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  • Example of tweeter compression and discussion

    Recently, as part of my testing, I've been trying to do compression power doubling. I set my system to 0db. My mic preamp to the lowest setting. And the test program (HolmImpulse in this case) to -24db. This is pretty close to 2.83V at the terminals iirc (I'd have to double check that). I then measure in 3db increments, ie. -24, -21, -18, etc. all the way to 0db. To compare, I then offset the -24db result the difference to see how they line up. So, to compare -24 to -12, I'd offset the -24 result +12db. Then the should overlay.

    I recently tested a new tweeter that is relatively pricey (>$75) doing this. I won't say which tweeter to keep the discussion on topic and I'm also unsure if what I got is considered good or bad. So I don't want to unintentionally defame a tweeter that may perform good.

    The tweeter is in the center of a 12x12 baffle with a 1/2" chamfer. It was flush mounted. My current soundcard calibration isn't good hence the blip at 20khz. I get that on all measurements. I put a 5.9uF and 0.44mH filter on it, randomly, to protect it. It's not a cross over slope, just a random filter.

    Here are my results:

    First to show the tweeter stays linear within the first few runs, here is -24 vs -18db. So this would be 1watt vs 4watts roughly.



    Stays pretty linear.

    Ok, lets skip all the rest and jump right up to -9 vs -24db.



    And then -6db.



    And now -3db.



    0db was just nasty. Distortion quadrupled and the sound was disgusting. My equipment was maxed out, so it may have been that. I'll leave it out.

    What I'm interested in is the fact that output increases at lower frequencies and decreases at higher frequencies. I assume the lower frequency output is actually distotion, and the higher frequency reduction is compression.

    Please discuss and feel free to ask for additional information about the results. Is this good, bad, what causes it, are people doing tests like this, am I doing the test properly, etc.
    https://www.youtube.com/channel/UCm2...oSKdB448TTVEnQ

  • #2
    Re: Example of tweeter compression and discussion

    Interesting subject!
    I've thought for years that people grossly underestimate the amount of thermal compression experienced in typical use and have also wanted to do similar testing.

    I'd like to know at least the physical characteristics of the tweeter. VC diameter, ferrofluid or not, etc.

    I agree that the increase at LF is distortion, however 4 kHz seems high for it to start increasing.

    Is it possible that your amp was clipping when the sound became real bad as you mentioned?

    Did you use an MLS test (does Holm offer anything else?) which would have broadband energy?

    Did you look at ARTA software at all, I believe it has features or at least an app note covering compression testing.

    I've wanted to do this in way where the SPL is constant since you cannot say that it was 90dB at all frequencies when the response is irregular.
    Edit: Perhaps this is not practical since then the input voltage has to be used to calculate the compression.

    There was an old paper by Gander in the AES that is very interesting, I'll try to look it up.
    Here it is and I noticed that he does not do them at constant SPL:
    http://www.aes.org/tmpFiles/elib/20120324/5251.pdf
    Edit: The above link stopped working but this one seems to work:
    http://www.aes.org/tmpFiles/elib/20120326/5251.pdf

    This is on motor geometry where constant SPL is also preferred for distortion tests, see Fig. 13 for example:
    http://www.aes.org/tmpFiles/elib/20120326/3936.pdf
    Last edited by Pete Basel; 03-26-2012, 05:21 PM.

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    • #3
      Re: Example of tweeter compression and discussion

      The sweep starts at low frequency, and goes up. Perhaps the heat builds up, and is hottest after a few seconds. The low frequencies start off cold. The High frequencies are affected by heat generated during the low frequency part of the sweep.

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      • #4
        Re: Example of tweeter compression and discussion

        Simple then. Change the sweep frequency start and stop, and only check compression over a narrow frequency window.
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        • #5
          Re: Example of tweeter compression and discussion

          Originally posted by Pete Basel View Post
          Interesting subject!
          I've thought for years that people grossly underestimate the amount of thermal compression experienced in typical use and have also wanted to do similar testing.

          I'd like to know at least the physical characteristics of the tweeter. VC diameter, ferrofluid or not, etc.

          I agree that the increase at LF is distortion, however 4 kHz seems high for it to start increasing.

          Is it possible that your amp was clipping when the sound became real bad as you mentioned?

          Did you use an MLS test (does Holm offer anything else?) which would have broadband energy?

          Did you look at ARTA software at all, I believe it has features or at least an app note covering compression testing.

          I've wanted to do this in way where the SPL is constant since you cannot say that it was 90dB at all frequencies when the response is irregular.
          Edit: Perhaps this is not practical since then the input voltage has to be used to calculate the compression.

          There was an old paper by Gander in the AES that is very interesting, I'll try to look it up.
          Here it is and I noticed that he does not do them at constant SPL:
          http://www.aes.org/tmpFiles/elib/20120324/5251.pdf

          This is on motor geometry where constant SPL is also preferred for distortion tests, see Fig. 13 for example:
          http://www.aes.org/tmpFiles/elib/20120324/3936.pdf
          Pete, how do you link to these articles? Aren't they copyrighted?

          John

          Comment


          • #6
            Re: Example of tweeter compression and discussion

            It looks like a case of the tweeter's motion serving as a cooling / air flow mechanism. At higher frequencies the tweeter moves a lot less.
            :blues: Flat frequency response, a smooth sound power response free of resonance, careful driver-integration, and high dynamic range both upward and downward :blues:

            Comment


            • #7
              Re: Example of tweeter compression and discussion

              Originally posted by Pete Basel View Post
              Interesting subject!
              I've thought for years that people grossly underestimate the amount of thermal compression experienced in typical use and have also wanted to do similar testing.
              I thought you might be interested given your comments in the iron speaker thread

              Originally posted by Pete Basel View Post

              I'd like to know at least the physical characteristics of the tweeter. VC diameter, ferrofluid or not, etc.
              ~25mm, no ferrofluid, dome, 8ohm nominal, Fs of <1000hz, raved by forum members, don't wanna give away to much. This isn't my tweeter either.

