar rectangular pulses or unipolar rectangular tone bursts,
2. with bipolar rectangular pulses or bipolar rectangular tone bursts,
3. with bipolar sine wave tone bursts;
4. with a spike pulse, if applicable.

The transmitter probe shall be driven as recommended by the manufacturer, depending on the intended use. The type of excitation (voltage, pulse length, burst signal length etc.) can have a strong influence on the recorded signals and on the sound field. For example, burst excitation reduces the bandwidth of the recorded signal. The choice of experimental equipment can have a significant influence too, because of the influence of the electrical coupling conditions like impedances of the pulser, probe, cables, and receiver. Therefore, the type of the excitation including all parameters describing the excitation as well as the used equipment shall be reported together with the applied setup.

Pulse-echo measurements can be performed with probes which can be used both as transmitter and receiver, only. The setup for pulse-echo measurements (see Figure 2 and Figure 3) includes a probe, which transmits and receives ultrasonic signals and a target, which reflects the ultrasonic signal. The reflecting target is described in 7.2.

The setup for stationary measurements (which are used to evaluate pulse parameters) and for the evaluation of the dead zone contains a large flat reflector as a target placed at the focal distance or the near-field length of the probe (see Figure 2). If the dead zone is larger than the focal distance, the distance between the probe and the reflector shall be selected to be larger than the dead zone and specified in the probe test report. The orientation of the probe shall be adjusted to maximize the received signal. If the focal distance is not known, the distance between the probe and the reflector shall be selected to maximize the reflected signal. This procedure is legitimate because for most probes the distance of the maximum signal is approximately equal to the focal distance.

The setup for scanning measurements (for the evaluation of soundfield parameters) includes a ball or a rod as a target. The target properties are described in 7.2. For these measurements the probe shall be moved relatively to the target (see Figure 3).

Key

1 ultrasonic instrument

2 PC

3 documentation

4 manipulator control

5 manipulator

6 probe

7 target

Figure 2 — Setup for pulse-echo measurements using a large flat reflecting target

Key

1 ultrasonic instrument

2 PC

3 documentation

4 manipulator control

5 manipulator

6 probe

7 ball reflector or rod

Figure 3 — Setup to measure the soundfield parameters of air-coupled probes using a ball or a rod as a target

This sub-clause describes characterization of pairs of probes. It can be applied to any pair of probes if one probe is applicable as a transmitter and the other one as a receiver.

For a characterization of a pair of probes, a setup as illustrated in Figure 4 shall be used. The distance between the two probes shall be equal to the sum of their focal distances or near-field lengths. The orientation of both probes shall be adjusted to maximize the received signal. If the focal distances or the near-field lengths are not known, the distance between the probes shall be selected to maximize the received signal.

Key

1 ultrasonic instrument

2 PC

3 documentation

4 manipulator control

5 manipulator

6 probe

7 transmitter probe

Figure 4 — Setup to measure the soundfield parameters of two air-coupled probes used as a pair

Characterization with a microphone can be performed only with probes which can be used as transmitter.

In a setup for stationary measurements (which are measurements of pulse parameters, see 8.3) using a microphone as a target, the microphone shall be placed in the middle of the focus of the probe, i.e. at the focal distance or near-field length on the acoustical axis. This position shall be found by maximizing the received signal starting from large distances to avoid optimization within the near-field. This setup is illustrated in Figure 5.

In a setup for scanning measurements (performed for the evaluation of the beam properties, see 8.4) using a microphone as a target, the the sound field of the probe shall be relatively moved to the microphone using a positioning system satisfying the requirements described in 7.2.

Care should be taken of the microphone effective aperture, which shall be reported, because its size influences the lateral resolution of the sound field image.

Key

1 pulse generator

2 PC

3 documentation

4 oscilloscope

5 manipulator control

6 manipulator

7 probe

8 microphone

Figure 5 — Setup for measurements with a microphone

Setups based on laser vibrometry can be used instead of a microphone to characterize transmitter probes. The advantage is the smooth spectrum. The disadvantages are the higher complexity of the measurement and its evaluation.

