Ultrasound Dictionary

The field of ultrasound is filled with technical terms, jargon, acronyms, and definitions. We hope you find this glossary useful!

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Acceleration, g

The time rate of change of velocity, usually measured in g and occasionally in metres per second per second (m/s2) in the SI system. It is interesting to note that 1g is not actually a unit of acceleration, but is the magnitude of the acceleration due to gravity at the earth’s surface. Using g to express a measurement of acceleration is therefore a comparative measurement by comparing the value measured with 9.81m/s². This is the same process used in the decibel measurement process.


A transducer whose electrical output is directly proportional to acceleration. The high-frequency response of an accelerometer is limited by its mounted resonant frequency. This is a design consideration. Most common accelerometers respond down to 1 or 2Hz, and some piezoresistive accelerometers respond to DC (zero frequency).

Acoustic camera

An acoustic analog of the Infrared camera. In this case a large number of MEMS sensors (80-120 typically) are arranged in an array to capture airborne sound. The spatial response from the sensors to sound is overlaid on a video image as a contour map to show amplitude. The Ultrasound versions of these cameras are typically used for leak surveys and electrical inspections.

Acoustic horn

A device designed to amplify sound. The physics of the horn have been down for thousands of years. By adjusting the length, shape, the mouth and the throat of the horn it is possible to vary the frequency response and provide natural, non-electronic, amplification. In terms of Ultrasound applications, commercially available horn attachments provide an amplification factor of 5-6x. This amplification is particularly useful when performing vacuum leak surveys or when there is a need to overcome the losses caused by the Inverse Distance Law.

Airborne leak detection

It seems that the first applications developed by most of the large manufacturers of Ultrasound systems today was airborne leak detection. This is performed using a range of different airborne sensors to locate leaks of gases such as compressed air, CO2, Nitrogen, Argon and Hydrogen for example as well as vacuum leaks.

Airborne sensor

In simple terms, any sensor used to detect airborne Ultrasound. There are three basic types: A microphone/ pressure transducer design, an acoustic horn and a parabolic dish.

Airborne Ultrasound

Ultrasound which is travelling through the air as opposed to structural Ultrasound. Airborne sources of Ultrasound include pressurized gas and steam leaks, electrical and rotating machinery applications such as belts, chains and open bearings.


An algorithm is a procedure which can be followed to perform a calculation or operation. In digital computers, algorithms for different purposes are stored and called into play when needed for certain operations. The procedure for calculating the FFT spectrum is an algorithm. Some manufacturers use algorithms in association with evaluating energy losses from Ultrasound readings of air leaks and for evaluation the lubrication status of a rolling element bearing.


To digitize an analog signal for processing in digital instruments, the signal first must be periodically sampled at a rate called the sampling frequency. Mathematically, as long as the sampling frequency is more than twice as high as the highest frequency in the signal, the sampled waveform will be a proper representation of the analog waveform. In practice, because filters do not have a infinitely sharp response, the minimum value used is typically 2.56x the highest frequency,

If, however, the sampling frequency is less than twice as high as the highest frequency to be sampled, the sampled waveform will contain extraneous components called “aliases”. The generation of aliases is called aliasing.

An example of aliasing sometimes occurs in motion pictures, as for instance when the wagon wheels in an old Western seem to be going backward. This is caused by the fact that the frame rate of the camera is not fast enough to resolve the positions of the spokes. Another example of optical aliasing is the stroboscope, where a moving object is illuminated by a flashing light and can be made to appear stationary, or move backward.

Aliasing must be avoided in digital signal analysis to prevent errors.

See also oversampling.


A condition where the components are coincident, parallel, or perpendicular, according to design requirements. Misalignment is the condition where the desired alignment is not achieved, and it causes abnormally high wear and power consumption in the machine. The procedure to correct misalignment is also called “alignment”.

Misalignment can be present between elements such as motor to pump though a coupling, or perhaps pulley misalignment. There is also internal and external misalignment possible with gearboxes. Hydraulic cylinders can also suffer from misalignment.


The magnitude, or amount, of a measured value. The amplitude of a sound can be expressed in terms of its Peak or RMS level. The measurement of amplitude is perhaps simple to conceive when dealing with DC values or indeed in the case of a single frequency sine wave. Measurement of amplitude becomes more complex when dealing with the clicks, pops and impulses frequently associated with an Ultrasound measurement.

See also Decibels.


