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 their customers and ask if they wanted the new release on a tape drive or a CD. The question was simple but I stuttered severely and struggled through each call.

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 imaged the brains 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. Fluency shaping therapy doesn’t address this problem. This was why I couldn’t produce slow, fluent speech outside of the speech clinic. I have an auditory processing disorder. I prefer to watch movies with subtitles on. At noisy events I can’t understand what people are saying and I can’t talk. 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. DAF should be a part of every stuttering therapy program.

Stuttering is a multifactorial disorder. The theme of this book is that different speech therapies treat different factors causing of stuttering. Stuttering therapy fails when a therapy treats only one factor. Stuttering therapy works when multiple therapies are combined to treat multiple factors causing 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 could “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 “wears 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.

Do your speech therapy in your most stressful conversations. Combine fluency shaping and DAF for slow, fluent speech. Tell the listener that you stutter to reduce stress. Then take the device off and 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 close to 80%.

The Edinburgh Masker

In 1981 the television series That’s Incredible! featured Herb Goldberg, a Chicago person who stutters 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%.(1) Another study found that the effectiveness didn’t diminish over a period of six months.(2)

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.(4)

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.(3)

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 (downloadable apps) to more than $5,000. Look for published studies about a device you’re considering buying. If DAF isn’t helping you, check that the device is doing what the manufacturer claims it should do.

A DAF device is relatively simple to build or program (FAF is not simple) but the microphone and earphones are anything but simple. Radio journalists use microphones that are about a foot long, and use big headphones. That’s what it takes to get great sound. As microphones and earphones are miniaturized, the low end of the frequency range is lost and microphones pick up more background noise. That’s the laws of physics. As a rule of thumb, any DAF device that you can hide from listeners will have poor quality sound and not do much for stuttering.

  • DAF should provide your voice to your ears delayed in the range 50ms to 100ms. Bluetooth headphones add a delay of 500ms or more. A delay of 500ms won’t help fluency.
  • You should hear your full vocal range. Adult male voices have a fundamental frequency around 125 Hz. Miniature microphones and earphones can’t pick up or reproduce these low frequencies. If you’re using FAF you’ll need one octave lower, i.e., the device should reproduce sounds down to 62.5 Hz.
  • You should hear a clear signal without distortion.
  • You should not hear background noise. I tried a DAF device that picked up the sound of a waiter stacking dishes a hundred meters away louder than my voice.
  • FAF requires advanced software running on a powerful computer optimized for signal processing. Mobile phones are optimized for data processing. Getting FAF is run 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.
  • I’ve seen countless Ph.D. speech-language pathologists confuse the sine waves produced by the Edinburgh Masker with white noise.
  • If you’re using a device outside of a speech clinic and not on telephone calls, the headphones must not block your hearing. You shouldn’t lose hearing to improve your speech. This is especially important for children in school. Bone conduction headphones are good because these don’t block your hearing and sound excellent at vocal frequencies.
  • Binaural (two ears) earphones are 25% more effective than monaural (one ear) earphones.

Use fluency shaping therapy with your DAF device. A speech-language pathologist should train you to use the device.

Use your device in stressful situations. Practicing in a speech clinic or at home alone won’t carryover to stressful situations.

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 from state special telephone equipment distribution programs, vocational rehabilitation, the Veterans Administration, or other programs. See the Discounts section of this website.

Device Reviews

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

(1) 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.

(2) 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.

(3) “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,

(4) Personal correspondence, September 9, 1994.

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