The measurement setup (see Figure 6) includes a thin light-reflecting membrane in the xy-plane whose velocity is measured using a laser doppler vibrometer. The sound pressure p at that point at the membrane is calculated from the measured velocity v_Foil taking into account the frequency-dependent influence of the membrane as given inFormula (2)^[1]:

(2)

where

is the angular frequency;

Z_Air is the specific acoustic impedance of air;

j is

m_Foil is the areal density of the membrane (kg/m²); and

d_Foil is the thickness of the membrane.

v velocity

v_Foil measured velocity in the membrane

In a setup for scanning measurements (performed for the evaluation of the soundfield parameters, see 8.4) using a laser Doppler vibrometer as a target, the probe shall be relatively moved to the foil using a positioning system satisfying the requirements described in 7.2.

Key

1 pulse generator

2 PC

3 documentation

4 oscilloscope / velocity decoder

5 manipulator control

6 manipulator

7 transmitter probe

8 membrane

9 laser Doppler vibrometer

Figure 6 — Setup for measurements of sound pressure using a laser Doppler vibrometer

As an alternative, it is also possible to measure the surface velocity directly on the surface of the transmitter probe using a laser Doppler vibrometer and to compute the sound field using a validated computational method, for example point source synthesis or finite elements method. In this case it is possible to quantitatively determine the sound field without any artifacts due to the receiver characteristic, which normally influences the sound field measurement.

Another application of laser vibrometry is refractovibrometry. The sound waves in air change its local density. The refractive index of air is a function of density. Changes in the speed of light caused by changing the refractive index can be measured with a scanning laser vibrometer. The transmitter probe is excited with a continuous wave excitation and the laser Doppler vibrometer scans the entire sound field of the transducer point by point. With this method, an image of the pressure distribution in front of the transmitter probe can be generated.

Another application of laser vibrometry is refractovibrometry. The sound waves in air change its local density. The refractive index of air is a function of density. Changes in the speed of light caused by changing the refractive index can be measured with a scanning laser vibrometer. The transmitter probe is excited with a continuous wave excitation and the laser Doppler vibrometer scans the entire sound field of the transducer point by point (seeFigure 7). With this method, an image of the pressure distribution in front of the transmitter probe can be generated.

Key

1 pulse generator

2 PC

3 documentation

4 oscilloscope / velocity decoder

5 manipulator control

6 transmitter probe

7 reflector

8 laser Doppler vibrometer

Figure 7 — Setup for measurements of pressure distrution using a laser Doppler vibrometer and refractovibrometry

The following sub-clauses describe the evaluation of measurements to evaluate the pulse properties such as duration, sensitivity, burst excitation saturation, dead time after transmitter pulse and frequency spectrum properties for transmitter and receiver. The measurements are performed using stationary measurement setups as described in 8.2.

The pulse properties are all measured at a fixed distance and position, with the exception of the measurements for the determination of the dead zone, see 8.3.4.

The measured unrectified signal (also called RF signal) shall be recorded. It is recommended to plot and document the transmitter pulse shape.

Evaluation procedure

The 10 % peak-to-peak amplitude value of the pulse defines levels symmetrically to the base line. The first and the last crossing point of the signal with these levels define the pulse duration L as shown in Figure 1.

Acceptance criteria

The pulse duration shall not be greater than the manufacturer's specification stated in the probe data sheet.

General

The evaluation procedure of the probes sensitivity depends on the measurement setup.

The evaluation of the pulse-echo and the transmission sensitivity is given in 8.3.3.2 and 8.3.3.3. The evaluation of the transmitter and the receiver sensitivity is given in 8.3.3.4 and8.3.3.5.

Evaluation procedure in time domain

Pulse-echo measurements can be applied only to probes which can used both as transmitter and receiver. The setup is described in 8.2.2. For these measurements, the peak-to-peak amplitude of the echo U_out shall be measured related to the transmit peak-to-peak amplitude U_in (ignoring the pulse overshot). The pulse-echo sensitivity is expressed in dB and defined as given in Formula (3).

(3)

where

U_out is the peak-to-peak voltage of the echo from a a large planar and smooth reflector, before amplification as measured in 8.2.2;

U_in is the peak-to-peak voltage applied to the probe with the ultrasonic instrument set to separate pulser/receiver mode.

Sensitivity evaluation in frequency domain

Use the set-up described in 8.2.2.

