Using PXI to Develop an Acoustic Camera

Author(s):
Anders Nordberg PhD - Nordborg Acoustics AB

Industry:
Transportation

Products:
PXI/CompactPCI

The Challenge:
Creating a data acquisition solution to measure noise control among multiple microphone channels.

The Solution:
Using NI PXI to develop a low-cost, high-channel-count data acquisition system.

Pinpointing Noise to Reduce Sound Pollution
Nordborg Acoustics AB provides innovative, intelligent, technical acoustic solutions to difficult problems such as noise control. Transportation such as motor vehicles, railway rolling stock, and aircraft produce most environmental noise, and legal noise emission limit values are becoming stricter to achieve an environmentally friendly transport system.

To support substantial global noise level reduction, we can easily capture dominating noise sources in sound-complex environments using our acoustic camera to localize which sources should be controlled.

Beamforming a Microphone Array to Minimize Capture Error
Combining signals from multiple microphones and accounting for their different positions results in an acoustic image. Because microphones and their corresponding measurement system must meet high standards, a mechanical arrangement keeps the microphones in their fixed positions to minimize measurement error.

During measurement, microphone signals are recorded and scanned to disk. The image generator program we developed produces an image out of the microphone signals and stores it on the disk. The different image colors show where the noise sources and strengths are located.

The program performs traditional delay-and-sum beamforming – sound signals are summarized, particularly accounting for the different propagation paths from the source and to the different microphones. The sources may pass the acoustic camera with high speeds (modern vehicles are still slow in comparison with the sampling speed), allowing the beam to focus and follow the bypassing source past the microphone array. The program zooms into any part of the studied object by computing a pixellated image of that part. More pixels equal better image quality, but longer computing times.

Ultimately, image quality depends on the number of microphones and their configuration. When they cover a big surface, the resolution gets higher; however, the image may contain noise, or specks, at high frequencies. Reducing microphone distances removes the specks, but at the same time worsens the resolution. So the best way to guarantee high quality images is to use many microphones.

After gathering sound and noise data, we combine the computed sound images from the microphones with a photo of the object to shows which parts radiate noise.

Powerful Data Acquisition over Multiple Channels
Our data acquisition system uses a PXI-1006 chassis filled with 14 eight-channel PXI-4472 cards, equaling 112 channels. Since we aimed for more than 100 microphone channels, this system offered some important features, including 24-bit A/D converters and integrated anti-aliasing filters to allow simple and inexpensive signal conditioning. Price-per-channel is also relatively low compared to other systems. Additionally, it is easy to expand the system by synchronizing two PXI-1006 chassis, resulting in 224 channels.

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