In aural rehabilitation, the Ling six-sound test, comprising /m, a, u, i, ∫, s/, is one of the most widely used tools for quickly assessing patients’ audibility of speech sounds (Ling. Speech and the hearing-impaired child: Theory and practice. Washington, DC: Alexander Graham Bell Association for the Deaf, 1976 http://bit.ly/1UGcgBq). However, when the test is applied to patients whose native language is not American English, its clinical validity is often challenged; numerous studies have shown that people perform less accurately when pronouncing or identifying nonnative sounds (Best. HLSRSR 1992; 110:89-108 http://bit.ly/1PpA9cm; Goto. Neuropsychologia 1971; 9:317-323 http://1.usa.gov/1UGbRiw).
RECENTLY DEVELOPED SOUND TESTS
To eliminate the adverse effects induced by linguistic differences and provide a more precise diagnosis, several sound tests for non-American English speakers have been developed, such as the Australian English Sound Test and the Chinese Sound Test (Agung. ANZJ Audiol 2005;27:33-41 http://bit.ly/1PpAhbL; Hung. J Speech Lang Hear Res 2016; 59: 349-358 http://1.usa.gov/1UGcOXU). The Australian English sound test has taken accent-dependent frequency distinctions into consideration. The authors have suggested replacing the American /u/ with the Australian /ɔ/ to represent the low-frequency range, because the Australian /u/ is generally higher in the second formant than that of the American /u/.
For the Chinese Sound Test, after analysis of the acoustic-phonetic characteristics of Mandarin phonemes, the six test sounds /u, ə, a, i, th, s/ were chosen because of their complementary frequencies that span the entire Mandarin speech spectrum.
As frequency-specific speech materials are widely used by various aural rehabilitation specialists, in addition to frequency range, Hung et al. have also considered speaker variability—that all test sounds show relatively low interspeaker variability. This indicates that the frequency components of the sounds remain consistent across speakers, which increases the test reliability. By capitalizing on the frequency specificity and the low degree of interspeaker variability of the Chinese test sounds, the clinicians are able to compare and cross-check the test results with more confidence, as shown in Figure 1.
PEDIATRIC AUDIOLOGICAL EVALUATION
Sounds such as the six test sounds or the Chinese test sounds can not only reliably function as test items for assessing patients’ auditory access to speech, but also serve as materials in pediatric audiological evaluation. From the psychological perspective, in contrast to pure tones or warble tones, the speech sounds can attract children's attention more easily and provide more accurate results regarding their auditory responses.
Consequently, numerous pediatric audiologists often utilize speech audiometry assessments such as speech detection threshold (SDT) and speech recognition threshold (SRT) along with conventional behavioral evaluation methods to either obtain more complete results or increase diagnostic accuracy. In particular, not only can the SDT facilitate validating audiograms, but it can also be used to cross-check electrophysiological assessment results. Concerning difficulties in obtaining complete behavioral audiological information, the SDT can serve as an initial estimation of the child's hearing thresholds so hearing aid fitting can be initiated promptly.
The SDT can also be obtained in unaided or aided conditions. In the unaided condition, the SDT shows the need for a child to have a hearing instrument to attain adequate auditory access to speech sounds. In the aided condition, it provides information on how well the hearing instrument is fitted and its performance across all frequency regions of speech.
As mentioned previously, all six sounds in the Chinese Sound Test are crucial in their own frequency regions. Completing the SDT evaluation using all test sounds can be a challenging task because of children's short attention spans. We have observed that the low-mid frequency sounds /u/ and /a/ are most effective in attracting the attention of Mandarin speaking infants and toddlers, and thus are helpful in conditioning the child and getting higher accuracy.
A second set of sounds used frequently during the assessment is the mid-high sounds /th/ and /s/. The SDT results of these sounds indicate how the child reacts to the high frequencies, which are directly related to speech intelligibility. For example, in the aided condition, the ability to detect the /th/ and /s/ sounds only at a level of 65 dB HL indicates that the child does not have sufficient auditory access to the high frequency signals through the hearing aids. Audiologists should be aware that further adjustment is needed or a cochlear implantation may be considered. However, employing one low frequency speech sound such as /a/ and one high frequency speech sound such as /s/ could facilitate an assessment to obtain the most useful information within the child's short attention span.
HEARING INSTRUMENT PERFORMANCE VERIFICATION
Speech sound tests can verify the performance of a particular hearing instrument in infants and toddlers who do not have the capability to perform word discrimination tasks. With rapid developments in technology, more features are now available in hearing instruments that are intended to provide additional auditory access to the users.
For example, frequency compression has recently become a widely used feature in pediatric hearing aid fittings worldwide because it may improve the audibility of high frequency speech for children with severe to profound hearing loss. Studies have demonstrated that children can benefit from this technology, which exhibits what parents consider a “good” aided audiogram (Simpson. Int J Audiol 2005; 44:281-292 http://1.usa.gov/1UGcSXC; Glista. Int J Audiol 2009; 48:632-644 http://1.usa.gov/1UGcOXV). In other words, children's aided responses fall within the speech banana.
A good audiogram, however, does not always guarantee adequate access to the speech sounds because of the static representation of dynamic speech processing in different hearing environments (Vermiglio. J Acoust Soc Am 2012;23:779-788 http://1.usa.gov/1UGbRiy). Thus, in conjunction with pure-tone audiometry, audiologists should use speech audiometry as a cross-check evaluation to ensure that the hearing instruments have sufficiently amplified all speech sounds. Any discrepancy between the audiogram and the SDT results indicates that the child's auditory performance may not be as good as the audiogram suggests. Audiologists should not ignore this discrepancy and parents should not be deceived by the purportedly “good” audiogram.
WISE USE OF SPEECH ASSESSMENTS
Speech sound test results provide information about children's auditory access to speech relevant to language development. As the cross-check procedure is a principle of pediatric audiological assessments, all audiologists should remember that speech sounds can verify the accuracy of an audiogram and electrophysiological test results. Despite the fact that speech assessments are valuable to audiologists, the materials are generally only available in American English. The increased awareness of hearing health worldwide should influence audiologists to start considering the needs of multilingual patients because they require a test that is adapted to their language. The initial step in using speech assessments wisely is to localize the test materials. The localization will significantly increase not only the diagnostic validity, but also the efficacy of the assessments.