The sensitivity band is defined as given Formula (4):

(4)

where

is the magnitude of the FFT of the received signal;

is the magnitude of the FFT of the excitation signal.

To extract these FFTs, the signal is windowed (as shown in Figure 8) with the purpose to select only the electrical excitation (window 1) and the echo (window 2), respectively. The length of window 1 and 2 shall be the same. Window 2 shall be delayed in respect to window 1. The length and the location shall be set in a way that window 1 shall only contain the excitation signal and that window 2 shall only contain the first echo.

The peak sensitivity is then given byFormula (5). See Figure 9.

(5)

Key

A amplitude

t time

t_W1 time window 1 for FFT

t_W2 time window 2 for FFT

U_in transmit peak-to-peak amplitude

U_out peak-to-peak amplitude of the first echo

1 excitation signal to the probe

2 first echo signal to the reflector

3 second echo signal to the reflector

Figure 8 — Sensitivity evaluation for pulse-echo and transmission

Key

A amplitude

f frequency

S_p peak magnitude of the FFT of the received signal

f_p frequency of the peak sensitivity

Figure 9 — Evaluation of the amplitude spectrum of the first echo

Sensitivity evaluation for the transmitter

For characterization of transmitter probes individually, a value for sound pressure p at a certain distance and U_in shall be determined using a calibrated microphone or vibrometer. The measurement setup is described in 8.2.4 for a microphone and 8.2.5 for a vibrometer. The sensitivity can be calculated as given in Formula (6).

(6)

and the measurement unit is Pa/V.

The sound pressure is usually not a linear function of the excitation voltage. In fact, sound pressure comes to saturation at a certain voltage level, so that the sensitivity as defined in Formula (3) drops with the growing voltage. Consequently, the measured sensitivity value is only valid for a specific excitation voltage. Therefore, it is necessary to report about the applied excitation voltage at which the sensitivity was measured. Furthermore, it is recommended to perform the measurements using the excitation voltage as it is intended in practice, which is often the excitation voltage recommended by the manufacturer.

Sensitivity evaluation for the receiver

For the characterization of receiver probes individually, a broadband excitation source is needed which can produce a known sound pressure at the location of the receiver. This source can be a thermoacoustic transmitter using one of the setups described in Annex A. To verify the sound pressure amplitude, the measurement described in the previous paragraph using a microphone as described in 8.2.4 or a laser vibrometer as described in 8.2.5 shall be applied.

The sensitivity of a receiver probe can be calculated as given in Formula (7).

(7)

Where S_receiver is the sensitivity, given in V/Pa.

Acceptance criteria

For probes manufactured in series, the sensitivity shall be within ±3 dB of the manufacturer’s specification stated in the probe data sheet. For individually designed or manufactured probes, the measured sensitivity shall be reported on the probe test report.

If the intended application of a transmitter probe includes the use of burst signals, the burst excitation saturation of the received signal shall be measured by increasing the number of burst cycles. The measurement of the peak-to-peak signal (see Figure 10) is performed in pulse-echo as described in 8.2.2 or in transmission as described in 8.2.3. The envelope of the signal shall be calculated by using the Hilbert transformation. For typical excitation voltages the number of burst cycles shall be increased until the increase of signal is less than 1 dB (see Figure 10). That number of cycles shall be reported as the number of burst cycles for typical excitation voltages for the burst excitation saturation.

Key

A amplitude

A₁, A₂, A₃, A₄, A₅ maximum amplitude of measured signal

t time

n number of burst cycles

Figure 10 — Burst excitation saturation

General

For the measurements of the dead time after transmitter pulse, the probe shall be moved along its acoustic axis towards or from a large flat reflector oriented perpendicular to the acoustic axis as specified in 7.2. The setup for this measurement is specified in 8.4.2. A B-scan shall be recorded, and the peak-to-peak signal amplitude shall be evaluated. Alternatively, the reflector may be moved relatively to the probe. The dead zone after transmitter pulse in air is given by the area in the proximity of the transducer where the envelope of the received reflected signal is smaller than the double of the envelope of the transmitter pulse (see Figure 11). The dead time after transmitter pulse is either the time measured directly or the time calculated from the dead zone after transmitter pulse as given in Formula (8).

