Announcement

Collapse
No announcement yet.

Two Seas DXT, dif. FS impedance

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Two Seas DXT, dif. FS impedance

    I measured the pair of DXT tweeters I got w/WT3 to make sure they're OK since I might not use them for a while. I didn't like the difference in impedance I see, so I measured four WaveCor tweeters to see if they had that kind of variance, not even close (much tighter):

    "...this is not a subwoofer" - Jeff Bagby ;)

  • #2
    Re: Two Seas DXT, dif. FS impedance

    Arguably, the difference becomes more negligible when we're talking peaks - because a circuit you use to tame it is not affected in a linear fashion. If you even out that peak, the impedance of the two drivers will again be very similar. Now, if that's affected FR greatly, there's a concern .. though I doubt the difference will be audible.

    Ex: 15 ohms in parallel with 6 ohms is 4.285 ohms; 14 and 6 ohms is 4.2, a difference of only .085 Ohms. At 5 ohms nominal impedance, 5 ohms is -6dB; .085 Ohms is .1 dB, I believe.
    I am trolling you.

    Comment


    • #3
      Re: Two Seas DXT, dif. FS impedance

      Originally posted by critofur View Post
      I measured the pair of DXT tweeters I got w/WT3 to make sure they're OK since I might not use them for a while. I didn't like the difference in impedance I see, so I measured four WaveCor tweeters to see if they had that kind of variance, not even close (much tighter)
      I've seen that in DXT tweeters also. It can be fixed. It's caused by uneven density of stuffing in the pole piece. Unlike the 27 standard series that has a punched foam plug with consistent density, the DXT has a folded over piece of cotton which is seemingly formed by hand. Just remove material from the denser one until the peak matches the other.

      Comment


      • #4
        Re: Two Seas DXT, dif. FS impedance

        Ah, great tip John! Based on looking at the parts of the sweep that are the same, I would guess that all other factors in the tweeter build are nearly identical and your fix should be just the ticket.

        I think (hope) these don't use ferrofluid. I was hesitant to just take these apart right away as they're more expensive that most of my other tweeters (I guess I am just too cheap!).

        I'll try it in a bit and report back!

        Accidentally removed some fluff from the wrong one first... Now here is a sweep of both after I've removed even more fluff from the one with the higher FS peak:



        I will tweak slightly more later, now time for bed.
        Last edited by critofur; 08-11-2010, 11:09 PM.
        "...this is not a subwoofer" - Jeff Bagby ;)

        Comment


        • #5
          Re: Two Seas DXT, dif. FS impedance

          The looser the stuffing is, the lower and broader the peak is. If I take the stuffing out, a cavity resonance in the vent causes a double peak.

          It seems to be polyester fiber that they're using now. It's slightly troublesome trying to keep all the loose thin strands tucked in and out of the gap when you pull the stuffing out then try to put some of it back in. The stuffing was packed VERY tightly in the tweeters I got so the peak was a lot sharper and higher than the manufacturer's spec sheet and DLR's 2008 measurements on his web page.

          I may try some other stuffing materials. I have various foams and different fiber materials - cotton, wool, celulose fiber batting, felt...

          I'm not sure what the ideal would be - but I guess I'll try to aim somewhere between the spec sheet and what I started with.
          "...this is not a subwoofer" - Jeff Bagby ;)

          Comment


          • #6
            Re: Two Seas DXT, dif. FS impedance

            Originally posted by critofur View Post
            The looser the stuffing is, the lower and broader the peak is. If I take the stuffing out, a cavity resonance in the vent causes a double peak.

            It seems to be polyester fiber that they're using now. It's slightly troublesome trying to keep all the loose thin strands tucked in and out of the gap when you pull the stuffing out then try to put some of it back in. The stuffing was packed VERY tightly in the tweeters I got so the peak was a lot sharper and higher than the manufacturer's spec sheet and DLR's 2008 measurements on his web page.

            I may try some other stuffing materials. I have various foams and different fiber materials - cotton, wool, celulose fiber batting, felt...

