EEVblog Electronics Community Forum
Electronics => Open Source Hardware => Topic started by: FenderBender on September 17, 2012, 07:51:13 pm
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Hey everyone,
Well after reading the OSHW Priority List I was reminded that one of the reasons people don't get proper healthcare is because they can't afford it. And what's one thing that is grossly overpriced: Hearing aids.
Now...Modern hearing aids are actually pretty complex little devices. The top of the range ones are definitely pretty cool. Good example of electronics miniaturization...However, I think there is still a lot of people out there that would much rather have their hearing back then care about whether the device fit's in their ear. Could be wrong..
I've seen them on Amazon, but all of them have some sort of problem..They are called "hearing assistants". They cost anywhere from $20-200. In fact, my grandpa had one. It was a real peace of garbage though. Wouldn't it be nice if there was a very solid solution that offered very respectable performance for a very modest price. I would not plan on making it something that would fit on your ear, rather a device that would fit in your pocket and use a standard earbuds/phones for sound.
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Of course, this could be nothing but a fleeting thought, but I thought I would put it out there. Here would be some goals
Small form factor (fit into a pocket)
Customization (3 band active equalizer)
Long battery life (TBD)
High quality sound (no static/noise/distortion)
Simplicity (make it easy to use for the intended audience (65+))
Of course those are pretty basic goals, but important ones..
As far as the circuit goes...
Electret mic --> Mic Preamp --> 3-Band equalizer --> Headphone amplifier --> Headphones
Unsure of how batteries and power would work. Rechargeable would add some issues possible.
The whole thing would probably just be a ton of op-amps. Simple and effective.
Any ideas?
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Straightforward to understand, perhaps, but not necessarily that effective, and certainly not small.
I'd use a chip more like this:
http://www.wolfsonmicro.com/documents/uploads/data_sheets/en/WM8904_2.pdf (http://www.wolfsonmicro.com/documents/uploads/data_sheets/en/WM8904_2.pdf)
This device integrates a stereo codec, programmable gain, equaliser, dynamic range compression and headphone amplifier, all into a device 4mm square. Add a simple microcontroller such as a PIC to program it up at power-on, and maybe provide a few different settings for different environments. The whole thing would be the size of a thumbnail and would operate off 1.8V for ages.
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Very interesting. It seems awfully complex for such a simple operation of mic -> earphone amplification.
Half way through the datasheet. Phew. Hell of a chip. My serial communication skills pretty much suck.
Is there any similar type of device that does not have so many features? Say 1 mic input, 1 output, volume control, and equalizer? Anything like that available?
I've been looking at the TI OPA332 op amp family. http://www.ti.com/lit/ds/symlink/opa4322.pdf (http://www.ti.com/lit/ds/symlink/opa4322.pdf)
I don't know. Looking around.
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AD, DSP, EQ, DA, 1.3V
http://www.onsemi.com/PowerSolutions/product.do?id=GA3216-E1 (http://www.onsemi.com/PowerSolutions/product.do?id=GA3216-E1)
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Thanks. Just out of curiosity...
Does anyone know how to choose a proper telecoil/T-Coil?.. you know the things that are in hearing aids that pick up magnetic field and translate it into sound (has to be from a telecoil transmitter, or from your phone)
I'm looking around at different telecoil manufacturers. Some have inductances of 50uH, some have 100mH, or bigger or less. Varying sensitivities etc.
Any idea how to choose one?
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I'm an audio engineer rather than an electronics engineer, so I'm not sure if I'm going to be able to give much advice to you. Apologies if I go into too much detail. I'm just trying to give some info on things you might not have looked at that might help with your design, either to make it cheaper or more effective than the crappy ones you can buy nowadays. There are so many simple designs out there that have probably never taken the audio aspect into consideration.