(8)

where

v is the speed of sound

Key

1 envelope of the transmitter pulse

2 envelope of the reflected signal

A signal amplitude

A_R amplitude of the reflected signal

t time

t_D dead time after transmitter pulse

Figure 11 — Schematic representation for the determination of the dead time after transmitter pulse

Acceptance criteria

The dead time after transmitter pulse shall not be greater than the manufacturer's specification stated in the probe data sheet.

Evaluation procedure

For the evaluation of the centre frequency and bandwidth from the amplitude spectrum, a frequency spectrum analyser/digitiser or an oscilloscope shall be used. The lower and upper cut-off frequencies f_l and f_u shall be determined at a 6 dB or a 3 dB drop from the maximum value in the frequency spectrum. The 6 dB drop shall be applied to pulse-echo measurements and transmission measurements specified in 8.2.2 and 8.2.3, while the 3 dB drop shall be applied to measurements with a microphone, laser Doppler vibrometer or a thermoacoustic source specified in8.2.4, 8.2.5 and Annex A respectively.

From these lower and upper cut-off frequencies f_l and f_u, the center frequency f_c, the bandwitdth Δf and the relative bandwidth Δf_rel shall be calculated as given in Formula (9), Formula (10) and Formula (11).

(9)

(10)

(11)

Acceptance criteria

The measured center frequency shall be within ±10 % of the frequency stated in the probe data sheet.

The measured bandwidth shall be within ±15 % of the bandwidth stated in the probe data sheet.

If the spectrum between f_l and f_u has more than one maximum, the amplitude ratio between adjacent minima and maxima shall not exceed the value stated in the probe data sheet, e.g. 3 dB.

This subclause deals with the properties of the air-coupled probe's soundfield. The soundfield properties are determined by scanning a target relative to the probe’s soundfield, either by moving the target or the probe. This target is a small, ideally a point-like reflector (i.e. a reflector being much smaller than the wavelength), or a microphone receiver or a laser Doppler vibrometer.

The following properties are investigated:

• Axial beam properties (described in 8.4.3)
• focal point z_F
• focal distance d_F
• near-field length
• focal length Δz_F
• axial beam profile, i.e. a scan along the beam axis (z-axis)
• Transverse beam properties (described in 8.4.4 and 8.4.5):
• Focal width Δx_F, Δy_F
• beam divergence Ω_x,Ω_y
• transverse beam profile, i.e. a scan perpendicular to the beam axis (z-axis).

The measurements shall be performed using a scanning system and automated collection of A-scan data during scanning. The soundfield parameters are computed from these scans. Two equivalent procedures are given for soundfield measurements. They differ only in the methods used to record the measurement results:

1. Measurements performed using one-dimensional (1D) scanning. This technique is based on readings along specific lines with respect to the beam (see Figure 12). These measurements yield B-Scans.
2. Measurements performed using two-dimensional (2D) scanning. This technique is based on readings along specific planes with respect to the beam (see Figure 13) and yields C-Scans.

Key

X, Y, Z directions

S_x, S_y, S_z scanning directions

Figure 12 — Scanning movements for one-dimensional scanning

Key

X, Y, Z directions

S_xy, S_yz scanning planes

Figure 13 — Scanning movements for two-dimensional scanning

Automated recording of the signal amplitude synchronized to the scanner movement shall be used. The raw data of these scans as well as the generated B/C-Scans shall be saved. The scan figures shall include a coordinate system and a colour scale or a greyscale appropriate to visualize the –3/–6 dB levels. The focal distance, the near-field length, the focal length, the focal width, the transverse profile and the beam divergence shall be deduced from the B- and C-Scans as described in 8.4.3, 8.4.4 and8.4.5.

For the measurements determining the soundfield properties, the beam axis coincides with the z-axis of the scanning setup. In order to achieve this, the beam axis shall be aligned with the z-axis before scanning measurements are started. The squint angle is compensated by setting the beam axis perpendicular to the xy-plane as illustrated in Figure 14, Figure 15 and Figure 16. This operation is performed by adjusting both θ and ψ of the probe holder to maximize the magnitude of the incoming signal.

