Frequency-Shifted Auditory Feedback

Frequency-shifted auditory feedback (FAF) shifts the pitch of your voice in your earphones. A FAF upshift makes you hear your voice sounding like Mickey Mouse. A FAF downshift makes you hear your voice sounding like a gravel-voiced radio announcer saying his station's call letters.

A quarter-octave pitch shift reduces stuttering about 35%. A half-octave pitch shift reduces stuttering about 65-70%. A full-octave pitch shift reduces stuttering about 70-75%. Combining DAF and FAF reduces stuttering about 80%.

Shifting pitch up or down is equally effective in short-term studies. But there may be long-term differences between up- and downshifts. FAF causes non-stutterers to speak at a higher or lower vocal pitch, depending on whether the device is set for an up or down frequency shift. This higher or lower pitch vocal pitch results from changing vocal fold tension. In other words, FAF induces changes in vocal fold tension in non-stutterers.

A study found that my company's FAF devices, set for a half-octave downshift, didn't cause a change in vocal pitch in stutterers. But speech clinics have reported that my FAF devices induce vocal fold relaxation in stutterers. Usually, stutterers need a greater pitch shift, between one-half and one octave down. Also, the study used older headphones which lacked the bass response of today's headphones. A new study might find that current devices, set to one-half or one octave down, induce vocal fold relaxation.

I've also seen FAF downshifts induce a slower speaking rate, similar to DAF. If this effect is consistent, then a FAF downshift should produce long-term carryover fluency.

Figure 3: FAF Stuttering Reduction

Conversely, a FAF upshift (the Mickey Mouse voice) appears to induce vocal fold tension. I've seen FAF upshifts induce faster speaking rates. If this effect is consistent, then a FAF upshift should result in poor long-term performance (e.g., no carryover fluency, and possibly "wearing off).

All published studies of FAF used octave-scale FAF. Octave-scale FAF requires lots of computing power (a fast Fourier transformation). My company's devices use octave-scale FAF. When you set my devices to a one-octave upshift, the 125-Hz fundamental frequency of an adult male voice is shifted up to 250 Hz. The 250 Hz first overtone of your voice is shifted to 500 Hz. The 500 Hz second overtone of your voice is shifted to 1000 Hz. And so on.

If you instead use a one-octave downshift, your 125 Hz voice is shifted in your earphones to 62 Hz. Your 250 Hz first overtone is shifted to 125 Hz, and so on.

Figure 4: Octave-Scale vs. Frequency-Compression FAF

But some devices made by other companies don't have enough processing power to produce octave-scale FAF. Instead, a simpler process uses frequency compression/expansion FAF. This sounds like the ring modulators used to make robot and alien voices in old science fiction movies. The upshift adds 500 Hz to your voice (or 1000 Hz or 2000 Hz, depending on the setting). Thus, your 125 Hz fundamental frequency is shifted to 625 Hz—more than two octaves up! Your 250 Hz first overtone is shifted to 750 Hz. Your 500 Hz second overtone is shifted to 1000 Hz.

When you downshift or subtract 500 Hz from your voice, your 125 Hz fundamental frequency vanishes. 125 Hz minus 500 Hz is nothing (there are no negative frequencies). The 250 Hz first overtone of your voice also vanishes. And the 500 Hz second overtone of your voice vanishes. You can only hear the weak third (1000 Hz) and higher overtones of your voice. When I tried another company's anti-stuttering device, I heard my voice in my ear rise in pitch as the FAF was adjusted lower!

No published studies have investigated whether frequency compression/expansion FAF has an effect on stuttering. A speech-language pathologist who works with such devices reported that frequency expansion (downshifting) "does not enhance fluency."

Long-Term Effects of DAF Combined with FAF

Nine stutterers used a DAF/FAF device about seven hours per day, for twelve months. The delay was set at 60 milliseconds and the frequency compression FAF at 500 Hz up. The subjects received brief speech therapy, specifically to prolong vowels and use "starter sounds" such as "um" and "ah."

At the start of the study, the DAF/FAF device reduced stuttering about 85%. Twelve months later, the subjects experienced no statistically significant "wearing off" of the devices' effectiveness. The subjects' speech without the devices didn't improve.

Figure 5: Long-term effectiveness of DAF/FAF device

Another study of the same type of device raised questions of whether long-term use could make stutterers' speech worse. Of six stutterers who used the device 10-23 months, two had speech about the same and four had speech much worse than before using the device. On average, stuttering increased about 50% after 18 months.

Why did one anti-stuttering device produce 55% carryover fluency (page 16), when another anti-stuttering device produced no carryover, or possibly made the subjects' speech worse? Speculatively, upward FAF has positive immediate effects but negative long-term effects. Hearing your voice shifted up may improve your auditory processing but make your speech motor activity worse (i.e., make you speak with tighter vocal folds). If the auditory processing effect goes away when the device is removed, but the speech motor changes are retained, then no carryover would result.

Figure 6: Long-term effectiveness of DAF/FAF device

Or possibly the subjects using anti-stuttering devices for thirty minutes of practice per day slowed down their speech, improving their fluent speech motor skills; while subjects wearing anti-stuttering devices all day spoke at normal speaking rates, possibly making their auditory processing worse (page 15).

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