            I'm not sure what the ideal would be - but I guess I'll try to aim somewhere between the spec sheet and what I started with.
            There's a compromise to me made most often, it's a balancing act. The denser it is, the better the midband performance. Too dense and the Fs moves up with higher Q to the SPL at the knee.

            I've only used lamb's wool, not wool felt, though it should work similarly for the same density. I've tried cotton quite a bit recently and find it to be nearly if not just as good. The problem in the past was uniformity of density. It's not easy to fluff up dense cotton b@lls to the same level. Now I use cotton from aspirin bottles, when they have it (they're getting cheaper and leaving it out at times). The benefit is very uniform density and it's very clean of course. The density seems to be nearly optimal for cotton as well in that is it far less dense than cheap cotton b@lls. It allows for compression if denser is needed.

            Start with a moderately higher density, allow it to "spill out" of the top just a bit for more distance to pass through.

            dlr

            p.s. I can't believe I have to write cotton b@lls cryptically.
            WinPCD - Windows .NET Passive Crossover Designer

            Dave's Speaker Pages

            Comment


            • #7
              Re: Two Seas DXT, dif. FS impedance

              I applaud you, Critofur, for seeking perfection, but I have to ask you, and anyone else if interested, if you had designed a "perfect" crossover for only one of the DXT tweeters based only on its measurements, would you have heard any difference had you used the other tweeter instead in that crossover?

              Originally posted by critofur View Post
              I measured the pair of DXT tweeters I got w/WT3 to make sure they're OK since I might not use them for a while. I didn't like the difference in impedance I see, so I measured four WaveCor tweeters to see if they had that kind of variance, not even close (much tighter):

              Comment


              • #8
                Re: Two Seas DXT, dif. FS impedance

                Originally posted by Paul K. View Post
                I applaud you, Critofur, for seeking perfection, but I have to ask you, and anyone else if interested, if you had designed a "perfect" crossover for only one of the DXT tweeters based only on its measurements, would you have heard any difference had you used the other tweeter instead in that crossover?
                Well, maybe MarkK will share his measurement files for his ER18DXT and we can model how much difference there actually would be if I built his design with my [unmodified] DXT tweeter with it's FS peak both higher and sharper than the manufacturer's specification, it's output at FS should be significantly less controlled, perhaps causing noticeable distortion?

                http://www.audioheuristics.org/proje...XT/ER18DXT.htm

                Or what of DLR had done a design using his measurement, (assuming he took advantage of the DXT's ability to handle a low and shallow slope crossover) and we tried to use DXT tweeters that measured like mine?:



                Do you think it's reasonable with a tweeter that costs more than $50 to have that much variation in the FS peak? Look at the plots of the four little WaveCor tweeters (four randomly selected) I included for comparison, that's the kind of tight QC I'd expect from Seas.

                Impedance sweeps are the quickest [I know] way to check tweeters for production consistency before you bother to use one in your next build. If you build a lot of speakers: if a tweeter you measure has significant variation from the normal sweep, it either gets tossed or returned. In at least one case I know of a manufacturer switched driver brands because variation from tweeter to tweeter was so significant and inconsistent that it made over 10% of the tweeters unusable. And it wasn't a matter of being nit-picky, the bad ones flat out sounded bad.
                "...this is not a subwoofer" - Jeff Bagby ;)

                Comment


                • #9
                  Re: Two Seas DXT, dif. FS impedance

                  Let me clarify a point I mentioned earlier: once you get much over 1Khz the unit-to-unit consistency looks outstanding - that leads me to believe that production controls are very tight, and inserting the stuffing maybe the only step significantly subject to human error.
                  "...this is not a subwoofer" - Jeff Bagby ;)

                  Comment


                  • #10
                    Re: Two Seas DXT, dif. FS impedance

                    Originally posted by critofur View Post
                    Let me clarify a point I mentioned earlier: once you get much over 1Khz the unit-to-unit consistency looks outstanding - that leads me to believe that production controls are very tight, and inserting the stuffing maybe the only step significantly subject to human error.
                    Yours do have curiously high peaks in them. The Seas measurements show a curve similar to mine, peaking at about 12.5 ohms. I remember being a bit surprised at the shape, but it is useful if this thing may be used down to 1.5K or whatever is considered the low end. Given it's high sensitivity due to the special horn loading, it is a good candidate. Hard to say if that peaking would cause enough difference, it would depend highly on the crossover used.

