Dipole Help

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  • BigSky
    Junior Member
    • Mar 2003
    • 8

    Dipole Help

    Hi,
    This seems like a great forum.I've been reading lots of posts,and found a great deal of helpful information,especially on dipoles,which is just what I need.So,now that I'm looking for some advice,I thought I'd ask here.

    I have a pair of Lowther PM6's,which I'd like to use in a dipole design,where they'd cover everything above,say 200hz,or whatever would be suitable in that range.
    I'm trying to decide on a suitable driver to cover the frequency range up to this point,and would be very grateful for your opinions.

    To clarify things a little -

    This would be a "2-way",actively driven,and equalized,system,loosely based on the Pheonix project,at Linkwitzlabs.com.

    I've been working my way through Linkwitzlabs' material,and searching the various 'speaker forums as I go,but there are a couple of things that I can't find answers to.

    eg : The Lowthers are very fast drivers,famous for their speed,in fact,but they sound thin from 200/250hz,on down - possibly even higher - even in a back-loaded horn.This is why I don't want to crossover at,for example,100hz.I hope that the woofers will add some body in this region.However,there don't seem to be many 12",or 15",units that can handle these freqencies well,without a very steep xover,which I'd like to avoid,if possible.

    Other than the Lambda's,are there any other good,reasonably priced,15" woofers that don't break up before 1 or 2 khz? I haven't been able to find any.

    Also,I can't find anything definite about the "speed" of these large units - some say there's nothing can keep up with the Lowthers,others say that a good driver can handle it easily.( some *really* expensive diy horn systems use 15" TAD drive units,which can be even more expensive than the Lowthers,so I guess it can be made to work,if you've got loadsamoney).

    How bad would it sound ? ,(ie:a woofer not keeping up).

    I imagine similar "speed-matching" problems might arise in mating a bass unit(s) to a good ribbon tweeter,though I'm not certain about this.If so,that might be a guide.Anyone confirm this? Did you find a woofer that worked well,in the end?

    I've heard good things about the Lambda Acoustics drivers,and could just about afford 2 of these,at the reduced prices.Are these really as exceptional as claimed ?

    Aother,possibly better,alternative was to use several less expensive 10" woofers per side,to handle 40 hz to 200hz approx.I thought I'd found the answer in the MCM 55 - 1555's,till I discovered that they only have an xmax of 1.5mm.I doubt this would be enough unless I had about a dozen per side.MCM have an even cheaper Aluminum 10" with an xmax of 4.5mm.Four of these per side might do,but how to tame the ringing?One thing I thought of - slitting the cone,then glueing the slits,like Scan-Speak do with their Slit-paper woofers,which are supposed to be great.But I don't know if this would work with a metal cone. Any ideas?

    sfdoddsy,

    Hi.I visited your site - and jonmarsh's - among others.You guys have built some amazing speakers.It's very encouraging to see what can be done.Steve,could you tell me the xmax of the 10" Focals you initially used in "BOB",just as a guide.You said that they only occcaissionally bottomed out,when rolled off at 40 hz,if I remember correctly.That would be fine for now.

    So,overall,anybody have any suggestions?

    Anyway,I think that's enough for now.Sorry to be asking so many questions,but I've no idea how else to find this info.

    Thanks in advance for any help you can give.

    BigSky.
  • sfdoddsy
    Senior Member
    • Sep 2000
    • 496

    #2
    The bass units of mine you mentioned are Focal 10L6411. They were a custom design for the top of the range Hales Alexandra speaker. Unfortunately, this means you cannot buy them. I got mine through eBay when QSC Audio were selling off the last of the Hales stock.

    However, they do come up on eBay every now and again. The best thing is the price - I only paid $40 each. I would imagine that be upwards of $150 if Focal still sold them.

    The spec sheet I have doesn't list xmax, but the other 10 inch 6411 Focal drivers list an 8mm xmax. Certainly they would appear to have at least that.

    I bottomed them running full range, but not when crossed over at 40Hz using a 24 L/R.

    I think I'm too dumb to answer any of your other questions, except to encourage you to go dipole.

    Cheers

    Steve




    Steve's DIY Dipoles
    Steve's OB Journey

    Comment

    • BigSky
      Junior Member
      • Mar 2003
      • 8

      #3
      Hi Steve,

      Thanks for answering.Now I know why I couldn't find the Focal driver anywhere.I couldn't afford Focals anyway.Just wanted to get a realistic guide to xmax requirements,in real life.

