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holger-d(at)shadowbrush.c
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 Posted: Tue May 14, 2013 11:40 pm Post subject: Microphone gain tuning |
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Hi all,
I seem to have a microphone gain tuning problem on the second DIY headset, following these instructions:
http://n999za.com/2011/02/04/roll-your-own-in-ear-headset/
For the first headset I slaughtered an old GA headset and used its wires and microphone. Works beautifully. Now on the second headset I am using new shielded wires and this microphone (packed into a wind screen):
http://store.acousticom.com/m7a-amplified-electret?keyword=5720-CA&category_id=0&description=1&model=1
The microphone is now overly sensitive and picks up everything. In a quiet environment, my transmissions are crystal clear, but in flight the surrounding noise pretty much drowns out my voice. I tried it with two radios: an ICOM IC-A6 (on the ground) and in the air with a KX-135A through an RST 422 intercom. In the hangar with everything quiet but a radio playing soft music 20 feet away that music makes it into the transmissions.
Since I bought the microphone new, I asked the vendor, and Joel writes back:
What you will need to do is adjust the microphone gain control within your radio to adjust it to fit the microphone properly. All radios are different and require proper tuning when replacing the microphone. Please refer to your radio's manual for adjustment procedures.
We do sell a microphone that allows you to tune the microphone with a screwdriver. This is a common occurence that microphones need to be adjusted to the radios to perform properly. Your mic currently is set too high in sensitivity/gain.
I can see how the combination radio and microphone needs to be tuned, I don't want to tune the radio to just this new microphone, and thus make all other headsets unusable in my plane.
Their adjustable gain microphones apparently need a 9V power supply - not ideal.
Should I try this trick posted to this group a while back:
Got a great idea from somebody, and the cost is free! Get the plastic 35mm
film containers. Cut an "X" in the lid and slide it on the mic arm. Then
wrap some foam around the mic and slide on the 35mm can after you have
drilled a 1/4" hole in the side. Line up the hole with your mouth. Better
than sliced bread. Just keep the hole tight against your mouth to keep out
engine noise.
My engine noise keeps my intercom open, but makes for a pleasant ride with
me as the tour guide.
walt evans
NX140DL
=============
Oscar Zuniga
Medford, OR
Air Camper NX41CC "Scout"
A75 power
But this probably addresses mostly wind noise, and I'm in an all enclosed cabin.
Any other thoughts?
Thanks!
Holger
[quote][b]
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klehman(at)albedo.net
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| Posted: Wed May 15, 2013 4:40 am Post subject: Microphone gain tuning |
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Oscar
I've played a version of this game. I adjusted the radio to work well
with a David Clarke non adjustable mic. Then I adjusted the mic on a
Lightspeed to match. While the light speed uses batteries to power the
active noise suppression, I often did not have batteries in it and the
mic worked fine anyway. I'm suspicious of the claim that the adjustable
mic needs a separate power source. All radios feed a DC bias voltage to
the mic anyway. Makes little sense to me that any would need additional
power. I'm sure others are more familiar with that but it would greatly
curtail the usefulness of such a mic.
I have found that the foam cover on the mic helps a bit but note that
most mics also sense background noise from the backside so that they can
cancel that out of the signal going to the radio. Makes me wonder if the
film canister you mention would work all that well although it would
certainly reduce the signal strength.
Ken
On 15/05/2013 3:38 AM, Holger Selover-Stephan wrote:
| Quote: | Hi all,
I seem to have a microphone gain tuning problem on the second DIY
headset, following these instructions:
http://n999za.com/2011/02/04/roll-your-own-in-ear-headset/
For the first headset I slaughtered an old GA headset and used its wires
and microphone. Works beautifully. Now on the second headset I am using
new shielded wires and this microphone (packed into a wind screen):
http://store.acousticom.com/m7a-amplified-electret?keyword=5720-CA&category_id=0&description=1&model=1
The microphone is now overly sensitive and picks up everything. In a
quiet environment, my transmissions are crystal clear, but in flight the
surrounding noise pretty much drowns out my voice. I tried it with two
radios: an ICOM IC-A6 (on the ground) and in the air with a KX-135A
through an RST 422 intercom. In the hangar with everything quiet but a
radio playing soft music 20 feet away that music makes it into the
transmissions.
