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  • 2 quick questions

    1. Xmax - RS180S-8 specifically - published spec is 6 mm. Unibox shows a peak of 7 mm for my box design. How far can you push it? Will that 1 mm of over excursion be audible or will it destroy my drivers?

    2. Max port air velocity - Unibox shows 2 thresholds 17 and 26 m/s. It this the practical limits? How bad would it sound if I exceeded that by 20% with a rear firing port?
    Craig

  • #2
    Re: 2 quick questions

    Originally posted by PWR RYD View Post
    1. Xmax - RS180S-8 specifically - published spec is 6 mm. Unibox shows a peak of 7 mm for my box design. How far can you push it? Will that 1 mm of over excursion be audible or will it destroy my drivers?

    2. Max port air velocity - Unibox shows 2 thresholds 17 and 26 m/s. It this the practical limits? How bad would it sound if I exceeded that by 20% with a rear firing port?

    1. 7mm of excursion will neither be an audible problem, nor harm your woofer. You are fine.

    2. Exceed which number by 20%? I wouldn't use a port that's hitting 31 m/s if that's what you mean. Besided chuffing, it will become restrictive at higher volumes as well. (Now, if you mean 20 m/s, then you will probably be OK for a rear port).
    Click here for Jeff Bagby's Loudspeaker Design Software

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    • #3
      Re: 2 quick questions

      Cool! Thanks Jeff.

      So my MTM design has 2 RS180S-8's in a 2 cuFt enclosure tuned to 38Hz which dictates a port that is 4" diameter by 8.173" long. That will keep the peak port velocity at about 20 m/s and Xmax at 7 mm.
      Craig

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      • #4
        Re: 2 quick questions

        Originally posted by PWR RYD View Post
        Cool! Thanks Jeff.

        So my MTM design has 2 RS180S-8's in a 2 cuFt enclosure tuned to 38Hz which dictates a port that is 4" diameter by 8.173" long. That will keep the peak port velocity at about 20 m/s and Xmax at 7 mm.
        That should be a fine set-up. Keep the group posted on your progress.
        Click here for Jeff Bagby's Loudspeaker Design Software

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        • #5
          Re: 2 quick questions

          Keep in mind the port speed calculations assume "worst case scenario". In my latest subwoofer build (Dayton SDVC 8" in 1.15 cubic feet tuned to ~24 hz with a 2" port), the software was yelling at me because the port was too small. I have yet to hear anything out of the ordinary, even at volumes louder than I normally listen.

          Front ported, too.

          I suppose if I listen to electronica at high volumes, port noise can become an issue...

          If you have a habit of listening looouuuuud, then port size becomes more important I suppose. I decided to build to my personal taste this time, and not what the software tells me to do
          Don't listen to me - I have not sold any $150,000 speakers.

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          • #6
            Re: 2 quick questions

            Originally posted by johnnyrichards View Post
            Keep in mind the port speed calculations assume "worst case scenario". In my latest subwoofer build (Dayton SDVC 8" in 1.15 cubic feet tuned to ~24 hz with a 2" port), the software was yelling at me because the port was too small. I have yet to hear anything out of the ordinary, even at volumes louder than I normally listen.

            Front ported, too.

            I suppose if I listen to electronica at high volumes, port noise can become an issue...

            If you have a habit of listening looouuuuud, then port size becomes more important I suppose. I decided to build to my personal taste this time, and not what the software tells me to do
            You should change your name to Johnny Yuma, you rebel you.
            Last edited by Pete Schumacher; 04-20-2010, 12:04 AM.
            R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio
            Byzantium Project & Build Thread || MiniByzy Build Thread || 3 x Peerless 850439 HDS 3-way || 8" 2-way - RS28A/B&C8BG51

            95% of Climate Models Agree: The Observations Must be Wrong
            "Gravitational systems are the ashes of prior electrical systems.". - Hannes Alfven, Nobel Laureate, Plasma physicist.

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            • #7
              Re: 2 quick questions

              You should change your name to Johnny Yuma, you rebel you.

              hehehe
              Don't listen to me - I have not sold any $150,000 speakers.

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              • #8
                Re: 2 quick questions

                For a front-mounted port, I wouldn't exceed 10 m/s, and for a rear-mounted port, I'd not exceed 17 m/s. But it's not just the absolute air velocity, it's also at what frequency the peak occurs and its relationship to the music you play.
                Paul

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                • #9
                  Re: 2 quick questions

                  The peak occurs at 36 Hz and I like to listen to some rock and also some 80-90's alternative. Probably nothing that has any sustained notes anywhere near that low.
                  Craig

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                  • #10
                    Re: 2 quick questions

                    Originally posted by Jeff B. View Post
                    1. 7mm of excursion will neither be an audible problem, nor harm your woofer. You are fine.
                    It should also be noted that it will manifest itself as an audible problem before it becomes a question of mechanical harm. I think Darren Kuzma must have been a fan of having more Xmech than Xmag--things like bumped back plates and progressive suspensions became more common in Dayton products while he was at the helm of new prod dev. With more suspension travel than Xmax, that means that there's a fairly wide safety margin, during which audible distortion will rise noticably. You'll hear the limit before the driver has a criticality event.