The flow of electricity through the air. Often a violent event lasting several seconds. Arcing produces a lot of Ultrasound and causes a lot of damage to anything being arced to – especially people.


Reduction in the amplitude of a signal caused by separation distance or barriers in terms of airborne Ultrasound signals and caused by material changes in structure borne signals.

See also Specific Acoustic Impedance.

Audible sound

The frequency range from 20Hz to 20,000Hz is nominally defined as the audible range of sound. Above 20kHz is Ultrasound and below 20Hz is Infrasound.


Parallel to the centerline of a shaft or turning axis of a rotating part.

Background Noise

When using Ultrasound, most of the noise in the site environment is only present in the audible frequency range. Your Ultrasound device should not be detecting that if it is set up correctly. There may be some components of that noise which are also generating Ultrasound – any impacting noise creating audible sound is likely also to be creating Ultrasound. The same can occur in contact measurements – steam flow near a bearing on a drying cylinder is a classic example. It is important to be aware of these possibly unwanted noises and to know how to reduce their influence on your measurement.

Ball Pass Frequency

The frequency corresponding to the rate at which the rolling elements in a bearing pass a particular location on one or other of the races. The inner race and outer race ball pass frequencies are different and are dependent on the geometry of the bearing and the rotation speed of the bearing. They are generally not harmonics of the turning speed, and are difficult to predict exactly due to variations in bearing geometry, contact angle, and load. The two frequencies are abbreviated BPFI for inner race and BPFO for outer race.

Ball Spin Frequency, BSF

The frequency at which the balls or rollers revolve about their own centerline in a bearing. This frequency is dependent on bearing geometry and the running speed of the bearing, and is seldom a harmonic of turning speed. It is difficult to predict accurately because of variations in bearing geometry, contact angle, and load.

Band-Pass Filter

A filter that only passes energy between two frequencies which are called the lower and upper cut-off frequencies. Band-pass filters can be fixed, where the cut-off frequencies are constant, and can be variable, where the cut-off frequencies are varied with time.

The key use of the band-pass filter in Ultrasound is in the heterodyning process.


Two variants of barrier behaviour in Ultrasound – airborne and structure-borne. In airborne, a rag, gloved hand or any other object of more than 2.5mm (0.1”) thickness can be positioned between a sound source and a sensor to block that sound.

In contact Ultrasound, every change in material along the path of signal from source to sensor is a barrier. The greater the different in Specific Acoustic Impedance between the materials, the greater the effect of the barrier to attenuate the transmission of that signal.

Blade Pass Frequency, BPF

In the case of a fan or turbine, the rate at which the blades pass by a fixed position is called the blade pass frequency. It is equal to the number of blades multiplied by the rpm of the rotor. Blade pass can also produce turbulent air flow which will be detected by Ultrasound and this BPF may produce pressure pulses.


See Barriers

Ball Passing Frequency Inner race, BPFI

For rolling element bearings, the Ball Pass Frequency Inner Race (BPFI), is the rate at which a defect in the inner race encounters a ball. BPFI usually is lower in amplitude than BPFO because the source is further from the sensor – the signal must pass through the rolling elements and the outer race before being detected. The BPI will be about 0.6 x RPM x No. of rollers.

Ball Passing Frequency Outer race, BPFO

For rolling element bearings, the Ball Pass Frequency Outer Race (BPFO), is the rate at which a ball passes over a fault in the outer bearing race. It is very commonly found in bearing signatures. The BPFO will be about 0.4 x RPM x No. of rollers.


The indentation of a race in a ball bearing due to a large static force or continuous vibratory force applied to the bearing when stationary. A brinnelled bearing will show large amounts of ball pass frequencies in its Ultrasound and Vibration data. This damage will lead to premature failure.

This same problem can occur in hydraulic cylinders which are vibrated in a set position.

Damage can also be caused by storing parts or machines on shelves or floors which are prone to vibrate.


For very small rates of leakage of, for example, compressed air. Creating bubbles can be helpful to the Ultrasound inspector because the popping of those bubbles is easily heard with Ultrasound. The use of bubbles is therefore extremely useful when dealing with very small leaks.


The verification of a measurement against a known source is called calibration. It is important that measurements are taken with instruments of known calibration and that the calibration is traceable back to a physical quantity – a vibration, a temperature, a pressure…

Annual calibration intervals are commonly recommended, but if the measurement devices can be mistreated, a more frequent calibration should be considered.

