Need bass roll-off for a hearing-assistance system

[Gweeble]

4. Never use the BoB at or close to zero gain.

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Hi Mic thread!

I'm a noobie to this forum. I'm using my HiMD as a hearing-assistance system, since both ears are down 30 dB from 800 to 8k, or so my audiologist tells me, though I find that hard to believe. She showed me through the usual hearing aids, and hearing assist devices, but the aids were too expensive and used odd-ball unrechargables, and the assists were all mono 1970's tech-level, with ugly proprietary earbuds. I asked "What do you have that has lots of stereo audio storage, and cool-looking earbuds? They gulped, "Why, nothing." I said to myself, "Ok, lets build me one from off-the-shelf parts instead"

So I bought a HiMD, a Sony stereo lapel-mic, a set of Koss sound-isolating earbuds, and a Boostaroo.

I was using the Sony stereo lapel-mic to record my school lectures as I monitor them thru the earbuds, boosted by the Boostaroo, but wanted to eliminate the clothing noise and try for binaural sound, mounting mics on my head so when I turned to face a person speaking the mics turned too. So I bought a set of Core low-cost binaurals.

BUT, the Sony mic has a tiny switch marked "Voice or Music" (a high-pass circuit) which the Core mics do not. I always had the Sony switched to Voice. I notice with the Core's that there is a heck of a lot of bass, I mean a big old lot! Some is so low and powerful that it makes me nauseous, especially wind and outdoor heavy trucks etc. Also, I don't want to have the recorder busying itself reproducing these frequencies that I do not want anyway, since it will be a definite drain on the battery.

I've been 20 years in pro-audio as a church and theatre install tech, with no formal electronics schooling. I know a lot, but its all just picked up in the field and in trade mags. Can anyone help me by giving me a high-pass filter design for these mics which will be small and cut-off around 150 Hz at like 6dB per? It would be nice if the whole thing was small enuf to fit inside a mini-xlr or TRS in-line but I could always figure something else out. Size isn't really a problem, if it won't fit a connector, it can go inside a plastic mini-altoids case or some such similar.

Next week I try the Shure EC2 sound-isolating earbuds! Depending where on my head the mics are placed I get feedback from the Koss buds when I crank it. The Koss are an open-back design though, and I haven't tried blocking the holes to cut out the mics from hearing them. If I can get this system to work my other doctor, a general practitioner, who is also a moderate hearing loss sufferer, wants to put a similar system together. For me it is great since I have a boosted real-time signal, and since it is being stored, I can listen to it again and again later, with the EQ on the HiMD set to better fit my ears, and not have to interrupt people during lectures or tutuorials with "Excuse me, what did you say?" I can let it slide and try to get it later from repeated listenings using a Custom EQ setting. At home I have a set of Bose active sound-isolating headphones, which also really help in that department.

NE-Way, thanks for being here!

Cheers

Jp in Toronto

[greenmachine]

All you need for a first order filter is one capacitor for each channel connected in series to the microphone.

The cutoff frequency fc of a first order low pass is given by

fc = 1 / (2 * Pi * R * C)

If you rearrange the formula you can calculate C

C = 1 / (2 * Pi * R * fc)

Assuming that fc should be 150Hz and the mic input impedance is 10000 Ohm, your cap would have to have about 0.1061 µF.

But the problem would be that you block the dc bias voltage from the recorder when using a capacitor in series. Thus the microphones need to be powered separately when you want to use such a filter (as your Sony mic is).

This microphone powering scheme is often called a 'battery box'. By using the calculated capacitance, you set the desired roll-off frequency. 2.2 µF is a commonly used value for a non-audible roll-off (approx. 7 Hz, 10 kOhm mic-in impedance).

There are several practical instructions floating around in the net how to materialize such a thing but with some experience and imagination you won't need any.

[greenmachine]

<pre>

--------------||---------------------------hot--- (left)

| input capacitor

from left mic |

|

---hot--------+----10k ohm-- "+"

\

---ground----------------------)---+--------------------shield-----

/ |

---hot--------+----10k ohm-- "+" "-"

| resistor 9 volt

from right mic| battery

|

--------------||---------------------------hot--- (right)

input capacitor

</pre>

[Gweeble]

Hey, thanks Greenmachine!

But I never used a battery inside the Sony mic, though there is a compartment on it for one. Hmmm.

I'll have a look inside the Sony mic and see if I can figure out what they are doing in there. Maybe I can mount a female 1/8 trs inside, bypassing the Sony mics whenever a trs (my Core's) is inserted. Fat chance, but worth a little exploration.

Cheers

Gweeb

(Hope I didn't post in the wrong thread; it sort-of is about eliminating background noise.)

[greenmachine]

I'd be interested to see it's inner life as well, please share your results with us.

Maybe a moderator could split the thread, it's indeed pretty much off-topic.

[Gweeble]

I'd be interested to see it's inner life as well, please share your results with us.

Maybe a moderator could split the thread, it's indeed pretty much off-topic.

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Good-day Greenmachine,

I want to thank you again for all your work in the reply you posted, especially the circuit diagram! It is very readable, and I will print and keep it.

I opened up the Sony stereo lapel. Its a pretty impressive bit of work. Nice green epoxy silkscreened pc-board, with maybe 15 components per channel of surface-mount devices. One set of components are rather large for surface devices. The mics themselves are large too, larger than the Core capsules, each is more like a couple of stacked aspirin tablets, while the Core's are basically sawed-off .22 ammo shells.

It looks possible to desolder the two mics, if done under a magnifier using a 22 gauge (telephone-wire) as a solder-tip. I did not check for continuity between the two -ve mic poles, though, but since it appears that the Core's use separate conductors for each pole of each mic, paired-up only once they arrive inside the 1/8 TRS-jack, this shouldn't be a problem.

I double-checked that there was no battery in it and put the mic back together. I double-checked that the Voice - Music switch was making a difference, and Yes, this is the case. Wind-noise especially was eliminated completely when in Voice. I also checked to see if the gain drops and if the high frequencies are affected between the two switch positions, but could hear no differences, in the crude test set-up here at home.

Maybe they are doing some fancy signal processing in there, following frequency and using phase-reverse to dump the gain, or some such. In checking into hearing-aids before going the minidisc route, I learned they are now doing digital signal processing in high-end hearing-aids which allows a lot of parameters to be accessed by a laptop. Bluetooth hearing-aids as well.

Again, thanks for your help!

Gweeb