Ultrasound listens above ~20 kHz, where friction, tiny impacts, turbulent leaks and electrical discharge all emit. An instrument heterodynes it down to audible sound and reports a dB level you trend — and it catches faults earliest on the P-F curve.
Its signature wins: finding compressed-air, gas, vacuum and steam leaks (big energy savings), checking steam traps, spotting electrical corona/arcing in switchgear, and early bearing distress before vibration shows it.
The killer routine is acoustic lubrication: grease the bearing while listening, and stop the instant the dB level bottoms out — neither starved nor over-greased. The model below lets you do exactly that.
Listening above hearing
Human hearing tops out around 20 kHz. Plenty of machine faults make most of their noise above that, where it's inaudible — but also where it's clean, because background plant noise is mostly low-frequency. Ultrasonic instruments pick up these high frequencies (airborne with a microphone, or structure-borne through a contact probe), then heterodyne them down into the audible range so a technician can hear the character of the fault, while a meter logs the decibel level (dB) for trending.
Three things make ultrasound special among the CBM techniques: it is directional (you can pinpoint a source), it is quiet-background (high frequencies don't travel far or through walls, so what you hear is local), and it is early — friction and the first surface distress emit ultrasound before they produce measurable heat or low-frequency vibration.
What it's used for
| Application | What it hears |
|---|---|
| Bearings (early) | Friction and the first micro-spalling — rising dB before vibration; and lubrication state |
| Acoustic lubrication | Grease to the point of minimum friction — no more over/under-greasing |
| Compressed air / gas / vacuum leaks | Turbulent flow through an orifice — directly findable and costed (energy) |
| Steam traps | Failed-open (continuous rush) vs working (cyclic) vs failed-closed (silent) |
| Electrical | Corona, tracking and arcing / partial discharge in switchgear — through enclosures |
| Valves | Passing / internal leakage across a closed valve |
The leak-detection use alone often pays for the kit: compressed air is one of the most expensive utilities in a plant, and a survey routinely finds leaks worth thousands a year — invisible and silent without ultrasound.
Grease by sound: acoustic lubrication
From bearings & lubrication, recall that most bearings die of lubrication problems — and over-greasing is as harmful as under-greasing. Ultrasound solves the guesswork. A correctly lubricated bearing has a low, steady ultrasonic level. Let the grease film thin and friction rises, so the dB climbs. Add grease and it falls — until it bottoms out at the right film. Keep pumping past that and it climbs again as the over-packed bearing churns and overheats.
So the routine is: attach the probe, watch the dB, add grease slowly, and stop when the level stops dropping. The model lets you find that point — and shows how a developing defect raises the floor the lubrication can't fix.
Interactive — Acoustic lubrication
Live modelUltrasonic dB vs grease added
Where it fits with the others. Ultrasound is the earliest alarm — friction and lube first, then defects. As damage grows it hands off: vibration resolves the bearing defect frequencies, thermography sees the heat, and oil analysis catches the wear debris. A complete programme layers them along the P-F curve.
Key takeaways
- Ultrasound listens above 20 kHz — directional, local, and the earliest warning for friction and lubrication.
- Leaks and steam traps are its bread and butter — often self-funding through energy savings.
- Acoustic lubrication ends over/under-greasing — grease to the dB minimum and stop.
- It leads the P-F curve and hands off to vibration, thermography and oil as damage develops.