Annual calibration does not eliminate the need for a functional test on equipment at the start and end of each measurement day.


Cavitation is a phenomenon that can occur in pumps and in valves and is a common source of damage. It is characterised by the formation and subsequent implosion of fluid vapor bubbles. This implosion produces pulses which are randomly spread in time and produce strong Ultrasound signals.


When the incoming signal to an amplifier saturates the input, the tops of the peaks of the signal are cut off – they are clipped. Clipping results in a distortion of the signal – producing both amplitude and frequency distortion.

Clipping causes the real Peak value to be reduced, the RMS value to increase and therefore the Crest Factor to be reduced.

If you have a clipped signal, it is a bad reading. It should be thrown away and re-taken.


Condensed steam, essentially therefore water, in a steam system. This will naturally be denser, cooler and slower moving than the steam flowing in the pipes. The conflict between the steam and the condensate frequently results in Water Hammer which can cause high levels of Ultrasound and vibration  resulting in damage to the steam system.

Contact sensor

An Ultrasound sensor used for measuring structure-borne Ultrasound. This type of sensor may be hand-held with a needle to make contact, magnetically mounted or permanently installed via a glued or welded stud or drilled and tapped.

Contact leak detection

Leaks can occur in valves , steam traps or filters. These are internal leaks and their presence is detected using a contact sensor.


Corona or surface Partial Discharge is a phenomenon in electrical systems resulting in the ionisation of the air. This will exhibit a constant burbling or popping sound at line frequency or 2x line frequency.

Crest Factor

The Crest Factor of a waveform is the ratio of the peak value of the waveform to the RMS value of the wave form. It is in other words the “peak-to-RMS-ratio”. The Crest Factor of a single-frequency sine wave is 1.414; i.e. the peak value is 1.414 times the RMS value.

The Crest Factor is one of the important measures of machine condition. Random noise will exhibit a Crest Factor of roughly 3, a bearing with poor friction might see this increase to 4-4.5 but a bearing with intermittent clicks and pops of early stage failure might show a reading for 4.5 and up.


One complete period of a periodic waveform is called a cycle. The units for Frequency used to be “cycles per second” until the ISO standardized on the term “Hertz”, in honor of Heinrich Hertz, the noted German scientist who was an early investigator of radio wave transmission.

When analysing time signals it is useful to be able to break it down to understand what might be happening during each repeating cycle – for example each revolution of a shaft.


A method of amplitude measurement common in Acoustics and Electronics sometimes used in other fields such as Vibration. In the case of Ultrasound, it is a measure of Voltage rather than a physical quantity. The dBµV scale is written as dBµV = 20log10(V/V0) where V0 is the reference voltage level of 1µV.

Decibels work in a different way to normal numbers because they are logarithmic. Perhaps the commonest mistake made is to multiply or divide them, this is not allowed. There is no such thing as a % of a decibel. Only add them and subtract them.

A difference of 6dBµV between two numbers means that the higher number has twice the amplitude of the lower number.

Decibels can always be converted back into a voltage by using µV=10(dBµV 20)


Sound has an amplitude and a direction. Lower frequency sound tends to be omni-directional – travelling in all directions with equal amplitude. As the frequency increases towards Ultrasound, the sound becomes more and more directional. We use this fact to our advantage when trying to locate the course of an Ultrasound signal.


Distortion is the presence of signals in a measured signal generated by the measurement system and not present in the original signal.

Dynamic Range

The dynamic range of an instrumentation device such as an amplifier or an analyzer is the ratio between the smallest signal it will sense without noise contamination to the largest signal it will accept without an overload occurring. Dynamic range is usually expressed in decibels, and most instrumentation used for Ultrasound measurement would have a dynamic range of 80 to 100 dB.

Engineering Units, EU

The units in which a measurement is made; for instance, velocity may be expressed in millimetres per second or inches per second.

It is very important when working with a decibel scale that the reference value in engineering units is known: dBµV or dBV are very different. 0dBV is 120dBµV because 1V = 1,000,000µV.

Fast Fourier Transform (FFT)

The FFT is an algorithm, or digital calculation routine used in the FFT analyzer, which calculates a spectrum from a time waveform. In other words it converts, or “transforms” a signal from the time domain into the frequency domain. The mathematics of Fourier stipulates the need for the signal to be continuous. Early stage bearing detects are frequently not continuous and tend to produce good information in the time signal but poor or misleading information in the FFT.

