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Re: Effects of Diaphragm Thickness

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  • ctown000
    Thanks, Eric. That s very helpful. It would be interesting to implement this approach in a standard large diaphragm condenser mic, but I guess the same
    Message 1 of 12 , Nov 24, 2012
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      Thanks, Eric. That's very helpful. It would be interesting to implement this approach in a standard large diaphragm condenser mic, but I guess the same question of how to actually reduce the damping would be an issue.

      -Chris

      --- In micbuilders@yahoogroups.com, Eric Benjamin <ebenj@...> wrote:
      >
      > >> "Microphone System for Extremely Low Sound Levels"
      > >> I read that one a while ago, although I don't fully understand it.
      >
      > I have a few spare moments while I'm formatting a disk so I'm going to try to
      > make the article more understandable.
      >
      > The first thing to realize is that all microphones make noise, even if they
      > don't have active circuitry inside. If nothing else, the thermal agitation of
      > the air is a source of noise in a microphone. The article from Bruel and Kjaer
      > starts out by analyzing the noise in their standard 1" measurement microphone,
      > the 4145. The 4145, with the B&K 2619 preamplifier, has a self noise of about
      > 11 dBA. If you look at the spectrum of the noise of the system and separate it
      > into contributions due to acoustic noise sources and electrical noise in the
      > preamplifer, you will see that between about 400 Hz and 12.5 kHz the noise of
      > the system is dominated by the noise of the capsule. That means, of course,
      > that no matter how much better you make the preamplifier you can't really
      > improve the audible self noise of the system.
      >
      > So B&K went a bit further, to create the 4147, and analyzed the various
      > contributions to the noise in the capsule. It turns out that there are three
      > principle contributions; R2, the equalization vent; Ra, the acoustic noise from
      > the air in front of the diaphragm; and R1, the noise from the damping behind the
      > diaphragm. The equalization vent is there to allow air to escape or enter
      > behind the diaphragm so it doesn't rupture due to changes in atmospheric
      > pressure. R2 is only significant at very low frequencies, say below 100 Hz.
      > That's not an issue for listening, or even for normal measurement purposes
      > because it's far lower than the threshold of hearing and lower than normal room
      > noise in any conceivable situation. The noise from the air in front of the
      > diaphragm is significant only at the very highest frequencies.
      >
      > What B&K did was to alter the construction of the microphone to reduce the
      > amount of damping due to the structure behind the diaphragm. This is what JLM
      > is complaining about; they really don't tell us exactly what they did. They do
      > say that they lowered the damping resistance by 16 dB, and that in turn lowered
      > the noise contribution due to R1 by 16 dB. But that damping was designed into
      > the 4145 to give it a flat response by overdamping the diaphragm resonance to
      > the point where it compensates for the boost due to diffraction. That gives the
      > microphone a remarkably flat +/-1dB response from about 5 Hz up to 15 kHz.
      > Anything that changes the damping undoes that careful balance that creates the
      > front response. Decreasing the damping as much as they did creates a huge peak
      > in the response of the capsule so they put an equalizer with a compensatory dip
      > in the preamplifier to flatten the response back out. That has the added
      > advantage of further attenuating the noise from the capsule and the front end of
      > the preamplifier.
      >
      > I did measure the EQ in the preamplifier and it was quite extreme, as one might
      > expect. I no longer have those measurements but I do recall that there was more
      > than 10 dB of cut. The net result is that the microphone response of the 4147
      > system is not as flat or as extended as the 4145. It only makes it up to 13
      > kHz.
      >
      > As a result, the 4147 capsule with 2660 preamplifier has a self noise that is -4
      > dBA as compared to the 11 dBA of the 4145. That's pretty impressive. I had one
      > of these available to me in my previous employment but I was never able to
      > really verify the noise performance because I couldn't create an environment
      > that was quiet enough.
      >
      > For me there are several take-aways from that article. One of them is that it
      > requires a very careful balance between diaphragm mass, diaphragm tension, and
      > the amount of damping behind the diaphragm to get a flat response in an omni
      > microphone. The second one is that you can create a low noise microphone by
      > decreasing the damping. To do that one might drill more holes in the backplate
      > because all of those holes represent resistances in parallel.
      >
      >
      >
      > ________________________________
      > From: ctown000 <ctown000@...>
      > To: micbuilders@yahoogroups.com
      > Sent: Thu, November 22, 2012 11:43:51 PM
      > Subject: [micbuilders] Re: Effects of Diaphragm Thickness
      >
      >
      > Thanks guys. I read that one a while ago, although I don't fully understand it.
      > This one also looks pretty good:
      >
      > Thermal Noise in Microphones and Preamplifiers
      > http://www.bksv.com/doc/TechnicalReview1972-3.pdf
      >
      > -Chris
      >
      > --- In micbuilders@yahoogroups.com, Eric Benjamin <ebenj@> wrote:
      > >
      > > You can find it here:
      > > http://www.bksv.com/doc/bv0015.pdf
      > >
      > > If you have difficulties downloading it, or if you find that their scan is too
      >
      > > difficult to read, shoot me a direct email and I'll send you a copy.
      > >
      > > Eric
      > >
      > >
      > >
      > > ________________________________
      > > From: Jerry Lee Marcel <jerryleemarcel@>
      > > To: micbuilders@yahoogroups.com
      > > Sent: Thu, November 22, 2012 8:43:56 PM
      > > Subject: Re: [micbuilders] Re: Effects of Diaphragm Thickness
      > >
      > >
      > > Hi, I've looked in the files and on B&K website; couldn't find it. can
      > > you give me a roadmap?
      > >
      > > Thanks
      > >
      > > Le 22/11/2012 16:40, userno232000 a écrit :
      > > >
      > > >
      > > >
      > > > --- In micbuilders@yahoogroups.com
      > > > <mailto:micbuilders%40yahoogroups.com>, "ctown000" <ctown000@> wrote:
      > > > >
      > > > > I've >
      > > > > On side note, years ago I used a U87, which had a Stephen Paul
      > > > modified capsule with 3 micron diaphragm, for a work project and we
      > > > compared it with a unmodified U87 (6 micron I believe). Although we
      > > > didn't do extensive listening tests, we concluded that for our
      > > > purposes they pretty much sounded the same. What difference we could
      > > > hear seemed well within the range of what might occur for any two not
      > > > necessarily matched microphones of the same type.
      > > > >
      > > > > Thanks,
      > > > > Chris
      > > > >
      > > > You may want to read the B&K paper on their extremely low noise 1"
      > > > mic. There is a discussion on diaphragm mechanical properties and
      > > > frequency response and noise. It's in our files section or a free
      > > > download from B&K.
      > > >
      > > >
      > >
      > > [Non-text portions of this message have been removed]
      > >
      > >
      > >
      > >
      > > http://www.bksv.com/doc/bv0015.pdf
      > >
      > > [Non-text portions of this message have been removed]
      > >
      >
      >
      >
      >
      > [Non-text portions of this message have been removed]
      >
    • Jerry Lee Marcel
      Interesting. Now correct me if I m wrong. The acoustic resistance can be separated in two entities: acoustic resistance of the front of the diaphragm, which is
      Message 2 of 12 , Nov 25, 2012
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        Interesting.
        Now correct me if I'm wrong.
        The acoustic resistance can be separated in two entities: acoustic
        resistance of the front of the diaphragm, which is the "active" side in
        an omni, and acoustic resistance of the rear of the diaphragm, which is
        exaggerated and tuned in order to get proper damping.
        In view of the results claimed by Neumann, that would suggest that the
        "passive" acoustic resistance is about 5-6 times smaller than the
        active. That's a serious loss, but it seems to be corroborated by other
        litterature (I've seen graphs where the undamped midrange peak is
        squeezed by about a factor 10).
        The Neumann trick (that's how I qualify it because I don't see a patent
        attached) seems to indicate an underdamped diaphragm with some
        electrical EQ. Doing that on a standard midrange tuned diaphragm would
        conduct to drastic LF AND HF boost, which would probably counteract the
        benefit of increased sensitivity, so they had to tune the diaphragm in
        order to peak at the highest band of interest. EQ being a roll-off
        starting at LF, noise penalty is reduced.
        So sintered brass offers high acoustic resistance while being
        electrically conductive.
        Have I got it right?