The system shall have sufficient dynamic range to collect the high-amplitude signals (obtained at the focal point) without saturation and the low-amplitude signals with a sufficient signal-to-noise ratio. The axial scan width (xz- or yz-direction) as well as the transverse scan width (x- or y-direction) shall cover at least the areas of the beam where the measured signal amplitude is 6 dB (if the target is a reflector) or 3 dB (if the target is a microphone or a laser vibrometer) lower than at the focal point.

Evaluation procedure

The experimental setup shall be as described in 8.2. The beam axis shall be aligned with the z-axis and the measurement shall be performed as described in 8.4.2.

The position of the sound exit point of the probe is marked z₀, as shown in Figure 14. The target shall be placed on the probe's beam axis.

In a 1D measurement, the target shall be moved relative to the probe along the z-axis, varying the probe-target distance, as shown in Figure 12. The distance z_F at which the signal has its maximum shall be determined as shown in Figure 15.

The focal distance is given by Formula (12).

(12)

Key

1 flat probe

2 focused probe

3 beam axis

4 distance

z₀ sound exit point

Figure 14 — The point z_o of the coordinate system for non-focused or focused probes

Key

A amplitude

S distance

V_P amplitude at focal distance

Z₁, Z₂ boundaries of the focal zone

Figure 15 — Axial profile of an air-coupled probe measured with a small reflector

By increasing and reducing the distance between the probe and the target, the near-field length or the limits of the focal zone shall be found, i.e. the two points where the amplitude is reduced by 6 dB, if the target is a reflector, or by 3 dB, if the target is a microphone or a laser vibrometer. Z₁ and Z₂ are the coordinates of these points on the z-axis. The length of the focal zone is given by Formula (13).

(13)

In a 2D measurement, d_F and ΔZ_F are deduced from the resulting C-scans analogously. The scan shall be performed in x-z or y-z plane as indicated in Figure 13.

An example of this plot is given in Figure 16.

Key

A amplitude, in dB

S distance

x, y directions

Figure 16 — C-scan image of the sound beam profile of a non-focusing probe as the result of a 2D scanning procedure using pulse-echo measurements on a rod or a sphere

Acceptance criteria

The near-field length or the focal distance and the length of the focal zone shall be within ±15 % of the manufacturer’s specifications stated in the probe data sheet.

Evaluation procedure

The measurement setup shall be as described in 8.2. The beam axis shall be aligned with the z-axis and the measurement shall be performed as described in 8.4.2.

In a 1D measurement, the focal width in x direction is measured by moving the probe relatively to the target in x-direction, keeping the distance to the probe equal to the focal distance, as indicated in Figure 12. The two points x_F1 and x_F2, where the amplitude level of the signal in comparison to the signal level at the focal point has decreased by 6 dB if the target is a reflector or by 3 dB if the target is a microphone or a laser Doppler vibrometer, determine the focal width, as illustrated in Figure 17.

The focal width in x-direction is then computed using Formula (14).

(14)

To measure the focal width in y-direction , return the x-position back to the focal point and proceed analogously in the y-direction to find by using y_F1 and y_F2 with Formula (15).

(15)

In a 2D measurement the values are deduced from the resulting C-scans analogously. The C-scan shall cover a transversal xy-area at the focal distance as shown in Figure 13.

Analogously to the 1D measurement, Δx_F and Δy_F shall be determined as diameters of the zones measured in the x- or y-direction where the displayed amplitudes are 6 dB (if the target is a reflector) or 3 dB (if the target is a microphone or a laser vibrometer) lower than on the beam axis (see Figure 17 and Figure 18 for an example).

Key

A amplitude

x probe position on x-axis

y probe position on y-axis

Figure 17 — Transverse profiles in the focus of an air-coupled probe

Key

A amplitude, in dB

x probe position on X-axis

y probe position on y-axis

Figure 18 — C-scan image of the sound beam of a focusing probe as the result of a 2D scanning procedure with indicated focal width Δx_F and Δy_F

Acceptance criteria

The focal widths shall be within ±15 % of the manufacturer’s specification stated in the probe data sheet.

Evaluation procedure

The measurement of the beam divergence is only required for non-focused probes. Therefore, it is not required for acoustic lenses or curved piezoelectric transducers.