See also spectrum


Flashing occurs on flow systems like valves and steam traps and is generally associated with a restriction in flow causing the pressure in the liquid to drop below the vapour pressure and causing the liquid to vaporise resulting in vapor bubbles. When this vapor flow impinge on piping or valve components, the bubbles burst causing erosion of the surfaces. Flashing produces impulsive signals in Ultrasound, like cavitation, but in this case can be detected some way downstream from the initiator of the flashing.

Flattop Window

The flattop window is a Window function used in some FFT processing in addition to the more common Hanning or Rectangular windows. The flattop window does not allow as fine a frequency resolution as the Hanning window, but it will accurately measure the level of a signal at any frequency, even if the frequency is between the lines of the FFT analysis. It is used in transducer calibration systems to increase amplitude accuracy.

Fluid-Film Bearing

A fluid film bearing is a sleeve bearing which supports the shaft, or journal, on a thin film of oil. The oil film layer may be generated by the rotation of the journal itself (hydro-dynamic bearing), or it may be generated by externally applied pressure (hydro-static bearing).


The supporting structure for a machine is generally called the foundation, and it is vitally important to the proper operation of the machine. Loose, flexible, or cracked foundations are the cause of many machine problems, especially misalignment.

Fourier, Jean Baptiste

A French mathematician and physicist, one time president of Egypt, who devised the Fourier series and Fourier Analysis for the conversion of time functions into frequency functions and vice versa.

Fourier Transform

The mathematically rigorous operation that transforms from the time domain to the frequency domain and vice versa.  See Fourier Analysis and Fast Fourier Transform.

Fourier Analysis

Fourier analysis is the overall process of modelling a function as a trigonometric series which was first published in 1807. The Transform processes the continuous signal in the time domain into the frequency domain.


Frequency is the reciprocal of time. If an event is periodic in time, i.e. if it repeats at a fixed time interval, then its frequency is one divided by the time interval. If a vibrating element takes one tenth of a second to complete one cycle and return to its starting point, then its frequency is defined to be 10 cycles per second, or 10 Hertz (Hz).

Frequency Domain

Motion exists in time, and it is said to be in the “time domain”. When that time signal is passed through spectrum analysis, the result is that data now represented in the Frequency Domain.

See spectrum.

Fundamental Train Frequency, FTF

The rotation rate of the “cage” supporting the rollers in a rolling element bearing. The FTF is always less than half the speed of the shaft. If there is a high amplitude of the FTF, it does not mean the bearing has a bad cage, but rather means that one of the rollers is cracked or otherwise deformed.


In pure physics terms this is defined as “resistance to motion”. The physics of friction as explored by Stribeck considers that friction is proportional to speed and load and viscosity at the same time. For a given combination of those three variables there will be a sweet spot which results in the lowest level of friction. Insufficient lubrication will increase friction. Too much lubrication also increases friction. It must the objective of the lubrication person to achieve this minimum friction state.

The frictional rubbing of molecules during flow in a pipe is also important in Ultrasound because it is this friction which allows the inspector to identify flow in the pipe.

See also Stribeck

Fundamental Frequency

The spectrum of a periodic signal will consist of a fundamental component at the reciprocal of the period and a series of harmonics of this frequency.


G is the acceleration due to gravity at the surface of the earth, and it is used as a unit of acceleration. G is not a constant, it varies over the earth’s surface, so an average value of  9.81m/s2 (32.2 feet/s2) is used..

Gear-Mesh Frequency

The gear mesh frequency, also called “tooth mesh frequency”, is the rate at which gear teeth mate together in a gearbox. It is equal to the number of teeth on the gear multiplied by the rpm of the gear. A gearbox will always have a strong dynamic components at the gear mesh frequencies.


Harmonics are frequencies which are integer multiples of another frequency.


The unit of frequency in the SI measurement system is the Hertz, abbreviated Hz. One hertz is equal to one cycle per second. The name is in honor of Heinrich Hertz, an early German investigator of radio wave transmission.


At the heart of an Ultrasound system is the process which converts an inaudible Ultrasound signal into an audible one. This is the heterodyning circuit. A centre or mixer frequency is defined, for example 38.4kHz, on either side of which is a band-pass filter which is typically 2.5 or 3kHz wide. The heterodyning process subtracts the mixer frequency from the output of the band-pass filter. For example, a signal at 40.4kHz heterodyned with a miser frequency of 38.4kHz would produce an output of 2kHz. The inaudible thereby becomes audible.

