Frequency transposition
People with steeply sloping or severe to profound hearing losses face a number of communicative challenges because they do not have access to the entire range of speech sounds.
Clients with high-frequency hearing loss may have extra requirements. Steeply sloping and severe to profound hearing losses are often accompanied by so-called dead regions. Dead regions are areas of the cochlea with no or very few functioning inner hair cells - typically where hearing thresholds have dropped below 60-70 db HL.
With a basal cochlear dead region, many high frequency sounds will not be audible, which can lead to misunderstandings.
Even though hearing aids can help hearing-impaired people hear sounds that would otherwise be unavailable to them, amplification is sometimes not enough. People with steeply sloping or severe to profound hearing loss often require more help.
The Audibility Extender feature can be relevant in those cases.
The Audibility Extender
The Audibility Extender feature exploits a frequency transposition technique to make high-frequency sounds audible.
Frequency transposition features basically move high frequency information like birdsong or the speech sound /s/ to a lower frequency region where it will be audible to the client.
There are basically two different techniques for frequency transposition: Linear transposition and frequency compression:
Linear transposition
The linear transposition technique shifts an entire high frequency region to a lower region. All components in the area are shifted by the same amount of Hertz, which means that the high-frequency area will be broader than the lower frequency area to which it is shifted.
It is therefore necessary to remove the least important part of the original signal to make room for it in the target frequency area.
As linear frequency transposition keeps the distance between the harmonics intact, the resulting sound will be subjectively very close to the original sound, only played at a lower frequency.
Frequency compression
Frequency compression does not remove any part of the sound. Rather, it pushes a broad area of the high frequency region into a more restricted lower frequency region.
In other words, the technique involves squeezing the high frequencies into a smaller lower frequency region.
This compression of the signal affects the distance between the harmonics, meaning that the resulting sound will differ more from the original than it does with linear transposition.
High sound quality and easy recognition
The Audibility Extender from Widex uses linear frequency transposition to ensure a high sound quality and easy recognition of external sounds.
Research has shown that linear frequency transposition has the potential to help clients regain access to high frequency information in speech and environmental sounds and that the users learn to utilize the extra information when it is made available to them (e.g, 1, 2).
References:
1. Auriemmo J, Kuk F, Stenger P (2008). Criteria for Evaluating Performance of Linear Frequency Transposition in Children, Hearing Journal, 61(4), 50-54
2. Kuk F, Keenan D, Korhonen P, Lau C (2009). Efficacy of Linear, Efficacy of Linear Frequency Transposition on Consonant Identification in Quiet and Noise, J Am Acad Audiol , 20(8)10 | 
