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 thought: 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 too patronising. If you want more info, try to get hold of the excellent book: Hearing Aids by Harvey Dillon or Andi VonLanthen’s one.