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You might reasonably expect that this problem can be eliminated by using the “free field over a reflecting plane as specified in ISO 3744” , referenced in IEC 60601-1-8 and a number of particular standards. But it turns out that the strongest reflection will actually come from the “reflecting plane” (the floor), and this reflection alone can cause a large interference pattern. The above graphs are actually simulated using only a single reflecting plane (floor). The graph on the left is the expected variation as a meter is kept at 1m above the floor and moved away from the sound source. The graph on the right is the variation with the sound level meter 1m away, and the height from the floor for both the source and meter is varied. The location of the peaks and valleys will also change with the frequency of the source.

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