Ultrasonic

What Is Ultrasonic?

Ultrasonic refers to any sound or vibration whose frequency exceeds 20,000 Hz (20 kHz), the approximate upper threshold of human hearing. While the term is used broadly in medicine, food processing, and manufacturing, in the context of asset reliability it specifically describes the detection and analysis of high-frequency acoustic signals that reveal mechanical, fluid, or electrical faults inside equipment.

Industrial ultrasonic instruments work by using piezoelectric transducers or resonant sensors to pick up these signals, then converting them into a frequency range that can be heard through headphones or displayed as a waveform. The technology is valued because many fault signatures, such as the early friction of an under-lubricated bearing or the turbulence of a pinhole gas leak, appear in the ultrasonic range long before they manifest as changes in vibration, temperature, or performance.

How Ultrasonic Technology Works

Every mechanical event that involves friction, impact, turbulence, or electrical discharge generates broadband acoustic energy. A portion of that energy falls in the ultrasonic range. Ultrasonic instruments are tuned to this range specifically to filter out the low-frequency background noise typical of industrial plants, making weak fault signals easier to isolate.

Two principal scanning modes are used:

• Airborne (non-contact) scanning: A directional sensor picks up ultrasonic waves propagating through the air. This mode is used for locating gas and compressed air leaks, detecting electrical discharge (arcing, corona, and tracking), and scanning sealed components without physical contact.
• Structure-borne (contact) scanning: A probe is placed directly on equipment surfaces. Sound waves travel through the material to the sensor. This mode is used for bearing condition assessment, lubrication monitoring, valve leak-through detection, and ultrasonic thickness measurement.

The raw signal is heterodyned (frequency-shifted) down to an audible range, allowing trained technicians to recognize characteristic patterns by ear. Many modern instruments also display waveforms, dBuV amplitude levels, and spectral data that can be logged and compared over time.

Key Applications in Industrial Maintenance

Compressed Air and Gas Leak Detection

Leaks in pressurized systems generate turbulent flow that produces a strong ultrasonic signature. Technicians scan pipework, fittings, and valves with an airborne detector to pinpoint the leak source, even in loud environments. Studies from industry sources consistently show that unmanaged compressed air leaks account for 20 to 30 percent of compressor output in typical plants, so systematic leak surveys deliver measurable energy savings.

Bearing Condition Monitoring

Rolling element bearings produce characteristic ultrasonic emissions as rolling surfaces contact the raceway. An under-lubricated bearing generates friction sounds that increase in amplitude and change in character before any measurable temperature rise or low-frequency vibration increase occurs. Technicians use contact probes on bearing housings to assess lubrication adequacy and detect early fatigue. This complements vibration analysis, which captures the same bearing faults at a later, lower-frequency stage.

Electrical System Inspection

Arcing, corona discharge, and electrical tracking on switchgear, bus bars, and insulators all produce ultrasonic emissions. Airborne scanning of electrical panels and substations can identify failing components before they cause outages or fires. This application is particularly valued because it can be performed safely at a distance, without de-energizing the equipment.

Valve and Steam Trap Inspection

Fluid turbulence caused by a leaking valve seat or a failed steam trap produces a distinct ultrasonic signal. Contact probes placed upstream and downstream of a valve or trap allow technicians to detect internal bypass flow that would otherwise require a thermal camera or invasive inspection to find.

Non-Destructive Testing

In manufacturing and structural inspection, high-frequency ultrasonic pulses are sent into materials and the reflected signals are analyzed to detect internal voids, cracks, inclusions, and thickness variations. This is the domain of formal non-destructive testing and is governed by international standards such as ASTM E2375 and ISO 16810. Phased array and time-of-flight diffraction (TOFD) techniques extend the method to complex geometries.

Ultrasonic vs. Other Condition Monitoring Technologies

Ultrasonic in a Predictive Maintenance Program

Ultrasonic inspection fits naturally into a predictive maintenance strategy. Because it detects faults earlier than many complementary technologies, it increases the lead time available for planning corrective work, reducing both unplanned downtime and the cost of emergency repairs.

Effective programs typically combine ultrasonic with vibration monitoring and thermography so that each technology covers the gaps of the others. A bearing fault, for example, may first appear as an ultrasonic amplitude increase, then develop into a detectable vibration signature weeks later. Trending both measurements over time gives maintenance teams greater confidence in remaining useful life estimates.

For continuous monitoring of critical assets, permanently mounted ultrasonic sensors can feed data to a software platform alongside vibration and temperature channels, supporting automated alerting and failure mode classification.

Ultrasonic Testing Standards and Certification

Formal ultrasonic testing for structural inspection is governed by internationally recognized standards. Key references include:

• ASNT SNT-TC-1A: The US standard for personnel qualification in non-destructive testing, covering UT Level I, II, and III certification.
• ISO 9712: The international equivalent for NDT personnel certification.
• ASTM E114 and E2375: Standards for pulse-echo immersion and contact UT techniques.
• ISO 16810: General principles for ultrasonic testing.

Maintenance technicians using handheld ultrasonic instruments for leak detection and bearing assessment typically follow manufacturer training programs and internal reliability standards rather than formal NDT certification, unless their role involves structural integrity assessment.

Practical Tips for Ultrasonic Inspection

• Establish baselines: Record initial dBuV readings on healthy bearings and equipment so that deviations are meaningful rather than arbitrary.
• Use the correct probe type: Airborne sensors work for leaks and electrical faults; contact probes are needed for bearings and valves. Using the wrong probe type produces unreliable results.
• Work at consistent distances: For airborne scanning, maintain a consistent distance from the target to ensure amplitude readings are comparable across inspections.
• Combine with lubrication routes: Ultrasonic bearing monitoring is particularly effective when paired with precision lubrication. Technicians listen while lubricating to stop at the correct amount, preventing both under- and over-lubrication.
• Document and trend: A single reading is less valuable than a trend. Log data with timestamps, asset IDs, and operating conditions so that changes can be tracked as part of a broader asset condition monitoring program.

Frequently Asked Questions

The Bottom Line

Ultrasonic technology gives maintenance teams access to fault information that is invisible to human senses and often undetectable by lower-frequency methods. Whether the goal is finding a pinhole leak in a compressed air header, catching an under-lubricated bearing weeks before it fails, or confirming the integrity of a weld, ultrasonic instruments provide early, actionable evidence that supports data-driven maintenance decisions.

Integrating ultrasonic inspection into a structured condition monitoring program, alongside vibration analysis and thermography, creates a layered approach to asset health that extends equipment life, reduces unplanned downtime, and improves the return on maintenance investment.