Delayed Auditory Feedback (DAF)

In 1992 a temp agency sent me to work at a tech company. They’d released a new operating system and needed someone to call all their customers and ask if they wanted the new release on a tape drive or a CD.

On a call the office telephone malfunctioned and I heard my voice back in my ear, delayed slightly. I’d done fluency shaping therapy so I knew how to slow my speech to match the delay. I spoke fluently on that call. Then I had improved fluency for the rest of the day.

The temp job ended a few days later. I started building a DAF telephone.

Rewards and Punishments

In 1965 Israel Goldiamond, a behavioral psychologist, wanted to see if he could make stutterers fluent via rewards and punishments. I don’t know what rewards he offered for fluent speech but the punishment was that whenever a stutterer stuttered, Dr. Goldiamond switched on delayed auditory feedback (DAF). DAF provides your voice to your ears delayed a fraction of a second. It’s annoying and disruptive to people who don’t stutter. DAF makes non-stutterers repeat or skip words.

With DAF the stutterers talked fluently. They asked Dr. Goldiamond to leave the DAF on.

Fluency Shaping Therapy

When speech-language pathologists (SLPs) tried DAF with stutterers they saw that the stutterers slowed down their speaking rate by stretching their vowels. DAF devices at that time were tabletop tape recorders so couldn’t be used outside of a speech clinic. During the 1970s Richard Curlee, William Perkins, and Bruce Ryan developed fluency shaping therapy to mimic the effects of DAF, without the devices.

I completed two fluency shaping therapy programs. I learned to slow down and talk fluently. My speaking rate was within the range of normal speech and I sounded relaxed and confident. I loved talking like this. But as soon as I stepped out of the speech clinic I went back to stuttering.

The Neurology of Stuttering

In the 1990s neuroscientists did brain scans of adult stutterers. They discovered two abnormalities:

  • Overactivity in the speech motor control area. Stutterers have too much muscle movement when talking.
  • Underactive auditory processing. There’s something wrong with how stutterers hear our voices. In particular, we seem to have problems integrating how we hear our voices with how we feel our muscles moving.

Fluency shaping therapy reduces overactive speech motor activity. It was no surprise that brain scans found overactivity in stutterers’ speech motor control area.

Underactive auditory processing was a surprise. No stuttering therapy addresses this problem. This was why I couldn’t do fluency shaping speech outside of the speech clinic. I have an auditory processing disorder. I prefer to watch movies with the closed captioning on. At noisy events I can’t understand what people are saying. I can’t talk at noisy events. I got A’s in Spanish because we were tested on reading and writing, but when I travel to Latin America I can’t understand what people are saying.

Altered auditory feedback (AAF)—DAF, FAF, or the Edinburgh Masker—appear to stimulate stutterers’ underactive auditory processing. No other stuttering therapy does this. AAF should be a part of every stuttering therapy program.

Stuttering is a multifactorial disorder. At least three neurological disorders underlie stuttering. The theme of this book is that different speech therapies treat different underlying causes of stuttering. Stuttering therapy fails when a therapy treats only one underlying cause. Stuttering therapy works when multiple therapies are combined to treat multiple underlying causes of stuttering.

Adaptation and Carryover Fluency

A Korean engineer bought one of my DAF/FAF devices. He loved it! He said that it made him completely fluent and he’d adjusted the controls to increase his speaking rate. He’d called his relatives in Korea and now he could talk as fast as his relatives!

I told him to put the controls back to the slow settings. If he continued to talk fast he’d experience adaptation, i.e., the effectiveness would “wear off.” If he used the device to slow down then his speech would improve over time and he’d develop carryover fluency.

DAF has two effects:

  • DAF stimulates auditory processing, which is underactive in stutterers, enabling you to talk any way you want.
  • DAF reduces speech motor activity, which is overactive in stutterers, enabling you to do fluency shaping therapy outside of the speech clinic.

If you do the former without the latter, talking fast without doing speech therapy, then over time you may develop adaptation or a device’s effectiveness “wearing off.”

But if you use both effects to slow down and use fluency shaping techniques then over time you should develop carryover fluency. Your speech should improve and you’ll need the device less and less.

Try to use DAF and fluency shaping techniques in your most stressful conversations. Then take the device off when the conversations become less stressful. Work on your carryover fluency.

Frequency-altered Auditory Feedback (FAF)

Joseph Kalinowski and his colleagues investigated stutterers’ auditory processing. In 1993 they tried DAF at different delays from 50 milliseconds (ms) to 200 ms. They found that DAF reduced stuttering at slow, normal, and fast speaking rates. Slowing down wasn’t necessary to improve fluency.

Dr. Kalinowski and his team then tried shifting the pitch at which adult stutterers heard their voices, up or down. They called this frequency-altered auditory feedback (FAF). FAF reduced stuttering around 70%, the same as DAF, at slow, normal, and fast speaking rates. Combining DAF and FAF boosted effectiveness to around 80%.