              Originally posted by Pete Basel View Post

              Is it possible that your amp was clipping when the sound became real bad as you mentioned?

              Did you use an MLS test (does Holm offer anything else?) which would have broadband energy?

              Did you look at ARTA software at all, I believe it has features or at least an app note covering compression testing.

              I've wanted to do this in way where the SPL is constant since you cannot say that it was 90dB at all frequencies when the response is irregular.
              Edit: Perhaps this is not practical since then the input voltage has to be used to calculate the compression.

              There was an old paper by Gander in the AES that is very interesting, I'll try to look it up.
              Here it is and I noticed that he does not do them at constant SPL:
              http://www.aes.org/tmpFiles/elib/20120324/5251.pdf

              This is on motor geometry where constant SPL is also preferred for distortion tests, see Fig. 13 for example:
              http://www.aes.org/tmpFiles/elib/20120324/3936.pdf
              It was a sweep. I've never looked into Arta.

              I won't be doing this testing with constant SPL. To hard to do. I suppose I could program my minidsp to provide an essentially flat response, but not necessary. I prefer it this way.

              Thanks for the papers, I'll have a look at them later.
              https://www.youtube.com/channel/UCm2...oSKdB448TTVEnQ

              Comment


              • #8
                Re: Example of tweeter compression and discussion

                Originally posted by rpb View Post
                The sweep starts at low frequency, and goes up. Perhaps the heat builds up, and is hottest after a few seconds. The low frequencies start off cold. The High frequencies are affected by heat generated during the low frequency part of the sweep.
                The sweep is less than 1 second. Perhaps near a full second, but no more.

                I feel that it's mechanical compression rather than thermal. There was about 30 seconds between each sweep. And not much current was passing through the voice coil at less than 1000hz due to the filter. It's an agressive filter for this tweeter. Others on the forum are using a much looser filter (if they only knew :eek: ).
                https://www.youtube.com/channel/UCm2...oSKdB448TTVEnQ

                Comment


                • #9
                  Re: Example of tweeter compression and discussion

                  Originally posted by GranteedEV View Post
                  It looks like a case of the tweeter's motion serving as a cooling / air flow mechanism. At higher frequencies the tweeter moves a lot less.
                  Interesting thought. Again though, I believe it's mechanical. Where we see a similar output in the 4khz range, could this be a combination of distortion and compression yielding what appears to be equal output?
                  Last edited by ryanbouma; 03-25-2012, 10:39 AM. Reason: Changed 5khz to 4khz.
                  https://www.youtube.com/channel/UCm2...oSKdB448TTVEnQ

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                  • #10
                    Re: Example of tweeter compression and discussion

                    Some more info I probably should have posted.

                    Here is the full response vs the filtered response. Remember the baffle is a terrible choice for diffraction, this is a nicer tweeter than it appears.



                    You can get a sense for the transfer function from that.

                    And here is the -12db vs -24db.



                    I would call that acceptable, but at -9db I would call that unacceptable. That's a blanket statement as I'm not sure how this relates to real world listening. Now, if -9db is to much distortion/compression, then -12db is 16 watts. This tweeter puts out around 88 to 90db /w/m. Ok, if we are 4m back that's about 78db, plus some room loudness, say 82db.

                    16 watts means we'll have peaks of ~94db at the LP.

                    This tweeter is rater much higher than 16 watts.

                    Anymore thoughts on this? Sorry it's so bogged down with info and details.
                    https://www.youtube.com/channel/UCm2...oSKdB448TTVEnQ

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                    • #11
                      Re: Example of tweeter compression and discussion

                      Originally posted by OlderMongrel View Post
                      Pete, how do you link to these articles? Aren't they copyrighted?

                      John
                      I've found some AES papers online, Leach, Keele, Harman, and a few others post a limited number of papers. I never know if I'm going to find them online but I usually just search for key words and happened to stumble onto these that seem to be put up by the AES. Seems there are just a limited number of papers available from that archive. It is hit or miss when searching.
                      You might notice that the path includes tmpfiles so they might not be up forever.

                      Does anyone know what the deal is with the AES making some of the papers available? Perhaps for educational purposes?

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                      • #12
                        Re: Example of tweeter compression and discussion

                        Can you post the actual distortion graphs?

                        Btw, it's pretty easy to figure out which tweeter this is.
                        "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche

                        http://www.diy-ny.com/

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                        • #13
                          Re: Example of tweeter compression and discussion

                          Originally posted by Face View Post
                          Can you post the actual distortion graphs?
                          Like this?



                          Note, I'm not exactly sure how to interpret Holm's distortion readings. I can post more of anything specifically that would help the discussion and determine the usability of the tweeter.


                          Originally posted by Face View Post

                          Btw, it's pretty easy to figure out which tweeter this is.
                          That's fine. Just please don't say it in public please. I don't want something negative I say to be used the wrong way. Thanks.
                          https://www.youtube.com/channel/UCm2...oSKdB448TTVEnQ

                          Comment


                          • #14
                            Re: Example of tweeter compression and discussion

                            Originally posted by ryanbouma View Post
                            Like this?
                            Ouch!
                            "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche

                            http://www.diy-ny.com/

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                            • #15
                              Re: Example of tweeter compression and discussion

                              Well, I'm not sure how representative it is. That's THD. Also, Holm is kind of funny about distortion. There was some discussion somewhere about it. That's also at insane SPL and at 1m listending distance.
                              https://www.youtube.com/channel/UCm2...oSKdB448TTVEnQ

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