High-Pass Filter

A filter that passes signals above a specific frequency called the “cut-off” frequency. High pass filters are used in instrumentation to eliminate low-frequency noise.


Defined in IEEE141 as voltages in the 100k-230kV range. Typically used at 115, 138 and 230kV


A condition of a rotating part where the center of mass does not lie on the center of rotation. Imbalance of a rotor causes a centripetal force at the frequency of the rotation rate to be applied to the bearings. If it is large, it can severely shorten the life of the bearings, besides causing undue vibration of the machine. Forces caused by imbalance are proportional to the square of the RPM, and this means that high-speed machines must be balanced to a higher standard than low-speed machines.

The high vibration associated with an imbalance condition will alter the load on the lubrication resulting in a periodic change in the friction state of the bearing. This results in a cycling increase and decrease in the Ultrasound reading with each revolution of the machine shaft.

Impacts or Impulses

Short duration impulsive signals containing lots of high frequency energy which is detectable by Ultrasound and can be captured in the time signal. This type of signal could come from a bearing, or an electrical signal or from a source of cavitation or flashing.


See Specific Acoustic Impedance


Sound in the frequency range below 20Hz is generally inaudible to humans.

Inverse Distance Law

When dealing with airborne Ultrasound, the relationship with distance from the source is an inverse distance law. When doubling the distance, the sound pressure will halve and when halving the distance, the sound pressure will double. In terms of out Ultrasound measurements, this means that doubling the distance will reduce the amplitude by 6dBµV and halving the distance will increase the amplitude by 6dBµV.


Kurtosis is a statistical measure of the amplitude distribution of a signal, and is very good at identifying significant outliers, large amplitudes if you will, in a data sample. In that respect it

Similar in some ways to Crest Factor but is a much more complicated statistical analysis of the data than just Peak-to-RMS ratio.

Laminar flow

Laminar flow in a fluid means that the fluid travels smoothly and in a regular path. Laminar can be considered the opposite of turbulent flow. Laminar flow does not generate any Ultrasound.


When a fluid, gas or liquid, passes from a high pressure to a low pressure through a hole that is not supposed to exist, usually accompanied by an irreversible loss of material and / or energy.

Longitudinal wave

A mechanical wave in which the particle motion is in the direction of travel of the wave. Sound is a mechanical wave.

See also Transverse Wave.


A human subjective response to sound. How loud a sound is perceived is based upon many factors such as how pleasant you perceive the sound to be, the impulsive and the predictable natures of the sound. Loudness should not be confused with the objective term amplitude. A sound of the same amplitude may be perceived to have different levels of loudness by different people.


Defined in IEEE141 as voltages up to 1,000V. Typically used at 380V, 415V and 480V


The introduction of any of various substances between sliding surfaces to reduce wear and friction. Prior to the petroleum industry, animal fats were the common lubricants. The friction caused by under- and over-lubrication is detectable using Ultrasound (mainly contact). There are various different states of lubrication.

See also Stribeck.


Magnetostriction is a property of magnetic materials that causes them to change shape in the presence of magnetic fields. This causes a vibration at the frequency of the field fluctuations, and this is part of the cause of 1x and 2x line frequency vibration found in electrical machines such as motors and transformers.


Defined in IEEE141 as voltages in the 1k-100kV range. Typically used at 4.16, 6.9, 12, 13.8, 34.5 and 69kV


Microelectromechanical system (MEMS), mechanical parts and electronics combined to form miniature devices, typically on a semiconductor chip. MEMS sensor use has exploded since their early applications in the 1990s. MEMS sensors can be used both as vibration and acoustic sensors – they are seen in on-line monitoring systems and acoustic cameras.

Mixer frequency

The centre frequency in a heterodyning circuit.


Strictly speaking, noise is any unwanted signal, but the term generally is used to indicate a random signal. Thinking in terms of an acoustic measurement, there could be electronic noise from the sensor, cable and measuring instrument as well as unwanted acoustic noise associated with the application. Good measurement practice requires an awareness of noise sources and how to control or minimise them.

Noise Floor

The noise floor is the residual noise level of an instrumentation system when nothing is being measured. The smallest signal must be above this noise floor if it is to be measured.

