Noise Reduction in Hearing Aids, Adding Comfort Not Necessarily Clarity
The most apparent effect of noise for people with hearing loss, is its impact on their ability to converse. It impacts their ability to communicate every time they try to have a conversation with somebody in a noisy place. On a daily basis we are confronted with desired and un-desired noise. Desired noise like music or un-desired like a noisy crowd or background traffic noise. The nature of the noise doesn't matter, whether it is considered desirable or undesirable, it can still interfere with a person's ability to understand.
People with hearing aids want to be able to communicate in all situations, just like people without hearing loss. There are a number of hearing aid features that work in a number of different ways to improve speech perception in the presence of competing noise for a hearing aid user. Directional microphones are one and noise reduction is the other. In this post we would like to try to explain how noise reduction works.
Noise Reduction in Modern Hearing Aids
Most modern hearing aids have some form of noise reduction or noise management feature. Noise Management or Reduction assists the hearing aid user in noisy situations by attempting to lower the volume of loud and intense noise. in most hearing aids the feature works on an automatic basis but turns itself off or down when the situation becomes quiet or if you need to hear because there is a speech signal present. This is an intelligent feature that makes changes in the output of the aid in order that you don't have to.
How it works
Most noise reduction systems are modulation based systems, modulation-based Noise Management analyses the overall level and the presence of sounds that change in level over time (such as speech). Speech is a very different signal than noise, this difference is what allows the system to analyse the sounds and make decisions. If the noise is loud enough, and speech isn't detected, then the noise reduction will kick in and reduce the overall level of output to prevent the situation becoming uncomfortable.
If speech is detected, then the level will be turned back up to ensure audibility of these important and desirable sounds. These adjustments are done in different frequency bands to turn down frequencies where noise is dominant, while leaving the frequencies that are useful and important for speech understanding alone.
In an ideal noise reduction system, the hearing aid will only reduce the undesired noise while leaving desired speech signals completely intact. In this ideal scenario, the hearing aid will understand what sounds it is expected to pass on to the listener and what sounds it should reject. In order to reach this goal, the focus has been on the acoustic differences between speech and noise. Noise can vary in an almost infinite number of ways, however the nature of speech is constant in its overall structure.
Attacking Noise in Multiple Channels
If we were to use this feature in a broadband manner, it would cause difficulties. While this gain reduction reduces the amplification of the competing noises, it would also decrease the energy in the speech signals. In other words, noise reduction would affect both speech and noise in exactly the same way. This is where splitting the amplification into channels or bands (depending on Manufacturer) comes in. Modern hearing aids are multi-channel devices with anything from three to thirty two channels. The noise reduction feature can work separately within each separate channel. It is therefore possible to reduce the amplification in the noisiest channels only, in the hope of avoiding the channels that are carrying important speech information.
How Much Reduction
The amount of gain reduction depends upon a number of factors, not just in the sound environment but also dependent on the hearing aid manufacturer. Different manufacturers use not just different systems but also their own decision and management rules. Whilst it depends upon how much noise there is, other factors, such as the overall levels of the speech and noise, are usually also considered. It is common to vary the amount of gain reduced depending upon a band's frequency. Frequencies more important for the understanding speech are not reduced as much as frequency bands less important for the understanding of speech.
All The Same?
They are most definitely not all alike, the different manufacturers features vary in effectiveness and the amount of gain reduction applied, even at the same frequencies. They also may differ in how rapidly they increase and decrease the gain (called attack and release times) and how they affect different types of sound. It has been an accepted in the audiology world for many years that noise reduction does not actually affect signal to noise ratio on its own. In other words, noise reduction improves comfort in noise but does not affect or increase speech clarity.
Improving Speech Clarity?
Except for Widex, who have presented a paper on their Speech Enhancer feature that shows an affect on signal to noise ratio, most manufacturers admit that their noise reduction has no affect on speech clarity. The Speech Enhancer feature in Widex hearing aids has been shown to affect signal to noise ratio between 2 and 6 db dependent on the noise source. However, while most noise reduction features just reduce noise, people do prefer listening in a noisy situation while wearing a hearing aid that includes noise reduction.
Better To Have It
Hearing aid users report that the background noise does appear to recede and that incoming speech does sound better. They also report that the entire listening experience is not quite as stressful or as fatiguing as it ordinarily would be. However these subjective reports are not backed up during objective testing, when speech perception scores with and without noise reduction are compared. There appears to be no improvement overall in speech perception with noise reduction except in the case of the Widex feature.
It has been speculated by researchers that speech perception scores would demonstrate an improvement after long-term listening. This postulation is based on the fact that with noise reduction in a noisy situation there would be less fatigue. More relaxed people do listen better than people who are tired or under stress. While this is purely speculative, it may help to explain why many people wearing such systems report an improvement in their ability to understand speech.