Since I bought the microphone new, I asked the vendor, and Joel writes back:
/What you will need to do is adjust the microphone gain control within
your radio to adjust it to fit the microphone properly. All radios are
different and require proper tuning when replacing the microphone.
Please refer to your radio's manual for adjustment procedures.
We do sell a microphone that allows you to tune the microphone with a
screwdriver. This is a common occurence that microphones need to be
adjusted to the radios to perform properly. Your mic currently is set
too high in sensitivity/gain./
I can see how the combination radio and microphone needs to be tuned, I
don't want to tune the radio to just this new microphone, and thus make
all other headsets unusable in my plane.
Their adjustable gain microphones apparently need a 9V power supply -
not ideal.
Should I try this trick posted to this group a while back:
/Got a great idea from somebody, and the cost is free! Get the plastic 35mm/
/film containers. Cut an "X" in the lid and slide it on the mic arm. Then/
/wrap some foam around the mic and slide on the 35mm can after you have/
/drilled a 1/4" hole in the side. Line up the hole with your mouth. Better/
/than sliced bread. Just keep the hole tight against your mouth to keep out/
/engine noise./
/My engine noise keeps my intercom open, but makes for a pleasant ride with/
/me as the tour guide./
/walt evans/
/NX140DL/
/=============/
/Oscar Zuniga/
/Medford, OR/
/Air Camper NX41CC "Scout"/
/A75 power /
But this probably addresses mostly wind noise, and I'm in an all
enclosed cabin.
Any other thoughts?
Thanks!
Holger
*
*
|
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Wed May 15, 2013 7:02 am Post subject: Microphone gain tuning |
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| Quote: |
I have found that the foam cover on the mic helps a bit but note that most mics also sense background noise from the backside so that they can cancel that out of the signal going to the radio. Makes me wonder if the film canister you mention would work all that well although it would certainly reduce the signal strength.
Ken
|
Porous envelops and enclosures are simple
attenuators of sounds. They can also present
tailored effects upon the frequencies of
sound allowed to pass.
A 'noise canceling microphone' is another
critter entirely. The following is purloined from
Wikipedia:
The development is a special case of the differential microphone topology most commonly used to achieve directionality. All such microphones have at least two ports through which sound enters; a front port normally oriented toward the desired sound and another port that's more distant. The microphone's diaphragm is placed between the two ports; sound arriving from an ambient sound field reaches both ports more or less equally. Sound that's much closer to the front port than to the rear will make more of a pressure gradient between the front and back of the diaphragm, causing it to move more. The microphone's proximity effect is adjusted so that flat frequency response is achieved for sound sources very close to the front of the mic – typically 1 to 3 cm. Sounds arriving from other angles are subject to steep midrange and bass rolloff. Commercially and militarily useful noise-canceling microphones have been made since the 1940s by Roanwell, [1][ dead link] Electro-Voice and others.
As you might guess, the mechanical differential microphone
presents some real design problems that get more difficult
as the size of the microphone is reduced. There are
electrical differential configurations that simply places
two identical microphones back to back amplified by circuits
that balance the two signals against each other in opposite
phase so as to achieve a degree of cancellation.
Then, my speaking closely to one of the two cartridges,
the differential balance is upset and the voice predominates
over background noise.
When we ran the airport, I had head-sets that could
be loaned to renters. I did some tests with the various
brands we had hanging on the pegs and found marked
differences in their ability to reject background noise.
