Masking

Masking

Eighty percent increase in fluency
Eighty percent increase in fluency

Masking

Another consistent finding is that people who stutter become more fluent when their own auditory feedback, presented  through headphones, is masked by external sound, although this effect only becomes significant if the sound level of the masking noise is sufficient to block out the speaker’s speech signal. Maraist and Hutton (1957) found their 15 subjects who stuttered consistently increased their fluency as the loudness of the masking noise was increased, and there is corroborating evidence from Burke (1969) and Murray (1969) for these findings. Cherry and Sayers (1956) in a series of seminal experiments manipulating auditory feedback found a strong fluency enhancing effect when a pure tone was presented at a level near the threshold of pain. A key finding was that stuttering decreased significantly when the subjects only heard lower frequencies (below 500 Hz) as opposed to only hearing higher frequency sounds (above 500 Hz). This, Cherry and Sayers argued, demonstrated the differential role of bone conduction, which more effectively transmits lower frequency sounds, and air conduction which was assumed to be associated with a broader range of frequencies. In fact, Cherry and Sayers’ findings of differential effects in stuttering reduction as depend-ent on the frequency ranges (high or low) of the masking noise have not been verified in subsequent experiments Barr & Carmel, 1968; Conture 1974

Nonetheless, the masking effect itself seems to be a consistent one and like DAF and more recently FAF it has found use as a commercially available fluency aid, most notably through the production of the Edinburgh masker, marketed in the 1970s and 1980s 

Despite the general agreement that masking auditory feedback can be an effective way of reducing moments of stuttering, opinions as to what forces are responsible for the effect differ widely. Some saw the early findings, par-ticularly those of Cherry and Sayers, as evidence for an auditory-perception theory of the disorder. However, findings from a number of early studies have suggested that increased fluency could be attributed to reparameterization of vocal strategies under masking, rather than to factors directly relating to auditory processing. For example (like DAF) masked speech was found to be associated with slower speech rates and slower syllable durations (Hanley & Steer, 1949; Ringel & Steer, 1963), greater vocal intensity (Atkinson, 1952; Garber, Siegel, Pick, & Alcorn, 1976), and increased fundamental frequency or higher pitch of the voice (Atkinson, 1952; Ringel & Steer, 1963). In other words, some claimed that the cause for the increase in fluency lay in changes within the realm of speech production rather than within auditory process-ing. Interestingly, slower speech rate, increased loudness and increased pitch are all features that have been associated with the speech of the hearing impaired

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