Equal Loudness Contour:
Listener fatigue:
Thank you. Regarding Equal Loudness Contour. So from 2kHz to 5kHz, humans perceive these frequencies to be much louder than another frequency with a constant SPL. Interesting. But now what does that mean in the sense of EQ? So what frequency range should be boosted? Let's say that <1000Hz does not need to be touched based on aging does not affect low frequencies. But now from around 1k-10kHz what should be boosted and what should be attenuated? Should the 5k-9kHz be amplified? Does that make it sound clearer?
Regarding Listener fatigue. So perhaps it would not make sense to use a super fancy huge bandwidth op-amp because it will potentially pick up on the parts of a signal that a slower op-amp might miss? By audio compression do you mean digitally compressed audio, like some hearing aids which use a DAC/ADC combo to do the signal processing?
Electret mic -> Buffer -> BP Filter -> EQ -> Headphone amp -> Headphones
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Here's the idea
I would probably use 2-3 slide potentiometers for the EQ so you can visualize it. However, I think I will already "pre-bias" the EQ underneath that. I'll explain what I mean. Even though I might have 3 potentiometers that show that no frequency range is being boosted, as in they are all right in the middle, on the inside, I will already have done some EQing that would be helpful for older ears. So like a resistor in series with that EQ potentiometer. Even though it looks like it's flat response, it will already be a little EQed out of the box.
Sorry if that made little sense. I want it to be customize-able, but I also want it to work well without much adjustment. If people still feel like they can't hear something, they can change the EQ or whatever.
Does that sound like a bad idea? Should it just have a preconfigured EQ?
Thanks
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I worked for some time repairing hearing aids--they are a nightmare!
The main types I worked on were analog types with digitally programmable characteristics.
These devices could be programmed for the required frequency response,dynamic range,gain etc,as determined by the Audiologist examining the wearer.
They use an "earphone" unit,in the body of the aid,with the sound fed to the earmould via plastic or rubber tubing.
The "in ear" devices used very short lengths of tube,& the "behind the ear" & the (very few) "pocket" units,a short tube between the earphone & an external tube cast into the body,which then had a long tube to the earmould.
The main things that went wrong,were the tubes coming loose in the earmould,(which caused weird echoing sound with the "in ear" units,& loss of sound in the other types),& earwax blocking of the earmould outlet,& in some models,the earphone & microphone .
Sometimes such problems would not be diagnosed & the Audiologist would re-program the aid,so that if you later repaired it,they would need to restore the previous program.
Just before I left,they brought in fully digital aids,which initially were very unpopular among the clients,due to high background noise & reliability problems.
Those problems seem to have been overcome now.
The Telecoils I recall were wound on small "dogbone"ferrites.
I have no idea of their inductance,but I would guess greater than 50uH.
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I might have meant 500uH. Anyway, very interesting.
I think they are definitely cool devices. Just very expensive ones. I guess the price is somewhat justifiable but not to everone I guess is my point.
Well! I guess I have to start prototyping. Eh, this probably won't go very far but who knows. If it doesn't get off the ground, already there is a wealth of knowledge right in this thread (none of which is from me! :p). Better start scrounging up the NE5532s and see what we can do. Hey that rhymed. ::)
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Jim, that was very interesting.
It would be interesting to see a device with each of the criteria he noted. A "T" slider for example or wind reducer and so on. Of course that would almost have to be digital. A bonus would be be that if it was digital you could save settings based on location or person, one for the wife and no sound at all when visiting the mother-in-law. Mix it with a gps and it could even be self adjusting based on location. Sorry I'm just thinking out loud.
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>>Thank you. Regarding Equal Loudness Contour. So from 2kHz to 5kHz, humans perceive these frequencies to be much louder than another frequency with a constant SPL. Interesting. But now what does that mean in the sense of EQ? So what frequency range should be boosted? Let's say that <1000Hz does not need to be touched based on aging does not affect low frequencies. But now from around 1k-10kHz what should be boosted and what should be attenuated? Should the 5k-9kHz be amplified? Does that make it sound clearer?
