Video: Acoustic camera

Video transcript

How a 100 microphones turn a sound into a place on a picture, and how to run a leak survey and a discharge survey with it. A leak in a compressed air line, and a discharge on an insulator, have something in common: both of them are making a sound right now. You cannot hear either of them, and even if you could, the plant around them is far louder. The way round it is where you listen. Plant noise is loud but low:

it collapses above a few kilohertz. A gas escaping through a small hole, and an electrical arc, both keep radiating far above that. Put the listening window up in the ultrasonic band, and the noise is gone while the thing you are hunting is still there. And the way it is located is simple arithmetic done a great many times. A 112 microphones hear the same sound at slightly different moments. Shift each one by the delay that would come from a given direction,

add them up, and only the sound from that direction reinforces. Do that for every direction in the field of view, and you have an image. For leaks, it does not stop at finding them. It estimates how much is escaping, and what that costs you over a year. For electrical work it does something else. It builds a phase-resolved discharge pattern in real time, and classifies what it is seeing into corona, surface, floating or noise. And it was made to be carried round a plant all day rather than set up on a tripod.

The sensitivity is what makes the range possible. Down at about one decibel of sound pressure across the middle of the band, which is why a leak of half a litre a minute is visible from across a bay. Everything you find leaves with you. Images, video and the sound itself, tagged with where you were standing when you took them. And the numbers, for the specification. Start with what is in your hands.

The whole front face is the instrument: a 112 microphones in a disc, an optical camera in the middle of them, a distance sensor and two lamps. Everything else is a screen and a battery. Three ports, and each one is there for a different job. The audio output is the one people forget, and it is worth knowing: it lets you hear the ultrasound, shifted down into the range your ears actually work in.

Before touching a setting, it helps to know what the instrument is doing. The microphones all hear the same sound, but at slightly different moments. Delay each one by the amount that a given direction would produce, add them together, and only that direction reinforces. Repeat for every direction in the field of view. The main screen has 7 areas and you will use 4 of them. Down the left is the quick settings strip, across the top the level and the distance, and down the right the frequency analysis with the two band handles.

The first choice is how many sources you want to see, and it changes what the screen is for. One source for hunting, 3 for comparing, and the full heat map for surveying a whole panel at once. The second choice is the one that decides whether the survey works at all: the frequency band. Audible and ultrasonic are presets. User lets you drag the two handles yourself, and the technique is simple: raise the lower handle until the plant disappears from the screen, and no further.

Then two settings that people confuse. Image range changes the span of the colour scale. Display threshold decides how quiet a sound has to be before the camera stops drawing it. Raise the threshold and the picture gets cleaner, but you will also stop seeing the faint thing you came for. For leak work, the instrument needs to know what your leak actually costs, and that means telling it about your plant.

For compressed air it wants the compressor and the electricity price. For a bought gas, it simply wants the price per litre. And then the measurement itself, which has one rule that matters more than any other: put the leak in the middle of the picture. The estimate is calculated from the distance and the level at that point, and off-centre it is simply wrong. Partial discharge work has its own rule, and it is just as easy to get wrong:

set the mains frequency first. The whole pattern is built against the phase of the supply, so on a 50 hertz network a camera left at 60 will classify nonsense. Files, and the habit worth forming before you start rather than after. Create the folder for the job first, and name it. 21 gigabytes is a lot of survey to sort out afterwards. And the app, which is what turns a folder of pictures into a report with locations on it.

Finally, the little it asks of you. One of these is worth doing at the start of every campaign: the microphone self-test. A 112 microphones is a lot of things to go quietly wrong. Acoustic imaging, from Amperis.