David Clarks were always good . . . but a bit pricey
compared to the Telex Echelon headsets that were about
half the price and still quite effective in their ability
to cancel noise and only $120 a pop in 1990 dollars.
A note of caution about adding 'stuff' to the outside
of a microphone . . . if it is a true noise-cancelling
design, then adding external features may work against
the original design goals for the product. If the mic
is not noise-canceling design and achieves improvement
of signal-to-noise ratio by proximity of the speaker's
lips, then added features are less problematic . . .
Foam socks are most useful in the reduction of wind
noise . . . REALLY important for open-cockpit flight.
But the nature of the foam's 'bubbles' can produce
unexpected results. Use a 'sock' crafted by a supplier
of microphones that is advertised for the reduction
of wind noise.
Balancing the ship's audio system to the differences
between brands is a separate issue. Not all microphones
are the same but as long as the airplane is fitted
with two or more of the same model, those differences
are a wash.
| Quote: | | Quote: | We do sell a microphone that allows you to tune the microphone with a
screwdriver. This is a common occurence that microphones need to be
adjusted to the radios to perform properly. Your mic currently is set
too high in sensitivity/gain./
I can see how the combination radio and microphone needs to be tuned, I
don't want to tune the radio to just this new microphone, and thus make
all other headsets unusable in my plane. |
|
EXACTLY. When dealing differences between microphones,
the 'fix' is to attenuate the hotter microphones to a
level consistent with the cooler ones.
| Quote: | | Quote: | Their adjustable gain microphones apparently need a 9V power supply -
not ideal. |
|
But the nature of the beast. If you want a mic
with the greatest applicability in the market, you
deliberately design for 'hot' and then provide
controls for attenuating the mic down to system
friendly levels.
| Quote: | | Quote: | /Got a great idea from somebody, and the cost is free! Get the plastic 35mm/
/film containers. Cut an "X" in the lid and slide it on the mic arm. Then/
/wrap some foam around the mic and slide on the 35mm can after you have/
/drilled a 1/4" hole in the side. Line up the hole with your mouth. Better/
/than sliced bread. Just keep the hole tight against your mouth to keep out/
/engine noise./ |
|
What ever works for you . . . assessment of
'elegance' is in the eye of the beholder. There
ARE electronic solutions transparent to the
appearance of the thing hanging out in front of
your face.
Bob . . . [quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Wed May 15, 2013 8:26 am Post subject: Microphone gain tuning |
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| Quote: |
http://store.acousticom.com/m7a-amplified-electret?keyword=5720-CA&category_id=0&description=1&model=1
The microphone is now overly sensitive and picks up everything. In a quiet environment, my transmissions are crystal clear, but in flight the surrounding noise pretty much drowns out my voice. I tried it with two radios: an ICOM IC-A6 (on the ground) and in the air with a KX-135A through an RST 422 intercom. In the hangar with everything quiet but a radio playing soft music 20 feet away that music makes it into the transmissions.
Since I bought the microphone new, I asked the vendor, and Joel writes back:
What you will need to do is adjust the microphone gain control within your radio to adjust it to fit the microphone properly. All radios are different and require proper tuning when replacing the microphone. Please refer to your radio's manual for adjustment procedures.
We do sell a microphone that allows you to tune the microphone with a screwdriver. This is a common occurence that microphones need to be adjusted to the radios to perform properly. Your mic currently is set too high in sensitivity/gain. |
This off-the-shelf cartridge is a two-wire 'electret'
device that contains what ever electronics are necessary
to make it look like the legacy, radio-powered, aviation carbon
microphone.
You cannot reduce it's audio output with a potentiometer
in it's leadwires. While the pot would indeed adjust
the audio output FROM this device, it would adversely
affect the DC power being conducted TO the device.
You can place a resistor-capacitor network ACROSS the
microphone's leads thusly.
[img]cid:.0[/img]
Adjust the resistor value such that the microphone produces
a signal commensurate with others in your system. The
capacitor keeps the audio attenuator resistor from shunting
the DC power.