The equal loudness curve is just a rough guide to show which frequencies tend to need boosting to make sounds sound 'flat'. Where the line goes up, that's generally where hifi hardware boosts, and where it goes down, that's where they usually cut (classic 'smile' EQ). I wouldn't bother with too much EQing for a hearing aid, as it might over-complicate things. I'd focus more on getting the S's and T's to be more intelligible. That's usually around 10kHz, but I have found in my experience that 10kHz is far too high for boosting S's and T's in most cases. Between 5-9kHz gives good results depending on the vocalist.
These are the EQing features I would implement if I was building it:
· HF Boost switch with three positions: Norm / +3dB / +6dB ( 8kHz boost with about 0.7Q )
· Rumble filter switch: ON / OFF ( 70Hz HPF -6dB/octave )
You could even go as high as 100Hz or 120Hz with the rumble filter - it might work better in hearing aids. 70Hz is just the compromise in the broadcast world, so things don't start sounding too tinny.
>>Regarding Listener fatigue. So perhaps it would not make sense to use a super fancy huge bandwidth op-amp because it will potentially pick up on the parts of a signal that a slower op-amp might miss? By audio compression do you mean digitally compressed audio, like some hearing aids which use a DAC/ADC combo to do the signal processing?
Transparent sound from the best op amps is usually regarded in audio circles as sounding "clinical" and "too perfect" compared to older analog equipment where the natural high frequency rolloff prevented harsh transients from coming through. This was what I was thinking about in terms of bandwidth. Rather than choosing a wide bandwidth of say 100kHz or more, you could roll off the audio spectrum around 15kHz. It's probably not a necessary feature, but it might be fairly easy to implement a -3dB rolloff point on the audio output, or limit the amplifying bandwidth of the op amp somehow.
Oh, and by compression, I mean dynamic range compression. It's probably an expensive feature to install anyway, so I wouldn't worry about that. The only problem you might have is dealing with distortion and clipping in loud areas. Maybe you could implement some very light automatic gain control? about -12dB for louder environments. Not sure how difficult that would be.
>>Here's the idea
I would probably use 2-3 slide potentiometers for the EQ so you can visualize it. However, I think I will already "pre-bias" the EQ underneath that. I'll explain what I mean. Even though I might have 3 potentiometers that show that no frequency range is being boosted, as in they are all right in the middle, on the inside, I will already have done some EQing that would be helpful for older ears. So like a resistor in series with that EQ potentiometer. Even though it looks like it's flat response, it will already be a little EQed out of the box.
Sorry if that made little sense. I want it to be customize-able, but I also want it to work well without much adjustment. If people still feel like they can't hear something, they can change the EQ or whatever.
Does that sound like a bad idea? Should it just have a preconfigured EQ?
I think a 3-band EQ might be a little complicated for some. Some people won't have a comprehension of which frequencies are meant to do what and what's the best way to configure it. I do like your "pre-configured" idea, though, along with the additional option of tailoring it for their hearing. What do you think of the switch idea with 3 positions? Then it's just a case of "Nothing->More->Even more". Quite easy for people to decode, and they will find the positions they like the best for different situations.
I like it. Very solid ideas. I thought the same thing. People might not understand exactly what those sliders mean, especially those of the older generation. Reducing options is one way to do it.
All the filters and options are pretty feasible.
I feel that automatic gain control could be done simple enough. I'd have to figure out the exact logic to it though.
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Hearing loss is something I understand well. My dad had severe hearing loss, it was to the point I would lose my voice after having short conversations because I literally had to shout for him to understand me. Hearing loss also impacted his life in a big way because people often thought he was suffering some sort of age induced dementia, but anytime I would talk with him I knew what was going on , he wasn't hearing all the words of the conversation. It is easy to misinterpret what someone is talking about if you only hear every 3rd word and so they took it as his mind was aging and so was his comprehension.
