The Signal to Noise Ratio (SNR) is a very important factor when trying to hear the content (the signal) when there is background noise. When the SNR is poor, it is difficult to understand what is being said. If the SNR is good, it is easier to understand.
The above table illustrates the difference in speech discrimination for people with normal hearing and people with a hearing impairment.
It has long been recognised that if we can achieve a 60% score for single word discrimination, then we usually achieve a 100% score in sentence discrimination. This higher result is not based on making sound louder, but hearing the words in context.
With even what is considered a good SNR in normal situations (+12 to +6dB), the hearing impaired person can only achieve 30-50% discrimination.
Even as amazing as Cochlear Implants are, they do not restore normal hearing. This means that an implantee is still left with poor discrimination (30-50%) even when the SNR is considered to be good.
Increasing the SNR is clearly of considerable benefit to all.
The Truth about ALDs - Signal to Noise Ratio
There are four simple ways to increase the SNR: 1. Increase the level of the signal without alteration of the noise level ALDs can increase the level of the signal without changing the noise level by using a microphone (cabled or wireless) placed closer to the sound source or by using a specialist microphone such as a conference microphone that picks up speech and not background noise due to the design of the microphone capsule and inlet ports.
Hearing aids may also amplify the background noise more than the sound of interest because the background noise is often softer and a hearing aid is normally designed to give softer sounds more gain.
2. Transmit the audio signal of interest only (no noise) ALDs can be used to transmit/receive the audio signal only by using a direct connection to the sound source. Hearing aids alone cannot be directly connected to audio sources, however, using an accessory (or an ALD!), this can be done.
3. Decrease the noise level without altering the signal This can only be achieved by turning down any competing background noise (TV, radio, air conditioning, etc.) or asking others to be quiet!
4. Remove the noise Modern hearing aids usually have some form of noise reduction strategy to increase the SNR (theoretically...). Unfortunately this is always a subtractive process that often removes sounds/frequencies that are so important for understanding.
So, while on paper, there is a 'technical' increase in SNR, what is left for the hearing impaired person to process is far less than ideal. Depending on the nature of the background noise (e.g. other people talking), this subtractive approach can often make the 'real' SNR worse.
For the best chance at hearing, an ALD will be the preferred choice as it delivers as much 'information' as possible without subtracting valuable and vital components of the sound spectrum.