Excursion limits and LSPCAD

Discussion in 'Home Theater Projects' started by Mike.Hjorten, Sep 5, 2003.

  1. Mike.Hjorten

    Mike.Hjorten Auditioning

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    I'm planning on a 80 to 100L ported sonotube sub tuned to 19-20 hz using a Stryke AV-12 driver. I was using WinISD to help me figure volume, port length and tuning. I keep hearing a few people warn about the excursion limits of the driver in certain alignments. So I down loaded LSPCADlite and input the parameters I settled on from WinISD. Low and behold LSPCAD calculates a peak in excursion of 40mm at around 25 hz when the box is 85L @20 hz. To lower the excursion peak the box must be significantly smaller which causes the low freq. extension to suffer!

    The AV-12 has an Xmax of 23mm (one way) which I believe means that it is capable of up to 46mm of excursion. Is this true and is 6mm of headroom enough?

    Is the driver going to be distorting a lot that close to the Xmax?
     
  2. Andrus_R

    Andrus_R Stunt Coordinator

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    You are shooting for 40mm? Hell, at 20mm walls will start to shake. I don't think you need to be too anal about building a DIY sub unless you mean to mass produce them for resale. 20 or below tuning is over rated. Mid bass is where the fun's at.[​IMG]
     
  3. Mike.Hjorten

    Mike.Hjorten Auditioning

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    I'm not too worried. To drive the speaker that far I'd really have to be cranking it. I was just wondering whether my understanding of the xmax…etc. was correct.

    You’re probably right. Once you have the driver and amp it’s not too hard to build a new enclosure if it didn’t work out.
     
  4. Richard Little

    Richard Little Stunt Coordinator

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    Hey Mike.Hjorten, have you found any answers for what kind of headroom if any is considered the norm? I don’t mean to be "anal" but if its not worth doing right then its not worth doing at all. I am using lspCAD lite to design a 330L box tuned to 15.8hz, with one 6"(15.2mm) port for an AV15 with 250Watts (at 8ohms, so I’m thinking that means 500 at 4ohms). This shows me (a first time DIY sub builder that could really use some help with this stuff:b ) a max excursion of 46mm on the nose at about 22hz and an output of almost 113dB at 16hz with a very slow increase in db with Hz up from there. Is designing this thing to just touch max excursion when max'd out ok, or are the more knowledgeable doing it a little different?
     
  5. Mike.Hjorten

    Mike.Hjorten Auditioning

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    Richard,

    I’m in the same boat! I’m trying to nail down my understanding of design before I start hacking away at an enclosure. Two things that I learned or figured out since I posted is that near the excursion limits the driver will not be behaving linearly. Xmax is defines as a drop of 30% in the magnetic field (Xmag) or 75% drop in the restoring force of the suspension (Xsus), whichever is less. (usually Xmag) So it would be best not to expect to run it right up to xmax.

    That being said LSPCAD plots the SPL at full power. So if your getting 112db at 1m at full power, you’ll get ~109 db at half that power. If you re-plot and look at the excur. when you reduce the power the excur reduces quite a bit. So if you are operating at sane volumes the danger of bottoming the driver should be reduced quite a bit. So I’m thinking that 6mm of headroom will be plenty.

    I think that real manf. might have more headroom than needed to prevent morons from bottoming the driver and generating warrenty claims.

    BTW I don’t think that you’ll get double the power into 4 ohms that you get into 8 ohms unless you have a real high quality amp. Most really decent amps can’t double their output into 4 ohms. For a [email protected] amp you might get 350-400W @4ohm. Most manufacturers will include the rating in there specifications.

    The other thing is that if you end up tuning around 25 hz or less plan on using some sort of EQ like the BFD. The sub will probably sound very boomy without equalization.
     
  6. Shawn Solar

    Shawn Solar Supporting Actor

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    I use unibox and for 85L it shows a max one way xmax of 17mm with 500watts.Also it is with a single 4"port tuned to 20hz. I have two sealed av12's and have put 500watts into them and really have not heard even the slightest titter or complaint[​IMG] Also you'll usaully ever come 6mm close to xmax on bass peaks.

    I put my meter on one of them with some heavy bass stuff and one meter corner loaded hit 115db. the spl is lower at the listening position though.
     
  7. Paul Spencer

    Paul Spencer Stunt Coordinator

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    With a driver like the AV12 I wouldn't be worrying about excursion limits!!!! ... are you serious?