Nyquist frequency

In the process of analog to digital conversion, the input signal must first be sampled.  If the signal contains any information at frequencies above one-half the sampling frequency, the signal will not be sampled correctly, and the sampled version of the signal will contain spurious components due to the phenomenon of aliasing.  The maximum frequency that can be correctly sampled is called the Nyquist frequency in honor of Harry Nyquist.


In practice, because filters do not have a infinitely sharp response, the minimum value used is typically 2.56x the highest frequency,


An octave is a doubling in frequency. Middle C on a piano is 256Hz. An octave above is 512Hz and an octave below is 128Hz.

Octaves and 1/3 Octaves

Octave and 1/3rd octave bands are a commonly used method of expressing frequency in acoustics.


In rotating machines, orders are multiples or harmonics of the turning speed.  In comparing spectra of rotating machines, it is convenient to express the frequency axis of the spectra in orders, especially if the machine speed varies between measurements.

Overall Level

A imprecise way of referring about RMS.


When digitally sampling an analog signal we have the consideration of Nyquist. This sets the minimum requirement of the sampling frequency, There are benefits to working with much higher sampling frequencies if the data processing power and memory capacity of the measuring instrument can cope. In audio we see internet radio channels and streaming services using 320k sampling. A 20kHz signal would normally require 20×2.56=51.2kHz sampling, so at 320k we have 6x the number of samples required by Nyquist. The music is 6x oversampled.

The beauty of oversampling is to provide more detailed information on the behaviour of rapid transient events. A 4kHz heterodyned Ultrasound signal sampled at 256kHz would be 25x oversampled.

Parabolic dish

An airborne Ultrasound sensor used to detect sources at a longer separation distance. Parabolic dishes are designed to focus plane waves to the focal point of the dish. It is therefore important that a dish is not used too close to the source otherwise the incident sound waves will not be plane and so will not be detected by the sensor at the focal point. Typically a parabolic dish will offer around a 26x increase in sensitivity over a standard sensor, This then affords the ability to be 26x further away to listen to the same source with the dish than with the normal sensor.


A valve or steam trap failure mode. This is another word for leaking. Essentially there should be no flow, but there is. Passing in a valve can also be a cause of cavitation.


The peak value of a signal is the maximum excursion in one direction from the zero point. The actual value can be changed significantly by increasing or decreasing the sampling frequency.

A peak is also used as a term to describe a sharp point in a spectrum.

Peak-to-Peak (Pk-Pk) Value

In measuring the amplitude of a signal, the peak-to-peak value is the difference between the highest positive peak and the lowest negative peak amplitudes.


A signal which repeats the same pattern over time is called periodic, and the period is the length of time encompassed by one cycle, or repetition.  The period is measured in time, in seconds.

1/Period gives Frequency in Hz. An event happening 10 times per second would have a frequency of 10Hz and a period of 1/10th of a second.


A signal is periodic if it repeats the same pattern over time.


Phase is a relative time difference between two signals. It is usually measured in units of angle rather than units of time, and it makes sense only if the two signals being compared are of the same frequency. One cycle of a periodic signal represents a complete circle, or 360 degrees of phase angle. A phase difference of 180 degrees is thus a difference of one half cycle.

Phase Angle

See Phase


Certain substances, especially some crystals such as quartz and man-made piezoceramics such as lead zirconate titanate (PZT), will develop an electric charge when they are mechanically stressed. This generation of a charge is called piezoelectricity.

Piezo-electric Transducer

Any transducer that uses a piezoelectric substance as an active element. Examples are force transducers, accelerometers, pressure transducers, and phonograph pickup cartridges.

Polar response

An airborne sensor will listen to sound in a particular way, just like a microphone does. The sensitivity changes as a function of the direction of the incoming sound. This is called the polar response. Generally, the maximum response (the highest sensitivity) is in the axis perpendicular to the diaphragm face of the sensor. 30° off axis the sensitivity could be down by 8dB or 60% of that value.

Pressure Wave

Sound is a mechanical pressure wave. Sound will travel as this pressure wave through gases, liquids and solids.


Radial means in a direction toward the center of rotation of a shaft or rotor. Think of radius or spokes on a wheel. Contact Ultrasound on machines and bearings requires a radial measurement.


A random signal is one whose future condition cannot be predicted. It is a term often applied to noise.

Rectangular Window

In spectrum analysis, the rectangular window is actually no window at all.  It is also called a uniform window, and is selected when the signal to be analyzed is a transient rather than a continuous signal. See also Hanning Window.


