Ultrasound is one technology, but the way you collect the signal—contact vs airborne—completely changes what problems you can see and how you use the data.
One Technology, Two Very Different Methods
Most plant teams talk about “ultrasound” like it’s a single tool. In reality you’ve got two:
- Structure-borne (contact) ultrasound – You couple a probe to the asset with a magnet, stinger, or clamp and listen to friction and impacts traveling through the machine structure.
- Airborne (non-contact) ultrasound – You scan the air with a horn, cone, or parabolic mic and listen for high-frequency sound from turbulence or electrical discharge.
Both typically work in the ~20–40 kHz band and heterodyne that ultrasonic energy down into audible sound plus a numeric level (dB). The trick is matching the method to the job:
- What failure mode are you chasing?
- Can you safely touch the asset?
- Do you need a trendable number or just a find-and-fix screen today?
What Each Method Is Best At
Structure-Borne (Contact) – Strengths
- Trendable, repeatable readings on rotating components – Fixed contact point and consistent pressure give you stable baselines for bearings, gears, and pumps.
- Earliest friction detection in rolling element bearings – Excellent for condition-based lubrication and early fault detection, including slow-speed bearings.
- Steam traps and valves – Body contact lets you “hear” disc cycling, blow-through, and seat leakage.
- Machine dynamics clues – Rubs, light looseness, and gear tooth distress when vibration access is poor or not yet set up.
- High signal-to-noise ratio in loud plants – Structure coupling rejects airborne clutter so you can still trend in noisy ultrasonic environments.
Airborne (Non-Contact) – Strengths
- Fast leak hunting – compressed air, gas, and vacuum leaks located quickly from a distance. Ideal for ROI-focused leak projects.
- Electrical arcing, corona, and tracking – scan energized equipment non-invasively through vents and seams.
- Screening where contact is unsafe or impossible – overhead, at height, behind covers, or inside energized gear.
- Finding turbulent sources – relief valves venting, pneumatic exhausts, bypass valves, and other “hissing” events.
Application Quick Reference
Both methods work on a lot of the same assets, but one tends to be the better primary choice. Use this as a route-planning cheat sheet.
| Asset / Problem | Acceptable Methods | “Better” Method & Why |
|---|---|---|
| Rolling element bearings (general trending) | Contact; Airborne for screening | Contact – Repeatable baselines and better severity calls. |
| Precision lubrication of bearings | Contact | Contact – Add grease in small shots while watching dB fall; stop at the minimum. |
| Slow-speed bearings (<60–120 rpm) | Contact (long dwell); Airborne for screen | Contact – Friction and impacts transmit well through the structure; airborne can miss subtle changes. |
| Gearboxes (mesh / bearing distress) | Contact on housing | Contact – Stable trend. Combine with vibration if available. |
| Steam traps (condition) | Contact on inlet/outlet/body; Airborne for quick pass | Contact – Confirms cycling versus blow-through; airborne is a fast pre-screen. |
| Valves (seat leakage / bypass) | Contact on body/bonnet | Contact – Seat leak noise couples into the body reliably for repeatable checks. |
| Pumps (cavitation, bearing) | Contact on housing; Airborne near suction | Contact for cavitation and bearings; airborne for suction leaks pulling air. |
| Compressed air / gas / vacuum leaks | Airborne | Airborne – Detects turbulent jets and pinholes quickly; no practical contact path. |
| Electrical (arcing, corona, tracking) | Airborne | Airborne – Listen through grills and gaps; no contact with energized gear. |
| Pneumatics (solenoids, quick exhausts) | Airborne; Contact on body | Airborne to find the noisy device; contact if you need to prove internal chatter. |
Choosing by Objective
Think less about the instrument menu and more about what you are trying to accomplish.
- Need a number you can trend over months? Use contact.
Fix the point, pressure, filter band, and dwell time (typically 5–10 seconds). Build a baseline at normal load and set alarms for that specific point (for example, +8 dB alert, +12 dB action tuned to your site).
- Need to rapidly locate losses or discharges today? Use airborne.
Sweep lines, flanges, cabinets, and enclosures. Confirm, tag, and push work orders into the CMMS with estimated loss or risk level.
- Access is hazardous or energized? Favor airborne for stand-off distance and zero contact.
- Noisy ultrasonic environment (steam, leaks everywhere)? Favor contact if you need trendable data and a clean signal.
How the Physics Drive the Results
Contact / Structure-Borne
Contact ultrasound travels through the metal. It sees friction and impacts conducted through the housing, which is why it reacts early to lubrication problems and small surface defects, often long before vibration RMS or temperature move much.
The tradeoff: technique matters. Change contact pressure, angle, or exact spot and your dB numbers can wander. That is not a flaw in the technology; it is a repeatability problem that you solve with discipline.
Airborne / Non-Contact
Airborne ultrasound listens to turbulence and electrical discharge in the air column. It is ideal for leaks and partial discharge and lets you scan from several feet away.
The tradeoff: reflections, wind, and other ultrasonic sources can contaminate readings. It is great for screening and localization, but generally weaker for long-term trending compared to a rigid contact point.
Field Method That Actually Works
Contact (Structure-Borne) Ultrasound – Setup and Use
Point Selection
- Choose a rigid, repeatable path to the component such as the outer race path for bearings, an inspection pad on gearboxes, or the valve or trap body.
