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DIY but exceptionally high end semi portable HA system made with pro recording studio equipment

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Old August 18th 05, 12:32 AM
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Default DIY but exceptionally high end semi portable HA system made with pro recording studio equipment

I have been trying to help my father who is suffering age related
hearing loss (very typical.. no high end left)... and he has used an
NHS dispensed HA (He lives in the UK), with some success; but it leaves
him unable to make out speech often on broadcast TV. (I should say he
listens to the TV via some Spendor BC1s and Quad amps as obviously
conventional TVs are "un-listenable to" even by non HA users).

Anyway; I have a good scientific background, and understand a fair bit
about the physics of sound, and its reproduction, and also the biology
of the ear, and electronics and computing. (I read maths at Oxford). I
used to be involved in recording classical music, so have a passion for
audio clarity. It strikes me that all hearing aids that I have so far
seen, seem to be battling to squeeze as much battery life, oh yes-and
quality out of components that are essentially too small to do the
job. As Scotty said "Ye canny change the laws of physics".

When producing semi-pro classical recordings, I worked out a long time
ago, that to achieve "acceptable" sensitivity from microphones, you
need 1 inch or so diameter condenser capsules. Anything less produces
horrible dictaphone type results. (OK I exaggerate.. but it's not far
off.). So: If one sticks to the premise that the device has to fit
invisibly within the ear; you are forever handicapped in the design

Now for my father, I have 2 DIY tasks in mind: (1) to concoct a
headphone style listening device for watching TV: He doesn't care
what's connected or the size... no one will see him!
(2) Closely following on the heels of this one... It struck me that if
we could pair a fairly unobtrusive in-ear; sound isolating earphone (At
the mo. the etymotic Er4s appear sadly the best one can do; though they
leave a lot to be desired), and perhaps use something like some AKG 414
mics that one sits on the seat in front during a concert; plugged into
a battery powered mic. pre amp that is of high quality (so few are);
and suitable "processing" box. In a restaurant one could stick the mics
on the table, having the obvious advantage in the noisy restaurant
environment of simply harnessing the inverse square law; to separate
the person talking; from background hubbub, by putting the microphone
nearer to them, than would be possible with all in one -within the ear
solutions. In both circumstances I feel it should be easy to surpass
the quality achievable with a hearing aid; at the expense of having to
be happy to lug some equipment with one, that is only semi-portable.

For these two very special circumstances; which hearing aids are just
not optimised for; his pleasure could be improved significantly;
judging by our experiments so far.

Before anyone says: I know one can never regain what's lost in terms of
his ability to perceive; but as long as one pours the highest quality
stimuli achievable into his ear canals; one should be able to maximise
the best of a bad starting situation. As an aside- my father has never
really been interested in hifi, but his ability to pinpoint the
"quality" hifi components recently as we started to play with this idea
within seconds is uncanny; in A-B comparisons when asked to. Just
because he has impaired hearing; doesn't mean he can't use what's left:
and he himself has observed; surely we should try and maximise the
quality of every component in the chain.

When I recently put my father under a pair of "listenable to"
headphones (AKG electret K340s, which retailed for approx. =A3150 UK 10
years or so ago, as I remember), fed from his DVD player; his eyes lit
up. Simply delivering good quality reproduced sound, allowed his
admittedly now poor hearing, the best chance of extracting enough
information for him to tell what is going on. He hasn't given the
headphones back, preferring them to his dispensed hearing aid.

I am wondering whether it is worth going the whole hog, and getting
some serious Stax ear speakers (e.g. the 4040s or gulp the 007s); and
couple them with some kind of DSP/ filter box; that would normally live
in a recording studio; and suitably calibrate the boxes, to give both
mutli band compression, clipping/limitting function, and frequency
filtering to try and compensate for the shape of the sensitivity of his

It is notable that a good friend of his, who is a retired GP (so an
intelligent; thoughtful chap); is thoroughly fed up having got through
3 or so really expensive "digital" hearing aids, (I think one cost many
thousands of pounds) with similar results-just ending in frustration.
He similarly noted that good hi-fi; seemed to help him more; and this
was before trying anything clever with filters etc., for this special
application of sitting at home in the lounge.