When a sound wave travelling through a medium reaches a boundary where there is a change in Specific Acoustic Impedance there will be some reflection and some transmission. The angle of reflection is the same as the incident angle “angle in equals angle out”. In airborne Ultrasound tests such as air leak detection, reflections can be a problem which makes it harder to find the source of a sound.


RMS stands for Root Mean Square, and is a measure of the amplitude of a signal. It is calculated by squaring the instantaneous value of the signal, taking the mean the squared values over time, and taking the square root of the mean value. The RMS value is the value which is used to calculate the energy or power in a signal. The RMS value of a single-frequency sine wave is 0.707 (√2/2) times the Peak value. The RMS value of a more complex signal needs to be measured.

If the signal varies with time (higher amplitude then lower amplitude) care needs to be taken to ensure that the RMS measurement has been done over a representative period of time in order not to be too low or too high.

Running Speed

The speed, usually expressed in revolutions per minute (rpm) at which a rotating machine runs. It may also be expressed in Hz.

Sampling Frequency

The frequency used to sample dynamic data. This frequency will be equal to or greater than the Nyquist frequency.

See also oversampling


The sensitivity of a transducer is the voltage output produced for a given, known, physical quantity. For example mV/g for an accelerometer or mV/bar for a pressure transducer.


The electronic output of a measuring channel could be considered a signal. A signal can be dynamic (AC) or static (DC).

Signal Conditioning

Signal conditioning is the manipulation of a signal from a transducer by such instruments as preamplifiers, filters, etc. Analog to digital convertors and heterodyning circuits could also be considered as signal conditioning elements.


A signature is the term used to describe a characteristic or behavior in a dynamic signal.  Signature analysis and signature comparison are tools frequently used in condition monitoring.

Sine Wave

A sine wave or sinusoid, is a representation of a trigonometric function. A graph of y=sin x will produce a sine wave if drawn. A single-frequency sine wave is the basic building block for Fourier analysis.


Slip is the difference between the actual speed of an induction motor and the synchronous speed, which is the speed at which a similarly wound synchronous motor would run. For instance, the slip of a motor turning 1760 RPM would be 1800 – 1760 = 40 RPM. The slip is dependent on the load on the motor, greater loads producing more slip, and hence lower speeds.

Sound barrier

When the velocity of an object exceeds the velocity of sound it is said to have gone through the sound barrier. At this point, there will be a noise such as a crack or a boom. The tip of a whip or the muzzle velocity of a bullet in a gun barrel are also examples of exceeding the velocity of sound.

See also barriers.

Specific Acoustic Impedance

A physical property of a material which is related to the density of the material and the velocity of sound in that material. The behavior of sound as it impinges on a barrier between two materials is directly controlled by Specific Acoustic Impedance.


Spectra is the plural of spectrum.


The spectrum is the result of transforming a signal from the time domain to the frequency domain.  It is the decomposition of a time signal into a collection of sine waves. Spectrum Analysis is therefore another term for Fourier Analysis.

Spectrum Analyzer

A spectrum analyzer converts a signal from the time domain into the frequency domain, and the FFT analyzer is the most common type today.

Standard Deviation, σ

A statistical term used to describe the spread of the distribution of the sampled data population. Mean value +1 standard deviation (1σ) can be expected to be exceeded by 16% of the data and mean+2σ by just 3%.

Steam Trap

An automated valve designed to open to purge air, CO2 and condensate from a steam system but close to prevent the loss of live steam to the condensate return system.


The Stribeck curve is a fundamental in tribology and therefore in lubrication, named after Richard Stribeck and first presented in 1901. The Stribeck curve identifies three lubrication states: Boundary, Mixed and Hydrodynamic. The curve shows the relationship between friction, speed, load and viscosity.

Structural Ultrasound

The transmission of Ultrasound through a solid structure.

Tightness testing

A test method using an Ultrasound tone generator. The generator can be placed inside or outside a nominally sealed volume and the inspector scans on the other side (outside or inside resp.) for the sound from the tone generator at a localised and elevated amplitude which thereby indicates an air path through, a leak.

Time Domain, Time Signal

The representation of a variable as a function of time. A time signal would show the variation in amplitude of an Ultrasound signal over time.

Time signals are the core element of dynamic signal acquisition and when captured correctly will contain all of the behaviour of that signal.


