Acoustic Cameras: Noise Detectives in Action

Author: Michael Vogel

Oct 13, 2021 Innovation

Occupational safety, building acoustics, industry: When noise becomes a problem and conventional analysis methods fail, acoustic cameras can help. The cases are as diverse as life itself.

It’s impossible to switch off your ears. At most, you can try to ignore unwanted sounds. A whisper is as quiet as the light rustling of leaves in the wind. A bottle tossed into the bottle bank is as loud as a gasoline lawnmower – except it’s over much faster. And whether we’re dancing at a club or disco or standing next to a chainsaw, both are similarly loud and can permanently damage our ears. But the perception is different: One is considered music, the other is noise. The concept of noise is therefore very subjective, but the fact that noise can make people ill is a matter of scientific knowledge. A person’s body and psyche suffer when permanently exposed to noise.
For this reason, legal regulations apply in various countries to protect the population from damage to their health. In Germany, the Technical Instruction on Noise Abatement (TA Lärm) regulates the protection of the general public from industrial noise, while the Occupational Health and Safety Ordinance (Arbeitsschutzverordnung) takes effect in professional life. In addition, “International Noise Awareness Day” intends to raise awareness of the issue. In Germany, it has been held as the “Day Against Noise” since 1998.
Searching for the sound source with the acoustic camera
For Ilja Richter, every day is a “day against noise”. He’s an expert for noise immissions at DEKRA and uses an acoustic camera to track down the causes of noise. “With the usual noise measuring devices, you can only record the sum of sound sources at a certain location. You know how loud it is, but not necessarily what the relevant sources are,” explains Richter. “The acoustic camera, on the other hand, makes the sources visible in images.” It uses an array of three plates, each with 128 microphones, to capture noise, which is then color-coded and superimposed on the optical image: particularly loud areas are red while quieter ones are blue. This is reminiscent of a thermal imaging camera that can detect thermal bridges on house facades.
“We’ve been using the camera for two years,” says Richter, who’s had a wide variety of measurement tasks. For example, there was a hotel in northern Germany, a new building. No matter how hard the staff tried, they received heavy criticism on online review portals. “The rooms were so badly soundproofed, it was as if there were no walls at all,” Richter recalls. “Whether the residents were watching TV or frolicking in bed, the room neighbors overheard more than they would have liked.” Richter’s measurements revealed several acoustic bridges, through which the noise spread into neighboring rooms.
The walls were of lightweight construction and the TV sets of two rooms always hung back to back on the same wall. The outlets for the connections were also at the same height. “This meant that low-frequency sounds were easily transmitted from one room to the other,” Richter explains. But it wasn’t just low-frequency sounds that penetrated to the neighbors. “The transition of the wall to the concrete ceiling via a shadow gap was also incorrectly executed,” says the expert, “so high-pitched sounds could spread.” After this report, the hotel finally had the means to force improvements.
Expert opinions to enforce further improvements
But sleep can be robbed not only in a hotel, but also on one’s own property. “A resident whose house was 300 meters away from a cement plant in southern Germany kept complaining about noise at night,” Richter says. During the day, the plant was proven to comply with TA Lärm, but not always at night. “A cement plant like this consists of countless installations, some of which are indoors, some outdoors. In addition, there are chimneys, towers – many potential causes,” he explains. Richter’s acoustic camera was predestined for this case.
However, vegetation obscured the direct view of the cement plant, which was 80 meters lower than the house. The camera needs a clear view for the image. The problem was solved with a lifting platform that lifted the camera 30 meters into the air to see the cement plant. “The acoustic image showed that the main sources were two buildings in the plant,” Richter says. “But it was impossible to determine more precisely from that distance, so I had to take acoustic images on the plant grounds as well.” That cleared things up: The bricks used in the building facades only insulated against heat, not sound. In addition, a lot of noise was escaping between a roll-up door and the facade because the space was poorly insulated. “To arrive at this result with a conventional sound meter, measurement and evaluation would have been much more tedious and would therefore have cost at least four times more,” says Richter.
Another “noise case”: The workforce of a sheet metal processing company in Lower Saxony was exposed to extreme noise. More precisely, to a shrill whistling sound in a huge hall. It was Richter’s job to find out which machine was the source and what was causing it. “The source turned out to be a machine for cutting sheet metal,” he says. A massive machine: it cut sheet metal that was unwound from one-meter wide and two-meter thick rolls. To make the cut, it used vacuum to suck the uncoiled sheet metal onto a rotating drum. “With detailed camera footage and the machine manufacturer’s design drawing, I was ultimately able to identify parts of the axle suspension as the source of the noise,” Richter says. “Air turbulence at a specific location inside the drum was causing the shrill whistle.” The company called in the machine’s manufacturer to remedy the problem together. And Ilja Richter was able to turn his attention to new cases.