ISO/FDIS 29821

ISO TC 108/SC 5
Date: 2025-12-13
ISO/TC 108/SC 5
Secretariat: SA
Date: 2026-xx
Condition monitoring and diagnostics of machinesmachine
systems — Ultrasound — General requirements, guidelines,
procedures and validation
Surveillance des conditions et diagnostic d'état des machines — Ultrasons — Exigences générales, lignes
directrices, procédures et validation
FDIS stage
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ii
Contents Page
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle of the airborne and structure-borne method . 2
4.1 General. 2
4.2 Application of airborne and structure-borne ultrasound within condition monitoring
programmes. 2
4.3 Correlation with other technologies . 3
5 Ultrasound equipment . 4
5.1 General. 4
5.2 Types of sensor . 6
5.3 Airborne sensor choice . 7
5.4 Structure-borne sensor choice . 7
5.5 Instrument characteristics . 7
6 Data collection guidelines. 8
6.1 General. 8
6.2 Comparative ultrasound . 8
6.3 Baseline method — Quantitative ultrasound . 9
7 Training requirements . 9
8 Assessment criteria . 9
8.1 General. 9
8.2 Error sources, accuracy and repeatability . 11
9 Interpretation guidelines . 11
10 Diagnosing ultrasonic problems . 12
10.1 Principles of diagnostics using ultrasound . 12
10.2 Generation of ultrasound . 12
11 Sensitivity validation guidelines . 13
12 Monitoring interval . 13
13 Alternative CM techniques . 13
14 Reporting . 13
Annex A (informative) Example of a compressed air leak survey . 15
Annex B (informative) Typical examples of ultrasound test reports . 19
Annex C (informative) Example of a generic sensitivity validation procedure — Ultrasonic tone
generator method . 23
Bibliography . 25

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
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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
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This document was prepared by Technical Committee ISO/TC 108, Mechanical vibration, shock and condition
monitoring, Subcommittee SC 5, Condition monitoring and diagnostics of machine systems.
This second edition of ISO 29821 cancels and replaces the first edition (ISO 29821:2018) which has been
technically revised.
The main changes are as follows:
— Clause 5— Section 5 has been revised to include Formula (1)equation (1) describing the decibel level;
— Clause 5— Section 5 has been revised to include MEMS sensors and a reference to ISO 1683;
— Subclause 5.3— Para 5.3 has an additional note describing parabolic reflectors;
— Figures B.1— Figures B1 to B.4B4 have been improved;
— — Minor editorial changes have been made.
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.
iv
Introduction
This document provides specific guidance on the interpretation of ultrasonic readings and wave files or
frequency and time domain printouts (sometimes called sound characteristics) as part of a programme for
condition monitoring (CM) and diagnostics of machines.
Airborne (AB) and structure-borne (SB) ultrasound can be used to detect abnormal performance or machine
anomalies. The anomalies are detected as high frequency acoustic events caused by turbulent flow, ionization
events, impacts and friction which are caused by incorrect machinery operation, leaks, improper lubrication,
worn components, and/or electrical discharges.
Airborne and structure-borne ultrasound is based on measuring the high frequency sound that is generated
by either turbulent flow, friction, impacts or ionization created from the anomalies. Personnel carrying out
ultrasonic inspections or measurements therefore require an understanding of ultrasound and how it
propagates through the atmosphere and through structures as a prerequisite to the creation of an airborne
and/or structure-borne ultrasound programme.
Ultrasonic energy is present with the operation of all machines. It can be in the form of friction, turbulent flow,
impacts and/or ionization as a property of the process, or produced by the process itself. As a result, ultrasonic
emissions are created and these are a useful parameter for monitoring the condition of machines and for
detecting machine anomalies.
v
Condition monitoring and diagnostics of machinesmachine systems —
Ultrasound — General requirements, guidelines, procedures and
validation
1 Scope
This document
— — gives guidelines for establishing severity assessment criteria for anomalies identified by airborne (AB)
and structure-borne (SB) ultrasound,
— — specifies methods and requirements for carrying out ultrasonic inspection, testing, measurement and
monitoring of machines, including safety recommendations and sources of error, and
— — provides information relative to data interpretation, assessment criteria and reporting.
2 Normative references
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 13372, Condition monitoring and diagnostics of machines — Vocabulary
ISO 13379--1, Condition monitoring and diagnostics of machine systems — Data interpretation and diagnostics
techniques — Part 1: General guidelines
ISO 13381--1, Condition monitoring and diagnostics of machine systems — Prognostics — Part 1: General
guidelines and requirements
ISO 17359, Condition monitoring and diagnostics of machines — General guidelines
ISO 18436--8, Condition monitoring and diagnostics of machines — Requirements for qualification and
assessment of personnel — Part 8: Ultrasound
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13372 and the following apply.