A signal at a specific frequency. Essentially a musical term used in acoustics. There can be a monotone – a singular frequency or there can be a multitone. Tonal quality is also sometimes referred to – though this can be rather subjective.


Surface conduction of electricity over rather than through an object. Often caused by the presence of dirt, dust or moisture providing an alternative path. Tracking produces short bursts of Ultrasound and results in characteristic tree or river delta marks on the surface.


A transducer is any device that converts one type of energy, such as vibration or sound, into a different type of energy, such as a voltage. Transducers can be reversible – a loudspeaker can be used as a microphone and vice versa.


A transient is another way of describing an impulse. Transients are typically going to be short duration events like pops or crackles. They could also be repeating – such as the tick of a watch or clock. These impacts may well be producing Ultrasound and are therefore detectable – even as defects on very slow running bearings.

It can also have a different meaning – a behavior could be transient in nature meaning it comes and goes.


How sound travels through one or several media. Transmission and Reflection are controlled by Specific Acoustic Impedance differs at the barrier between two materials. For good transmission of Ultrasound, you want to minimise the number of material changes and avoid the used of polymers.

Transverse wave

Vibration in a solid is a transverse mechanical wave. In a transverse wave the molecules move perpendicular to the direction of travel.


A trend is a plot of amplitude versus elapsed time. It is used to show the historical variations in a measured parameter.

Tunable Frequency

Refers to the ability to move the mixer frequency of a heterodyning circuit.

Turbulence, turbulent flow

Flow in which the fluid undergoes irregular fluctuations and mixing. Ultrasound detects turbulence. It is this turbulence which is detectable in an air leak or in flow through a pipe or valve.


Sound in the frequency range above 20kHz. Has no practical upper limit.


A device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries). Valves come in in all shapes and sizes and work in many different ways. The common problems associated with valves which makes the interesting for Ultrasound inspectors are defects such as passing or leaking, blocked, cavitation and flashing.

Vane Pass Frequency

In a centrifugal pump or a fan, the vane pass frequency is the number of fan blades or impeller vanes times the turning speed of the rotor. The vane pass always shows up as a relatively strong component in the vibration spectrum of a pump or fan. Thus, a pump with 7 vanes will have a spectral component at 7 times the RPM, or “7 X”.

There can also be associated pressure pulsations associated with this frequency which will result in Ultrasound generated as a series of pulses.

Velocity of Sound

The speed with which sound travels through a medium. The more dense the material, the more efficient the transfer of the energy between the vibrating molecules will be and so the higher will be the speed of sound. The velocity of sound is therefore higher in metals than in liquids which in turn is higher than in gases. Because of the relationship to density, the velocity of sound can change with temperature.

The nominal velocity of sound in air at 21℃ at sea level is 344m/s (1238km/h or 770mph). At 0℃ this would drop to roughly 331m/s.


Vibration is the oscillation of a point, an object, or a part of an object around a fixed reference, or rest, position. An object can vibrate as a unit, in which case it is called “whole body vibration”, or, as is usually the case, an object can vibrate in a complex way where it deforms and different parts of it vibrate at different frequencies and amplitudes.

Vortices, singular Vortex

Vortices are eddies, or “whirlpools” sometimes formed at the ends of airplane wings, fan blades, propeller blades, pump vanes, and the edges of other structures in a fluid flow such as power lines. Vortices absorb energy, and they cause turbulence.

Water Hammer

Excessive vibration in steam distribution pipework caused by the presence of low velocity, denser, water with high velocity, lighter, steam fighting to occupy the same pipe. This is the result of one or more defective steam  traps and needs attention in order to prevent pipe rupture.


A wave is a disturbance propagated in a medium, and it results in local motion of the medium. Waves transmit energy in the medium, and travel at characteristic speeds depending on the medium and the type of wave.

Sound waves are longitudinal waves, vibration travels as transverse waves.


The graphical representation of a time signal or a spectrum.

Wavelength, λ

Measured in units of length, the wavelength of a sound wave can be calculated by λ=v/f that is wavelength is velocity divided by frequency.

Wavelength is very important when considering how to block a sound wave. There is a minimum requirement for a barrier to be at least a quarter wavelength – roughly 2.5mm (0.1”) for Ultrasound.

White Noise

White noise is defined as a random signal with equal energy in all frequencies. The noise heard in an FM radio when tuned off station is approximately white noise. This is also the typical noise that might be associated with a healthy bearing or an air leak.