Understanding Ultrasound Analysis

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Portable balancer & Vibration analyzer Balanset-1A

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Dynamic balancer “Balanset-1A” OEM

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Ultrasound Analysis — also called airborne and structure-borne ultrasound — is a condition monitoring technology that listens for high-frequency sounds well above the range of human hearing. People can typically hear up to about 20 kilohertz (kHz); ultrasound instruments are designed to detect sounds in the 20 kHz to 100 kHz band. These high-frequency emissions are generated by friction, turbulence, and electrical arcing — three things that almost always accompany a developing fault. The instrument detects the ultrasound, translates it into an audible signal heard through headphones, and measures its intensity (amplitude), which is displayed as a decibel (dB) level. In effect, it lets an inspector “hear” problems that are otherwise completely silent, making it a powerful complement to vibration analysis and thermography in a modern predictive maintenance programme.

1. Definition: What is Ultrasound Analysis?

At its core, ultrasound analysis is about capturing acoustic energy that the human ear cannot register. The physics matters here: ultrasonic waves are short-wavelength and highly directional, and they attenuate quickly with distance and through solid barriers. This is precisely what makes the technique so useful for inspection — because the sound dies away rapidly, the loudest reading reliably points back to the source, letting an inspector pinpoint a leak or a faulty contact with confidence.

Ultrasound is generated wherever there is friction (a dry or damaged bearing), turbulence (gas escaping through a small orifice), or electrical discharge (arcing, tracking, and corona). The instrument detects this emission with either an airborne sensor (an ultrasonic microphone) or a contact sensor (a waveguide pressed against a surface to capture structure-borne sound). The captured signal is then conditioned and presented to the inspector as both an audible tone and a numerical dB level, so the diagnosis combines a trained ear with an objective, trendable measurement.

2. How It Works: Heterodyning

The core technology inside an ultrasound instrument is called heterodyning. This is an electronic process that accurately converts the very high-frequency, inaudible ultrasonic signal into a lower-frequency signal within the audible range, without changing the sound’s original character. The “hissing” of a compressed-air leak still sounds like a hiss in the headphones, and the “crackling” of an electrical arc still sounds like a crackle. That faithful translation is what makes the diagnosis so intuitive: an inspector learns to recognise the signature of each fault by ear.

Heterodyning works by mixing the incoming ultrasonic signal with a stable reference frequency generated inside the instrument. The mixing produces a difference frequency that falls within the audible band. Because the original amplitude relationships are preserved, the decibel reading on the meter remains a meaningful, repeatable quantity that can be logged and trended over time — turning a subjective “it sounds worse” into a documented increase in dB that supports a maintenance decision.

3. Key Applications in Maintenance

Ultrasound analysis is a versatile technology with several high-value applications:

a) Leak Detection

This is the most common and financially beneficial application. The turbulent flow of a gas — compressed air, steam, nitrogen, or any pressurised medium — escaping from a pipe, valve, or vessel creates a large amount of broadband ultrasound.

• Procedure: An inspector uses a handheld ultrasound device with an airborne sensor to scan an area. The instrument is highly directional, so as it is brought closer to a leak the audible signal in the headphones grows louder and the dB reading on the meter rises, guiding the inspector straight to the source.
• Benefits: Finding and fixing compressed-air leaks can save a plant tens or even hundreds of thousands of dollars per year in wasted energy. Compressed air is one of the most expensive utilities in a factory, and a single, audible leak left unaddressed runs up cost every hour the compressor is loaded to make up for it.

b) Electrical Inspection

Electrical faults such as arcing, tracking, and corona in medium- and high-voltage equipment all produce ultrasound, often before they produce enough heat to be seen by an infrared camera.

• Procedure: An inspector can safely scan enclosed electrical cabinets from the outside. The ultrasound generated by a fault escapes through air gaps in the cabinet seals, so the panel never has to be opened to find a problem.
• Benefits: This is an excellent, non-contact way to detect serious electrical faults before they lead to an arc-flash event, directly enhancing plant safety. It is also an ideal screening step to perform before opening a panel for thermography, helping decide whether the panel is even safe to open. Both methods sit alongside other non-intrusive techniques such as non-destructive testing.

c) Mechanical Inspection (Condition-Based Lubrication)

Ultrasound is also highly effective for assessing the condition of rolling-element bearings and for guiding lubrication practice — a discipline often called acoustic or condition-based lubrication.

• Procedure: A contact ultrasound sensor is placed on the bearing housing, capturing the structure-borne sound the bearing radiates as it turns.
• Interpretation:
  • A healthy, well-lubricated bearing produces a low, steady “hissing” sound.
  • A bearing that needs grease shows a higher dB reading. A technician applies grease slowly, stopping the moment the dB level begins to drop — preventing the over-lubrication that itself causes bearing wear and seal damage.
  • A bearing with a developing defect such as a spall produces a repetitive “crackling” or “popping” sound as the rolling elements strike the flaw, giving very early warning of bearing failure.

4. Ultrasound vs. Vibration Analysis

For bearing analysis, ultrasound and vibration analysis are complementary rather than competing. Ultrasound is often better at catching very early-stage failures (Stage 1) and lubrication issues, because the first sign of distress is a faint high-frequency emission long before the defect is large enough to move the bearing measurably. Vibration analysis is better at diagnosing the exact nature of a later-stage fault — for example, distinguishing a ball pass frequency outer race defect from a ball pass frequency inner race defect — once the flaw is visible in the vibration spectrum and identifiable through bearing fault frequencies. Many vibration analysts use envelope analysis to extract those same early bearing impacts from the vibration signal, narrowing the gap between the two techniques.

5. Where Ultrasound Fits in a Field Programme

Ultrasound, infrared, oil analysis, and vibration each see a different slice of machine health, and the strongest reliability programmes layer them together. Ultrasound flags a leak, a sparking contact, or a starved bearing in seconds; vibration then quantifies the mechanical condition and tells you why. When a route screen reveals a rising bearing tone or elevated 1× unbalance, the natural next step is to put a true two-channel instrument on the machine. A portable analyser and balancer such as the Balanset-1A measures the 1× amplitude and phase in the machine’s own bearings at operating speed, so once ultrasound has pointed to a rotating-machinery problem you can diagnose an imbalance and correct it on-site — closing the loop between detection and repair without sending the rotor to a shop.

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