ISO and IEC maintain terminologicalterminology databases for use in standardization at the following
addresses:
— — IEC Electropedia: available at https://www.electropedia.org/
— — ISO Online browsing platform: available at https://www.iso.org/obp
3.1 3.1
airborne and structure-borne ultrasound
non-destructive test method detecting airborne and structure-borne ultrasound above 20 kHz created from
or through a medium
3.2 3.2
background noise
unwanted noise present in a signal which cannot be attributed to a specific cause
Note 1 to entry: This ultrasonic noise can emanate from the area surrounding the measurement position, which can
cause false indications.
3.3 3.3
scanning
moving a receiving transducer or an array of transducers around a suspected source of ultrasound to verify
its location
3.4 3.4
sonic reflection
airborne ultrasound reflected off a solid surface possibly indicating a false reading
3.5 3.5
contact module
waveguide in the form of a rod attached to an ultrasonic transducer conducting ultrasound through physical
contact with the structure or machine
4 Principle of the airborne and structure-borne method
4.1 General
Airborne and structure-borne ultrasound is a physical wave that occurs within the test subject (material or
machinery component) or in the atmosphere and is detected externally either close to or at a distance from
the test subject and is based on the detection of high-frequency sounds.
Most ultrasonic instruments employedused to monitor equipment detect frequencies above 20 kHz which is
above the range of human hearing (20 Hz to 20 kHz). The differences in the way low-frequency and high-
frequency sounds travel help to explain why this technology can be effective for condition monitoring since:
— — low-frequency sounds maintain a high intensity of sound volume and travel further than high-
frequency sounds, and
— — high-frequency sounds are more directional. As high-frequency sound waves propagate from the point
of generation, their intensity level decreases rapidly with distance depending on the elasticity and density
of the medium traversed which helps to identify the origin of a sound source.
Airborne ultrasound is propagated through an atmosphere (air or gas) and detected with an ultrasonic
microphone while structure-borne ultrasound is generated within and propagated through the structure and
is usually detected with a contact module, although other sensors may be used. These contact modules do not
require any coupling agent, as the detection frequencies are low enough that, unlike traditional pulse-echo
ultrasound, small air gaps between the contact probe and the structure under test do not significantly
attenuate the received signal. If permanently mounted sensors are used, careful mounting techniques should
be utilized to avoid signal attenuation or resonances, or both. The structure can be a machine or any
component of a machine or a system.
4.2 Application of airborne and structure-borne ultrasound within condition monitoring
programmes
Ultrasound is a non-invasive measurement technique which can identify a range of faults in many applications.
Table 1Table 1 shows typical examples of ultrasound applications to machine condition monitoring.
4.3 Correlation with other technologies
Airborne and structure-borne ultrasonic is often used in a condition-monitoring programme to detect failure
modes that have been previously identified by another technology. There are instances where airborne or
structure-borne ultrasound is the first indicator of a failure mode, such as in the detection of faulty slow-speed
bearings and/or insufficient lubrication in rolling element bearings. Airborne or structure-borne ultrasound
can also be used to identify a potential safety hazard to a practitioner using an alternate technology, for
example, in the inspection of enclosed electrical systems. Airborne and structure-borne ultrasound can be
used to determine if an arc flash hazard is present before opening the cabinet for an infrared thermographic
inspection.
Acoustic emission is the phenomenon of radiation of acoustic (elastic) waves in solids that occurs when a
material undergoes irreversible changes in its internal structure. Acoustic emission is traditionally utilized to
monitor items that are under stress to detect the formation and location of cracks including structures,
pressure vessels and pipelines. Many of the acoustic emission applications are similar to the structure-borne
ones described in this document.
NOTE For information on the use of acoustic emission for condition monitoring refer to ISO 22096.
Table 1 — Ultrasonic application examples
Pressure or vacuum leak
a a
Machine description Mechanical Electrical
a
detection
Heat exchangers AB — —
Boilers AB — —
Condensers AB — —
Control air systems AB — —
Valves SB — —
Steam traps SB — —
Motors — SB SB
Pumps AB SB SB
Gears/gear boxes — SB —
Fans — SB —
Compressors AB SB SB
Conveyors — SB and AB SB
Switchgear — AB and SB AB and SB
Transformers — SB AB/SB
Insulators — — AB
Junction boxes — — SB
Circuit breaker — — SB
Turbines AB SB —
Generators (utility) AB SB AB/SB
Lubrication — SB —
High-speed bearings — SB and AB —
Low-speed bearings — SB and AB —
Pressure or vacuum leak
a a
Machine description Mechanical Electrical
a
detection
a AB: airborne; SB: structure-borne.