I'm guessing that a value in the 100 to 1000 ohm
range will prove satisfactory. You might temporarily
wire a potentiometer into the resistor's slot, adjust
the pot for desired gain, then measure with ohmmeter.
Substitute a fixed resistor of the appropriate value.
Bob . . .
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holger-d(at)shadowbrush.c
Guest
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| Posted: Wed May 15, 2013 11:47 pm Post subject: Microphone gain tuning |
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Hi Bob, Ken,
Many thanks for the very detailed explanation of the situation. A slap on the forehead is in order: I have covered up the front-facing side of the microphone, and thus might have disrupted the noise reduction mechanism. Interestingly, I mentioned this to the vendor, but it wasn't commented on in their response.
Let me first run tests with the front side uncovered. Then try your resistor-capacitor network solution, Bob. I really like the simplicity of the latter, just what I was hoping to hear from you!
I'll report back with further findings. Once this is figured out yet another DIY headset is going into production for the daughter. The pooch stays on Mutt Muffs as he rarely contributes anything useful to the conversation.
Thanks again!
Holger
On May 15, 2013, at 9:25 AM, Robert L. Nuckolls, III wrote:
| Quote: | You can place a resistor-capacitor network ACROSS the
microphone's leads thusly.
|
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dlj04(at)josephson.com
Guest
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| Posted: Wed May 15, 2013 11:53 pm Post subject: Microphone gain tuning |
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Nearly all noise-canceling microphones in aviation headsets these days
(including the Acousticom mentioned) use a single diaphragm electret
capsule with front and back ports just as described in the Wikipedia
entry Bob quoted (I know, I wrote it...) There's a simple circuit,
usually one or two transistors, to interface with the aircraft radio's
carbon mic circuit, provide some gain, and power the FET in the electret
mic. You need to be able to adjust the gain of this circuit. There is a
version of this same mic, the Acousticom 5715, that is adjustable. It
would really be better to get the adjustable version rather than try to
kludge an attenuator.
A resistor and capacitor across the mic output could cause some
distortion, as the mic would then be loaded with a lower impedance than
just the radio input.
Trying to muffle the microphone with some thick material over it is
seldom successful as the frequency response and noise cancelling
properties will be altered significantly.
--
David Josephson
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu May 16, 2013 6:44 am Post subject: Microphone gain tuning |
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| Quote: |
A resistor and capacitor across the mic output could cause some
distortion, as the mic would then be loaded with a lower impedance
than just the radio input.
|
I've not found this to be the case. The artificial
lowering of radio input impedance has no non-linear
contributions. The amplifiers common to electret-
to-carbon interface are pretty high output impedance
to begin with and 'adjustment' of loading to bring
two microphones into agreement with each other is
not severe.
To be sure, having an adjustable mic cartridge is
the ideal condition, but when a non-adjustable
is what you have in hand, then a little 'jiggering'
of the gain is a low risk experiment.
| Quote: | Trying to muffle the microphone with some thick material over it is
seldom successful as the frequency response and noise cancelling
properties will be altered significantly.
|
To be sure, effective noise cancelling design is not
a trivial task. Exploiting the designer's skills
is not well served by altering the environment that
the designer targeted. Even so, I've seen some
wide variation in the levels of noise-cancelling
success.
I think the best cartridges I've ever encountered were
the very low impedance, dynamic cartridges common to
military headsets. We were working with aviation
headsets and intercoms on the Cessna O2A and T41
programs back about 1968. I did a dynamic-to-carbon
interface module to connect the military mic to a
121.5 emergency transceiver on the T41. Those were
VERY good microphones but with an output in the 1-2
millivolt range. An interesting task.
Bob . . .
