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.
1 · 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.
2 · 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.
3 · 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.