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Thread: Phase Response vs Frequency Graph

  1. #1
    Tommy Yonley is offline Gold Club Member (1000+ posts)
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    Phase Response vs Frequency Graph

    I have looked at the specs of microphones for a while but I have not seen many of these:
    http://www.studioreviews.com/images/phase-large1.jpg

    It is a graph of the phase vs. frequency of a microphone. In this case, it was used to compare the RFT M16 to the Apex 460 microphones--according to the article ( http://www.studioreviews.com/m16-460.htm )they are essentially the same thing.

    So I did a bit of searching around and this kind of measurement is actually fairly common. You can find them published for DPA microphones (http://www.core-sound.com/dpa4003.html)
    You can get phase/frequency respons graphs for your microphones done:
    (In Germany) http://www.ibf-acoustic.com/2005/index05.html
    Software is out there that will produce it: http://www.telebyte.com/pioneer/

    Here is an interesting article about phase and delay (this ties a bit into the discussion about the Little Labs IBP). http://www.libinst.com/tpfd.htm

    Here is an incredible discussion at Klaus' microphone lab that started out as a discussion of transformer vs transformerless mic designs but wandered into some very interesting discussions about phase. http://recforums.prosoundweb.com/ind...msg/4703/0/0/0

    It seems like a having simple phase response graph of a microphone may not tell you whether or not it sounds good but it seems like a step in the right direction--one that more manufacturers should publish.

    It would be interesting to see if there are any general trends in the phase response of ribbon microphones vs condenser.

  2. #2
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    Tommy, frequency/phase response graphs are part of the standard documentation portfolio for acoustic measurement microphones, but aren't generally published for studio microphones. That might be a blessing, however, because the information might not be as useful for the purposes of most studio engineers as you might imagine, and not very many people would really know how to interpret the data. The most important skill would be knowing how to ignore the part of the information which doesn't relate directly to any audible phenomena whatsoever--a large fraction of the data in this case.

    The audibility of phase distortion isn't exactly a simple topic. But it's curious: The actual research that's been done on this topic seems not to be believed very widely in the on-line audio community. The beliefs which many people seem to have about this subject don't come from any real-world experience or experimentation. Rather, the beliefs that I see expressed most vehemently and most often on line are apparently just beliefs, to which people are attracted because ... well, they're attractive beliefs, which let us think well of ourselves.

    Number one among these is the claim that "our ears are more sensitive than the most sensitive scientific instruments." Any such claim is simply not borne out when it comes to phase. Above about 1.5 kHz, we humans are all essentially deaf to ordinary amounts of phase distortion/group delay in monophonic signals. It also turns out that we are far more sensitive to phase distortion in specially-generated, artificial test signals than in ordinary program material such as speech or music--not (as many people insist) the other way around.

    Now, under certain circumstances we can also be astonishingly sensitive to variations in the phase relationships between stereo channels. But no graph of the phase characteristics of a single microphone would tell us anything relevant to that fact. On the contrary, I suspect that such graphs would lead to many incorrect conclusions. If honest, detailed frequency/phase graphs for good dynamic microphones and good condenser microphones were printed side by side, audiophiles would gasp; some would have fainting spells. This isn't something we want to do during the summer, when so many people's psychotherapists are on vacation. There might even be mass demonstrations demanding that no engineer ever use a dynamic microphone again, and calling for the permanent destruction of all recordings ever made with them. Likewise with directional microphones in general--some people who trust their eyes more than their ears would instantly swear off cardioids forever, especially the large-diaphragm types.

    I really think it's best when the published specifications for a microphone bear some clear relationship to the practical issues of using that microphone. As far as missing specifications for microphones are concerned, impulse response measurements (which include the phase response, indirectly) might be more useful than frequency/phase response measurements as such. But there again, I think a lot of people who worship at the altar of the "vintage classics" would turn green in the face if they saw what they're being so reverential about; it ain't pretty.

    --best regards
    Last edited by David Satz; 11-07-2006 at 04:07 AM.

  3. #3
    Tommy Yonley is offline Gold Club Member (1000+ posts)
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    David,

    The specifications for microphones generally include a frequency/ampltude plot, sensitivity, pattern shape, maximum spl, and noise. My problem is that this information tells you next to nothing about the quality of the sound that the microphone is capable of producing. It tells you very little about what applications the microphone is useful for (other than is it "bright" or "dull"). You can find many microphones of different types at very different price points that have virtually identical specifications. One of them might sound "thin and nasal" while one of them has a "beautiful high end sheen".

    Of course the same principal applies to preamps. There are many preamps that are flat within .2 dB from 10Hz to 100+kHz but they all (reportedly) sound different (though I don't have the experience of actually hearing these differences).

