When fitting hearing aids, audiologists are well aware of the importance of hearing high frequencies (such as birdsong and speech sounds like ‘s’ and ‘sh’) and try to ensure that they are as audible as possible. However with most hearing aids, they are often limited by the extent of the high-frequency loss, as well as the upper frequency range. To deal with this challenge, some hearing aids move high frequency information down to regions where it can be audible using frequency transposition.
There are basically two different techniques for frequency lowering transposition – linear transposition and frequency compression. Linear transposition shifts an entire high frequency region to a lower region and means that the resulting sound will be very similar to the original sound but with deeper, lower pitch. Frequency compression squeezes the high frequencies into a smaller lower frequency region, thus resulting in sound that differs more than the original. Linear transposition is used in a feature known as the Audibility Extender from Widex to move sounds down to lower frequency areas. It selects a ‘start’ frequency based on the user’s thresholds and hearing loss configuration. This start frequency works as a cut-off and information above this is moved down by one octave.
Francis Kuk, VP Clinical Research, ORCA-USA explains:
“The Audibility Extender (AE) receives information on the user’s hearing loss to decide which frequency region will be transposed. The frequency where transposition begins is called the start frequency. Typically, one octave of sounds above the start frequency will be transposed. This is called the source octave. The AE picks the frequency within the ‘source octave’ region with the highest intensity (for example, peak frequency), and locks it for transposition. As the peak frequency changes, the transposed frequency also changes.”
For example, let’s say the peak intensity is 4,000 Hz. This frequency and the sounds surrounding it will be transposed linearly by one octave to 2,000 Hz. In addition, every frequency will be shifted down by 2,000 Hz. For example, 3,000 Hz will now be at 1,000 Hz and 4,500 Hz will be at 2,500 Hz.
“In this way,” says Francis Kuk, “the transposed signal is likely to be placed in a region where the hearing is aidable. To limit the masking effect from the transposed signal and any potential artefacts, frequencies that are outside the one octave bandwidth of 2,000 Hz will be filtered out. The level of the transposed signal will be automatically set by the AE so it is above the threshold of the wearer.”
Research reveals that linear frequency transposition has the potential to help people with severe to profound hearing loss gain access to high frequency information in speech and environmental sounds. Research also suggests that these listeners learn to utilize the extra information when it is made available to them.
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 American Academy of Audiology, 20(8)10