Influence of Amplification on Word Intelligibility and Phonetic Contrast Errors in Dysarthria

By Turner, Greg; Martin, Hilary et al. | Journal of Medical Speech - Language Pathology, December 2008 | Go to article overview

Influence of Amplification on Word Intelligibility and Phonetic Contrast Errors in Dysarthria


Turner, Greg, Martin, Hilary, de Jonge, Robert, Journal of Medical Speech - Language Pathology


Voice amplification can improve speech intelligibility when only reduced loudness underlies the intelligibility deficit (Adams, 1997). However, a recent study by Cariski and Rosenbek (1999) documented amplification-based improvements in speech intelligibility for a speaker with dysarthria exhibiting deficits in both articulation and loudness. To better understand the relationship between amplification and word intelligibility, 70 prerecorded words spoken at a conversational intensity level by three speakers with dysarthria exhibiting intelligibility deficits resulting from segmental and suprasegmental speech deficits were artificially amplified. Each of the 50 listeners was seated in a single-walled sound booth and listened to the 70 words produced by each speaker. For each word set, the listener was assigned randomly to one of five different amplification conditions (35, 45, 55, 65, & 75 dBHL) presented via a calibrated audiometer. Word intelligibility and phonetic contrast errors were evaluated through a force choice paradigm (Kent, Weismer, Kent, & Rosenbek, 1989). Results indicated little change in both word intelligibility and phonetic contrast errors across the five intensity levels for all three speakers. These findings support the limited benefit of amplification for those speakers with word intelligibility deficits resulting from impairments in the speech production system beyond respiration and laryngeal functioning.

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Respiratory and/or phonatory impairment to the speech production system can result in reduced vocal intensity for speakers with dysarthria. Reductions in intensity can adversely influence speech intelligibility (Pickett, 1956; Ramig, 1992). Behavioral treatment techniques aimed at increasing vocal intensity lead to increases in intelligibility (Yorkston, Hakel, Beukelman, & Fager, 2007). However, for some speakers exhibiting reductions in loudness, traditional therapy aimed at reducing impairment or teaching compensatory strategies may not be successful or is not recommended. One alternate form of intervention to target vocal intensity for this population is the use of voice amplification (Adams, 1997; Cariski & Rosenbek, 1999; Green & Watson, 1968; Spencer, Yorkston, & Duffy, 2003).

Voice amplification typically involves a portable amplification system consisting of a microphone, amplifier, and loudspeaker, worn by the speaker with dysarthria (Duffy, 2005). Typically portable amplification systems provide up to 15-20 dB of amplification between 100 Hz and 10 kHz. The level of amplification remains relatively constant across the frequency range; however, some amplifiers offer the ability to alter amplification for certain frequencies. Finally, some high-end amplifiers like the Speech Enhancer (Electronic Speech Enhancement, Inc.) not only amplify but enhance the clarity of the voice (see Bain, Ferguson, & Mathisen [2005] for a description of how a speech processor improves the clarity of user's speech).

In a review paper on management techniques for treating respiratory/phonatory deficits in dysarthria, Spencer and colleagues (2003) found that voice amplification is recommended only for individuals with dysarthria who exhibit "reduced loudness and adequate articulation" (p. lviii). Based on a typical amplifier, speech sound distortions resulting from articulatory impairment will be amplified but not repaired, leading to little change in such measures as speech intelligibility. This is in contrast to the effects of training individuals with dysarthria to speak more loudly (Sapir, Spielman, Ramig, Story, & Fox, 2007; Tjaden & Wilding, 2004). In studies such as these, increases in intensity are associated with temporal and spectral changes, and quite often improvement in speech intelligibility.

Cariski and Rosenbek (1999) completed one of the only published studies quantifying change in intelligibility as a result of amplification for an individual with reduced intensity, hypernasality, and "very imprecise articulation" (p. …

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