Delayed Auditory Feedback

Delayed auditory feedback (DAF) seems to have two distinct effects, depending on whether the delay is short (25 to 75 milliseconds, or about a twentieth of a second) or long (75-200 milliseconds, or about a tenth of a second).

A short delay immediately reduces stuttering about 70%,(13) without training, mental effort, or abnormally slow or abnormal-sounding speech. You just put on the headphones and talk. Hypothetically, this effect results from correcting a central auditory processing abnormality. While this effect is impressive, it doesn't 100% eliminate stuttering, and the effect goes away when the headphones are removed. A short delay appears to correct one factor in stuttering.

A longer delay induces a slower speaking rate with stretched vowels (continuous phonation) target (page 33). This requires training and sounds abnormally slow and monotonic.

The chart shows that at normal speaking rates, a short delay can reduce but not eliminate stuttering. A longer delay can reduce stuttering further, at the expense of speaking rate.(14)

Using DAF in Therapy

DAF stuttering therapy begins with training a stutterer to use the slower speaking rate with stretched vowels target, without using DAF. When the stutterer can complete a simple speaking task, such as counting to ten, using this target correctly, then he can use a DAF device. DAF therapy then has several goals:

• To increase the length and complexity of sentences while using the DAF device to support on-target fluent speech.
• To increase the stress of the speaking situation while using the DAF device to support on-target fluent speech.
• To reduce the need for the DAF device, until the stutterer no longer needs the device.

In other words, the stutterer first uses the DAF device for short phrases in the speech clinic. Typically this is one or two seconds per syllable, with the delay set at 200 milliseconds. He must achieve all fluent speech targets, e.g., all syllables stretched equally, all syllables stretched to one second, no pauses between words, and no dysfluencies.

The stutterer then uses the device in longer conversations in the speech clinic, again achieving all the fluent speech targets. Then he uses the device in more stressful speaking tasks, such as role playing with his speech-language pathologist (page 93).

When the stutterer achieves these goals, then he decreases the delay and increases his speaking rate. But if he has any dysfluencies he goes back to the longer delay and slower speaking rate.

The stutterer can also decrease the volume, and use the device in one ear instead of both ears. He can use the device at the beginning of conversations, and then turn it off when he feels capable of speaking on target with the support of the device. He can discontinue using the device in low-stress conversations; then in medium-stress conversations; and finally reserving the device only for stressful conversations such as public speaking. Eventually he should need the device only occasionally.

Mistakes in DAF Use

Don't use DAF at normal speaking rate with a long delay. If you want to talk at a normal speaking rate, set the DAF delay between 50 and 75 milliseconds.(15) Don't use a delay longer than 75 milliseconds unless you're using closed-loop slow speech (page 38).

I've seen this scenario over and over. A stutterer gets a 50% fluency improvement at 50 milliseconds. He gets a 75% improvement at 75 milliseconds. He sees that the dial goes up to 200 milliseconds. He thinks, "I'll crank up this baby! I'll redline it! I'll turn it up all the way to 200 milliseconds and I'll be 200% fluent!"

200 milliseconds is for speech five to ten times slower than normal. Non-stutterers can't talk normally with a 200-millisecond delay (with rare exceptions due to a linguistic abnormality) but most stutterers are capable of forcing themselves to "tune out" the delay. This appears to be due to our auditory processing underactivity. In other words, if you use DAF incorrectly you may make your auditory processing underactivity worse. This may explain why some stutterers have reported that a DAF device lost effectiveness or "wore off" over time.

Another mistake is to use a DAF device in low-stress situations (such as reading aloud) and expect carryover to high-stress situations. Carryover works the other way. Use an anti-stuttering device in situations in which you stutter, and don't use it in situations where you speak fluently.

Long-Term Effects of DAF

Nine adult stutterers used DAF devices thirty minutes per day, for three months. The thirty minutes consisted of ten minutes reading aloud, a ten-minute conversation with a family member, and a ten-minute telephone call. The subjects received no speech therapy.

The device used was the School DAF, made by Casa Futura Technologies (my company), with a binaural (two ears) headset. The subjects were allowed to set the delay where they wanted. Most selected delays around 100 milliseconds.

At the start of the study (0 months), the subjects stuttered on 37% of words, on average. With the DAF device their stuttering dropped to 10%. In other words, the device improved their fluency about 70%.

Three months later the subjects stuttered on 17% of words, when not using the DAF device. When wearing the DAF device they stuttered on 13% of words.

This shows that, when not wearing the devices, the subjects' stuttering diminished from 37% of words to 17% of words, or a 55% improvement. This is "carryover fluency," that is, the device training users to need the device less and less.

Figure 2: Long-Term Effects of DAF

The increase (from 10% to 13%) in stuttering when wearing the devices wasn't statistically significant. Examining this more closely, stuttering when wearing the device increased only for "automatic speech," such as reciting days of the week, and for repeating words and sentences after the examiner. No change in effectiveness was found in conversations or in a "picture description" task. This suggests that any "wearing off" effects occurred in less-important speaking situations.

The "carryover fluency" effect was the same across all speaking tasks.

In another study, an eleven-year-old boy received fourteen hours of structured therapy with mediated learning and a Casa Futura Technologies School DAF. His stuttering diminished from 9% dysfluencies to 5% dysfluencies (when speaking without the device, a 47% improvement). One year later he still had 5% dysfluencies. Another fourteen hours of treatment reduced his stuttering to 4% dysfluencies.

Two other studies combined speech therapy with a DAF device. One study was of adults, the other of children. Both studies found that combining DAF and stuttering therapy trained the subjects to speak fluently (less than 2% stuttering) and no longer need the devices.

Back to Table of Contents