Re-evaluating the Efficacy of Frequency Transposition

Abstract

Patients with a precipitous high-frequency hearing loss pose a unique challenge for dispensing audiologists. Hearing aids rarely provide sufficient high-frequency gain without over-amplifying the lower frequencies, which results in poor sound quality or significant sound distortions. The occlusion effect and upward spread of masking may also occur. For these patients, the high frequencies are not easily reachable. Recently it has been suggested that a precipitous loss may make the high frequencies non-functional, "dead," or unaidable. Some studies suggest a decrease in word recognition scores when these "dead" regions are amplified. This result casts doubt on the usefulness of amplifying such a region. For people with either an unaidable or unreachable high-frequency hearing loss, information carried in those frequency regions would not be available despite the use of amplification. What Is Frequency Lowering? Audibility of unaidable or unreachable high-frequency information is achieved by converting that information to the lower frequencies where hearing is aidable or reachable. This conversion process is called frequency lowering. For example, information carried at 4000 Hz may be "lowered" to 2000 Hz so that it is heard as 2000 Hz. It is important to recognize that frequency lowering doesn't restore hearing in the original high frequencies. Instead, the signals are heard at a lower frequency. For this reason, a frequency-lowered signal will initially sound "unnatural" to most listeners. Historical Results The concept of frequency lowering is not new. Braida, Durlach, Lippmann, Hicks, Rabiowitz, and Reed (1979), in an excellent review of frequency-lowering techniques, dated studies on the topic beginning in the 1950s. Early attempts at frequency lowering implemented on analog technologies made it difficult to achieve the desired signal-processing results without introducing significant artifacts, such as unnatural-sounding speech and distorted temporal and rhythmic

Effects of Frequency Compression and Frequency Transposition on Fricative and Affricate Perception in Listeners With Normal Hearing and Mild to Moderate Hearing Loss

“…It is important to note that neither technology was designed for mild to moderate losses. In fact, the use of FT for the individuals in this study is inconsistent with the recommended criteria established by the manufacturer (Kuk et al ., 2009a). For both NH and HI listeners, FT degraded the perception of /s/ and /z/, resulting in a significant increase in place of articulation confusions with all the voiceless (except /t∫/) and voiced phonemes, respectively, in the stimulus set.…”

“…Historically, attempts to implement frequency lowering have been limited to adults and children with severe to profound sensorineural hearing loss, SNHL, (e.g., Braida et al ., 1978; Parent et al ., 1998; McDermott et al ., 1999; Simpson et al ., 2005, 2006; Kuk et al ., 2009a). For these individuals, arguments for the use of frequency lowering are relatively easy to make since speech perception deficits increase as high-frequency audibility decreases (Turner & Robb, 1987; Dubno et al ., 1989; Ching et al ., 1998; Horwitz et al ., 2002).…”

“…This study was conducted to determine if two modern frequency-lowering techniques, frequency transposition (FT) as implemented in Widex hearing aids and nonlinear frequency compression (NFC) as implemented in Phonak hearing aids, which were both originally designed for listeners with severe to profound SNHL (Simpson et al ., 2005; Kuk et al ., 2009a), have the potential to improve fricative and affricate perception for listeners with mild to moderate SNHL. A brief overview of the two techniques is provided here; see Alexander (2013a) for a more extensive discussion and the methods below for illustrative examples.…”

Assessing the effect of noise-reduction to the intelligibility of low-pass filtered speech

“…Statistical significance was determined by using the percent recognition score as the dependent variable, and LPF cut-off frequency, SNR level and signal processing condition as the three within-subject factors. Three-way analysis of variance (ANOVA) with repeated measures indicated a significant effect (F [1,9]=8.59, p<0.05) of LPF cut-off frequency, SNR level (F [1,9]=8.59, p<0.001), and signal processing condition (F [4,36]=11.43, p<0.001), a non-significant (p>0.05) interaction between LPF cut-off frequency and SNR level, between LPF cut-off frequency and signal processing condition (p>0.05), between SNR level and signal processing condition (p>0.05), and among LPF cut-off frequency, SNR level and signal processing condition (p>0.05).…”

“…For instance, a new trend, i.e., frequency compression (FC) technique, is emerging in clinical hearing aids fitting. Frequency compression delivers the inaudible information from unaidable HF regions to the aidable LF regions [1], aiming to help HI listeners to use their residual hearing in LF regions to hear HF sounds. In addition, the contribution of low-frequency residual hearing for speech recognition was demonstrated in studies of combined electric-and-acoustic hearing, whereas HI patients were implanted with electrodes to restore their HF hearing, and utilized their low-frequency residual hearing (typically 20 to 60 dB hearing level up to 750 Hz, and severe-to-profound hearing loss at 1000 Hz and above).…”