At the time our family had next to no funds to even keep food in our own homes, and when we checked for getting him a hearing aid they wanted $2300 USD, so we couldn't do it and insurance , what little we had, couldn't cover the cost. Some family members tried things like those TV commercial super hearing devices to learn they are nothing but junk, not even good for people that have good hearing. I started to design something myself, I was out of work at the time so little funds too, around the dspic line of chips with an WM8731 as codec chip. Unfortunately my father passed in July of this year and I wasn't able to give him the hearing assistance that he deserved. I have since made it a goal in life to put as much of my electronics knowledge as possible towards helping disabled people to have better quality of life. There is so much that electronics can do and if someone can come up with a low cost hearing aid, that would be awesome and a tremendous help to so many people.
I haven't done much with the design since his death, his death still impacts me quite a bit and I guess working on that project, I just think of him the whole time I am working on it. What I had worked okay, but the biggest issue I faced was how to amplify sounds without amplifying everything in the room, it requires quite a bit of dsp work and I'm afraid I am very weak in that area.
External design would be welcomed by lots of people, I was going for something like the old shirt pocket type, possibly fitting it inside an altoids can. Really when you have a loved one that cannot hear you don't care if it all fits in the ear.
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Thanks for sharing. I'm very to hear about your father. I'm sure he was a great man.
In regards to the hearing aid, if someone with a good deal of engineering knowledge wants to help humanity, I think this is a good place to start.
I know a completely analog approach would be pretty difficult in terms of getting good sound reproduction (of the sounds you want to hear!) as you say. I honestly have little signal processing experience myself, which might make designing one difficult. Some of the problem might be due to quality of the parts used. A cheap mic will sound like a cheap mic, etc.
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One of the terrible "SOCIALIST"(Boo!,Hiss!) things in Australia,is that people on the Aged Pension can get their hearing aids free!
If you are a "self-funded" retiree,you can get a discount,but I'm not sure how much it is worth.
Obviously,the Government funded ones are not the most "high tech",but they are still very good.
Every year,in the Autumn,there would be a rush by clients to get their Aids sorted out,prior to them becoming Grey Nomads" & heading North for the Winter.
The old codgers would enjoy themselves,& be back in the Spring,with Hearing aids they had dropped in the sea while fishing!
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but the biggest issue I faced was how to amplify sounds without amplifying everything in the room, it requires quite a bit of dsp work and I'm afraid I am very weak in that area.
An omnidirectional microphone can be used to capture the background noise, and used in a noise cancellation configuration with a more directional microphone (preferably located close to where one's ear would be.)
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One of the terrible "SOCIALIST"(Boo!,Hiss!) things in Australia,is that people on the Aged Pension can get their hearing aids free!
If you are a "self-funded" retiree,you can get a discount,but I'm not sure how much it is worth.
Obviously,the Government funded ones are not the most "high tech",but they are still very good.
Every year,in the Autumn,there would be a rush by clients to get their Aids sorted out,prior to them becoming Grey Nomads" & heading North for the Winter.
The old codgers would enjoy themselves,& be back in the Spring,with Hearing aids they had dropped in the sea while fishing!
Well you CAN get the same here in the States but there's a lot of little nooks and crannies that a lot of people get gypped by. Not the right coverage or don't meet a certain requirement. So there's a good amount of people that don't fall into a category that will land them a free/reduced price hearing aid...but might not be able to afford a $1k aid. I think it's worth while.
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but the biggest issue I faced was how to amplify sounds without amplifying everything in the room, it requires quite a bit of dsp work and I'm afraid I am very weak in that area.
An omnidirectional microphone can be used to capture the background noise, and used in a noise cancellation configuration with a more directional microphone (preferably located close to where one's ear would be.)
That is a very nice idea I must say. What would you use? A differential amplifier circuit perhaps?
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Obviously,the Government funded ones are not the most "high tech",but they are still very good.
