Click to view
(Color online) Representative data from one participant demonstrating an unintended intensity change following a frequency shift. The upper panel illustrates the modified broadband root-mean-square (left axis) and frequency (right axis) of the feedback signal. Note the rise in frequency of the feedback F 0 signal during the F 0 shift. In the lower panel, the intensity of the waveforms for the voice (boom microphone) and both probe tube microphone channels are plotted. Note the approximately 9 dB unintended intensity change pre- and post- shift. Intensity changes also occurred during the shift due to mixing of the shifted and unshifted sounds in the ear canal, which are depicted as pulsating intensity levels.
Click to view
(Color online) In-ear calibration shows a graphical display of the recorded gain level in the ear canal during vocalization, with unweighted SPL displayed on the x axis and time on the y axis. The participant's voice is measured with the calibrated microphone at a target intensity of 75 dB SPL. Intensity in each of the participant's ears alternated between feedback “off” (i.e., trough) and feedback “on” (i.e., peak). The gain is depicted as the distance between the peaks and the troughs (∼10 to 15 dB). Right and left channels are shown as balanced, with the right and left probe tube signals stacked (<2 dB difference). The F 0 level, as well as the target intensity, were monitored throughout testing.
Click to view
(Color online) (A) Phase relationship between voice at the microphone and feedback output of the effects processor pre- and post-shift. Each of the channels was scaled independently so that they all have approximately the same peak-to-peak height in order to more clearly show the phase relationships. A change in timing relationship is evident between voice and feedback signals pre- and post- shift. (B) Resulting in ear sound waveform as recorded with the probe tube microphone, illustrating the reduced waveform amplitude pre- and post- shift. This channel was also scaled to have approximately the same peak-to-peak height as waveforms in (A).
Article metrics loading...
Effects of frequency-shifted feedback are typically examined using Eventide Harmonizer Series processors to shift the fundamental frequency (F 0) of auditory feedback during vocalizations, eliciting compensatory shifts in speaker F 0. Recently, unexpected intensity changes were observed in speakers' ear canals, corresponding with F 0 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.
Full text loading...