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Thread: Ribbon Mic Frequency Response

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  1. #1
    Tommy Yonley is offline Gold Club Member (1000+ posts)
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    Ribbon Mic Frequency Response

    Does anyone have any information on what causes the frequency response of ribbon mics to vary? More specifically, what causes the high frequencies to be attenuated?

    For example, the RCA SK-46 is "flat" to about 6Khz, then starts dropping fast to be down 15 db at ~10 kHz and gone after that (see the pic).

    It is pretty common in a lot of old ribbon microphones to have that kind of frequency drop.

    So what factors go into making a ribbon mic with less high end drop off? Is it possible to modify a ribbon microphone to reduce the high frequency drop off?

    Also, I am curious how the proximity effect can be modified.


  2. #2
    Tommy Yonley is offline Gold Club Member (1000+ posts)
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    I suppose that there is nothing wrong with answering your own questions.

    We all know about http://www.coutant.org/contents.html, but I dare say that few have looked at all of the links. Most relevant to this discussion is:
    http://home.vicnet.net.au/~macinc/news9.htm

    This actually does explain a lot about the frequency response of a ribbon microphone, but in reading it quickly through, I don't quite understand all of it.

    Of course, this doesn't get into anything other than the ribbon and magnet, but it is a pretty good discussion.

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    Thanks for that link. Very interesting.

    So I guess we won't see ribbons out to 20K any time soon.

    Except for Johnny Carsons SM33 desktop ribbon mic. It went to 50K. At auction, that is. $50,750 to be exact. That has to be the most expensive mic ever sold.
    Lynn Fuston
    3D Audio

    Making beautiful music SEEM easy since 1979.

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    Ribbon mics have a warmer sound to them because the frequencies are rolled off on the upper range.

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    Tommy Yonley is offline Gold Club Member (1000+ posts)
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    I don't know if I understand it correctly, but according to this, the frequency response of a ribbon is flat up to a high frequency limit. That limit is a function of the container that the ribbon is enclosed in, specifically the distance from the front to the back of the area around the ribbon where the pressure wave is traveling? I don't think I quite get it. Couldn't you just leave a little bit of space directly around the ribbon so that the distance from the front to the back is the distance from the front of the ribbon to the back of the ribbon (that would be plenty small to get 20kHz)?

    Also, here is another link that isn't specifically about ribbons but does discuss various microphone response types including "pressure gradient". Also, discussed is the "SPHERICAL EXPANSION GRADIENT" which is the key to understanding the cause of proximity effect.
    http://www.poppyrecords.co.uk/other/mics.htm

    Another link explains how various kinds of microphones work and actually includes math: http://www.du.edu/~jcalvert/tech/microph.htm

    Even more (with much more math). These are good:
    http://www.acoustics.salford.ac.uk/s...20complete.pdf
    http://www.acoustics.salford.ac.uk/s...hones%20L5.pdf

    - Tommy

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    Originally posted by Tommy Yonley:
    <STRONG>I don't know if I understand it correctly, but according to this, the frequency response of a ribbon is flat up to a high frequency limit. That limit is a function of the container that the ribbon is enclosed in, specifically the distance from the front to the back of the area around the ribbon where the pressure wave is traveling? I don't think I quite get it.</STRONG>
    That's not how I read it.

    Look closely at this:

    What determines the top frequency limit of the microphone then? As the frequency in the example above increases, a frequency is reached at which the acoustic path length from front to back is one-whole wavelength. With the 1-inch physical dimension quoted, this would occur at 10kHz. Then the pressure wave arriving at the back of the microphone is 360 degrees out of phase, and the microphone output drops to zero. Before this frequency is reached, the response starts to fall. In this case, the 1-inch path difference is rather too big to give high fidelity results, and the output would be starting to fall at 5kHz, with a null at 10kHz.

    So the limitation in upper frequency is the thickness (thin-ness) of the ribbon.
    Lynn Fuston
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    Quote Originally Posted by 3daudioinc View Post
    That's not how I read it.

    Look closely at this:

    Then the pressure wave arriving at the back of the microphone is 360 degrees out of phase....

    So the limitation in upper frequency is the thickness (thin-ness) of the ribbon.
    How did I not notice this before? 360 degrees out-of-phase? Wouldn't that put it back IN phase?
    Lynn Fuston
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    Quote Originally Posted by 3daudioinc View Post
    How did I not notice this before? 360 degrees out-of-phase? Wouldn't that put it back IN phase?

    No it wouldn't. But you know that. ;-)
    Andreas Lassak
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    Originally posted by 3D Audio Inc.:
    <STRONG>So the limitation in upper frequency is the thickness (thin-ness) of the ribbon.</STRONG>
    I read it to indicate the length of the ribbon is more important.
    Jim Dugger
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    {{{WARNING: NO MATH JUST SPECULATION POST}}}


    I always thought ribbons had a much lower top end because of their being velocity/pressure oriented...

    I was taught to believe hi frequencies have the same velocity, but tend to have less pressure, and "die out" relatively quickly when compared to low end...on a much larger scale, you can hear the low end rumble of thunder several miles away, but have to almost directly under the producing clouds to hear that high end crackle and snap....

    Same principlal applies (I thought...now it appears I was wrong) to the recent "Thunderbox Cars" thread....in that higher frequencies dissipate and die much faster than their low end couonterparts...

    FWIW: I have and use said SK46 a lot...
    Ken Morgan
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