A machine on your line has started to whine. Or a new building has a hum that nobody can place, and someone suggests an acoustic camera. It is the right tool for the job. It lays a colour map of sound over a photograph of the scene. The source shows up as a hot spot on the panel, bearing or duct joint that is radiating it.
Which camera will find your source depends mostly on one number: the lowest frequency it can locate from the distance you will work at. Settle that first, and the questions about weight and software follow from your workflow. Merkel Technologies offers SevenBel's Sound Scanner in Israel, and its four sensors show each of these trade-offs on a real instrument.
What an acoustic camera measures
An acoustic camera is an array of microphones built around a camera. Sound from one source reaches each microphone at a slightly different moment. Beamforming software turns those differences into a direction and a level for every frequency band. You get a heat map over the image. You can filter it by frequency and by time, for example to separate a fan's tone from the rush of air around it.
It shows you where a sound comes from. Your engineers still work out why that part is noisy, starting at the component the map points to.
The lowest frequency decides the array size
Locating a source has a physical floor. As a rule of thumb, the array should be roughly as wide as the wavelength of the sound you want to locate. Wavelengths grow quickly as frequency drops: about 34 cm at 1 kHz, 69 cm at 500 Hz, 1.37 m at 250 Hz and 2.74 m at 125 Hz. SevenBel's buyer's guide puts a workable ratio of array diameter to wavelength between 0.6 and 1.5. Below 0.5, it calls the array likely inadequate for locating sources from a distance.
In practice, a 35 cm handheld array locates sources from roughly 800 Hz upwards, which suits many leaks. It misses the band from 100 to 500 Hz, where most building-acoustics problems sit, from HVAC noise to sound travelling through walls, windows and doors. Before you compare models, write down the band you need to see. Then check the array diameter against it.
Reading the frequency range on a spec sheet
A single frequency range on a brochure can mean three different things. The microphones' frequency response says what they can hear. The far-field beamforming range says what the array can locate from half a metre to several metres away, the distance most industrial and building work needs. A near-field or holography figure can go as low as 30 or 40 Hz. It holds only with the array within about 15 cm of a vibrating surface, and it usually comes from several measurements stitched together under stable conditions.
So ask every supplier the question SevenBel recommends: what is the far-field lower frequency limit at one metre? Its guide warns about brochures that lead with a low number and do not say which of the three ranges they quote.
Four SevenBel sensors for four kinds of work
SevenBel gets a large aperture without a heavy array by moving the microphones. The Sound Scanner samples the sound field with five microphones while the sensor rotates. Its algorithm reconstructs 400 virtual microphone positions over a measurement surface up to 2.5 m in diameter. The acoustic image is computed in the cloud or on a laptop on site, and you analyse it in a mobile app. The battery lasts up to six hours.
The sensors split the frequency range by job. The P254 reaches down to 125 Hz and suits environmental noise. SevenBel's guide rates its 254 cm aperture for every building-acoustics frequency, including the low ones.
From 250 Hz, the P132 is the all-round sensor for construction sites, test laboratories and anechoic chambers. The handheld P50 starts at 700 Hz and fits vehicle interiors and other confined spaces. The P12, from 2.8 kHz, is designed for gas and compressed-air leaks and for partial discharge on electrical equipment.
Steady sounds, repeatable events and one-off noises
A scanning sensor builds its image while it moves. It works best on sounds that stay steady through the measurement, such as a running fan, a pump at constant speed or a leak. For sounds that come and go, SevenBel's software locates transient events when they repeat, like a valve that clicks on every cycle. A single noise that never returns is better captured by a static array that records all its microphones at the same instant.
Time sets a second limit. An acoustic camera tells you where a sound comes from during a measurement. To follow a bearing as it wears over weeks, you need continuous monitoring, which we covered in our post on acoustic predictive maintenance.
Weight and setup decide where the camera can go
An instrument that needs two people and a tripod tends to stay in one room. SevenBel's guide puts a 35 cm handheld array at 1.5 to 2 kg, with under a minute of setup. A 120 cm static ring weighs more than 10 kg and takes about 20 minutes to set up. SevenBel's own 254 cm rotating sensor weighs about 5 kg and takes roughly a minute. If your camera will travel between roofs, production halls and test sites, weigh those figures as seriously as the frequency range.
Measuring your own noise before you choose a camera
Three answers narrow the choice: the lowest frequency of the sound you care about, the distance you can measure from, and whether the source is steady, repeatable or a single event. Once you have them, the best test is a measurement of your own source. Merkel Technologies can run a proof of concept with the Sound Scanner on your machine, product or building. Your decision then rests on an acoustic image of the problem you actually have.
The Sound Scanner sits in our material analysis and NDT line. Contact us to plan the measurement. No cost and no obligation for a first consultation.