The measurement shall be performed at the focal distance and additionally at the far end of the focal zone at z₂. The value for z₂ is determined as described in 8.4.3. The corresponding values x₂₁, x₂₂ and y₂₁, y₂₂ shall be recorded, i.e. the target (or probe) positions on x-axis and on y-axis where the amplitude decreases by 6 dB (reflector) or 3 dB (microphone) from the maximum value, which is obtained on the beam axis. This is illustrated in Figure 19. The width of the sound field in x- and y-direction and shall be calculated as given in Formula (16) and Formula (17).

(16)

(17)

The angles of beam divergence can be computed using the values Δx_F and Δy_F at , which is the focal width as obtained in 8.4.4, and and at using Formula (18) and Formula (19).

(18)

(19)

Key

A amplitude

x, y probe position on x- or y-axis

Figure 19 — Transverse profiles at the near-field length , measured using pulse echo technique

To avoid that the measurements of beam divergence produce strongly underestimated divergence angles, the measurements should be performed as far as possible from the probe instead at the far end of the focal zone.

NOTE Use a position as far as possible from the probe with sufficient signal to noise ratio (e.g. at twice the focal distance).

Acceptance criteria

The angles of divergence shall not differ from the manufacturer’s specified values as stated on the probe data sheet by either ±10 % or by 1 °, whichever is larger.

1. 
   (informative)
   
   Characterization of receiver probes using thermoacoustic excitation

For the characterization of receiver probes, a broadband ultrasonic source covering the amplitude spectrum of the receiver can be applied. Such a broadband source can be a thermoacoustic transmitter. Thermoacoustic transmitters emit short ultrasonic waves containing a broad range of frequencies when excited with short electric pulses. A receiver to be characterized can be placed in their sound field. The recorded signal can be analysed analogously to the acoustic signal of a transmitter recorded using a broadband microphone or a laser vibrometer. Factors influencing the received signal include the excitation signal, the geometric and material properties of the source, the relative position to the receiver and the receiver itself.

For the purposes of stationary measurements (measurements of pulse parameters, see 8.3), several possibilities for the measurement setup are being discussed in the scientific community. There is an agreement that the thermoacoustic transmitter and the characterized receiver probe should be oriented facing each other, so that their acoustical axes overlap. The excitation signal should be a square pulse with the length smaller than 1/(2f), where f is the expected frequency of the receiver probe. There are still open discussions about the exact properties of the transmitter and the recommended distance. These discussions are currently guided by the goal to come as close as possible to the properties of the receiver, independently on the thermoacoustic transmitter.

Regarding the distance, these options have been proposed among others:

1. The distance transmitter-receiver should be equal to the focal distance or near-field length of the receiver probe. This might be a valid recommendation if the focal distance or near-field length of the receiver is already known and if the thermoacoustic transmitter is approximately a point source.
2. The distance transmitter-receiver should be equal to the focal distance or near-field length of the thermoacoustic transmitter as measured with a broadband microphone or laser vibrometer. This way the center of the receiver will be exposed to the shortest possible acoustical excitation, but its edges will not.
3. The distance transmitter-receiver should be chosen so that the two foci overlap, i.e. the

distance should be equal to the sum of focal distances or near-field lengths.

1. The distance should be adjusted so that the amplitude of the received signal is maximized.

For the purposes of scanning measurements, which are performed to determine the sound field, a perfectly broadband point source would be ideal. However, with decreasing size the acoustic energy of the source also decreases, which poses difficulties for measurements. A compromise between various requirements is needed, which is still a subject of research.

Whichever transmitter, excitation parameters, measurement setup and data evaluation procedure are chosen, they shall be exactly reported.

In order to use thermoacoustic transmitters for the characterization of receivers, the transmitters’ sensitivity and their spectrum shall be known. For this purpose, the resulting sound pressure of thermoacoustic transmitters shall be measured at the same distance as the distance between the thermoacoustic source and the characterized probe. The measurement shall be performed using a broadband microphone or a laser vibrometer as specified in 7.1, using the setup specified in 8.2.4, similar to Figure 5.

Bibliography

[1] Q. Leclère and B. Laulagnet, “Particle velocity field measurement using an ultra-light membrane,” Appl. Acoust.69, 302–310 (2008)