      That 8mm's makes the 1.5 of the MCM look even less likely.Pity.It seems great,otherwise.It's difficult to find the normally recommended drivers in the UK,at decent prices,and shipping heavy units from the US costs an arm and a leg.I can get the MCM's here,cheaply,so they seemed irresistable.

      I'll look around for advice on this,and on damping the aluminum cones.

      I think I'm too dumb to answer any of your other questions, except to encourage you to go dipole.
      Modesty,modesty. Thanks for the encouragement.It was your BOB article/project that got me back on this track,though it was Mr. Linkwitz who first got me interesded in dipoles.

      Thanks again.

      BigSky.

      Comment

      • ThomasW
        Moderator Emeritus
        • Aug 2000
        • 10934

        #4
        I'm not familar with your specific drivers but most Lowthers have a ~ +/- 1mm Xmax. That's not necessarily a good thing for dipole driver where the output is excursion not thermal limited. If you're going to be listening near field or at quite low SPLs, or use multiple drivers/side, then dipole mounting may work......

        Cone breakup issues aside, I know of no large diameter woofers that will play up to 1-2K without severe beaming. It's just one of those laws of physics. The Lambda drivers (or other drivers with the phase plugs) will have better dispersion than drivers lacking phase plugs.

        Good (=expensive) metal cones drivers (Seas Excel for example) have very good performance characteristics. Cheap metal cone drivers in general sound terrible.

        BTW, the metal in a metal cone driver is virtually a 'foil'. Trying to 'slit' it will ruin the driver. The only reason SS gets away with slitting the Rev drivers is because the cones are quite thick.

        BTW, the Phoenix woofer has a 100Hz XO point. And it's not a good idea to use a higher XO point given that the "W" baffle design creates problems above 100Hz. Use of a standard 'flat' dipole baffle is recommended if the XO point is to be higher than 100Hz.

        It should be noted that an important physical property of drivers is acceleration not speed. All cone drivers are moving at the speed of the frequencies they are playing, not faster, or slower. The ability to acelerated and decelerate quickly (aka transient response) determine 'good' drivers. High efficiency drivers have no inherent acceleratation advantage over lower efficiency ones. They simply play louder for a given input power. Note that given 'adequate' motor strength (supercharged Dodge Hemi for example ) it's possible to accelerate a manhole cover as fast as any high efficiency cone driver......




        theAudioWorx
        Klone-Audio

        IB subwoofer FAQ page


        "Complicated equipment and light reflectors and various other items of hardware are enough, to my mind, to prevent the birdie from coming out." ...... Henri Cartier-Bresson

        Comment

        • BigSky
          Junior Member
          • Mar 2003
          • 8

          #5
          Hi Thomas,

          Thanks for the answers.Very helpful.

          most Lowthers have a ~ +/- 1mm Xmax. That's not necessarily a good thing for dipole driver where the output is excursion not thermal limited
          That goes for my Lowthers,too.I forgot to mention it,but it's another reason I wanted to keep the crossover point quite high,like 200 hz,or more.

          Cone breakup issues aside, I know of no large diameter woofers that will play up to 1-2K without severe beaming. It's just one of those laws of physics
          Funnily enough,Lowthers are notorious for beaming as well,despite their small size.I've heard many complaints about it.

          I'm disappointed to hear about the poor sound of the cheap metal cones.I'd read quite a few good "reviews" of the Tangband (?) units,and some others,but have never heard them.Maybe it depends on what you compare them to,and my expectations are quite high,I guess.Too good to be true,again,it seems.

          Is the poor sound due mainly to the cone resonances?I'd suspected that this was their main problem.

          If so,what about damping them with something like small pieces of Dynamat(?).I've read that a 1" square totally dampens an aluminum coke can.Of course that would add mass,but I don't know how much.

          BTW, the metal in a metal cone driver is virtually a 'foil'. Trying to 'slit' it will ruin the driver. The only reason SS gets away with slitting the Rev drivers is because the cones are quite thick.
          That's very interesting.The main reason I was hopeful about the metal cones,was the lightness.I'd hoped that with such a light cone,even a modest magnet would be able to impart some serious accereration - you're right,this is what I meant by speed,and that is the Lowthers' most famous (good) quality.There are a few bad one's,too.

