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Hearing Thresholds of OAEs
dala, 2015). Even though SFOAEs are considered to be the steady-state version of TEOAEs, there is great promise in the ultimate clinical usefulness of SFOAEs.
In contrast, within a decade after the discovery of TEO- AEs, reasonably affordable and simply operated commercial equipment based on averaging procedures used for evok- ing conventional auditory brainstem responses (ABRs) was available. Because the TEOAE is measured after the tran- sient-eliciting stimulus occurs, ears from different individu- als produce a response that exhibits unique spectral patterns (see Figure 3C,D). This idiosyncratic property makes it dif- ficult to develop a set of standardized metrics that describe the average TEOAE for normal-hearing individuals. Due to this difficulty in determining “normal” TEOAEs in terms of frequencies and level values, they are most often described as being either present or absent. Thus, one of the most pop- ular uses of TEOAEs clinically is as a test for screening audi- tory function, particularly in newborns.
DPOAEs commonly measured at the 2f1−f2 frequency (an arithmetic formula in which the DPOAE frequency is equal to two times the value of the lower frequency f1 primary tone minus the value of the higher frequency f2) and elicited by pre- senting simultaneously two long-lasting pure-tone bursts at the lower (f1) and higher (f2) frequencies are likely produced by the nonlinear aspects of the OHC transduction process in which new frequencies are generated that are not present in the input signal. The frequencies and levels of the tone bursts or primary tones are important in that the largest DPOAEs are elicited by f1 and f2 primaries that are within about one- half octave of each other (e.g., the optimal frequency ratio used for humans is f2/f1 = 1.22) and with levels (L1 and L2) that are offset. For example, typical clinical protocols measure the 2f1−f2 DPOAE, which is the largest DPOAE in human ears, in response to primary-tone levels of L1 = 65 and L2 = 55 dB sound pressure level (SPL; Stover et al., 1996).
DPOAE levels in the form of the DP-gram are measured, as a rule, from about 800 Hz to 8 kHz at 6-10 frequency points/ octave. The test frequency is usually represented by the f2 frequency, which is assumed to correspond to the frequency region on the BM at which the TWs of the primary f1 and f2 tones maximally overlap. This assumption is based on a combination of theoretical considerations, experimental studies, and observations of the generation of DPOAEs in pathological ears. Although typically measured to 8 kHz to match the upper limit of the conventional audiogram, the high-frequency limit for measuring DPOAEs can be extend-
ed to 20 kHz (Dreisbach and Siegel, 2001). Given that high- frequency hearing for test stimuli > 4 kHz is most vulnerable to cochlear pathologies such as pharmaceutical-induced ototoxicity, it is notable that well-developed monitoring pro- grams have incorporated test frequencies up to at least 10 kHz (e.g., Konrad-Martin et al., 2014).
Hearing Screening
As noted above, one of the most widely used applications of evoked OAEs is in newborn hearing screening. Because early identification and habilitation (treatment) are directly linked to the successful development of the language and speech skills of a hearing-impaired child, a reliable method for identifying newborn hearing loss is essential.
Traditionally, the ABR has been used to screen infants iden- tified as being “at risk” for hearing loss. However, such high- risk registers identify, at most, only one-half of the children eventually proven to have significant sensorineural hear- ing loss (Mauk et al., 1991). During the 1990s, the notion of using TEOAEs to test all newborns was proposed in the form of developing “universal newborn hearing screening” programs. In this manner, a rapid pass/fail decision could be achieved that would reveal whether a given threshold of hearing was less than or greater than ~30 dB hearing loss (Brass and Kemp, 1994). Indeed, currently, TEOAEs are be- ing used in many national (e.g., Prieve and Stevens, 2000) and international (Grandori and Lutman, 1996) settings as indicators of hearing difficulties in newborns.
Overall, TEOAEs have been used much more extensively than DPOAEs in newborn hearing screening. However, with the introduction of specially designed DPOAE instrumenta- tion, they have become an acceptable estimator of newborn hearing capability (Gorga et al., 2000). Most certainly, the Rhode Island Hearing Assessment Project (Vohr and Maxon, 1996) has been a model program in the application of OAEs to the early hearing detection and intervention (EHDI) ap- proaches developed in the United States. However, screen- ing protocols that optimally combine TEOAEs or DPOAEs and automated ABR (AABR) testing in a two-stage protocol (e.g., TEOAE or DPOAE testing followed by AABR testing of initial failures) are still being evaluated for their optimal effectiveness (Akinpelu et al., 2014). Furthermore, results showing that 23% of those identified with permanent hear- ing loss at approximately 9 months of age passed the AABR but failed the OAE test at birth (Johnson et al., 2005) illus- trate the need for continued surveillance of hearing status
48 | Acoustics Today | Fall 2017