(http://www.topfermented.com/wp-content/uploads/2010/01/EarHorn-300x287.jpg)
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Obviously,the Government funded ones are not the most "high tech",but they are still very good.
(http://www.topfermented.com/wp-content/uploads/2010/01/EarHorn-300x287.jpg)
Ah that's last year's model!!
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Well after reading the OSHW Priority List I was reminded that one of the reasons people don't get proper healthcare is because they can't afford it. And what's one thing that is grossly overpriced: Hearing aids.
As a one time hearing aid user ( I now use a cochlear implant ) I would like to point out that the price of privately bought hearing aids include the fees to the audiologist for making & fitting earmoulds and subsequent maintainance / adjustment.
Now...Modern hearing aids are actually pretty complex little devices. The top of the range ones are definitely pretty cool. Good example of electronics miniaturization...However, I think there is still a lot of people out there that would much rather have their hearing back then care about whether the device fit's in their ear. Could be wrong..
You will be amazed at the number of people with a real hearing problem who are too vain to wear visible hearing aids - I wore mine from when I was a toddler so I was well used to them. But I still remember being ribbed for being the only pupil in a deaf school to be still using a body worn aid when everyone else were using behind the ear aids!
I've seen them on Amazon, but all of them have some sort of problem..They are called "hearing assistants". They cost anywhere from $20-200. In fact, my grandpa had one. It was a real peace of garbage though.
Have you torn it down to work out why it was garbage? I think it was because the frequency response of that gadget did not match your grandpa's response closely enough.
Wouldn't it be nice if there was a very solid solution that offered very respectable performance for a very modest price. I would not plan on making it something that would fit on your ear, rather a device that would fit in your pocket and use a standard earbuds/phones for sound.
Most old people with hearing difficulty have a falling frequency response above 2000Hz. So much speech sounds ( like constanants such as S P T Z ) have fundamentals above 2000Hz that speech becomes more difficult to listen to. So you would have to provide gain above that frequency.
Also I would suggest finding out what kind of audiogram ( http://en.wikipedia.org/wiki/Audiogram (http://en.wikipedia.org/wiki/Audiogram) ) such old people have and have a look at this wiki page about audiology ( http://en.wikipedia.org/wiki/Audiogram (http://en.wikipedia.org/wiki/Audiogram) ) to get an idea of the kind of response needed.
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TheWelly,
I'm going out right now, but thanks for the input.
I partially think that a body worn aid would be almost less of vanity problem because you could just slip it into your pocket and make it look like you are just wearing a earpiece. But, I am not psychologist and I have little experience. So I could be wrong.
I did tear down his hearing "assistant". I think the design was actually okay. It used 100% discrete construction. Nothing but BJTs, Diodes, Resistors, and Capacitors in the whole thing. It actually kept the noise floor decently low, but actual sound quality was pretty poor. Very boomy-echoey sounding. It was louder, sure, but not necessarily more clear.
The components used were also very low quality. Every capacitor was electrolytic except for a few SMD caps. They literally used a 0.1uF electrolytic. That's just being cheap. It was built down to a price.
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Obviously,the Government funded ones are not the most "high tech",but they are still very good.
(http://www.topfermented.com/wp-content/uploads/2010/01/EarHorn-300x287.jpg)
I used to think some of the old codgers would have been better off with a pair of those!
They might not have found a way to break them!
Dear old ladies specialised in having their Aids devoured by their dogs! ;D
Of course some of the faults were due to a vacuum in the heads of the design Engineers.
On one series,they had a tiny toggle switch that you had to 'pump"repeatedly to change volume.
The wearers assumed that if you pushed the switch either way & held it there, the volume would change accordingly.
After all,just about everything else works that way!
They would bring them back & complain,& be told the right way to use them.
The problem was that if you use it correctly,the switch carks it in no time--& it is a total mongrel to replace!
The "Ginger Beers" saved a few components,& incurred unending hatred by the Techs who had to fix the things!
