TL;DR. DAF (delayed auditory feedback) plays your voice back with a short delay; FAF (frequency altered feedback) plays it back at a shifted pitch with no delay. Both can reduce stuttering immediately for many adults, and the two combined sometimes help more. DAF is the common default. Neither is a cure — each is a practice tool, not a replacement for a speech and language therapist.
Two ways to alter what you hear
When you speak, you constantly monitor your own voice through your ears. Both DAF and FAF deliberately tamper with that returning signal, which is why they sit under the same umbrella term: altered auditory feedback (AAF). The two differ in what they change.
- DAF — delayed auditory feedback. Your voice comes back through headphones a fraction of a second late, typically around 100 ms. It changes the timing of what you hear.
- FAF — frequency altered feedback. Your voice comes back at a different pitch — shifted up or down, commonly by roughly half an octave — but in real time, with no delay. It changes the sound of what you hear.
Both are well documented as producing an immediate reduction in stuttering for a substantial share of adults, though, as with everything in stuttering, response varies from person to person.
What DAF does
DAF inserts a deliberate gap between speaking and hearing yourself. That delayed echo acts as an external pacing cue. Two things tend to follow: speech slows down, because the speaker waits for the echo before pushing on, and articulation softens, because the auditory system stops fighting a desynchronised internal signal.
The delay is the only setting that really matters. Around 100 ms is the common starting point in research and practice; smaller delays feel more natural and transfer better to ordinary speech, while larger delays slow you down more dramatically. For the full picture, see delayed auditory feedback and how DAF works.
What FAF does
FAF leaves the timing alone and changes the pitch. As you speak, an app or device raises or lowers the frequency of your voice — say, half an octave up — and feeds that shifted version straight back into your ears.
There is no slowing-down mechanism here. The leading explanation is that hearing a different voice rendering your own words partially recreates the choral speech effect: stuttering drops sharply when people read in unison with another speaker. A pitch-shifted echo is, in a loose sense, a second voice. FAF is widely studied alongside DAF, and reviews of altered-auditory-feedback research report measurable immediate effects for many users — though the size of the effect, and who responds, is individual.
Combined AAF: delay plus pitch shift
You do not have to choose. Combined altered auditory feedback applies a delay and a frequency shift at the same time. Several research devices and apps offer this, and a subset of people find the combination more effective than either setting alone.
The honest summary from the evidence is that no single configuration wins for everyone. Some speakers respond strongly to delay, some to pitch shift, some to both together, and a minority find altered feedback unhelpful or distracting — which is fine, because there are plenty of other useful tools. The practical takeaway is to experiment: try DAF on its own, FAF on its own, and the two combined, and notice which one your speech actually responds to.
It also helps to test each setting on different speaking tasks. Reading a paragraph aloud, talking about your day, and rehearsing a phone call all stress speech in different ways, and the configuration that feels best while reading is not always the one that holds up in conversation. A few minutes spent comparing them honestly is far more useful than assuming one mode must be superior.
The evidence, honestly stated
Public clinical sources are consistent and measured. The NIDCD describes electronic altered-feedback devices as one option among stuttering management approaches, not as a fix. ASHA frames technology as a support within therapy rather than a standalone treatment. Peer-reviewed reviews of altered auditory feedback document a genuine immediate fluency effect for many adults under both delay and frequency shift.
What the literature does not support is durable change from the feedback alone. The immediate effect is one thing; lasting improvement that carries over to ordinary, unaided speech is another, and that comes from regular practice with technique work — ideally guided by a speech and language therapist. For the research detail, see the science of DAF.
How apps implement DAF and FAF
In software, both effects are cheap to compute and run entirely on your phone. A DAF app buffers your microphone signal and replays it a set number of milliseconds later. An FAF app runs your voice through a real-time pitch-shifting algorithm and outputs the shifted result with as little processing latency as possible. Combined AAF chains the two.
Apps make this flexible in a way that hardware rarely matches: you can dial the delay in milliseconds, set the pitch shift in semitones or octaves, and switch a routine from DAF to FAF to combined in seconds. That makes a phone a sensible place to discover which configuration suits you before considering any dedicated hardware. For a wider comparison of options, see the best stuttering apps.
The wired-earbud caveat
This one matters for both DAF and FAF. Use wired earbuds, not Bluetooth. Wireless headphones add their own variable latency on top of whatever the app is doing. For DAF that corrupts the delay you set; for FAF the extra lag undermines the real-time feel that the pitch-shift effect relies on. The cheapest pair of wired earbuds you own will give a cleaner, more reliable result than premium wireless ones. If your phone has no headphone jack, a basic wired adapter is enough.
Where altered feedback fits
Whether you land on DAF, FAF or both, the framing is the same: altered auditory feedback is a tool inside a practice routine, not the routine itself, and not a substitute for professional help. A workable pattern is a few minutes practising a chosen technique with altered feedback on, then a couple of minutes repeating the same material with the headphones off — the transfer work where any gains start carrying into natural speech.
Neither DAF nor FAF cures stuttering, and no responsible provider claims otherwise. Used sensibly, alongside exercises and, where possible, a stuttering-specialist therapist, altered feedback is a useful and inexpensive thing to have in your toolkit.
Where to learn more
- Delayed auditory feedback (DAF) — settings, evidence and how to use delay for practice.
- How DAF works — the mechanism in detail, including the choral speech effect.
- The science of DAF — the evidence base, key papers and effect sizes.
- Best stuttering apps — how to compare apps that offer DAF, FAF and combined feedback.
