Acoustic cameras: Why does the number of microphones matter?

When comparing datasheets of different acoustic camera manufacturers, you quickly notice the number of microphones varies a lot. In this article we will explain the technology and science behind acoustic cameras and explain why having enough microphones really matters.

More microphones—better acoustic sensitivity

Bigger isn’t always better, but in acoustic imaging having enough microphones plays a critical role. In general, increasing the number of microphones improves acoustical performance. Acoustic cameras generally use MEMS type microphones (microelectromechanical systems) as they provide great performance, stability, low power consumption, and small size. Although MEMS microphones can typically pick up loud noises (typically over 120 dB(A)), they have high self-noise level, which means that the quietest sound levels can’t be picked up with a single microphone. This self-noise however can be attenuated by combining the signal from multiple microphones. By doubling the number of microphones roughly 3 dB of noise is automatically removed. Therefore the sensitivity to detect quiet sounds can be increased by maximizing the number of microphones.

By utilizing 124 state-of-the-art microphones, the NL Camera is able to locate 0.02 l/min air leaks in optimal conditions. It is not possible to receive this type of accuracy with a smaller number of microphones, especially since the NL Camera microphones have market leading sensitivity.

In this example there are two signals from two leaks. These leaks can be missed if the sensitivity of the acoustic camera is not good enough. In the example, the 16.5 kHz leak signal can be detected with a system consisting of 32 microphones and the 18.5 kHz leak signal with a system consisting of 124 microphones.

Better detection range and resolution

Another important technical fact on the data sheets one should pay attention to is detection range, where the number of microphones plays a critical role. As sound travels through air, it is attenuated by 6 dB every time the distance is doubled. By increasing the number of microphones, the maximum detection range can also be increased: the range is roughly doubled by quadrupling the number of microphones.

One might wonder why every manufacturer doesn’t increase the number of microphones. It is easy to think that apart from increased cost there wouldn’t be any cons. This is however not true – by increasing the number of microphones the number of audio data signals that need to be processed is also increased. This requires a lot of built-in processing power. Portable acoustic cameras with a lot of microphones haven’t been widely available earlier due to lack of available internal processing power.

A quick way to reduce the required processing power would be reducing the resolution of acoustic image. An acoustic camera essentially calculates the sound level from numerous different directions (acoustic image pixels), so decreasing the number of pixels reduces the required processing power. However, an acoustic image resolution is like the resolution of your computer’s display – a small resolution makes the image blurry. In a similar way, reducing the number of sound pixels hinders the performance of the device. 

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By increasing the number of sound pixels, faults can be located more reliably and from longer distances. As distance increases so does the gap between the acoustic image pixels. If the number of available pixels is limited, it is difficult to determine exactly where the sound is emitted from.

While developing the NL Camera, these factors as well as portable compact size of the device have been closely considered. The NL Camera packs 124 microphones and a lot of processing power. This combination enables industry leading detection sensitivity, excellent acoustic image resolution and great range.