Speech-Language Pathology/Stuttering/Sound Quality, Background Noise
A study comparing two DAF/FAF anti-stuttering devices made by different companies found one device to be more than twice as effective as the other. The difference in effectiveness was likely due to differences in sound quality. It's like the difference between listening to Beethoven at a concert hall, or as a cellphone ringtone.
Different anti-stuttering devices have different frequency ranges. Generally, the bigger the microphone and earphones, the wider the frequency range.
The range of human voices is about 125 to 5000 Hz. If you use one-octave FAF downshifting you'll need 60 to 5000 Hz frequency range. Some anti-stuttering devices have this range.
In contrast, hearing aids typically have a frequency range of 200 to 7000 Hz. The frequency ranges typically aren't flat, but instead are tuned to sound best between 3000 and 4000 Hz (where most people lose their hearing). Hearing aids can't reproduce the low range of human voices, especially the fundamental frequency of phonation that's key to stuttering therapy.
Monaural vs. Binaural Sound
Binaural (two ears) sound is 25% more effective than monaural (one ear) sound. Some devices can be either binaural or monaural. Other devices are monaural only.
A research based on eleven participants has tested which ear is better, if you use a monaural anti-stuttering device ("Effect of monaural and binaural altered auditory feedback on stuttering frequency", A. Stuart). Non significant differences in stuttering frequencies for right versus left monaural conditions has been found (p = 0.54). Nevertheless, the right ear connects to left side of the brain, which processes speech and other rapidly changing auditory signals, and, hypothetically, is the better ear for using an anti-stuttering device. In contrast, the left ear is better for tones and music. But brain scans have found that stutterers have more right-hemisphere activity during speech (in contrast, non-stutterers have more left hemisphere activity during speech). If stutterers process speech on the right side of their brains, then a monaural anti-stuttering device should be worn in the left ear. It would be interesting to investigate left- and right-ear altered auditory feedback with brain scans. This might shed light on the question of whether the unusual right-hemisphere activity of stutterers during speech is speech processing or heightened emotions (such as speech-related fears and anxieties.)
(Or maybe if you're gay and live on the east coast, you wear your anti-stuttering device in your left ear, but…)
Some anti-stuttering devices work well in quiet speech clinics, but are unusable in a noisy restaurant. A variety of features help solve the background noise problem.
Positioned correctly, a noise-canceling directional microphone eliminates background noise at the source.
In contrast, omnidirectional microphones pick up background noises. Hearing aids typically use omnidirectional microphones designed to pick up environmental sounds louder than your voice.
A "push to talk" button also eliminates background noise. You push a button and the device switches sound on. Llet go of the button and the sound switches off.
In noisy environments you're usually in a group. E.g., you go out to a restaurant with three friends. You talk one-fourth of the time. Most of the time you sit and listen, with clear hearing. When you have something to say, you push the button.
A push-to-talk button also works well for a child in school, who mostly listens and occasionally is called on by the teacher.
Most anti-stuttering devices have high-frequency filters to reduce noise above your vocal range.
Voice activation switches on sound when the user talks, and switches off sound when the user stops talking. Voice activation works well if the device has a noise-canceling directional microphone. If the device has an omnidirectional microphone, loud noises switch on sound.
Some anti-stuttering devices analyze your speech and switch sound on when (or before) you stutter, and switch sound off when you're speaking fluently (or not speaking). Typically such devices analyze vocal parameters such as pitch and volume to determine whether your vocal folds are tense or relaxed.
Some anti-stuttering devices have dynamic expansion. This makes loud sounds louder and quiet sounds quieter. If you're using a noise-canceling directional microphone this makes your voice louder and background sounds quieter. With an omnidirectional microphone it can make your voice quieter and background noise louder.
Listening to someone talk, while you wear a DAF device that's picking up the other person's voice, is like reading the following:
That says, "difficult to hear another person speaking." You hear the person speaking twice, with the words out of sync.
In contrast, small FAF pitch shifts, without a DAF delay, have little impact on your ability to hear. It's like hearing music played on a violin vs. on a viola. Or hearing your friend's voice drop a little when recovering from a cold. This is known as "acoustically transparent."
Warranties, Returns and Repairs
Read price sheets and warranties carefully. Are there hidden fees? How long is your trial period for returning the device? Do you get a full or partial refund?
Toss the device onto a hard floor (before you pay for it!). How breakable is it? How long is the warranty? Are repairs covered in all situations, or excluded if it was your fault that the device broke?
How easy would it be to lose a device, especially for a child?
Ask about the company's return rates.