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One hearing impaired lad I know was really ashamed to talk to girls because every time he tried their looks were glancing ever so often to the hearing aid behind his ear. If memory serves it was awkward enough for me even without such complications so I can't imagine how awful it must have been for him.
Maybe he would have felt better if the hearing aid used a standard iPhone earbuds with mic on the cord with the battery and electronics in the pocket?
On a separate line of thought, if we expect this aid to be used by the elderly an on-off switch and a volume control is the upper limit of complexity. Rumble and hiss filters etc. must work automatically. If practical physiologically even the volume should be automatic. Old tube radios used to have auto gain to keep the volume steady when the signal faded didn't they?
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I suppose it could be done. Though I feel like you should be able to adjust volume if you want to.
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I am Hard of hearing, and was recently tested. I'm about 65% loss in both ears, I have a miracle ear I was given (actually a pair but one was lost while working and was never turned in) but its failing, and it would cost me $250 to repair it, Hard to do when you are unemployed, and thats also causing me issues getting a job now, a new hearing aid would cost me $1300. so I need to figure something out some how..
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I'm a full time audio engineer & work weekly with in ear monitoring systems on stage. I'm learning the pic & avr microcontrollers as a hobby pursuit which is why I got on this forum :-)
Anyway, if there's anything I can do on the audio front, I'd certainly be happy to help.
todd
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The pure in ear devices are justifiably expensive ... they are also highly unoptimal.
Necklace worn microphones offer far superior information for source separation filters ... and avoid the need of trying to get all the DSP hardware into a tiny in ear device. Getting stuff like voice detection and blind source separation right though seems non trivial.
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Can someone point to some reading material about the nature of the signal processing that needs to be done?
For example when we are talking about hearing damage are we talking about decreased sensitivity across the bandwidth of for example loss of hearing in a certain part of the bandwidth?
Besides the standard LP HP and BP filters, do we need to consider compressors? For example if the person's hearing only works between 200 - 600 Hz do we need to compress the entire spectrum to this particular range or do we just apply filtering? Hell I don't even know if this selective bandwidth thing even exists..?
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Can someone point to some reading material about the nature of the signal processing that needs to be done?
Do you mean how DSP works, or what should happen in the 'real' analog world of creating clarity of sound for your ears?
do we need to consider compressors? For example if the person's hearing only works between 200 - 600 Hz do we need to compress the entire spectrum to this particular range or do we just apply filtering?
Compressors compress dynamic range..ie: turn loud things down in a very fast manner...like reshaping the sound of a snare drum hit, or leveling out volume between bass guitar notes. I'd imagine this is precisely what you wouldn't want to do with hearing assistance because it could kill off the clarity of consonants. Now, an expander + a limiter, that might be useful to make consonants louder & then limit them to a max loudness.
Moving a 2000hz sound down to a 200hz sound is pitch shifting. It's typically kinda mucky & bizarre sounding if you're being that aggressive with it.
Hell I don't even know if this selective bandwidth thing even exists..?
Seems to me you could hook a microphone to preliminary HP/LP/BP filters, then split the signal to a super snappy VCA chip, and send the rest of it to a dsPic/blackfin/whatever running an FFT routine or something like that. Once the dsPic deciphers sounds from 2khz-4khz, use it's DA converter to juice the gain on a VCA. Use that with some open air earbuds & only amplify the frequency range that needs to be amplified, and only when it is present. The whole board could be 2-3 chips + some passive components. Might even get someone to make a windows gui to write amplification behavior tables to the dsPic.
Just a thought.
todd
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Can someone point to some reading material about the nature of the signal processing that needs to be done?
Do you mean how DSP works, or what should happen in the 'real' analog world of creating clarity of sound for your ears?
I mean medical studies on patients. It wasn't very clear I confess.