- Mark the spot with a paint dot, label, or tag so every technician finds the same location.
Probe and Pressure
- Use a magnetic base or consistent spring-loaded stinger whenever possible.
- Avoid grease fittings unless they are your only path; otherwise you are listening to the zerk, not the bearing.
Instrument Settings
- Narrow the ultrasonic band, commonly around 30–40 kHz.
- Use fixed gain; do not keep chasing the level.
- Hold each point 5–10 seconds and record speed, load, and temperature conditions in your notes.
Trend Rules
- Compare each point to its own baseline, not to another motor or bearing.
- Trend in dB delta from baseline; do not obsess over absolute numbers.
- Use sound quality as a cross-check:
- Smooth hiss – Normal.
- Rushing or rough hiss – Under-lubed or beginning distress.
- Crackling or distinct impacts – Surface damage or more serious defect.
Lubrication Control
- Add small grease shots, then pause 10–20 seconds for the bearing to stabilize.
- Watch the dB level and stop when you hit the minimum.
- If the dB level rises immediately after adding grease, you are churning; stop and reassess.
Airborne (Non-Contact) Ultrasound – Setup and Use
Scan Setup
- Start with moderate gain so general background sits mid-scale.
- Use a cone or parabolic attachment for better directionality in noisy areas.
Sweep Technique
- Move slowly, typically 6–24 in (150–600 mm) off the surface.
- Use angle changes to triangulate where the sound peaks.
Confirming the Source
- Once you find a peak, reduce gain to isolate it from background noise.
- For leaks, use soap solution or a portable flow meter to confirm and estimate loss.
- For electrical findings, change angle and distance to rule out reflections; capture a sound file and escalate per your electrical safety rules.
Recording and CMMS Actions
- Tag the location and note approximate severity, such as small hiss versus roaring leak, or light sizzle versus heavy crackling discharge.
- Attach photos and sound files where possible.
- Push work orders into your CMMS with priority based on safety, energy loss, or downtime risk.
Where Each Method Under-Performs (and How to Fix It)
Structure-Borne – Watch Out For:
- Bad path to the component such as thin covers, loose guards, or long brackets.
Fix: Use a stinger to reach a stiffer path or install a small magnetic pad bonded directly to the housing as a permanent test point.
- Technique drift, such as different technician, different pressure, or different spot.
Fix: Standardize the probe, mark the point, and put a short “how-to” note on the route card.
- Hot surfaces over 80–100 °C.
Fix: Use a high-temperature waveguide or magnetic foot; do not handhold a stinger on hot gear.
Airborne – Watch Out For:
- High ambient ultrasound from steam, many leaks, or multiple pneumatic exhausts.
Fix: Use cones or parabolic mics, drop the gain, get closer, and when in doubt confirm with contact readings where possible.
- Wind or heavy airflow blowing leak noise away.
Fix: Shield the area with cardboard or your body, and scan from multiple angles.
- Reflections off shiny surfaces, especially in electrical rooms with metal-clad gear.
Fix: Change angle and distance; confirm with a sound file and correlate with infrared and visual inspection before you write a major work order.
Examples You Will Actually See in a Plant
- Bearing at +10 dB over baseline with a smooth rushing sound (contact): Lubricate in small increments while watching dB fall. Stop at the minimum and verify a slight temperature drop. If the dB does not change at all, suspect a blocked grease line or that you are not actually on a good path to the bearing.
- Gearbox with a buzzy tone and intermittent crackles (contact): Plan inspection for possible early tooth distress. Cross-check with vibration, such as enveloping or demod, and oil debris analysis. Adjust rescan frequency rather than waiting for a catastrophic failure.
- Hissing around a flange 3 m away (airborne): Close in, find the peak, confirm with soap, and estimate leak cost. Tag it and load a corrective work order with an energy-loss estimate so it competes fairly with other tasks.
- Switchgear cubicle with sharp sizzling that peaks near a cable termination (airborne): Escalate to an electrical inspection for possible tracking or corona. Corroborate with infrared and a safe visual once the electrical team signs off on access.
Decision Guide (Quick Picks)
- Trending rotating assets and controlling lubrication → Start with structure-borne (contact).
- Finding energy losses or electrical discharge today → Start with airborne.
- Access is risky, at height, or energized → Airborne first.
- Area full of ultrasonic noise → Use structure-borne to confirm and trend.
- Need to prove a valve or steam trap condition → Structure-borne across inlet, outlet, and body.
How We Teach and Use It in Practice
- Routes – Bearings, gearboxes, valves, and traps get contact points with baselines and alarm bands. Leak and electrical sweeps use airborne scans.
- Cross-validation – Significant contact ultrasound findings are cross-checked with vibration or oil where practical. Airborne electrical findings are corroborated with infrared and visual checks under proper safety controls.
- Reporting – One concise report shows what we measured, why we used that method, the dB change or qualitative result, and what to do next such as lubricate, repair, or adjust rescan interval.
Bottom line: use structure-borne when you need repeatable, diagnostic-grade trends on rotating and mechanical internals, and use airborne when you need speed, stand-off safety, and the ability to hunt leaks or partial discharge. The plants that win do not choose one; they combine them, screening fast with airborne, then proving and trending with contact.