5 Ultrasound equipment
5.1 General
Airborne and structure-borne ultrasonic instruments are typically hand-held, portable and battery operated
for ease of use in the field. Online, non-portable systems are also utilized mainly for condition monitoring
where an anomaly may occur and needs to be addressed at the inception rather than when a route-based
inspection is scheduled. Most online applications target a narrow range of applications where amplitude is the
primary parameter that is monitored and false indications are less likely to occur.
It is recommended that the system consist of an instrument, ultrasonic transducer and headphones. It is also
recommended that the demodulated output signal can be made audible using headphones to enable
discrimination between competing sources. This allows the practitioner to recognize and prevent the
acquisition of poor-quality data.
The system shall provide for the detection of acoustic energy that is either airborne or structure-borne in the
range above 20 kHz and shall translate (demodulate or heterodyne) this energy into an audible signal that can
be seen on a signal strength indicator and heard through headphones. The signal strength is usually displayed
in decibels and commonly referred to as decibel value. The demodulated or heterodyned signal is
representative of the amplitude and frequency characteristics of the original ultrasonic signal.
The ultrasonic physical pressure wave or pressure variation which is received and measured by the ultrasonic
instrument is demodulated and converted to a corresponding decibel level, L , given by Formula (1)::
p
L = 20 log r log r (1)
p 10 a 10 a
where
Lp, is the sound pressure level (dB));
ra is the relative voltage amplitude ratio, referenced to the threshold level of the instrument.

Instrument sensitivity may vary between different manufacturers. Each manufacturer establishes its own
threshold level (0 dB) If a condition monitoring application requires a comparison or trending of signal
strength readings over time, care should be taken to use instruments that have the same sensitivity so that
comparable data can be obtained. When making comparisons between instrument readings, the dB readings
shall be of the same type.
NOTE: Guidance on preferred reference quantities for acoustic levels is given in ISO 1683.
The main housing contains ultrasonic transducers that receive the ultrasound signal and convert it to an
amplified electrical signal. Next, this signal is fed into the main instrument where it is amplified again, then
demodulated or heterodyned. The demodulation or heterodyne principle is used to convert the non-audible
ultrasonic frequencies down to the audible level suitable for humans to hear and for interfacing with recording
and analysing devices. In the demodulation or heterodyne process, the audio signal is a direct translation of
the original signal and this demodulated signal is used for further analysis (see Figure 1Figure 1).).
The demodulated or heterodyned signal allows the practitioner to identify a relevant sound source and to
determine the event or condition producing the ultrasound (e.g. air leaks in the same area as an electrical
discharge can cause confusion to an unskilled operator). The demodulated signal can also be used to
determine the location of the irrelevant ultrasound that could lead to a false reading.
Therefore, the headphone output signal is not a divided signal where the audio frequency is multiplied by a
number and ends up with the ultrasonic frequency. In the demodulation (heterodyne) process, the incoming
ultrasonic signal is mixed with an internal oscillator signal and the difference is amplified and then sent to the
headphone output and the meter circuit. An analogy would be a piano key being struck once a second (1 Hz);
the resultant sound would contain the resonant frequency of the string that the piano key is linked to,
modulated by the 1 Hz of the key being struck. If the piano string signal (carrier frequency) were removed,
what would be left is the 1 Hz signal (modulation frequency) of the key being depressed.
The ultrasonic sensors only detect high-frequency noise caused by friction or turbulent flow and do not
respond to low-frequency acceleration, displacement or audible sounds. In the case of bearings, ultrasound is
created by the motion of the rotating elements. As a bearing deteriorates, imperfections form on the rotating
surfaces and when a rotating element interacts with the imperfection, it produces an acoustic event or
symptom. The actual fault frequencies of the affected bearing modulate the high-frequency components of the
generated ultrasonic noise or signal. The signal after the demodulation or heterodyning would only leave the
original modulation.
For example, in a bearing, if the fault frequency is 48 Hz, the instrument detects the ultrasonic component that
is modulated by the 48 Hz fault frequency. When that signal is demodulated or heterodyned, the audio signal
at the headphones does not contain the ultrasonic signal, but contains the 48 Hz fault frequency signal.
In high-speed bearings, when analysing the demodulated or heterodyned ultrasound signal with a spectral
(FFT) analyser, and comparing it to the signal from an accelerometer, the signals would be qualitatively
similar. With low-speed bearings typically below 10 r/min, comparison of ultrasonic to vibration signals
depends on the acceleration sensor type. For example:
— — Standard piezo-electric vibration accelerometers have low signal output at low excitation frequency.
(typically <5,0 Hz).
— — Micro-electromechanical systems (MEMS) accelerometers typically have good output response down
to 0 Hz.
— — Ultrasonic sensors can provide useful measurements from large bearings (e.g. >15 m diameter) on
machines operating at speeds < 1 r/min, with the sensor output then evaluated by spectral analysis (FFT).