                    If you're using Unibox, there's a field called "Linear Cone Overdrive". Set it to 1.15 and it will add 15% to the red line in the excursion graphs. 15% is generally a safe number. However, if you're ever modeling downfiring woofers, you'll notice that there's a "Downfiring Application" field. Unibox documentation suggests that it be used as the Linear Cone Overdrive value (to account for the Xmax lost due to suspension sag).
                    nothing can stop me now

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                    • #11
                      Re: 2 quick questions

                      FWIW, I made a pair of NatPs, an RS180-based MTM. I went for low end extension with very good results. The box is 65l, tuned to 32 Hz with (predicted) F3 of 32 and F10 in the mid 20's. In-room data is quite flat to 30Hz, below which output drops like a rock. I rarely use the sub...

                      The two criteria that led to that result were a) using half rated power to determine my Xmax limits, and considering room gain at the very low end. The predicted FR for flat response results from a box tuned to 37.5 Hz.

                      Room gain is a result of pressure build up at frequencies below those that can form standing waves. Withi an 8' celinig, that's below 70Hz. If you build flat, you can end up with boomy bass. I built in a roll-off, gaining low frequency extension along the way. and ended up flat in my placement.

                      Have fun,
                      Frank

                      PS neat idea, Dirk. I'd never noticed overdrive before.

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                      • #12
                        Re: 2 quick questions

                        Originally posted by fbov View Post
                        Room gain is a result of pressure build up at frequencies below those that can form standing waves. Withi an 8' celinig, that's below 70Hz. If you build flat, you can end up with boomy bass. I built in a roll-off, gaining low frequency extension along the way. and ended up flat in my placement.

                        Have fun,
                        Frank
                        I thought the onset of room gain began based on the length of the largest room dimension, not the smallest.
                        nothing can stop me now

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                        • #13
                          Re: 2 quick questions

                          Originally posted by Dirk View Post
                          I thought the onset of room gain began based on the length of the largest room dimension, not the smallest.
                          I believe you are correct sir.
                          R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio
                          Byzantium Project & Build Thread || MiniByzy Build Thread || 3 x Peerless 850439 HDS 3-way || 8" 2-way - RS28A/B&C8BG51

                          95% of Climate Models Agree: The Observations Must be Wrong
                          "Gravitational systems are the ashes of prior electrical systems.". - Hannes Alfven, Nobel Laureate, Plasma physicist.

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                          • #14
                            Re: 2 quick questions

                            Originally posted by Dirk View Post
                            I thought the onset of room gain began based on the length of the largest room dimension, not the smallest.
                            If that were correct, I'd have no room gain. The room's 8x16x22, so onset would be 26Hz. That's not the case; I have room gain with the speaker described, which has virtually no output at 26Hz.

                            Kevin Haskins does his application notes assuming a similar size room, and it diverges from driver response about 70Hz. In the absence of highly directional drivers, one would expect each major mode to pick up energy below its primary mode as it becomes availalble. The effect would increase as frequency drops and new modes start pressure loading.

                            Have you folks compared sims to in-room measurements, sans EQ? What did you see that leads you to your conclusoin?

                            Frank

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                            • #15
                              Re: 2 quick questions

                              Originally posted by fbov View Post
                              If that were correct, I'd have no room gain. The room's 8x16x22, so onset would be 26Hz. That's not the case; I have room gain with the speaker described, which has virtually no output at 26Hz.

                              Kevin Haskins does his application notes assuming a similar size room, and it diverges from driver response about 70Hz. In the absence of highly directional drivers, one would expect each major mode to pick up energy below its primary mode as it becomes availalble. The effect would increase as frequency drops and new modes start pressure loading.

                              Have you folks compared sims to in-room measurements, sans EQ? What did you see that leads you to your conclusoin?

                              Frank
                              Everyone's room is different, with different shapes, openings, and pressure leakages due to construction. However, room modes are not the same thing as room gain. Modes are based on the nodes that form from parallel walls forming standing waves with nulls and peaks. Room gain is caused by pressurization at frequencies whose wavelengths are too long to develop within the room's dimensions. This is defined by the longest dimension of the room. However, even in a room with one end open, it doesn't necessarily mean that modes won't still create significant peaks, and that some degree of pressurization won't still occur within the boundaries that are present. You just won't get anywhere near full pressurization, so the gain will be reduced signficantly.

                              Jeff B.
                              Click here for Jeff Bagby's Loudspeaker Design Software

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