How to fit hearing aids to patients with severe hearing loss?

Patients with severe hearing loss need to consider three factors: hearing threshold, uncomfortable loudness level (UCL) and hearing resolution.

The traditional solution is to use high-power linear  Hearing aid  In this type of hearing aid, the method of providing high gain is used to provide sufficient amplification to obtain speech audibility.

XNUMX. Features of Extreme Hearing Loss

Hearing loss is not only manifested in the decrease in sensitivity, but also affects the ability of various sounds; although the sound can be heard, it cannot effectively extract useful information from the signal.

XNUMX. How to choose hearing aids for severe hearing loss

1. Linear zoom:

In the past, only linear hearing aids could provide the required gain (50~70 dB) and acoustic output (130~140 dB SPL) to meet the needs of patients.Therefore, many patients who have worn hearing aids for 10 to 30 years use linear super high power Hearing aid , The hearing aid uses peak clipping, the most direct output limiting method.

In recent years, some ultra-high-power and high-quality hearing aids have entered the market, using clearer output control circuits.The output compression system is designed to limit the amount of amplification and the electrical loudness output of the receiver by highly compressing the signal.The advantage of this compression method is that it does not introduce distortion caused by peak clipping when limiting.

2. Non-linear amplification

Fully automatic non-linear amplification for patients to obtain better audibility for soft speech signals, and provide better sound quality for medium and strong sounds. As hearing loss intensifies, it is necessary to increase a certain gain and compress the full range of speech accordingly. Put it into the residual dynamic range.By analogy, putting the entire speech range into the residual dynamic range of patients with severe hearing loss requires a large compression rate.

3,Adaptive speech compensation

Adaptive speech compensation
speech
alignment, ASA) matching principle uses multi-channel, non-linear processing method to solve the hearing problems of most patients with sensorineural hearing loss.Hearing aids adopting this matching principle have made special adjustments to gain, compression and compression types to provide hearing aid users with as much speech information as possible.When the hearing loss is more than severe, the non-linear matching algorithm needs to be corrected.However, it is not enough to put as many amplified speech signals as possible into the residual dynamic range. The difference in signal utilization ability of the human ear due to the difference in resolution must be considered.When selecting hearing aids for patients with severe hearing loss, the useful dynamic range may not exceed 1000~2000
Hz 。

In fact, for more than 70 dB HLHearing loss, The compensation of gain tends to all.The simple loudness mode and the method of increasing the compression ratio to compensate for the narrower residual dynamic range are not suitable. Such processing cannot guarantee sufficient speech comprehension. Too much compressed signal will cause difficulty in understanding. It is recommended that the compression ratio not exceed 2.5:1 .

In view of the characteristics of severe hearing loss, the matching algorithm for severe hearing loss is not only an extension of the ASA algorithm, but also should be further modified.Some patients with very severe hearing loss need more linear components of the signal (we call them type 1 patients), while some patients can fully use the ASA algorithm for moderate compression of the signal (type 2 patients).Type 1 patients have severe inner ear damage and few remaining intact inner hair cells, so too much compression will have an adverse effect on them.Except for pure tones, their signal resolution is very low and can only use the peak part of the speech signal.And some patients with severe hearing loss can make good use of the signal compressed by the ASA optional algorithm (type 3 patients).The loss of outer hair cells in these patients is serious, but the degree of loss of inner hair cells is not serious (resulting in high hearing threshold), so they have sufficient signal resolution ability to extract useful information from compressed signals.Because of individual differences in patients, general matching methods like ASA may not be fully applicable.

Some hearing aids have added a software tool similar to the sound perception manager, enabling it to switch between three modes, changing the gain, compression rate, and compression type.

Mode 1:Compared with mode 2, the release time of the high and low frequency channels is prolonged, which can provide more linearity for the processing of fast-changing speech signals.In addition, in the high and low frequency channels, the gain for small signal input is reduced by about 8
dB.The result is equivalent to a reduction in the compression ratio.The overall effect of these processing is to give more linear processing to the rapidly changing input signal and provide the basic speech peak part to the patient.And continue to provide sufficient amplification for slower changing input signals.

Mode 2:The default matching principle is similar to the ASA matching algorithm, using fast syllable compression of the low frequency channel and slow adaptive gain of the high frequency channel.Its purpose is to provide a moderately compressed output signal to obtain a moderately strong voice signal and good audibility. The audibility of soft voice can be increased by adjusting the manual volume control.

Mode 3:Compared with mode 2, the release time is shortened in the high frequency channel, and the gain is increased in the low and high frequencies (thus increasing the compression ratio).To compress more speech signals in the residual dynamic range, this method is more effective for patients who have sufficient ability to distinguish speech from the compressed signal.

多数的患者助听器的设置可以采用模式2,表明他们可以利用中等压缩的信号,特别是当高频通道压缩释放时间不是很短(默认ASA原理是230ms)的情况。有些极重度听力损失患者可以使用更多的压缩(类型3),他们有足够的能力使用残余的动态范围中的言语信息,他们中的大多数对模式2的设置表示满意,但往往会寻求更好的言语理解度。这些患者在需要获得更好的分辨率时,才需要调整微调。而另一些患者听觉系统损失极为严重,即使是模式2中的中等压缩对他们来讲也是非常大的。他们会反映从线性转换到非线性处理使得言语变得“太混浊”或者抱怨声音“太嘈”。即使是使用模式2,他们也会对线性助听器更为青睬,使用模式1后,往往会感觉更好。

When choosing hearing aids for patients with severe hearing loss, attention should also be paid to determining the dynamic range of the spirit.For most patients with hearing loss, the discomfort threshold (UCL) can be estimated based on the hearing threshold (HTL) value through the optional software.Although this method is not completely accurate, the effect is good in actual matching work.However, for patients with severe to extremely severe hearing loss, it is necessary to make full use of the residual hearing range and measure the UCL at each frequency point.In addition, the selection personnel should check the UCL settings of the patient’s hearing aids. A gain increase of a few decibels may also be very helpful to them.

in conclusion

Many patients with severe hearing loss who have long-term experience in using linear hearing aids are potential users of new nonlinear super-power hearing aids.When choosing a new type of hearing aid for such patients, the first thing to do is to ensure that they are familiar with the “hearing feeling”.As long as patients are not very dependent on this type of signal, they will gradually adapt to the transition from linear to non-linear.Theoretically speaking, if patients with severe hearing loss can extract valuable information from the distorted peak clipping signal, they can get more information from the non-linear hearing aid. Of course, patients need time to find out under correct guidance. And use this information.

Link:      How to fit hearing aids to patients with severe hearing loss?

REF: Hearing amplifierHearing LossDigital Hearing Aids
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