        Le 25/11/2012 01:09, ricardo a écrit :
        >
        > > The second one is that you can create a low noise microphone by
        > decreasing the damping. To do that one might drill more holes in the
        > backplate because all of those holes represent resistances in parallel.
        >
        > Yes. But you also decrease the capacitance and 'working capacitance'
        > (the bit which changes sound into electric stuff); both of which make
        > the mike noisier
        >
        > You can also increase the diaphragm spacing with a similar effect.
        >
        > What you need to do is to maintain the 'working capacitance' while
        > decreasing the acoustic resistance. You do this with more and smaller
        > holes while keeping the area of the backplate directly behind the
        > diaphragm the same.
        >
        > Dunno if the Neumann pdf book tells you this secret.
        >
        > The ultimate is a backplate of sintered brass where the 'holes' need a
        > magnifier to see. STC had a design like this though it was a cardioid.
        > It was described in the Unobtainium 'Electroacoustics' - Gayford.
        >
        >



        [Non-text portions of this message have been removed]
      • ricardo
        ... I don t think Neumann use this in any of their present mikes. The trick I was hoping their PDF book would describe is the one about more & smaller
        Message 3 of 12 , Nov 26, 2012
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          > The Neumann trick (that's how I qualify it because I don't see a patent attached) seems to indicate an underdamped diaphragm with some
          > electrical EQ. Doing that on a standard midrange tuned diaphragm would conduct to drastic LF AND HF boost, which would probably counteract the benefit of increased sensitivity, so they had to tune the diaphragm in order to peak at the highest band of interest. EQ being a roll-off starting at LF, noise penalty is reduced.

          I don't think Neumann use this in any of their present mikes. The 'trick' I was hoping their PDF book would describe is the one about "more & smaller holes giving less resistance."

          B&K use "underdamped diaphragm + EQ" on the quietest mike in the world which Eric describes. Sennheiser use it on their very quiet MKH radio mikes.

          I played with this on the Fig-8 capsule I describe in my Files directory. I list some of the caveats in Fig8.doc but the noise advantage is very real.

          The STC/BBC cardioid with the sintered brass backplate wasn't a particularly LN design. It was quite thick so actually introduced more resistance as part of the 'back phase shifter'. I just put it forward as what you'd come up with if you took 'more & smaller holes' to the extreme.
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