At high frequencies a fraction-of-an-inch position change flips a comb-filter peak into a null

High-frequency sound has very short wavelengths, so the interference (comb) filtering caused by a nearby reflecting surface is acutely sensitive to position. When direct sound combines with a reflection, path-length differences of half a wavelength invert the interference — turning a reinforcement (peak) into a cancellation (null). Because a 9kHz wave is only about 1.5 inches long, moving a microphone just 0.75 inches (a quarter of the way toward a different geometry, but half a wavelength of path difference off a nearby surface) shifts that reflection’s phase by 180 degrees, converting a prior valley into roughly a 6dB peak. The practical upshot: any tuning or feedback filter set for one exact mic/speaker geometry is invalidated if the mic or cabinet moves even slightly, and HF measurements are inherently position-dependent.

Examples

A vocal notch dialed in at soundcheck stops working once the singer shifts the mic an inch: at 9kHz that inch is more than a full wavelength, so the comb pattern the notch targeted has moved.

Assessment

A 9kHz tone shows a null at one mic position. Explain, using wavelength, roughly how far you must move the mic to turn that null into a peak, and why the same move barely affects a 90Hz tone.