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Port Resonance - what should I be looking for?

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  • #16
    Originally posted by fpitas View Post
    I can't decide if the trap will make Trap music better, or filter it out
    I tried the trap on Trap and it did not trap anything, all the trap crap was no less than rap
    John H

    Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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    • #17
      Originally posted by monkish54 View Post

      This is really cool!

      How does the trap work, exactly? I would have thought the additional pipe would cause tuning issues.
      Thanks. No, the additional trap pipe did not affect the frequency or magnitude of my main tuning frequency (40Hz or so) throughout my tests. This was probably because it was filled with damping material and then closed at the end. As I understand it, the trap was acting as a tuned, damped resonating filter that let the main port resonance pass through unaffected.
      Plumber's Delight: http://techtalk.parts-express.com/fo...notech-winners
      Linehopper: http://techtalk.parts-express.com/sh...Esoteric-build
      Plan-Tanic: http://techtalk.parts-express.com/sh...ght=Plan-Tanic

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      • #18
        Originally posted by jhollander View Post
        I used a similar trap on this project http://projectgallery.parts-express....ojects/sls-85/

        A good starting point for the trap is 1/2 the port length located in the middle of the port. The theory is the trap creates an additional resonance which is out of phase with the port resonance. I have no reference that supports that statement, but the reduced port resonance is real.
        Thanks, John. I would add that my trap was "tuned" experimentally, with OmniMic positioned at the port terminus as the swept sine wave played continuously. I swapped out different lengths of the trap pipe material on the fly while watching the 1st and 2nd port peaks bounce around on the computer screen. When I "stumbled" onto "just" the right trap length and "just" the right density of trap damping material, the 1st and 2nd port resonances flattened out completely.

        Bill
        Plumber's Delight: http://techtalk.parts-express.com/fo...notech-winners
        Linehopper: http://techtalk.parts-express.com/sh...Esoteric-build
        Plan-Tanic: http://techtalk.parts-express.com/sh...ght=Plan-Tanic

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        • #19
          Originally posted by jhollander View Post
          I used a similar trap on this project http://projectgallery.parts-express....ojects/sls-85/

          A good starting point for the trap is 1/2 the port length located in the middle of the port. The theory is the trap creates an additional resonance which is out of phase with the port resonance. I have no reference that supports that statement, but the reduced port resonance is real.
          I may have a reference:

          "Higher resonances in the time domain"

          http://newt.phys.unsw.edu.au/jw/flutes.v.clarinets.html

          __________________________________________________

          The wave is out of phase because it is delayed by 1/2 wavelength (middle of the port)?

          The trap length is 1/2 port length because that gives the closed-open tube the same resonant frequency as the open-open tube (port)? That's brilliant, you guys! Why am I just hearing about this now?

          4thtry

          How large did your trap end up being in comparison to your port tube?

          I have a large 6" x 33" port I modeled for my new reference build (to avoid chuffing at 115db @ 1M) but the port resonance lies an octave below the theoretical crossover point. Maybe I will trap the port to avoid the resonance.

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          • #20
            Originally posted by Brian Steele View Post

            Yup, we may be on to something here.
            I wonder if my "port resonance" and "driver to top wall resonance" would cancel if I top ported with the port extending halfway between the top wall and the driver? Obviously that would be a silly port location in practice, but it's an interesting concept that may be useful like you said.
            Attached Files

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            • #21
              Not sure if it is along the same concept, but I had a commercial powered sub that always puzzled me.

              It had a single round port tube. Half way down it’s length it was split open a certain distance, wrapped in a breathable material like felt.
              Projects:

              Breezy Monitors: http://techtalk.parts-express.com/fo...reezy-monitors
              transcenD: http://techtalk.parts-express.com/fo...5035-transcend
              Summits: http://techtalk.parts-express.com/sh...75-The-Summits
              References: http://techtalk.parts-express.com/sh...-My-References
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              • #22
                Originally posted by monkish54 View Post

                The wave is out of phase because it is delayed by 1/2 wavelength (middle of the port)?

                The trap length is 1/2 port length because that gives the closed-open tube the same resonant frequency as the open-open tube (port)? That's brilliant, you guys! Why am I just hearing about this now?
                I believe that's the case. I started to experiment; as I should be able to place the trap anywhere along the length of the port and adjust the length of the trap to get the cancellation. I did not reach any conclusion other than the midpoint of the port length was the most effective (ran out of time). Bill's method of experimentally adjusting is the best method imo.
                John H

                Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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                • #23
                  a4, I wouldn't worry about it. I'd worry even less if you can make the vent go out the back.
                  Are you sure you can't fit a 3/4"id x 5" long tube in there, maybe w/an elbow?

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                  • #24
                    Originally posted by monkish54 View Post

                    4thtry

                    How large did your trap end up being in comparison to your port tube?

                    I have a large 6" x 33" port I modeled for my new reference build (to avoid chuffing at 115db @ 1M) but the port resonance lies an octave below the theoretical crossover point. Maybe I will trap the port to avoid the resonance.
                    A: Total port length was 26 inches. I started with a trap of about 13 inches (1/2) and trimmed it, in small 1/8" to 1/4" increments, down to about 10 inches to get it to work. As far as the inside diameter of the trap goes, I used 1-1/4" ID PVC pipe for the trap and 1-1/2" ID PVC pipe for the 26" long port. The trap was stuffed with a small amount of dacron & Acousta-stuff. - In my experiments, I tried 3 different trap positions along the length of the 26 inch long port. I tried the midpoint as well as 1/4th and 3/4 points. No difference in measurements. So I put my trap about 1/4th down the port and it worked just fine. Hope this helps.

                    Bill

                    Plumber's Delight: http://techtalk.parts-express.com/fo...notech-winners
                    Linehopper: http://techtalk.parts-express.com/sh...Esoteric-build
                    Plan-Tanic: http://techtalk.parts-express.com/sh...ght=Plan-Tanic

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