Why Don't Hearing Aids Cause Damage to the Ears?

With some people fitted with hearing aids that have certain frequencies amplified by 70 DB or more, why aren’t their ears permanently damaged?

Since the human voice produces an audible sound of about 60 SPL DB and if that frequency was amplified by 70 SPL DB by hearing aids, the ear would receive a total 130 SPL DB which is well beyond where permanent ear damage can occur.

I don’t think you will find anyone who this level of amplification is undamaging, but the option is to not hear. Overall, discussion of this topic is taboo (ignorance is bliss).

I’m not a pro but I don’t think most hearing aids produce that much sound. 60 DB sounds get boosted, as well as 30 DB and 80 DB sounds but there is a limit. My UCL (uncomfotable level) is set to around 117 but it would have to be a loud sound to get to that level. Most things come in somewhere between 40 and 105, depending on the frequency. I don’t need that much boost in the lower frequencies and there is not that much sound coming in, in the higher frequencies, in human speech. So those high frequencies are getting hit very, very briefly.

In your example, if a person had 90 DB of loss and there was speech at 60 DB, the hearing aid should be set up to boost that only up to where the person could hear it, maybe 100 DB (boost of 40DB), even if the hearing aid was capable of boosting things up 70DB.

Eh?

If I say: not true, does it save me having to explain a lot of stuff about conductive losses gain rules, compression and the like?

Also dB HL vs. dB SPL?

Yes, but that’s just the semantics (Also dB(Aeq)).

It’s the original statement that I object to - the one that infers that hearing aid manufacturers and audiologists are deliberately making people’s hearing worse in the long term. It’s ill-informed nonsense, but I don’t want to appear ‘combative’ or ‘lacking in class’ in terms of my response.

Hearing aids don’t function in terms of dBHL since that is a formula used to simply create an audiogram where normal can be represented by a nice, pretty, fairly straight line. They function in terms of dBSPL which is the sound pressure level (SPL) of a sound when it reaches the tympanic membrane (eardrum). Sounds that are at the very extremes of frequencies (very low and high) have to have a higher SPL in order for us to hear them as opposed to the ones in the middle due to the natural resonance properties of the ear and how sound waves travel through air. The ear is not a linear system so 1 decibel of sound in does not equal 1 decibel of sound out…it varies by intensity and frequency of the sound.

Even high-frequency emphasis hearing aids aren’t able to get 50 or 60dB of gain (volume) much beyond 3000Hz due to the limitations of the electronic components. Even if they could, people that would require this much volume at those frequencies wouldn’t get any benefit (most likely) because amplifying sounds that much at those frequencies would cause a significant amount of distortion. Plus, they most likely have little-no functioning hair cells in the ear to send the signal to the brain to begin with (cochlear dead zones) so trying to stimulate something that is probably not there won’t do any good.

If you combine all this and add in the limiting properties of the hearing aids, how the ear works, and the amount of gain needed to make speech audible, it results in very few people being amplified to a point where it causes further loss. Those that are, are generally in the severe-profound range in the low - mid frequencies and for them it doesn’t really matter because they need to hear. It’s a choice of hearing and having potential further loss or not hearing at all. I will also add that for most of these people the additional loss is minimal. I have a couple patients who, when I program their hearing aids, I get the dBSPL warning and their hearing hasn’t changed at all over the past few years. It’s a pretty high level also, usually around 125-130dBSPL…in order to exceed that their hearing has to be pretty bad.

The aids you see discussed on this forum are designed with limiting factors intended to keep sound from being amplified to damaging levels. Still, I take my aids out of my ears and wear foam earplugs when I’m using noisy power tools or in a very loud environment where speech comprehension is impossible in any event.

Search around the web or watch late night TV and you’ll see advertising for el cheapo analog hearing assistance devices that are designed to boost a wide range of hearing without limiting factors, and with those, I’m sure you can do some serious damage to your hearing.

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.

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.

As far as the limiting capability of hearing aids, typically a CIC hearing aid will limit the maximum SPL DB to about 95-105 DB but a behind the ear type can produce over SPL 130 DB output.

I don’t think those limiting capabilities are on purpose but just that the device can’t produce any more SPL DBs with current technology. I’m sure that manufacturers would produce CIC hearing aids that could output 130 SPL DBs if the technology would allow it.

As far as headphones/earphones are concerned, they appear to have a better SPL DB limiting capability than hearing aids. Headphones such as the Sennheiser HD448 and the Bose AE2 have a maximum SPL DB of about 115 DB and reasonable quality earphones such as the Sennheiser CX300 II have a maximum SPL DB of a little less. Most cheaper ear buds are down in the 105 SPL DB range.

