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differences in same model tweeter

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  • differences in same model tweeter

    I tested a pair of Seas 27TDC which I want to use for a pair of booshelfs with Dayton RS150 drivers. I see some significant differences in the measured values but not sure if this will translate to anything really audible. I'm thinking of crossing the tweeters over at around and I'm wondering how audible any of this might be if I cross the tweeter at around 2Khz or just above. Maybe what appears to be big variances in numbers might end up being inaudible in the real world usage but I figure I should ask before starting to do the build.

    Anyone with any suggestions or guidance, that would be great.







  • #2
    Re: differences in same model tweeter

    Looks like a pair of fine tweeters to me. The bump in more damped in one, could be due to amount of ferrofluid used, or if you were holding your hand over top when measuring.
    "I just use off the shelf textbook filters designed for a resistor of 8 ohms with
    exactly a Fc 3K for both drivers, anybody can do it." -Xmax

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    • #3
      Re: differences in same model tweeter

      Does this tweeter have a chamber? - I'm too lazy to look it up.

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      • #4
        Re: differences in same model tweeter

        It has chamber and no ferro.

        Difference in between is more likely due to some slight difference how the foam plug inside pole piece vent is placed. My guess, but not a wild one , the one that has more pronounced impedance peak at about 1.2Khz will have slightly less output in that area. It should be fine for 2Khz xover, I wouldn't worry.

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        • #5
          Re: differences in same model tweeter

          Thanks everyone. I feel a little better about the measurement. BTW I opened both tweeters VERY carefully (removed the diaphram) and the foam plug was kind of sticking out more on one. I put it back so it looks the same but in the end, it still measures the same as before. I wont worry about it then even though I was sort of neurotic about it

          I must say though that these tweeters sound really REALLY good to my ears even just connected by themselves with a simple cap connected to them. Quite delicate sounding but with the ability to go quite low.

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          • #6
            Re: differences in same model tweeter

            The double bump is analogous to a vented speaker where the cavity under the dome is the speaker box and the pole vent is the vent tube, the rear chamber would be the room - not a very big one of course.

            I was going to suggest that you very carefully open them and simply look for differences. You could try swapping parts if you are really curious.

            I'd be worried about a leak which could cause that. Check for flatness on the back side of the dome assembly and perhaps try some mortite around the rear cavity seal to see if it makes them the same. The deeper dip in the impedance is the higher Q resonance.

            I believe that the rear chambers often have felt or foam damping and you might see if anything looks different down the vent hole.

            The differences probably don't matter in the passband.

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            • #7
              Re: differences in same model tweeter

              You know, looking at the curves one more time, the main difference is actually not the second impedance hump, it is the first major one. I hadn't paid attention to that, I had looked at the different looking 2nd hump. But the 2nd hump is actually almost same, the way the first major jump joins to it is different. In the first graph the impedance peak goes above 15 ohms, in the second it is at 14 ohms, so second one has a more damped resonance and so joins more to the 2nd hump. As Pete wrote it could be difference in damping inside the second chamber, or air leak that gives more damping, or even the surround coating or something like that gives more damping on the main resonace.
              Good news is since the main difference is on the main impedance peak, the frequency response differences will be closer to there, which is further away from the xover frequency.

              Overall, actually these are highly matched tweeters, their Fs is almost same, not that common occurance in my limited experience. I really wouldn't worry.

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              • #8
                Re: differences in same model tweeter

                I can think of plenty of tweeters I've used whose Fs don't match within a pair.

                As a matter of fact, I can't think of a single pair that their Zs measure identically, but they end up always performing the same. So I wouldn't worry.
                Line Array: IDS-25 Clone, FE-83.
                2-2.5 Way:
                Zaph Audio's winning entry: ZA5+SB29. - Microliths: RS125+RS28. - Small Bangs: TB W4-1658SB+SEAS 27TBFC/G. - Monoliths: Peerless 830884+SEAS 27TBFC/G.
                3-3.5 Way:Miniliths: SEAS P21/CA21REX+Neo8 PDR+Neo3 PDR. - Megaliths: 2xDayton RS270+2xT-B W4-1337SB+SB29. - ZDT3.5 +: 2xDayton RS180+Dayton RS52+Vifa DQ25. Reflexos: OB 4xDayton RS150 + Neo3 PDR.

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                • #9
                  Re: differences in same model tweeter

                  SEAS advertises their precoating of fabric dome technique resulting in consistency, which these measurements show. I wish they had some tweeters with copper caps on the motors.

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                  • #10
                    Re: differences in same model tweeter

                    Well after all the feedback I got from people that know way more than me, I have zero doubts or worries now about starting this project. The part I dread is working in the garage in the winter. Even with a heater, its still cold. Sucks!

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