Unexpected intensity changes in the ear canal during a F(0)-shifted feedback experiment.

Effects of frequency-shifted feedback are typically examined using Eventide Harmonizer Series processors to shift the fundamental frequency (F0) of auditory feedback during vocalizations, eliciting compensatory shifts in speaker F0. Recently, unexpected intensity changes were observed in speakers' ear canals, corresponding with F0 shifts. An investigation revealed that feedback time delays introduced by the processor resulted in phase shifts between feedback and unprocessed voice signals radiating into the ear canal via bone conduction, producing combination waves with gains as high as 6 dB. Shifts of this magnitude potentially alter the interpretation of previously published results and should be controlled in future studies.