Announcement

Collapse
No announcement yet.

PR and air leaks

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

  • PR and air leaks

    Hi, i want to use 2 passive radiators in a build but when i press in the passives the speaker is moving out and slowly getting back in place wich shouldn't happen. If the enclosure really is air tight the speaker should stay 'out' when i press the passives inside. Or am i wrong? I have a jbl portable speaker with passives and when i press a passive, the other passive stays out, so it's absolutely 100% air tight!

    How does this effect the sound because my speaker get -6 db at full excursion measured with a imm-6. While measuring other speakers they al get there listed spl +- 1db. I also use this speakers (nd91) in a other vented project and they reach listed spl at +-2db, so it has something to do with the passives or the air leaks of the enclosure. Maybe the passives are to heavy so i will lose spl? But 6db is a lot. I don't hear air leaks but the enclosure is not 100% air tight.


  • #2
    A speaker should never be 100% sealed. It needs to be able to equalize the internal cabinet pressure with the outside atmospheric pressure.

    How does your model compare to measured? Where is the speaker -6 dB down?

    Comment


    • #3
      What devnull said. Unless the leak is simply enormous, the main danger is whistling noises.
      Francis

      Comment


      • #4
        The OP is correct. The cone should move back to its neutral position, but very slowly. The leak with a PR will have the same effect as with a sealed or ported cab, it will upset the tuning. The source of the leak could be in a number of places, from the cabinet joints to the driver and PR gaskets. The usual method to find leaks is with a test tone too low in frequency to hear, allowing one to locate the leak via the whistling it creates.

        To see the effect of a leak on response in WinISD look at the Ql in the Advanced tab of the Box window. The default value is 10, which is an average reasonably tight enclosure. A value of 100 would be absolutely airtight, a value of 5 moderately tight, a value of 1 very leaky.
        Last edited by billfitzmaurice; 07-10-2021, 12:53 PM.
        www.billfitzmaurice.com
        www.billfitzmaurice.info/forum

        Comment


        • #5
          Larger speaker drivers seem to be easier to get airtight.

          Push in quickly on a typical 8" to 15" subwoofer with a PR and you should see the PR's slowly come back to resting position... it should take maybe between 4 to 8 seconds as an estimate to get back to normal resting position.

          Then let go of the driver... it should take another 4 to 8 seconds to reach equilibrium again. That is pretty much normal for PR enclosures I've constructed. You'd think they would be more airtight than that but that's been my experience so far, and I don't think there were really any 'leaks' in any of these so to speak... really just some 'leaky' materials that allow the changes in air pressure.

          Now, I've done a few projects with ND91 size drivers and smaller, and they can be a bit of a bugger to seal completely. It's common for those little ones to 'bounce back' to resting position a lot quicker than that... partly because there is less air compressed or rarefied, but also because they are often TOUGH to seal up perfectly... flanges a few MM thick and terminals that stick out farther than the frame of the driver are two pretty good reasons this happens. It's actually pretty critical to seal up the small ones as an air leak can just KILL any bass response on those. Just glump up the openings with some gasket caulk or black foam mounting tape and do the best you can.

          One way to find an actual leak though is to run some bass test tones though the speaker at varying frequencies, using a frequency generator app or something similar, then you should be able to hear the 'leak' a little better.

          Hope this helps.

          TomZ
          Zarbo Audio Projects Youtube Channel: https://www.youtube.com/channel/UCEZ...aFQSTl6NdOwgxQ * 320-641 Amp Review Youtube: https://youtu.be/ugjfcI5p6m0 *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
          *Part 2 *Gluing multiple curved laminations of HDF

          Comment


          • #6
            Think of the rate that the PR is moving. If it were converted to a frequency, it would be probably something like 1 Hz or less. Not really a problem for actual bass response.

            Comment


            • #7
              Originally posted by devnull View Post
              A speaker should never be 100% sealed. It needs to be able to equalize the internal cabinet pressure with the outside atmospheric pressure.

              How does your model compare to measured? Where is the speaker -6 dB down?
              I just measure how loud music can be played. In previous builds i run like 10 songs and pick the highest spl wich is archieved after that. I get pretty close to a save maximum spl with this way. It's not that accurate, especially with dsp added, so i have to look at this as well.

              Comment


              • #8
                Originally posted by billfitzmaurice View Post
                The OP is correct. The cone should move back to its neutral position, but very slowly. The leak with a PR will have the same effect as with a sealed or ported cab, it will upset the tuning. The source of the leak could be in a number of places, from the cabinet joints to the driver and PR gaskets. The usual method to find leaks is with a test tone too low in frequency to hear, allowing one to locate the leak via the whistling it creates.

                To see the effect of a leak on response in WinISD look at the Ql in the Advanced tab of the Box window. The default value is 10, which is an average reasonably tight enclosure. A value of 100 would be absolutely airtight, a value of 5 moderately tight, a value of 1 very leaky.
                Thanks, i will look this up tomorrow in winisd. If passive designs are not 100% airtight, does this mean that a PR is designed with a 'breathing' surround or something?

