I would like to build a hearing aid, appreciate any professional advice

[ayeo99]

Hi All,

My hearing has deteriorated and I need to build a hearing aid as commercial hearing aids are very expensive.

Any advice how to do it myself?

Rgds,
Andy

 

[RadioRon]

First thought that comes to my mind is...forget about making it as small as commercial ones. Put the electronics in a small plastic enclosure that can be worn elsewhere and use a commercially made ear bud, and learn to live with a wire between the two.

Hearing aids can be all-digital, using DSP for filtering, or they can be analog and somewhat less flexible or complex. If you are not familiar with the technologies used, perhaps a simple analog one would be ok to start with. I don't think this is a trivial design exercise since you need to have a very good microphone, adjustable gain, tone control (a multi-band equalizer), automatic gain control to limit the maximum output and compress the range (so that quiet sounds can be heard along with loud sounds), and a driver for the earpiece.

 

[audioguru]

Most hearing-impaired people I have met a have loss of high audio frequencies. Some people think that speech sounds are in the narrow frequency band from 300Hz to only 3kHz but many voices go down to 80Hz and important consonant sounds go to 14kHz. Therefore equalization is important.

I agree that compression is important to limit the volume of loud sounds but I think AGC to boost quiet sounds might bother you (drive you crazy?).

 

[tvtech]

Hi Guys

I am also hearing impaired. I can rarely/barely hear a CRT Line stage start up anymore. And that is dangerous for me.
I will be watching this thread with interest. I could learn a lot here by seeing if someone here can actually do this.

Regards,
tvtech

 

[cct314]

First off, some weasel words: Do not construe what is said below as medical advice. Consult professional advice for YOUR own situation BEFORE taking any action! Do YOUR OWN research! Anything that is said, if you follow or use, you do so at your own risk, no warranty expressed or implied! Do not taunt Happy Fun Ball!

* * *

Having impaired hearing myself (significant conduction deafness since I was very young - but the nerves are still in excellent shape even though I'm well into middle age) I would urge you to get your hearing tested - mostly so that you know what sort of condition it is for which you are trying to correct - and make certain that it isn't something trivial and easily fixed. The obvious one - earwax - should not be overlooked as my mother (who is a retired nurse) can attest as that accounts for many cases of "deafness", often by those who absolutely insist this to NOT be the case until they are proven wrong!

Now, I'm not a spokesman for any hearing aid company (I should technically be wearing at least one of them, but I do not as I have adapted to fill in gaps by lip-reading in the presence of other people as well as interpolate/extrapolate with very good accuracy the missing words/vowels/consonants in context - a skill that is NOT easily acquired and has taken decades to do so!) but it would be a real shame to throw amplification onto an ear in which there was already nerve damage - and then cause additional damage with excess sound pressure level, wiping out/damaging what was left!

If one has conduction deafness, this is (probably) less likely to happen as you are (more or less) overcoming attenuation, but if it is actual nerve damage, one must be *really* careful to avoid blasting any sound pressure at a level that will cause physical damage - and without calibrated instrumentation and knowing what, exactly, the "inner ear attenuation" already is, you are essentially blind.

If/when you have ruled out that it is something that is trivial or something more serious that could actually be "fixed" (e.g. wax, fluid buildup, cholesteatoma, etc.) walking away with your own, private "frequency response" chart for each ear would be invaluable in applying a "correction factor." At this point, you would presumably know what sort of hearing impairment you have (e.g. conduction, nerve, combination, etc.) and know the risks that one might take when applying unknown (!) amounts of sound pressure to the ear.

* * *

The world is rather lousy with graphic equalizer-type schematics and for starters, a simple bass/midrange/treble diagram would be appropriate: If you are typical of most, your most problematic hearing loss likely occurs in the 500-2500 Hz range where the loss of distinction of unvoiced consonants makes distinction of words rather difficult at times, so a "midrange" and "low-treble" boost might be most beneficial.

The use of the aforementioned "AGC" circuit is also important. With nerve deafness, the available "dynamic" range is simply lost, which means that you will have to compress the quiet to the loud into a much smaller set of dynamics.

While there are many ways to do this, among the simplest and most effective are with a resistive opto-isolator - essentially a CdS cell coupled to an LED, with the CdS cell in the feedback loop of an op-amp.

These devices are described here:

https://en.wikipedia.org/wiki/Resistive_opto-isolator

and a real-world application may be found here:

https://en.wikipedia.org/wiki/Automatic_gain_control

I have used these devices many times and they are fairly cheap and work quite well, being nearly fool-proof! By placing a potentiometer in parallel with the CdS cell you can reduce the MAXIMUM gain that will occur under no-signal conditions while putting a resistor in series with the CdS cell can reduce, to a certain extent, the "compression" factor. In the "Automatic Gain Control" article, one could add a separate op-amp section to the LED, itself, so that it "lit up" under a much lower audio level, allowing the AGC to be triggered "sooner." There are versions of these devices with back-to-back LEDs so that asymmetrical waveforms will trigger these devices on the highest peak, but one could also precede them with a full-wave bridge, or simply wire up an op-amp based full-wave circuit as well.

These resistive opto-isolators are made by Vactec and may be found on the surplus market, but are easily made using LEDs and CdS cells - even dual-color (2-lead) units, sealing the lot in black heat-shrink tubing to shield from extraneous light.

Sure, there are more elegant devices out there to do this (e.g. a VCA based on an OTA) but this is a very quick, easy and simple circuit to build and implement.

* * *

Again, simply throwing amplification into ones ears to overcome hearing loss without known exactly what one is doing is NOT a good idea, but since I just know that someone reading this forum is going to do it anyway (human nature!) I just thought that I'd throw out a few warnings and ideas.

