What are the frequency bounderies for Far Field analysis?

Far Field analysis has a lower and upper frequency bound. The lower bound indicates from which frequency the sound image can show results. The upper bound indicates from which frequency spatial aliasing can happen.

Lower frequency bound

When you analyze below the lower frequency bound you get large sound blobs. In the most extreme case, your whole sound image shows one color.

The lower frequency bound depends on the measurement array size. A larger CAM has a larger measurement array and can visualize larger wavelengths (and therefore lower frequencies).

Lower frequency bound

CAM64 = 1000 Hz
CAM1K = 250 Hz

What are the frequency bounderies for Far Field analysis? -
Figure 1: Far field analysis of a fan for the frequencies 84 – 106 Hz at a distance of 2m.

Upper frequency bound

When you analyze above the upper frequency bound you should be aware of spatial aliasing. In your sound image, you would also see ghost sources. These ghost sources are not the actual sound source or a reflection of sound. An easy way to determine the appearance of a ghost source is to rotate your camera (in the measurement plane) and see if the source rotates according to your rotation movement, see Figure 2 and Video 1.

The lower frequency bound depends on the measurement array size. A larger CAM has a larger measurement array and can visualize larger wavelengths (and therefore lower frequencies).

The upper frequency bound depends on the microphone grid spacing (20 mm for a full microphone selection).

Upper frequency bound (for full microphone selection)

CAM64 = 8575 Hz
CAM1K = 8574 Hz

What are the frequency bounderies for Far Field analysis? -
Figure 2: Rotation of a CAM64 in the measurement plane to determine the ghost sources.

The lower frequency bound depends on the measurement array size. A larger CAM has a larger measurement array and can visualize larger wavelengths (and therefore lower frequencies).

The upper frequency bound depends on the microphone grid spacing (20 mm for a full microphone selection).

Upper frequency bound (for full microphone selection)

CAM64 = 8575 Hz
CAM1K = 8574 Hz

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