Now those readings are pretty high but if those headphone are used with mobile devices (iPods, MP3 players, or bluetooth headsets), the maximum SPL DB is probably reduced even further (especially bluetooth headsets) since they can’t drive the headphones/earphones at that SPL DB level.

Finally headphones such as the Bose AE2 and Sennheiser HD448 have a 32 ohm impedance and therefore it is more difficult to drive to the maximum SPL DB from a mobile device making them even safer. Even though those headphones are designed for mobile devices, it is very hard to drive those at the maximum SPL DB from a mobile device.

According to my investigation and testing of two headphones and six earphones, the Apple iPod ear buds drives the maximum output from my laptop 9 SPL DB greater than the Bose AE2 and the Sennheiser HD448 as well as the Skullcaddy Ink’d and Sony Ericcson ear buds at the same volume setting at 375, 500, and 750 Hz. The Sennheiser CX300 and CX300 II drive the output 6 SPL DB greater than those headphones and earphones at the same volume setting. Other frequencies are more difficult to compare since my ear produce consistently the same SPL DB for 375-750 Hz per headphone/earphone, is the plateau of the loudest sounds that I hear, other frequencies are not, and headphones and earphones can have different characteristics at those frequencies (especially the lows, high mid range, and high range).

If any of the headphones or earphones are overdriven, clipping will occur either by the mobile device or headphones or earphones.

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.

To mis-quote the Beatles ‘How many holes does it take to fill the Albert Hall?’

Like the engines in our cars it all comes down to the (inverse of the) amount of cubes you have.

If I play your 105dB into a 2cc coupler, it comes out as 105dB. If I play it into a 0.707 (Zwislocki) coupler it comes out at ? 120dB ? If I play it into a real ear with a narrow External Auditory Meatus due to Exostoses - measuring 0.3cc, 135dB??

The actual output is a function of the impedance of the receiving mechanism - ie both the resistance + reactance. Expressed in electrical terms as resistance and capacitance.

So, unless you have a REM system to tell you what is actually going on, you are just guessing, based on an average. I’m fairly sure that I don’t have an average ear, do you?

This is a fascinating discussion, Which leads me to another issue for wich I have never gotten a good answer.

What is the best way to keep safe in a noisy environment while still maintaining the ability to commuicate ?

I sometimes have to go into my noisy factory. There is one place where the average is 107 dB. I wear muff that according to specs, attenuate 23 dB. If I put these muffs over my hearing aids without turning down the volume, I get 23 dB reduction to 84 dB. Now as I read the stuff I got from Costco, at 84 dB my aids will amplify (at 80 dB input), 23 dB.

Which tells me I have just completely negated the value of muffs and I am hearing about 107 dB.

Now I turn my volume down 12 dB, but it seems to me, I’m still hearing 95 dB. Am I right ?

Not to belabor the point, but your explanation seems to be centered around the premise that the max output of the instrument is never that high, and if that is so, then why is the instrument designed for it? Is your point one of dynamic range, i.e. 20 ms at 130db to hear a trailing ‘s’ is not damaging, while a constant barrage at 130dB for a lengthy “ee” is damaging?

No, your aids will not reproduce the mechanised sounds or the peak values as loudly as they will speech, in addition there will be output limiting compression. Your use of the muffs is appropriate and offers protection from loudest sounds, while you have ‘help’ to overcome the extra quiet. You might benefit from a quiet program if you are suffering from auditory fatigue, or at least one with all the noise filter settings turned to full.

Max AVERAGE SPL.

The noise a motorbike helmet makes at 100mph is damaging in about 15minutes iirc, but that is continuous. Speech levels are way shorter, lasting only a few milliseconds. Longer vowel sounds are only 30-500 milliseconds - and depends on your language and emphasis: some Latinised ones are more sing-song. Music is way different.

However, even in a ‘normal’ sentence the time a receiver spends at ‘full’ power is tiny. So the peak value will only be obtained for incredibly short periods in between the rise and fall of the output.

So, Speech is unlikely to cause damage even with aids of 145dB output. And, if you are in need of 145dB to hear, it’s likely that a small 4KHz notch in your hearing is going to be the least of your communication issues.

Thank you , Um Bongo.

John,
If I was going into someplace producing 107db of noise, I’d take my HA out,put foam plugs in and then put the muffs on!