Acoustic Camera Frequency Range: Leaks vs Partial Discharge

Why Acoustic Cameras Work in the Ultrasonic Band

Most industrial faults emit sound across a broad spectrum, but the most diagnostic energy sits above 20 kHz — beyond human hearing. That is not an accident of nature: small orifices, high-pressure jets, and electrical discharges all create turbulence or ionization that radiates energy efficiently at ultrasonic frequencies, while the audible band is crowded with motors, fans, and voices.

An acoustic camera is, in essence, a microphone array that images this ultrasonic energy. It samples the sound field with dozens of microphones, computes the direction of arrival, and overlays a hotspot on the visible picture. The frequency range it covers determines what it can hear: a camera that only listens up to 40 kHz is excellent at gas leaks and useless at partial discharge. Choosing the right frequency range is therefore the first decision in buying an acoustic camera.

The GSW Series covers the full industrial band from 2 kHz to 100 kHz, which is why it is used across compressed-air, gas, corona, and PD programs. But let us break the band down by fault type so the choice is clear.

Gas and Air Leaks: The 20–40 kHz Band

Pressurized gas escaping through an orifice generates broadband turbulence noise with strong energy in the 20–40 kHz range. The physics is favorable for detection: the smaller the leak relative to the pressure, the higher the frequency content, and even small pinholes radiate enough ultrasound to be imaged from several meters away.

• Compressed air (6–10 bar): typically peaks between 20 and 40 kHz. The classic energy-audit target — a single 3 mm hole at 7 bar can cost hundreds of dollars per year in electricity.
• Natural gas and CNG: methane leaks from flanges, valves, and regulator stations sit in a similar band, usually 20–45 kHz, depending on pressure.
• SF6 in GIS and breakers: high-pressure SF6 leaks emit strong ultrasound around 20–50 kHz, detectable before the gas concentration becomes a safety or environmental issue.

For these jobs, a camera with a 20–40 kHz range and a reasonable array is enough — which is why entry-level ultrasonic detectors and compact cameras such as the FA611S are popular for leak teams. The limitation appears when the same survey must also find electrical faults.

Partial Discharge: The 40–100 kHz Band

Partial discharge and corona emit their strongest ultrasonic energy higher up, typically between 40 and 100 kHz. Surface discharges, internal voids, and loose connections in switchgear produce broadband bursts whose high-frequency content carries the diagnostic signature. A camera limited to 40 kHz will either miss PD entirely or show only a weak, ambiguous hotspot.

This is the single most important spec for utility teams: if you inspect switchgear, you need a camera that reaches 100 kHz. The GSW Series and SW136 both cover the full band, so one instrument handles leak surveys in the morning and PD walks in the afternoon.

“We bought an entry ultrasonic detector for leaks and it never found PD. When we moved to the 136-mic array with full-band spectrum, we found three PD sites in our own switchroom in the first week.” — Electrical Maintenance Engineer, Paper Mill, Colombia

Matching Camera Frequency Range to Your Assets

• Compressed-air and steam plants: a 20–40 kHz camera is sufficient; prioritize portability and battery life for long walkdowns.
• Gas distribution and CNG stations: choose up to 50 kHz with a good array and image clarity — you will be reading flanges and valves outdoors in daylight.
• Substations and switchgear rooms: full-band (up to 100 kHz) is mandatory. Pair the acoustic camera with a PD detector such as the SC Series or HPD200 to confirm airborne findings with TEV measurements.
• Combined utility crews: one full-band camera (GSW Series or SW136) replaces a leak detector plus a PD detector for the airborne role — and the GSW adds IR gas imaging for SF6 and VOC work.

For teams that also use thermal inspection, the EasIR or Hammer II finds the temperature anomaly while the acoustic camera localizes the airborne source — the two senses cover faults the other cannot see.

Spectrum Analysis: Turning Noise into Diagnosis

Knowing the band is not enough if the instrument cannot show you the spectrum. A live frequency display lets you distinguish a 30 kHz leak from an 80 kHz PD burst even when both are present at the same level. This is the difference between a camera and a tool: the SW136 plots the spectrum in real time so the operator sees the peak frequency, the burst pattern, and the harmonics — the fingerprints that identify the fault.

Field tip: when a hotspot appears, freeze the spectrum and look at the shape. A broad smooth hump around 25 kHz says leak. A series of sharp bursts above 60 kHz, repeating at 100 or 120 Hz (twice line frequency), says PD. The shape is the diagnosis.

GSW vs SW136 vs FA611S: Frequency Coverage

All three Unitech models reach 100 kHz, so none of them will blind you to partial discharge. The difference is scope: FA611S for pure leak work, SW136 when spectrum analysis matters, GSW Series when you need one tool for the entire acoustic program including SF6 gas imaging.

The Bottom Line

Frequency range is the first spec to check when buying an acoustic camera. Leaks live in the 20–40 kHz band; partial discharge and corona live between 40 and 100 kHz. If your job involves switchgear, transformers, or cables, buy a camera that reaches 100 kHz — the GSW Series or SW136 — and use the live spectrum to tell a leak from a discharge. The right frequency coverage is what turns a “noise detector” into a fault diagnosis tool.