EEL 6586, Project - Hearing Aids algorithms

We studied hearing loss algorithms in this project. As the conductive hearing loss is due to sound conducting problem in the ear canal, it can be treated with a linear sound amplifier. However, the other kind of hearing loss, sensorineural hearing loss, which is resulted from the disorder of the middleor inner-ear, can not be solved merely by a linear amplifier. Sensorineural hearing loss is what we will focus on in this project; and for convenience we mean sensorineural hearing loss simply by hearing loss or sensorineural impairment in later discussion. The symptoms of sensorineural hearing loss are relatively more complicated. First, the high frequency signal power is diminished in the sound signal perceived by people with sensorineural hearing loss. This makes some phonemes inaudible or can not be heard in a correct way, as the second formant or the higher order formants can not be detected effectively. This problem can be alleviated by frequency compensation in which high frequency component are boosted by different gains according to hearing impairment level (see, e.g., [1][2]). Another symptom of sensorineural hearing loss is that the dynamic range between the weakest sound that can be heard and the most intense sound that can be tolerated becomes much narrower. The weakest sound that can be heard by patient with hearing loss will have a much higher threshold. On the other hand, however, the most intense sound that can be tolerated does not rise as much as threshold of the weakest sound. This narrowed dynamic range will make the normal sound can not be heard to be continuous and smooth by people with sensorineural impairment. A linear amplifier will always make some sound level out of the range, while the non-linear amplification, namely dynamic range compression, can let pass all sound levels into the perception of patients with hearing loss (see, e.g., [1] [3]). So far we have discussed the two most important aspects about hearing loss, one in frequency domain, and the other in the time domain. Detailed methods to solve these two problems are discussed in Section II and Section III respectively. Additional symptoms of hearing loss include poor frequency resolution and poor time resolution. However, they are beyond the scope of this project for the complexity involved.