While frequency lowering to accommodate cochlear dead zones is relatively common, frequency raising is not—so far. 

By Marshall Chasin, AuD

Starting with the Davis et al. study1 in 1950 and later with the excellent work of Brian Moore2-3 starting in the 1990s, we now know that there can be “cochlear dead regions” where less may be more. For people with significant hearing loss, cochlear damage may be so great that amplification needs to be either reduced or transposed away to a healthier region of the cochlea. Most of this research has been focused on high-frequency hearing loss, and hence, the transposition has been downward. Our field has many names for this “frequency lowering” technology such as frequency compression, frequency transposition, sound recovery, extended audibility, and many other marketing terms used by major manufacturers. 

Further Reading: The Problem with Frequency Transposition and Music, Part 1

Frequency lowering can be very useful in order to improve speech intelligibility, and in some rare cases, for music appreciation as well. For music, a common clinical practice is merely to lower the amplitudes of the higher frequency harmonics (such as with a high-cut of the frequency response), which will assist in avoiding the distortion in a cochlear dead region. But what about frequency raising?

Why Frequency Raising?

An algorithm that is yet to be commercially available for some of our patients with a low-frequency (or reverse slope) sensorineural hearing loss would be “frequency raising.” Like its high-frequency cousin, frequency raising would take lower frequency sounds and relocate them to a region that has better cochlear function. And like frequency lowering, there would need to be strict requirements to ensure distortion would not occur; namely a one-octave linear frequency increase. With a one-octave linear frequency increase, all altered or raised harmonics would be shifted up to overlay an already existing harmonic of the original sound with no other “new” harmonics being created.

Reducing the Amplitude

One strategy is to simply reduce the amplitude of the low-frequency fundamental to a point where it does not cause cochlear distortion (Figure 1 and Audio file 1). This technology is available and represents the current state of affairs when dealing with reverse slope sensorineural hearing loss,  essentially a low-cut tone setting. The limitation of this approach is that the person may be receiving very little information about low-frequency cues in speech and in music, such that it is akin to not wearing any amplification at all.

Figure 1. One frequency-raising approach is essentially a low-cut tone setting., shown in this audiogram.
Figure 1. One frequency-raising approach is essentially a low-cut tone setting. You simply reduce the amplitude of the low-frequency fundamental to a point where it does not cause cochlear distortion.

                                   

Removing the Low-frequency Fundamental

However, another algorithm shows the spectrum where the low-frequency fundamental is removed and linearly frequency is raised exactly one octave (Figure 2 and Audio file 2). The associated audio file demonstrates this in an ABA format where the A portion is the unaltered music, and the B format is the processed sound with a linear one-octave frequency raising. There are some audible artifacts with this type of processing, but it is unclear whether this is merely an artifact of the simulation and not this proposed hearing aid technology. No additional harmonic or inharmonic components are created so, at least in theory, it should function as predicted.

Figure 2. Removing the low-frequency fundamental and raising linear frequency by exactly one octave, shown in this audiogram.
Figure 2. Removing the low-frequency fundamental and raising linear frequency by exactly one octave is an approach that may benefit cochlear implant recipients.
With this approach, the low-frequency fundamental is removed and linearly frequency is raised exactly one octave. This audio file (2) demonstrates this in an ABA format where the A portion is the unaltered music, and the B format is the processed sound with a linear one-octave frequency raising. HR 09 2024. Frequency Raising Back to Basics

         

Benefits for CI Recipients

Such a frequency-raising algorithm (Figure 2) may also have benefits for those fit with cochlear implants where there can be significant issues in transducing low-frequency elements of speech and music such as rhythm and an adequate bass response.

Further Reading: The Problem with Frequency Transposition and Music, Part 2: The One Octave Example

Currently there are two cochlear implant manufacturers that utilize a one-octave linear frequency lowering4, and extending this to a frequency-raising digital signal approach should be a rather straight-forward innovation that may be quite useful for both those people with low-frequency sensorineural hearing loss and for those with cochlear implants.

Marshall Chasin, AuD, is an audiologist and the director of auditory research at the Musicians’ Clinics of Canada, adjunct professor at the University of Toronto, and adjunct associate professor at Western University. You can contact him at [email protected]. 

Original citation for this article: Chasin M, Frequency Raising: Perhaps the Time Has Come? Hearing Review. 2024;31(9):06-07.

References

1. Davis H, Fernandez C, McAuliffe DR. The excitatory process in the cochlea. Proc Natl Acad Sci U S A. 1950;36(10):580-587. doi:10.1073/pnas.36.10.580

2. Moore BC. Dead regions in the cochlea: diagnosis, perceptual consequences, and implications for the fitting of hearing aids. Trends Amplif. 2001;5(1):1-34. doi:10.1177/108471380100500102

3. Moore BC. Psychoacoustics of normal and impaired hearing. Br Med Bull. 2002;63:121-134. doi:10.1093/bmb/63.1.121

4. Chasin M, Fabry D, and Kuk F, The Benefits of Linear Frequency Lowering for Music. Hearing Review. 2024;31(1):08-13.