                Comment


                • #9
                  Originally posted by tomzarbo View Post
                  Larger speaker drivers seem to be easier to get airtight.

                  Push in quickly on a typical 8" to 15" subwoofer with a PR and you should see the PR's slowly come back to resting position... it should take maybe between 4 to 8 seconds as an estimate to get back to normal resting position.

                  Then let go of the driver... it should take another 4 to 8 seconds to reach equilibrium again. That is pretty much normal for PR enclosures I've constructed. You'd think they would be more airtight than that but that's been my experience so far, and I don't think there were really any 'leaks' in any of these so to speak... really just some 'leaky' materials that allow the changes in air pressure.

                  Now, I've done a few projects with ND91 size drivers and smaller, and they can be a bit of a bugger to seal completely. It's common for those little ones to 'bounce back' to resting position a lot quicker than that... partly because there is less air compressed or rarefied, but also because they are often TOUGH to seal up perfectly... flanges a few MM thick and terminals that stick out farther than the frame of the driver are two pretty good reasons this happens. It's actually pretty critical to seal up the small ones as an air leak can just KILL any bass response on those. Just glump up the openings with some gasket caulk or black foam mounting tape and do the best you can.

                  One way to find an actual leak though is to run some bass test tones though the speaker at varying frequencies, using a frequency generator app or something similar, then you should be able to hear the 'leak' a little better.

                  Hope this helps.

                  TomZ
                  Thanks Tom, i will run a 20hz sinewave tru it. If i push the passives and holt it, the nd91 takes 1 or 2 seconds at max to gain back to normal position. So not really bad i gues??

                  I glued everything and i'm 99% sure it isn't the problem, so i gonna make a second(stereo) enclosure and if i have the same reaction i will put some extra glue behind the tweeter and use some isolation strip for the nd91 to see if there is some difference.

                  Comment


                  • #10
                    Originally posted by Scarface1 View Post

                    Thanks, i will look this up tomorrow in winisd. If passive designs are not 100% airtight, does this mean that a PR is designed with a 'breathing' surround or something?
                    It's not just a PR thing.You will see the same effect with a sealed MTM. Push in one cone, the other should extend. If there's minimal leakage the second cone will slowly retract. If there's zero leakage it won't retract. If there's a lot of leakage it won't extend.
                    www.billfitzmaurice.com
                    www.billfitzmaurice.info/forum

                    Comment


                    • #11
                      I'm surprised no one has mentioned the value of an impedance sweep. A PR box, since it's essentially a ported alignment, should have 2 sharp impedance spikes in the
                      bass region, with the actual box Fb in the minimum between them. If you have significant leaks, the peaks will be way lower than normal, and they will be rounded,
                      So first, make sure that the box is sealed up, go over all the seams on the inside with caulk.
                      Then, as Tom Z has mentioned, use some weatherproofing spongy tape under the drivers and PRs.

                      I think I hear a difference - wow, it's amazing!" Ethan Winer: audio myths
                      "As God is my witness I'll never be without a good pair of speakers!" Scarlett O'Hara

                      High value, high quality RS150/TB28-537SH bookshelf - TARGAS NLA!
                      SB13/Vifa BC25SC06 MTM DCR Galeons-SB13-MTM
                      My Voxel min sub Yet-another-Voxel-build

                      Tangband W6-sub

                      Comment


                      • #12
                        The effect on impedance of changing Ql is seen in WinISD. It has the most impact on the value of the upper peak, which goes down as Ql goes down.
                        www.billfitzmaurice.com
                        www.billfitzmaurice.info/forum

                        Comment


                        • #13
                          Originally posted by billfitzmaurice View Post
                          The effect on impedance of changing Ql is seen in WinISD. It has the most impact on the value of the upper peak, which goes down as Ql goes down.
                          Of course, but we are talking about measuring the actual possible leakage on a box that's been built.

                          I think I hear a difference - wow, it's amazing!" Ethan Winer: audio myths
                          "As God is my witness I'll never be without a good pair of speakers!" Scarlett O'Hara

                          High value, high quality RS150/TB28-537SH bookshelf - TARGAS NLA!
                          SB13/Vifa BC25SC06 MTM DCR Galeons-SB13-MTM
                          My Voxel min sub Yet-another-Voxel-build

                          Tangband W6-sub

                          Comment


                          • #14
                            You'd use the sim to diagnose the measured result. If you don't have the sim chart for comparison you have no way of knowing if the measured result is good or not.
                            www.billfitzmaurice.com
                            www.billfitzmaurice.info/forum

                            Comment


                            • #15
                              Thanks guys, i gues i know what i have doing wrong. I sealed everything and run a 20 hz test tone and there where no leaks that i can hear. I did some spl measurements and get like 89db sometimes even with distortion so bad that it can't be the driver wich is maybe past xmax. I looked for the peerless 3.5 passive radiator parameters and i saw a Qms of 10 wich is very very high, while the PR is very very stiff. So i looked up the parameters on the forum and the Qms was 1.44 and i get different results. I don't know why i put a Qms of 10 but mostly i do this with cheap PR because they are not stiff at all.
                              Attached Files

                              Comment

                              Working...
                              X