For example we need to know if some hearing deficiencies are frequency dependent. If a patient who has lost most of his hearing can retain some of it in a certain frequency range. Then we need to know whether it is useful or not to remap normal speech frequencies to this patient's hearing pattern. Can a patient get used to it and start to recognize speech better? Or maybe it has side effects? Wrong glasses can cause headache and nausea, how about wrong hearing aids?
All in all, I'd rather read some medical research instead of going on intuition alone.
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Gotcha.
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Can a patient get used to it and start to recognize speech better? Or maybe it has side effects? Wrong glasses can cause headache and nausea, how about wrong hearing aids?
My daughter has cochlear implants (direct electrical stimulation of the auditory nerves) and this sounds highly unnatural; everyone sounds like a Cylon . This does not cause nausea or headaches. At most it can be tiresome.
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Hi all,
Some ideas:
Years back i saw some studies with multiple microphone array's that gave a directional effect.
Spacing between the mic's would attenuate specific frequencies and amplify others.
A hearing problem does not always mean the absolute sound level hearing.
One can have a good conversation in a quiet room but as soon as environment sounds at specific frequencies appear following the conversation becomes a problem.
I have anticipated in some test where a tape was played with a newsreader at a normal conversation level.
Then the same tape was played but white noise was added until the signal noise ratio was 3 dB.
Very quickly the newreaders voice was difficult to follow.
I had a friend who was a shortwave radio HAM. He could pick up HAM conversations burried in noise where I could hardly discriminate a calling station. This was training the brain to do the DSP.
I admire people who can have a conversation in a Disco or on a factory floor with heavy equipment.
Altough i think they do more lipreading than hearing.
Understand that when you amplify sound it also amplifies the unwanted surrounding noise.
The trick is to discriminate those 2 and only amplify the sound you want.
The easiest way to do this is make the distance to the sound you want to hear short and the distance to the noise large.
If you decouple the mic from the earpiece and point it to your subject you will do better that fancy DSP algorithms.
Hide your micophone in a ring or watch and you can bring it discretely to the subject.
Maybe a bluetooth earpiece with some bluetooth microphones will get you going.
Using both ears and 2 channel audio will give a stereo image in your head. This will make it easier for the brain to conentrate on a single voice in the stereo image.
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I think no algorithm has been invented or found yet that can separate one voice from background noise like the human brain can. There's more then just simple frequency domain filtering as multiple voices occupy the same frequency band.
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With multiple microphones you can isolate sources. ICA is a famous algorithm for this for instance, but requires as many microphones as sources I think so is not the most practical one.
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Apart from the obvious voice band filtering, you could use directional microphones aimed to give the highest sensitivity at typical conversation distances.
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Apart from the obvious voice band filtering, you could use directional microphones aimed to give the highest sensitivity at typical conversation distances.
There are already hearing aids available with voice band filters and up to 6 microphones in some instances but way out of the price range of most people with hearing loss. Some of my former clients purchased $6 - 10,000 per hearing aid to achieve this and what concerns me about the practicality is why did they come and see me in the first place.
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How difficult is it to create a cheap open source version? Fast ARM boards like the Raspberry Pi are quite cheap nowadays.
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Most of the cost is in the non recurring part ... ie. find someone mad enough to actually do it and not charge for his time.
Also if you try to use advanced signal processing with multiple microphones for voice isolation or frequency compression I expect a contributory infringement lawsuit, so better not publish under your real name.
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As a practical note.... my daughter goes to a regular school and to help her hear the teacher better, the teacher wears a wireless microphone. My girl wears a receiver that retransmits the received signal to the hearing aids via a standard inductive ring? interface.
So even with four DSP cores they can't even begin to come close to the processing capabilities of the human brain, which after all, just has two microphones as input ;)
IIRC these hearing aids already have two microphones to help voice separation, one omnidirectional one and a unidirectional one. The algorithms is what sets the four different manufacturers apart, but they all function about just as good. It's subject of major marketing blahblah.