The Edinburgh Masker

In 1981 the television series That’s Incredible! featured Herb Goldberg, a Chicago stutterer who used an Edinburgh Masker. That show received more mail than any other episode of the series and Goldberg was invited back for a second show.

The Edinburgh Masker uses a throat microphone to detect vocal fold vibration (phonation). The device, which is pocket-sized, then synthesizes a sine wave (a humming sound) at the frequency of your phonation, and provides this to earphones. One study found that the device reduced stuttering 50%.[ref]Block, S., Ingham, R.J., & Bench, R.J. (1996). The effects of the Edinburgh Masker on stuttering. Australian Journal of Human Communication Disorders, 24, 11-18.[/ref] Another study found that the effectiveness didn’t diminish over a period of six months. [ref]Dewar, A., Dewar, A.D., Austin, W.T.S., Brash, H.M. (1979) The Long Term Use of an Automatically Triggered Auditory Feedback Masking Device in the Treatment of Stammering. British Journal of Disorders of Communication, 14(3), 19-26.[/ref]

Goldberg set up a non-profit to import Edinburgh Maskers from the manufacturer in Scotland. He sold about 2500 devices during the 1980s. He later told me,

I am in contact with over 500 people who use or have used the Masker. In most cases the end result is the person uses the device less and less as time passes due to less need for it.Personal correspondence, September 9, 1994.

What the Experts Think

A systematic review and meta-analysis examined 19 studies of electronic anti-stuttering devices:

Both DAF and FAF have been found to reduce stuttering during oral reading in the laboratory
with participants typically experiencing 40–85% reductions in stuttering.[ref]”Altered auditory feedback and the treatment of stuttering: A review,” Michelle Lincoln, Ann Packman, Mark Onslow. Journal of Fluency Disorders, Volume 31, Issue 2, 2006, Pages 71-89,[/ref]

Clinicians who treat stutterers like DAF. In 25 years I’ve sold thousands of DAF devices to schools. School SLPs typically have caseloads of thirty or more children, with a wide variety of speech and language disorders. The SLPs had one class in stuttering in grad school, or half a class, or no stuttering class. They don’t have time for lengthy training or time-consuming therapies. They take a School DAF out of its box and have the kid speaking fluently in minutes. You can read some of their reviews.

If You Tried DAF and You Still Stutter

There are dozens of AAF devices and apps. Prices range from free (apps) to more than $5,000.

  • DAF is technically not difficult to produce. It doesn’t even require a computer. Yet I’ve seen plenty of DAF devices and apps that produced more noise than signal. I tried one that didn’t make any sound at all. I’ve seen people confuse amplification with DAF.
  • FAF requires advanced software running on a powerful computer optimized for signal processing. Mobile phones are optimized for data processing; getting FAF is run without glitches on a phone is something only a few engineers in the world can do. One device that is advertised as producing FAF shifts frequencies, not pitch.
  • The Edinburgh Masker is a simple (engineers would call it “elegant”) design that doesn’t rely on a computer. Yet I’ve seen countless Ph.D. speech-language pathologists confuse the sine waves produced by the Edinburgh Masker with white noise.

If a device doesn’t help your fluency, check if it’s doing what the seller claimed it would do. Then check if you’re hearing the AAF or if you’re hearing noise.

Look for published studies about a device you’re considering buying.

When selecting a microphone, two parameters are important:

  • The microphone’s frequency range must pick up your full vocal range, especially your phonation frequencies in the 100-200 Hz range.
  • The microphone must pick up your voice clearly while rejecting background noise.

The best option is a throat microphone, which picks up your phonation directly and can be worn comfortably all day.

When selecting a headphones or earphones, three parameters are important:

  • The headphones or earphones must not block your hearing. You shouldn’t lose hearing to improve your speech.
  • The frequency range must reproduce your full vocal range plus FAF shifting, in other words, 50-400 Hz for the shifted fundamental frequency of phonation for men, women, and children.
  • Binaural (two ears) earphones are 25% more effective than monaural (one ear) earphones.

Bone conduction headphones and open sports earphones are excellent choices because these don’t block your hearing, and sound excellent at the vocal frequencies. Acoustic coil audio is an excellent choice for inconspicuous use; this is what television announcers and undercover police officers use (and the Edinburgh Masker).

Use fluency shaping skills with your DAF device. If it’s been years since you had speech therapy you likely need a refresher.

Use your device in both stressful and easy situations. Practice in an easy situation will make it easier to use a device in a stressful situation. Using a device to speak fluently in a stressful situation will let you take the device off in easy situations and develop carryover fluency.

Most important, talk more! Ask your employer for a position that requires talking to customers. Start a hobby that requires talking, for example, tutoring an immigrant to learn English. Or just get out and talk to strangers.

Discounts and Subsidies

Most Americans who stutter qualify for free DAF devices. See the Discounts section of this website.

Device Reviews

For more about specific devices please see the reviews section of this website.

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