Acoustic Camera dB Readings: How to Interpret Decibels

What the dB Number Actually Means

An acoustic camera measures sound pressure at its microphone array and displays the result as a decibel (dB) level on top of the visible image. That number is the instrument’s answer to one question: how much ultrasonic energy is coming from this direction right now? The higher the number, the stronger the acoustic source — a tight compressed-air leak, a corona discharge, or a partial-discharge (PD) site in a switchgear compartment.

But a dB reading is only useful when you know three things about it: which reference scale the instrument uses, what frequency band it is listening to, and how far the source is from the array. The same leak can read 35 dB at 5 meters and 55 dB at 1 meter. Two cameras from different manufacturers can display different numbers for the same leak because they reference different scales. Learn the scale, hold the distance constant, and the reading becomes a reliable tool instead of a confusing digit.

“The number on the screen is a starting point, not a verdict. We treat dB as a screening metric and the acoustic image plus spectrum as the evidence.” — Maintenance Superintendent, Substation Contractor, Mexico

dB, dBm, dBuV: Which Scale Is the Camera Using?

Manufacturers use different decibel references, which is the most common source of confusion when teams compare readings between instruments:

• dB SPL (sound pressure level). Referenced to 20 micropascals, the threshold of human hearing. This is the scale used by acoustic cameras that calibrate in absolute sound pressure. It is the closest to a physical, comparable unit.
• dBm / dBuV (electrical). Referenced to a power level (1 milliwatt) or voltage (1 microvolt) at the sensor. Many ultrasonic detectors and some acoustic cameras display these scales because the sensor output is electrical. They are internally consistent but not directly comparable to dB SPL numbers.
• Relative dB. The instrument displays the level above its own noise floor or above an ambient reference. Useful for comparing one joint to the next in a single survey, but the absolute value has no meaning outside that instrument.

Unitech Tools acoustic cameras — the GSW Series, SW136, and FA611S — display calibrated levels that stay consistent within the instrument family, so a reading taken with one Unitech unit can be compared with another. When you write the reading into a report, always note which instrument and scale you used.

Typical dB Levels for Leaks and Partial Discharge

The table below shows the level ranges a 136-microphone acoustic camera typically displays for common electrical and mechanical faults. Treat them as survey guidance, not laboratory constants — distance, pressure, and plant noise shift every value.

Notice the pattern: electrical faults live higher in frequency than gas leaks. That is why a camera with only a 20–40 kHz filter catches air leaks but misses PD. The SW136 shows the live spectrum so you can see where the energy sits before you decide what it is.

Why Frequency Matters More Than Level

Two sources can produce the same dB level but different frequencies — and that difference tells you what you are looking at. A 50 dB reading at 30 kHz is almost certainly a gas or air leak. A 50 dB reading at 80 kHz, repeating in bursts, is far more likely to be partial discharge or corona.

This is why a level-only reading is dangerous: a loud bearing at 25 kHz can mask a moderate PD burst at 70 kHz in the same machine. The inspector who records only the loudest number walks away from the most dangerous fault. Use the frequency axis — the GSW Series and SW136 both display the spectrum — and treat dB level as the how loud answer while frequency is the what is it answer.

Field rule: never classify a source by level alone. A reading above 40 dB on the high-frequency band, pulsing with the line frequency, is a PD suspect regardless of its absolute level.

Distance, Background Noise, and Other Field Errors

• Distance changes everything. Ultrasonic energy falls off quickly with distance. A reading taken at 5 meters cannot be compared with one taken at 1 meter. Keep the same inspection distance for every joint in a survey, or normalize the values in the report.
• Angle matters. Point the camera at the suspected source, not across it. Off-axis readings understate the level because the microphone array’s directivity rejects energy from the sides — by design, so it can localize the source.
• Background ultrasound. A running compressor, a steam vent, or a nearby discharge lamp injects ultrasonic noise into the measurement. Take a background reading away from the suspect, then subtract it mentally from the source reading.
• Reflections and barriers. Metal enclosures reflect ultrasound. A hotspot can appear where the sound bounces, not where it originates. Move around the source and confirm the hotspot follows the physical location.
• One-off snapshots. PD is intermittent. A single burst can look like noise. Record for 30–60 seconds or use the burst-detection view to capture the pattern before you trust the level.

A Field Workflow for Reliable dB Readings

1. Set the baseline. At the start of the route, take a background reading in open air away from equipment. Record it as the survey’s reference floor.
2. Standardize distance. Inspect every breaker panel, cable termination, or flange at the same distance (typically 0.5–1 m for enclosed equipment, 2–5 m for open substations).
3. Look at the spectrum first. Check where the energy sits before assigning a cause. Low band = leak/corona/mechanical; high band + bursts = PD.
4. Capture the image and level together. Save the acoustic image with the dB value overlaid. The photo is what makes the report credible.
5. Repeat suspicious readings. Re-measure after repositioning. A stable hotspot at the same level from two angles is real; a one-off spike is suspect.
6. Compare against the healthy twin. Read the identical asset next to the suspect one. The delta between healthy and faulty is the most reliable severity indicator in the field.

Which Unitech Tools Model Fits Your Survey

Teams that combine acoustic imaging with thermal and PD tools get the full picture: the EasIR thermal camera finds the heat anomaly, the SC Series PD detector confirms the electrical activity with TEV and AE sensors, and the acoustic camera localizes the airborne source. Cross-checking dB readings across these tools turns a number into a diagnosis.

The Bottom Line

A dB reading on an acoustic camera is a fast, repeatable screening metric — if you know the scale, hold the distance, and read the frequency. The GSW Series, SW136, and FA611S from Unitech Tools make that easy: calibrated levels, live spectrum, and an acoustic image that shows exactly where the sound comes from. Use the workflow above, and the same tool that finds your compressed-air leaks will also catch the partial discharge that thermal inspection alone cannot see.