Thursday, 8 September 2011

My answer to the question of how hearing aids 'damage' your hearing through wear.....

Originally Posted by zafdor View Post
No, educate me.

Agreed on the conductive losses, I can understand that as an exception. since the conducted loss is in effect an attenuator to what the cochlea is exposed to. I don't see how compression/gain rules or the like would change the fact that a power aid can easily put out 130dB if you allow it to.

Perhaps a more insightful discussion is what exactly are the parameters of loud sounds that can cause permanent hearing loss. I personally take the somewhat ignorant and conservative approach that if I'm exposed to anything over 80db SPL c weighted average I wear hearing protection.
Originally Posted by MikeF View Post
I'll admit that my example indicates extreme hearing loss but even a typical person with 30-40 SPL DB amplification for some frequencies falls into the danger zone of having their hearing damaged during normal conversation, watching tv, or normal daily activities.

It is generally accepted that 90 DB sound levels for 8 hours a day and 100 DB sound levels for 2 hours per day can cause ear damage. Therefore people that had 30-40 SPL DB amplification would likely be in environments daily that could produce those sound levels.

If those levels of amplification could produce ear damage, you would think that an ENT or audiologist would warn people to stay away from vacuum cleaners, dishwashers, clothes dryers, air conditioners, walking in city streets, noisy restaurants, or even background music without ear protection or possibly removing the hearing aids. Also you would think that the ENT or audiologist would recommend that a person use headphones instead of hearing aids when watching tv since headphones do not over amplify deficient frequencies and it is likely that the person will hear the tv better than with hearing aids.

So it appears that amplification for someone that doesn't hear certain sounds has a different effect on the possibility of damage to ears than over amplification of sounds for people with normal hearing. It would be nice to get some hard facts about hearing aids and ear damage instead of just speculation (lot of speculation on the Internet) so that each of us can try to create an environment that enhances our lives. I have yet to see an ENT indicate that properly adjusted hearing aids will either cause or not cause ear damage.
OK, it comes down to the application of gain.

First you have to understand that these numbers we throw around on here 80-90-100 dB aren't a linear scale. To put it simply, if one speaker puts out 80dB and you put another speaker right next to it you produce a whopping 83dB. So a doubling of 'power' equates to a 3dB rise in measured output.

The next thing to take into the equation is that there's a reasonability test applied to gain which forms the basis of all gain calculations. IE: how much sound is 'needed' to hear whilst staying within people's UCL or uncomfortable loudness levels.

The third aspect is that the inputs we have been discussing (speech) aren't 65dB AVERAGE, they are 65 dB 'peak' value. The average value is about 30-40dB depending on the mix of ambient and speech proportion.

The fourth part of this is that given all of the above, engineers determined in the 50's 60's 70's 80's and 90's refinements to the basic concepts so that you don't break some fairly fundamental 'rules' of gain.

Namely: 1/2 gain and 1/3 gain rules, basically that if you have a conductive hearing loss, you will receive Half that amount back in gain, at the appropriate frequencies. If you have sensorineural loss, that will be 1/3 or less. Now in our fancy digital age, there are systems that deviate from this to an extent, but that's the basic precept.

So, if you have a 60dB loss, on average, you'll get 20dB gain on average. Which, even on a 65dB input signal doesn't put you anywhere near the 85dB (Aleq), simply because the average dose of continuous speech at 85dB is going to be around 40-50dB. Any more than this is going to do your swede in as you'll get Auditory Fatigue from the long term exposure.

To combat this, for the last couple of decades hearing aids have been built with circuits that incorporate compression - compression is a technique which allows louder sounds to be given less gain while still amplifying the quieter ones. This has the beneficial effect of making the quieter sounds audible but not blowing your head off with the louder sounds. Now, you'll hear some people saying that compression is bad, and linearity is good, especially for music. Yes, this is true to an extent, but, non-believers, listen-up! you've had some degree of compression in your analogue aids for years - go and dig out the old Gennum and K-Amp specs if you like - or even ask Mead Killion (if he's still going around on his unicycle....) Basically all circuits compress the output to some extent to avoid the receiver clipping - overdrive distortion on the sound. ~(output-limiting compression)

And finally, just when you think that the manufacturers are really going to do you down, we have software. All the stuff you can see, and all the stuff you cant. Here's a though: wouldn't it be a really good idea to put a limiter into ALL hearing aids? Just to keep the Lawyers happy and to stop self-programmers from really doing damage? Well, what if we put in default UCL limiters at 105-110dB? So that the peak values of the output wouldn't do any real harm......You can override them of course, but that's at your risk.

Just think, if somebody fires a gun near your head with occluding hearing aids in - you'll experience less hearing damage than the person who fired it.....

So, there it is, in a nutshell. Hearing aids listening to speech are unlikely to damage your hearing further. I hope that doesn't sound to patronising. If you want more info, try to get hold of the excellent book: Hearing Aid by Harvey Dillon or Andi VonLanthen's one.

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