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Thread: Ribbon Mic Frequency Response

  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.

  3. #3
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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.

  5. #5
    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
    3D Audio

    Making beautiful music SEEM easy since 1979.

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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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    Wireline is offline 3D VIP 2004, '05, '06, '08, '09, '10
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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...
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    The length of the ribbon is important in two ways:

    1) longer ribbons will have more response aberration in the vertical direction because of phase interference over the length of the ribbon

    2) longer ribbons can be tuned to a lower resonance frequency easily, and therefore have more extended "solid" bass response

    The important thing when looking at the high frequency response of these mics is that there is always a tradeoff of high frequency response versus output. In other words, you could have a ribbon just sitting there by itself and it would have great high frequency response, but since the overall length of the "gradient" would be very small you would get very little usable output.

    Therefore the challenge is to get good high frequency response AND enough output to meet your noise requirements. The Coles 4038 is a good compromise from the older school designs, and the AEA R88 has very smooth and extended high frequency response for a newer design.
    Matt Ashman
    Ashman Acoustics

  10. #10
    Tommy Yonley is offline Gold Club Member (1000+ posts)
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    Wireline, how do you like the SK-46? (I am interested because they are one of the cheapest ribbon mics out there.)

    ---

    "So the limitation in upper frequency is the thickness (thin-ness) of the ribbon."

    This was my first thought. And it makes sense.

    However, read the following:

    "More typical designs would have a -3dB point between 10kHz and 20kHz. This is achieved by making the path-difference from the front of the ribbon to the back of the ribbon as small as practical."

    So if all we are talking about is the ribbon, why wouldn't they just say "to get the best high frequency response, make the ribbon as small as possible". Why do they talk about the "path difference"?

    If we simply consider the ribbon by its self, and the fact that it vibrates freely, the front to back difference is going to be the thickness of the ribbon (essentially nothing) plus the distance that the ribbon is distorted from sound waves (not very much), it seems like even an average ribbon should be able to get up to ~20kHz.

    Look at this quote:
    "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."

    Now, look at the frequency response of the SK-46. Interesting. The SK-46 is basically flat until ~5kHz and then drops to a null at ~10kHz. So according to the theory (and assuming that the frequency response is limited by the ribbon its self, not the transformer or something else) the path difference on this microphone should be somewhere around one inch? There is no way that the path difference around a ribbon its self is an inch. That is why I am having a hard time understanding this.

    I think that it is intuitive that the bass response (and overall output) should be better for a larger ribbon and that a smaller ribbon should have less smear in the high frequencies--this is exactly like a small vs large diaphragm condenser microphone.

    - Tommy

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