Patients with sensorineural hearing loss often complain that “the voice is loud enough, but not clear enough”, “sometimes the voice is too light to hear, sometimes it is too noisy to bear”, “it can be heard well in quiet environment, but it can not be heard clearly when there is noise”.
In fact, these complaints are caused by the influence of hearing loss on speech intelligibility. So why does hearing loss make speech intelligible?
The decrease of speech intelligibility caused by hearing loss mainly comes from four aspects:
1. Hearing loss
The spectrum information of different speech is different. With the increasing degree of hearing loss, the patients with sensorineural hearing loss often have the fuzzy feeling of “loud but not clear” because they can’t recognize some speech components.
The figure shows the spectrum analysis of OO and EE. It can be seen from the figure that the first formant information of OO and EE is basically the same, while the second formant is different so that the sound is different.
When hearing loss occurs, hearing loss will gradually affect the ability to distinguish sound information above 700Hz. Therefore, people with hearing loss will not be able to distinguish OO and EE (gray area in the figure) because they cannot hear the second formant.
2. Narrowing the dynamic range of hearing
The dynamic range of hearing refers to the effective range of hearing from the moment when the human ear can hear the sound to the moment when the sound cannot be tolerated. The normal hearing is generally 100dB HL.
With the continuous decline of hearing, the auditory dynamic range of patients with sensorineural hearing loss gradually narrowed, and thus lost the ability to perceive sound information of different intensities.
The wide blue area of (a) in the figure is the normal dynamic range of hearing, which means that people with normal hearing can not only sense the low voice, but also can not feel the loud intolerable; while the dynamic range of (b) in the figure is significantly reduced, so that people with hearing loss can only hear the loud voice, but can not sense the low voice.
3. Reduced frequency resolution
Different sound information contains different frequency components. Under normal circumstances, these different sound information can activate different parts of the cochlea, so that the human ear can distinguish speech from noise.
However, with the aggravation of hearing loss, the cochlear sensitivity of patients with sensorineural hearing loss will gradually decrease. When the frequency of noise is close to the frequency of speech, the lower cochlear sensitivity will not be able to distinguish the two frequency components, so that the brain loses the ability to recognize speech information from noise.
4. Weak time resolution
In daily life, the background noise fluctuates greatly. When the noise intensity is at a low level, the normal hearing people can use this “short” time interval to extract speech components from information, so as to obtain the ability to recognize speech in noise.
However, for hearing loss patients, this ability will gradually disappear with the increasing of hearing loss. When the background noise in the environment is larger, it is more difficult to extract speech information
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Link：How to deal with hearing impairment
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