                    An odd peaking of Z at Fs could also slightly complicate the highpass. The flatter one of mine and the one as shown in the Seas curve makes it a lot easier. I'm a bit puzzled at yours, but at least you now have the two close together. It has the appearance of the density being a tad low.

                    Still, it's unusual for Seas to have an issue such as this, one benefit of the foam plugs, but they may just not be as effective or simply cost more. I didn't try cotton at the time, only lamb's wool. Since you do have some peaking in yours, you may want to experiment.

                    dlr
                    WinPCD - Windows .NET Passive Crossover Designer

                    Dave's Speaker Pages

                    Comment


                    • #11
                      Re: Two Seas DXT, dif. FS impedance

                      Originally posted by critofur View Post
                      The looser the stuffing is, the lower and broader the peak is. If I take the stuffing out, a cavity resonance in the vent causes a double peak.
                      Just like stuffing an enclosure, the amount of stuffing changes the damping at resonance. Too much or not enough isn't good.

                      Same thing occurs with ferrofluid too. Different amounts of ferrofluid changes the height of the peak at resonance.

                      If your tweeters measure differently, they will sound differently too.
                      Obviously, with any given pair of tweeters you want the peaks to be the same.

                      Comment


                      • #12
                        Re: Two Seas DXT, dif. FS impedance

                        I'll mention the elephant in the room....

                        Why is a $50 plus tweeter from one of the most respected names in driver manufacturing susceptible to this? Is it even remotely acceptable that you have to spend over $100/pr on tweeters, only to disassemble them and fix them?

                        From what I have seen here, I will scratch the DXT off of my short list of future tweeter purchases, I guess.

                        Man, I thought stepping up to that price level from that manufacturer would give you some peace of mind.
                        Don't listen to me - I have not sold any $150,000 speakers.

                        Comment


                        • #13
                          Re: Two Seas DXT, dif. FS impedance

                          Originally posted by johnnyrichards View Post
                          From what I have seen here, I will scratch the DXT off of my short list of future tweeter purchases, I guess.
                          True, Seas dropped the ball on that one. It's obvious the pole vent stuffing, and the wadding of cotton in particular, is done by hand and therefor subject to severe consistency issues. This could be a problem for people that have no impedance measuring ability and just want to build a kit like the Idunn or Mark K's ER18 design.

                          However, this tweeter has some very good benefits that can't be had in very many other tweeters, and the fix is worth while.

                          Or, skip the fix and use a traditional L-pad with a slightly lower value shunt resistor for damping, and peak variances will be largely irrelevant anyway. This is not as big a deal as it seems.

                          Comment


                          • #14
                            OT but related story

                            All this inconsistency reminds of a work experience I had. We had placed an order for shearpins that would yield between two different forces. The delta was quite large. Yet, three batches in a row failed, from the approved supplier (samples were greatly scattered). As each batch was significantly expensive, the end result was to accept the batch that had the lowest average maximum shear force, assuming the low force was not an issue.

                            As a joke, we took a box of paper clips out of supply and all measured within a couple pounds of each other. If the diameter had been a little bigger, we would have used the 50 cent box instead of the $1000 box. In this case, the fault lied in the uniqueness of the product, and getting the metal consistent for the small batch (only a couple of pounds total weight) was proving difficult.

                            Maybe because the DXT is built through production variations, putting a different faceplate on motors pulled from an existing line, using a different method of stuffing was necessary.
                            - John

                            "Injustice anywhere is a threat to justice everywhere." - Dr. M. L. King
                            www.BuildTheDream.org

                            Comment


                            • #15
                              Re: Two Seas DXT, dif. FS impedance

                              Soft domes in general have quite a bit of variance, but Seas used to be known for excellent QC...

                              Shame on them, it's a premium product, and should be consistent as such.

                              Comment

                              Working...
                              X