Frequency Compression and Transposition

High-Frequency Speech Sounds

[Page 781]Sometimes hearing aids are limited in their ability to provide sufficient amplification for high-frequency sounds or cannot do so without audible squealing as the amplified signal is picked up by the microphone and reamplified (known as feedback). Therefore, at least three challenges limit the audibility of high-frequency speech sounds: (1) They are usually relatively soft compared to the levels other speech sounds, (2) hearing loss is usually worst at these frequencies, and (3) because of the first two challenges, significant amplification is needed at these frequencies.

Communication challenges can result when children and adults cannot hear high-frequency speech sounds. Speech sounds most impacted by high-frequency hearing loss are those that are produced by the noisy turbulence of air that passes through various partial constrictions in the oral tract. The noisy turbulence, known as frication, constitutes an entire class of consonants called the fricatives, which includes [s], [z], [S], [T], [D], [f], [v], and others. Frication is also present to varying extents in other consonant classes, namely, the affricates (e.g., [dZ], [tS]) and stops ([k], [g], [t], [d], [p], and [b]). Due to the acoustic properties highlighted earlier, [s] and [z] are particularly vulnerable to hearing loss, even when the severity is mild-to-moderate. At the ends of words, these sounds can carry a lot of information that affects the meaning of an entire utterance. For example, in English they can denote possession (“Jonathan’s”), plurality (“badgers”), tense (“plays”), and many other morphological markers. Adults who miss this low-level acoustic information may be able to recover the meaning of the utterance by using linguistic and acoustic contexts. However, the need to pull this information from other sources in an ongoing communication situation might make listening more effortful, especially when speech is degraded by challenges such as noise, reverberation, and an unfamiliar accent.

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