This example shows a small 12cm wide air pump. The hologram size is 27cm x 27cm, derived from 3cm spacing resulting in a maximum frequency of 5700Hz. Based on critical frequencies found in the spectrum, the various holograms pictured below show the rotating pump itself (left, 1150Hz), the air outlet (middle, 430Hz), and the air intake (right, 715Hz). This example demonstrates that it is possible to propagate to frequencies with a half-wavelength larger than the hologram size, as is the case for the middle hologram (½λ≈40cm). This still allows correct localization at the expense of less reliable quantification results. Besides that, with small structures, it is important to take the granularity of possible sound sources into account, and match microphone spacing accordingly. In this case, the 3cm spacing matches the size of the different small pump components.
Figure 1: Pump, air intake and air outlet analyzed with NAH, all at -8dB dynamic range
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