In addition to mechanical condition analysis, signal analysis of the heterodyned signals received from
electrical discharges can help identify the severity of the condition and can also help distinguish the difference
between loose components such as transformer windings and electrical discharges.
29821_ed2fig1.EPS
Key
1 transducer pre-amp
2 variable gain amplifier
3 demodulation circuit
4 mixer
5 oscillator
6 low-pass filter
7 audio amplifier
8 phone output
9 line output
10 RMS-to-DC converter
11 digital I/O
12 sensitivity/frequency adjustment knob
13 store button
14 CPU and digital controls
15 gain control
16 frequency control
17 converter input
18 display
Figure 1 — Block diagram example of an ultrasonic detector
5.2 Types of sensor
Airborne ultrasound is propagated through an atmosphere (air or gas) and detected with an ultrasonic sensor,
while structure-borne ultrasound is generated within and propagated through a structure and is usually
detected with a contact module, although other sensors may be used. A guide forExamples of which sensor
should be chosen can be found in Table 1Table 1.
5.3 Airborne sensor choice
An ultrasonic instrument with fixed sensors might have limitations with respect to field of detection and might
not be suitable for all applications. For ultrasonic instruments with interchangeable sensors, there is normally
a choice of two types of sensor configuration: wide-angle and parabolic.
NOTE A parabolic reflector microphone is a device that uses a parabolic dish to collect and focus sound waves from
a specific direction onto the microphone sensor.
For machine condition monitoring, wide-angle airborne sensors are particularly useful for gaining an overall
assessment of the machine condition utilizing the maximum machine area for comparison of ultrasonic
signatures. This allows the comparison of multiple components in a single machine. This module type is also
useful in confined-space areas where the access area can be very small.
Parabolic sensors are useful for remote detection such as elevated conveyors, equipment, vessels and outdoor
substations, where access may be limited and the machine, system, or component of either, may be at a great
distance. The narrow field of reception is helpful especially for pinpointing leaks in overhead piping or in
determining which phase in a high-voltage electrical tower has an electrical discharge.
5.4 Structure-borne sensor choice
Structure-borne sensors are used to non-invasively detect internal abnormal performance or machine
anomalies. There is normally a choice of hand-held contact, magnetically coupled or permanently installed
(threaded) sensors.
The contact sensor (stethoscope) is most commonly used when a machine, system or component needs to be
quickly scanned to determine where an anomaly or fault condition is located. It is also effectively used to get
into tight spaces to gain access to a good monitoring point. For measurement points that are just out of reach,
extension contact rods can be used. For measurement points that are in difficult to reach or in unsafe areas,
permanent remote contact sensors can be used.
Magnetically coupled contact sensors remove the measurement variation associated with hand-held contact
sensors. They are useful where a long sampling time is required or where there are multiple practitioners
taking readings on the same sampling point. For example: when measuring an electrical transformer, a slight
movement of a contact sensor may sound like partial discharge, which could cause a false indication of an
anomaly.
5.5 Instrument characteristics
5.5.1 General
When selecting an ultrasonic instrument, the sensitivity, frequency response and ability to record the
heterodyned (demodulated) ultrasonic signal output shall be carefully considered with respect to the
application. Some manufacturers recommend that applications require monitoring at different frequencies for
the best results. Other applications require a recording of the heterodyned (demodulated) sound signature
for further analysis and reporting.
5.5.2 Frequency response
When using an airborne or structure-borne ultrasonic instrument with heterodyned (demodulated)
frequency tuning capability, the practitioner should be aware of typical frequencies applicable to specific
applications. These monitoring frequencies are primarily due to the propagation of the ultrasonic wave
through different media, but can also be influenced by the resonance of the ultrasonic sensor. Examples of
typical monitoring frequencies and applications are shown in Table 2Table 2.
Table 2 — Typical monitoring frequencies
Acquisition Application Frequency
method
kHz
Airborne Leaks, electrical 40
Bearings, mechanical 30
Structure-borne Valves, steam traps 25
Electrical – sealed leaks – underground 20
6 Data collection guidelines
6.1 General
Several techniques are recognized and in use throughout the industry to collect data. With the most recent
advances, ultrasonic detectors have become much more sophisticated and have evolved from subjective
listening devices with hand-written data to systems that can store test data, record sound samples, analyse
the data through data management software and process the recorded sound samples with signal analysis
software. These instruments provide the capability to identify changes in the condition of monitored
equipment and to determine any further action to be taken.
Where ultrasound techniques are used as part of a condition-monitoring program, the program shall be
implemented in accordance with ISO 17359. Diagnosis and prognosis shall be carried out in accordance with
ISO 13379--1 and ISO 13381--1, respectively.
6.2 Comparative
...