    When you say "is 6mm headroom enough?" how are you going to measure that? How will you know that it isn't going to bottom out with a sudden burst of bass in a movie? ... I think you will just have to get a pretty good idea of how much you can turn up the volume on movies and music.

    The way I see it, the real issue is distortion. No driver sounds its best when pushed to its limits. The distortion increases and you get a significant loss in fidelity. They sound most accurate when loud enough to be loud yet are well within their limits. They will start to lose fidelity long before they are at risk of damage.

    Regarding LspCAD, some may disagree, but I don't think it's a good idea to use simulations for absolute figures - how much amp power is required, max SPL, excursion limits, etc. I find simulations work best with relative results rather than absolutes. eg. they let you compare design options and evaluate which design works best. I have found LspCAD appears to give misleading results if you are looking for absolute results. Computer models are rarely sophisticated enough to do that.

    eg. Shiva in a 100L vented sealed box
    12 watts rms input power
    6db eq @ 20 Hz with a Q of 1
    cone excursion: 32mm p-p!!!

    Clearly, more amp power would be needed to achieve the eq, but LspCAD does not allow for this.

    I agree with your learn as much as you can before you start approach. Why not first build a prototype? You can make a simple box in no time and you can do it cheap. This is the approach I'm taking with my AV12's - still waiting for 2 of them to arrive. I have already built a 70L test box from wood that was lying around, and I will be making some more. I have packed a lot of theory into my brain, but I won't decide til I have had a chance to listen to the different options I like and have heard them set up for a while.

    Here's a few of my conclusions on vented alignments so far based on discussions, simulations etc:
    * a moderate sized vented sub tuned @ 20 Hz can work well - say 70L (there's not much advantage in going the 85L option on stykes website)
    * a 140L EBS is worth considering for really deep bass and will work better with room gain than the 70L and it is much easier to fit in a large vent
    * try tuning an EBS lower and lower and you will find that the response theoretically becomes the same as a sealed box @ 0 Hz tuning
    * tuning the 140L EBS lower ... say 14 - 16 Hz will give you a response that may in fact work very well with your room gain and you may get away with little or no eq

    I will be testing all this when my sub arrives!!!
    If intersted, I have some of this type of stuff on my site,
    although keep in mind there are a few idea in there I have dispensed with but I'm having problems updating my site.

    cheers,
    Paul
     
  8. Mike.Hjorten

    Mike.Hjorten Auditioning

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    That was exactly my question. Should you really care if LSPCAD shows your approaching the excursion limit of your driver?

    In the DIY subwoofer forum you often see recommendations for lower box volumes than what models well in the various software packages. Many times they justify it by saying that you “too close bottoming your driver” or “you need to protect your driver” by lowering the volume to reduce the excursion.

    After I played with the numbers a little bit I realized that unless I was hitting a bass peak requiring a full 400W or greater I wouldn’t be very close to the xmax.

    What I was really after was a rule of thumb concerning either how much to trust LSPCAD’s calculations or if you could predict the excursion of the driver how close to xmax should you go in order to maintain low distortion? After all who cares if it hits 120 db if it distorting like crazy (>30%).

    Sounds like LSPCAD is very conservative. So I won’t worry too much. When I’m done I’ll post the results.
     
  9. John_MackieBass

    John_MackieBass Stunt Coordinator

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    if things arent FALLING off the walls, you arent having fun!!!! [​IMG]
     
  10. Paul Spencer

    Paul Spencer Stunt Coordinator

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    I don't believe that the design of a sub should be concerned with limiting the excursion of the driver!

    What you want is an amplifier that can drive your sub to maximum excursion with enough headroom to prevent clipping, and an alignment that allows you to do this without exceeding the thermal power rating of the driver. This is the way to get the most clean SPL out of your sub without risking damage. If you use a smaller box you will have less efficiency. More power will be required to reach the same SPL level as a bigger box, and if that means if you try to get the same SPL the amp might start to clip and then your problems begin. Added to that you will have more distortion!

    Choose the size of a sealed box based on the desired Q - usually either 0.7 or 0.5 critically damped. If its a vented box then its about size, tuning and extension.

    In a nutshell, I see no real reason to make a box smaller to limit excursion.

    When LspCAD tells you that you can exceed xmax with a given amp, consider it a good thing!!! It means that design is more efficient and you are less likely to damage the driver, which is caused by exceeding a drivers thermal power rating and amplifier clipping which results in overheating the VC.
     

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