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dlj04(at)josephson.com
Guest
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| Posted: Fri May 17, 2013 9:26 am Post subject: Microphone gain tuning |
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On 5/16/13 11:57 PM, AeroElectric-List Digest Server wrote:
| Quote: | > >A resistor and capacitor across the mic output could cause some
> >distortion, as the mic would then be loaded with a lower impedance
> >than just the radio input.
I've not found this to be the case. The artificial
lowering of radio input impedance has no non-linear
contributions. The amplifiers common to electret-
to-carbon interface are pretty high output impedance
to begin with and 'adjustment' of loading to bring
two microphones into agreement with each other is
not severe.
Bob, if that's your experience with the actual circuits in common use,
|
then by all means it should be fine. The circuit I saw was a simple
emitter follower and not particularly high impedance, asking for more
audio current than it was happy with would be a problem. The problem is
not a nonlinear element, it's just that the circuit runs out of output
current. There are two common input impedances for aircraft audio
circuits, set by the value of the resistor feeding "bias" current to the
mic at either about 150 or about 600 ohms. Some amplified dynamic mics
have difficulty driving the 150 ohm variant without clipping.
And yes, the dynamic noise-canceling mics, usually the M101 or the M87,
are excellent. It takes 40 to 60 dB of gain to bring one of these to
carbon mic level however.
And about the noise canceling element, covering either front or back
will make it into an omnidirectional microphone with a really odd
frequency response.
--
David Josephson
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Fri May 17, 2013 5:41 pm Post subject: Microphone gain tuning |
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At 12:24 PM 5/17/2013, you wrote:
| Quote: |
On 5/16/13 11:57 PM, AeroElectric-List Digest Server wrote:
>> >A resistor and capacitor across the mic output could cause some
>> >distortion, as the mic would then be loaded with a lower impedance
>> >than just the radio input.
> I've not found this to be the case. The artificial
> lowering of radio input impedance has no non-linear
> contributions. The amplifiers common to electret-
> to-carbon interface are pretty high output impedance
> to begin with and 'adjustment' of loading to bring
> two microphones into agreement with each other is
> not severe.
Bob, if that's your experience with the actual circuits in common
use, then by all means it should be fine. The circuit I saw was a
simple emitter follower and not particularly high impedance, asking
for more audio current than it was happy with would be a problem.
The problem is not a nonlinear element, it's just that the circuit
runs out of output current. There are two common input impedances
for aircraft audio circuits, set by the value of the resistor
feeding "bias" current to the mic at either about 150 or about 600
ohms. Some amplified dynamic mics have difficulty driving the 150
ohm variant without clipping.
|
Hmmmm . . . I think the examples I encountered were single
transistor devices. This goes back a bit. I think I was
working with some of first Electro-Voice hand-held electret
mics and matching them to head sets of a brand I don't
even remember.
Emitter follower? Oh yeah, PNP upside down so that they'll
bias up with the radio's output.
I guess the only work around with passive components would
involve some series inductance to maintain the DC path
and paralleled with a series resistor/capacitor network
to set attenuation in the AC path.
A bit messier but perhaps doable. But yes, in these days
of every guy and his dog making/selling accessories, interface
conventions are not very 'standard'. Good thing we're working
with OBAM aircraft.
| Quote: | And yes, the dynamic noise-canceling mics, usually the M101 or the
M87, are excellent. It takes 40 to 60 dB of gain to bring one of
these to carbon mic level however.
|
It seems that the trade offs for mechanical accommodation
for noise cancelling compromises sensitivity. Fortunately
the electronics are simple and cheap so I say "you take
care of the noise, I'll take care of the gain."
| Quote: | And about the noise canceling element, covering either front or back
will make it into an omnidirectional microphone with a really odd
frequency response.
|
Very few of our fellow travelers are aware of the significance
of the various noise cancelling techniques. Hmmmm . . . that
might make a good topic for a Kitplanes article. I've been
doing some writing for them. Would you like to do one on intricacies
of the aviation microphone? I'll be Paul Dye would be pleased
to have it offered.
Bob . . .
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