I feel quite strongly; that none of the signal processing systems I've
read about in commercial hearing aids seem to do offer any more than
the basic kind of functionality that I imagined existed 20 years ago
(but obviously didn't). None of the function appears difficult to buy
in completely configurable guise, if you're happy with a big box i.e. a
piece of recording studio 19 inch rack equipment. Indeed if you proceed
down this route I suspect one should be able to maintain significantly
better sound quality, if bolted together from off the shelf studio bits
of equipment, and being prepared to spend a lot of time carefully
calibrating everything to my father's hearing characteristics.
Conventional hearing aids seem to be hampered by apparently quite low
end audio components.

Etymotic for example, trumpet their products as being pretty
audiophile, with some justification. However, I own their top end ear
phones (ER4s) (sold for listening to music they are not HAs) which many
rave as being "the ultimate hi fi ear phones"-- well they strike me as
"just about ok"; but they are anything but great; when compared to say
sitting in front of a decent pair of electrostatic speakers or hi end
Stax ear speakers, both of which I always think are good starting
points, as audio references.

We are willing to throw money at the problem: the aim here is to
maximise quality. Has anybody else gone down the same route; and if so;
have any suggestions of particular pieces of kit I should put into my
hypothetical audio chain? Has anyone specifically got any suggestions
for studio equipment that is appropriate for the signal processing

I can't believe I'm the first to try this... any suggestions/
observations from those who've been here before?

Sorry this is such a long post, but I would truly value any feedback
any one has to offer.

Jonathan Histed
Old August 19th 05, 12:31 AM
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I would suggest an inexpensive electret pzm type mic as having the best
overall performance parameters as to reach and intelligibility. I'm not
convinced the gear needs to be expensive, just that it have enough
headroom to handle frequency shaping and the ability to comperess the
final signal. When I was a child hearing aids were box affairs that were
carried on a belt with an earpiece going from it and the mic was in the
box, did I mention it was tube powered. Chip amps that fit in a radio the
size of a pack of cards are common and could do the job nicely with some
sensitive headphones which are often in the 90 + db range. Doing the
frequency shaping using dsp in the research stage would be good with the
final assembly done using the amp chip with a compression ic and a
network. It might be useful to consider telephone technology where years
of research went into finding the amount of compression and frequency
shaping that provides the best intelligibility. I fear your presumption
about hifi gear and best intelligibility are two mostly unrelated factors
with divergent goals.
Old August 21st 05, 04:35 PM
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> Sorry this is such a long post, but I would truly value any feedback
> any one has to offer.
> Jonathan Histed

You've got a range of only loosely related problems to address.

The following needs/issues to be defined clearly before starting:

- the precise nature of your father's hearing impairment
- the sources of sound you expect to "channel" to him (fixed vs. mobile;
people vs. electronic)
- the "portability" issue - for use in a fixed location only?? or??
- something else that just eluded me... :- )

I'd look at the idea of using a stereo FM "transmitter" and a high
quality "walkman" reciver, for a fixed setup, locally. Any DSP or EQ
could be applied before the xmiter. This would make for an economical
and locally portable set up.

FYI, sound can also be "broadcast" within a loop via a direct connection
to an amplifier's output, and an AM receiver that consists mostly of an
inductor and amplifier.

There are around here some "40mm Digital" headphones for the whopping
sum of $2.00 US that are actually quite fine to listen to! (Coby is the
brand, FYI)Good bass, smooth highs. Comfortable too.

As far as mics, there are all sorts of imported large and small diameter
condenser mics these days, under $100US for certain.

I'd stay away from PZM, since that will pick up the whole room,
including the noise. In a fixed, living room conversation situation, a
stand and a small shotgun mic or two would do wonders. Imho.

For TV and other sources, a direct connection is preferable, imho.

Everything depends on what the hearing deficit is, as this determines
the requirements for augmenting the sound.

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