What is Frequency Lowering?
Frequency lowering is the process of shifting a range of input frequencies into a different and lower range of output frequencies. Fundamentally, this is an amplification technique that distorts the input signal, but for some hearing losses, particularly high-frequency hearing losses, this induced distortion may, in fact, prove to be useful.
When to use it?
For some hearing losses, the high-frequency region of the audiogram may prove to be unaidable directly, yet there may be some important speech cues that fall within that unaidable region. In such circumstances, it may prove to be beneficial to shift the input energy in this unaidable frequency range to reside in a lower range of output frequencies -- frequencies that are aidable. In so doing, the important high-frequency speech cues would then produce an audible output result for the patient. With practice, the patient can come to recognize these altered speech cues as representative of the original high-frequency phonemes that were produced.
Figure 1 shows an example of a Real Ear Aided Response, or REAR, obtained within Audioscan's Speechmap® verification tool. This REAR was obtained in the presence of a 65 dB SPL average speech input signal. Notice that in the area circled in green, the aided output remains below the threshold line shown in red. This is a clear indication that for this fitting, audibility has not been restored for these frequencies when 65 dB SPL average speech is present. If audibility cannot be restored by adding additional gain within this frequency region (perhaps because of the bandwidth and/or gain limitations of the hearing aid being used) or if audibility cannot be restored by selecting a different hearing aid, then this might be a situation where frequency lowering could be employed in an attempt to make important speech cues in this frequency range, audible in a lower aidable frequency range.
Figure 1. Real ear aided response (REAR) obtained with Audioscan's Speechmap verification tool.