 

[tvtech]

^^^Excellent advise cct314

This is more complicated than I thought.

I mean, in my case, I can perfectly well hear people speaking in a quite environment..looking at me and addressing me...as soon as there is background noise...I am lost.

There was a test on the Net a few Years ago. It involved progressive testing from the Left to Right ear. A Frequency response thing if you want.

My Right Ear could hear up to 16KHZ..
My Left Ear could barely hear around 12KHZ.

A CRT Line Stage runs @ 15.625 KHZ. Cant hear it.

That is where as a Tech it becomes dangerous... but you learn to adjust.

Heaters...got them...line is running.
Plus..job card near set Anode..and moves when set is switched on = line is running.
And a good dose of gut feel too.

At the rate I am falling apart...I could of been a really good blind Taxi driver.

Regards,
tvtech

 

[Inquisitive]

Using a white cane to tap the road while your arm is out the taxi window may scare your passengers.

 

[ayeo99]

cct314 - thanks for the advice on the other causes of hearing deterioration.

I did went for a hearing check and my hearing deterioration is not due to ear wax or infection.
Cannot remember the details of the test results but it showed a decline from midrange onwards.
I even got to test the hearing aid, I think it was Bentone (or something like this).
It costs more than USD1,500.
I'd like to make one myself, though I know I can't make one that is as small and advanced as the Bentone one.
But I believe I can make one that is useable for my case.

I would appreciate very much if anyone knows of any project kits ( I can do the soldering) where I can assemble a hearing aid.
Or provide a circuit diagram where I can buy the components and build one myself.

Thank you in anticipation

 

[ayeo99]

Found this from the web:
https://bestengineeringprojects.com/electronics-projects/hearing-aid/

How do I adjust the volume?

Anyone tried this before?

 

[audioguru]

I turn up the treble tone control when the TV news announcers are hiding their microphones (a mic on TV must NEVER be seen no matter how muffled it makes the voices). Then I turn down the treble tone control to normal for commercials and shows. FM radio sounds great and AM radio sounds awful.

I notice lately that when speaking, many women end each sentence with a growl sound like their voice has quit working. They did not speak like that in "the good old days".

 

[audioguru]

The hearing aid circuit in post #9 has many mistakes. It is extremely distorted and is much too sensitive. The dual opamp selected should NEVER be used for an audio preamp because it is too noisy and also produces crossover distortion. Look here:

 

[ayeo99]

audioguru - thanks for your professional advice on this circuit.

Any recommendations on other hearing aid circuits?

 

[4pyros]

I notice lately that when speaking, many women end each sentence with a growl sound like their voice has quit working. They did not speak like that in "the good old days".

Do you find that only happens when they talk to you. LOL

 

[4pyros]

The hearing aid circuit in post #9 has many mistakes. It is extremely distorted and is much too sensitive. The dual opamp selected should NEVER be used for an audio preamp because it is too noisy and also produces crossover distortion. Look here:

I can't believe the headphones are right across the power supply.
Well it seems to be quite common in these kind of circuits.

 

[ayeo99]

https://www.ti.com/product/lm4916 - seems like it is a 1.5V audio amp but I am not sure if the power output is high enough to use for a hearing aid

any expert advice on whether this IC can be used for the hearing aid circuit?
anyone has a hearing aid circuit designed using this IC?

 

[k7elp60]

Here is a schematic of a hearing amp I built a few years ago. It has a built in compressor so all the sounds are at one level. I used if before I was able to buy hearing aids. I have a built unit that I no longer need and I even have a spare circuit board and might have most of the parts if some one is interested. Contact me for details.

 

[Mosaic]

If you design one from transistors alone it can prob. work with much less voltage. The question I have is; how much dB amplification is required and what is the efficiency of the earphone unit? Also I imagine you're gonna need to have high end emphasis....anything over, say 4Khz, is gonna need boosting to enhance audibility. Then you have to deal with the feedback issue between the mic and the earphone. there's going to be a natural resonant frequency feedback that kicks in once you have any decent amplification going. So you're going to have to dampen that by mechanical modelling or electronic means.

hard to believe that circuit AG spoke about uses a drive transistor with no biasing....no negative feedback...that thing could saturate and pop the earphone coil.

 

[audioguru]

Do you find that only happens when they talk to you. LOL

I like it when a woman whispers in my ear, "Come and get it". OOps, that is Selena Gomez's song:

Actresses in movies do not talk to me. They growl at each other like wild cats. That's it!!

We talked about the LM4916 in another thread.
It has a lot of hiss and distortion so it needs a gain of only 1 plus a preamp.
It is an extremely tiny IC that must be soldered by a robot.
Its BTL output power to a normal 32 ohm earbud is too low to be a hearing aid and might also be too low for a person with normal hearing.

 

[Mosaic]

Here are some options with 'how it works' explained. Even a constant volume version!
http://www.talkingelectronics.com/projects/HearingAid-2/HearingAid-2.html

 

[audioguru]

Here are some options with 'how it works' explained. Even a constant volume version!
**broken link removed**

The description of how the first circuit works is completely wrong (it says the 6.8k resistor between the collector and emitter of the BC557 when it should say between the base and emitter) which makes it confusing. Its gain of only 50 is too low for a hearing aid. Where can anyone find an earphone that is 8 ohms today because they are 32 ohms now.

The second circuit puts DC through the earphones which might destroy them and will cause severe distortion. Their plug is wired so that if you simply connect them in series at their plug then they will be out-of-phase which sounds weird. Two earphones should produce stereo but the circuit is mono. The second circuit produces severe distortion.

Both circuits do not provide important equalization of the frequency response.