In a noisy environment, hearing impaired patients need more signal-to-noise ratio than normal people to obtain the necessary speech intelligibility.It is currently recognized that one of the methods to improve speech intelligibility is to improve the signal-to-noise ratio through directional technology.Experiments have shown that for every 1dB increase in signal-to-noise ratio, speech comprehension will potentially increase by 10%.However, non-directional hearing aids can amplify noise and speech at the same time and cannot achieve the purpose of improving the signal-to-noise ratio.
The development direction of hearing aid noise reduction technology is to rely on the design of the microphone, from single microphone noise reduction to multi-microphone noise reduction processing, to achieve the purpose of noise reduction and improvement of speech resolution.
As an important technology to improve the intelligibility of speech in a noisy environment, the directional microphone system is widely used in the design of hearing aids.In recent years, this technology has been further developed. Some research reports discussed the classification of directional microphone systems, their use effects in specific clinical applications, and related issues, such as how to choose different types of directional microphones The specific impact of these different directional microphone systems in practical applications, etc.
Some research reports show that, compared with the omnidirectional microphone system, the single-microphone system has an average improvement of 7dB, while the dual-microphone system has an average improvement of 7.4dB.
The directional microphone adopts the method of reducing the sensitivity (gain) of the side and rear sound to improve the signal-to-noise ratio, and its effect is reflected by the directional index.More precisely, in a typical noise environment, for sounds from the side and rear, a higher directivity index means that the microphone is less sensitive to these sounds (not from the front), resulting in a significantly improved signal-to-noise ratio.
另一方面，如果语言信号（或其它想听的声音）从这些方向传来，它们的可听性就会被方向性麦克风降低。在比较了全向性和方向性麦克风对于来自收听者后方声音的识别率，声音强度为50dBSPL和65dBSPL（安静环境下），相比而言，方向性麦克风的语言识别率低于全方向性麦克风。对轻声说话（50dBSPL）的理解度下降超过20%（全向性=37%，方向性=13%），这就证明了方向性麦克风在某些场合的表现并不是很好，有潜在的局限性。在这种情况下，使用者可能会错过一些重要的信息，导致理解或交流问题。所以应该还有一些额外的补偿措施来弥补这种损失。
A very simple way to solve this problem is to add a switch to the hearing aid, allowing the user to choose directional or non-directional.However, this method must require users to really feel the difference between directional and non-directional, and to understand under what circumstances they need to choose and how to choose.On the contrary, for users who do not have this ability, they cannot get convenience from this function.In addition, even those who can judge and choose for themselves cannot switch at the moment when they are most needed or most suitable.However, some new features of hearing aids can ensure the correct use of directional and non-directional microphones in this situation, especially for directional microphones with a high directivity index (DI).
Another solution is to use a combination of wide dynamic range compression (WDRC) circuit and directional microphone, because this circuit has a relatively low compression threshold (CT), which can partially compensate for the decrease in sensitivity.A microphone with a low compression threshold can get a higher gain for light sounds than a microphone with a high compression threshold, and it has a certain compensation effect for the sensitivity reduction of the directional microphone system.
Studies have proved that the speech test scores with a single microphone system (low CT) are higher than those with a dual microphone system (typical CT). When a soft voice comes from behind, the single microphone system gets 44% of the speech score, while the dual microphone system The system score is 37% in omnidirectional mode and only 13% in directional mode.Hearing aids using a single-microphone system can have such a high score, indicating that a low compression ratio system may also have the same effect as other hearing aids that use a single microphone to collect rear sound.The fact is that directional microphones reduce sensitivity, and low compression ratio systems can reduce this negative effect. It can bring benefits to all directional systems, regardless of whether directional microphone systems provide omnidirectional/directional switch .
The research results show that the signal processing system of the hearing aid combined with the directional function can obtain the desired signal-to-noise ratio improvement. https://www.jhhearingaids.com/