    I have a lot of faith in technology, and I believe that it is fundamentally possible to make scientific measurements that would discriminate between a crappy microphone and a good one--while the usual specs are the same--but this kind of measurement is well beyond the state of the art (as far as I know).

    Maybe a phase plot wouldn't be useful, but I don't think anyone would know this until they had a lot of experience comparing what they hear to the frequency vs phase (along with frequency vs. amplitude) data of various equipment. Since phase data is unavailable for almost everything, it is virtually impossible for anyone to ever gain the experience necessary to make an informed decision.

    I admit that I am not aware of the "psychoacoustic research" that has been done in this area (maybe you could provide some interesting links)--this is, after all, a fairly new topic to me; however, I think that in general there has been a lot of "research" that has "proven" things that are simply not true (in every area of science)--so a healthy skepticism of the current research on anything isn't always a bad idea. There are always going to be evil, stupid, shortsighted, and biased people out there and some of them are going to be scientists who will do bad research. Just as we look back and laugh at those foolish people of the 1400's who thought the world was flat, someday people will look back on the foolish people of the 2000's for some ridiculous notions that we currently regard as fact. (But this is another topic.)

    We need to figure out what kind of technical information about microphones and preamps (and other things too but this is the mic/pre forum...) that would be useful and then make an effort to get that information out there (maybe even supplied by the manufacturers).

    I like the impulse response stuff. We don't know how to really use it yet, but I can certainly see that becoming useful.

    Right now, we are pretty much in the stone ages regarding all of this--we can't even agree on what attributes make a cable sound good (that is, if cables do sound good/bad)!

  4. #4
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    Tommy, my own experience with microphone specifications is that they are very useful, to the extent that they're honest and to the extent that we really know how they were obtained. But I really don't think that it's reasonable to expect any specifications to tell us how a microphone will sound in actual use, and I don't think that it's worth searching for some missing holy grail of a specification that will do so. Certainly if there were such a thing, it would not be frequency/phase response--that is really an almost irrelevant specification as far as the sound of a microphone is concerned; I don't think that we're being deprived of anything useful there.

    When I was starting out as a location engineer I used to "tag along" with more experienced engineers on their jobs (with their permission, of course). I had the good fortune to be living in Cambridge, Massachusetts back then, and one of the people who was extremely helpful to me was David Griesinger. He didn't hide anything that he was doing, and he answered every question I could think of about what he was doing and why. Yet when I tried imitating his techniques--even using some of the same equipment--I never got anything that could be mistaken for the same sound as he got. Likewise whenever I lent my equipment to other engineer friends, which was fairly often, they would always get a different sound from anything I would have gotten with the same equipment.

    So the first step, it seems to me, is to understand how much of the result can be expected to come from the recording equipment and how much from the performers, the hall, and the choices that you or I make as engineers. To me one really crucial moment is when everything is set up and the first test "take" is made--you listen to that and make adjustments in the microphone positions/angles, or even change capsules or pattern settings or put up a different pair of mikes altogether. No two engineers do that exactly alike, because no two of us listen alike or have the same sense of musical and sonic proportion.

    Anyway, one "teachable moment" that I had recently with microphone specifications occurred when I recorded an audition CD for a very talented young soprano, in a bright room that was unfortunately too small for that person's wonderful voice. I chose a pair of Neumann KM 86 figure-8s, for reasons that I can explain separately if you're interested; my real point is that I was sad later on when I played the recording back over good loudspeakers and heard harsh-sounding off-axis room sound. I went and took another look at the polar diagrams for the KM 86, and realized that if I had only believed my eyes instead of my wishful thinking, I might not have made that particular choice--I ought to have chosen other figure-8s with a more even directional pattern at high frequencies (Schoeps or other Neumann, Royer or Beyer microphones for example). The KM 86s have huge high-frequency peaks at 45 degrees, which is front and center when you're using them for Blumlein stereo (coincident at 90 degrees)--and that's plain as day on the polar graph, but I chose not to see it until it was too late.

    So, until I've learned to make all the good use I can out of the basic, available specifications from honest manufacturers such as Neumann, I don't exactly feel that I'm in a position to demand further, more arcane specifications. And I think that I'm not alone--from the discussions I see on this board and others that I hang out on, I'm not convinced that most engineers really know how to read polar diagrams for all that they're worth--I suspect that a lot of people look at them and say, "Nope, nothing there" without realizing that that's often quite far from the truth.

    Still, as I said, graphs and specifications only give us information to help guide our choices, and from there on we have to use those funny flappy things on the sides of our heads, and the mysterious squishy gray thing that's supposed to be in the upstairs part of our heads as well.

    --best regards

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