          The carbon fibre cones appealed more strongly,for similar reasons,light,rigid,with much better damping,but I think the small xmax would stop them getting down to near 40 hz on a dipole,even with four per side.

          Am I right about this,do you think?

          Use of a standard 'flat' dipole baffle is recommended if the XO point is to be higher than 100Hz.
          That's my intention.I didn't mention it,and other things,since the post was already more than long enough,like this one.Sorry,on both counts.

          it's possible to accelerate a manhole cover as fast as any high efficiency cone driver......
          Finally,I've heard of adding magnets to a woofer to beef up it's own magnet.Ever tried this? Would that help with the acceleration issue?I could get some surplus neodynium magnets if it'd be worthwhile.

          Thanks for being so helpful already.The reason I'm trying so hard not to make a mistake is that my budget is so limited,and if I make a mistake it'll be a *long* time till I can try again,so I really appreciate all the help I can get.

          One serious drawback with dipoles appears to be that you can't really just get a couple of drivers to try out,due to the xmax-related issues,and it's quite costly to buy eight,even modestly priced units,only to find that they might be fine in a box,but are useless for dipoles.

          I'm really relying on the reduced acoustic energy output,for domestic reasons,so I can't just plump them in a box if they don't work out.

          So,thanks again.Best Wishes.

          BigSky.

          Comment

          • sfdoddsy
            Senior Member
            • Sep 2000
            • 496

            #6
            Bigsky,

            Thanks for the flattery. That'll get a response guaranteed.

            I'll leave Thomas or Jon to give technical stuff, but I'd also have to say the Lowther doesn't sound an ideal dipole driver.

            As an example, the Seas L22 8 inch driver (7mm xmax) would appear to produce more output alone than the Lowther. A few of these per side might be good. I have some L21s and they are good if you cross them over below 1K.

            I should also note here that while I appreciate using good (expensive drivers), my playing around seems to indicate that the biggest improvement is using an open baffle in the first place, with the good drivers then being incremental.

            But since open baffle is cheap, just build a test baffle and check it out.


            Cheers

            Steve




            Steve's DIY Dipoles
            Steve's OB Journey

            Comment

            • BigSky
              Junior Member
              • Mar 2003
              • 8

              #7
              Hi Steve,

              No flattery,just the truth.Just think how impressive your list of commercial speakers is,and you put together a set of dipoles,with speakers you just happened to have lying around,that sounded better.That takes some beating as an impressive prototype.

              Thanks for the tips on drivers etc. Regarding the lowthers,I'd really like to use them,for several reasons,if it's at all possible.

              eg : Most importantly,I already have them,and couldn't even think about getting anything else that touches them,in terms of their strengths.If I try to sell them I'll take a big loss,and will then have to buy mids and tweeters that aren't very great.
              However,maybe I'll have no choice.

              Could anybody tell me what would the lowest reasonable xover point be for the Lowthers,bearing in mind their 1mm xmax,and my taste in music?Few examples follow -

              I'd like to be able to listen at reasonable levels - ie:loud enough to have to shout a little,but nowhere near club/disco levels - to the likes of Peter Gabriel,Fleetwood Mac,Chris Rea,Kate Bush,Springsteen etc.,as well as gentler stuff,and quite a bit of classical,esp. piano,choral and so on.

              If there's no chance of this with the Lowthers in dipoles,I'll have to try selling them,but I really hope it doesn't come to that.

              If I could get my hands on some decent,but not too pricey, 10"drivers I could build a baffle and experiment,but I can't really do much till I make a choice/guess.(I really don't like poly cones).

              I'll be using chip-amps,based on LM1875/3875's and 3886's.

              Thanks for all the help so far.

              Comment

              • ThomasW
                Moderator Emeritus
                • Aug 2000
                • 10934

                #8
                Adding magnets to existing drivers isn't a good way to increase the BL product.

                If you really want a high efficiency system using MCM drivers seriously consider the line array Jon's designing. The only drawback is that the B&G RD-50's from Parts express won't be cheap.

                There have been some QC issues with TB drivers.

                Other issues with any metal driver is the peaking from cone breakup. Use of low order XO slopes isn't advised, because one should get out of the driver long before the peaks can impact the FR

                Metal cone drivers are extremely light/stiff and have excellent acceleration.

                Without seeing a real world FR plot of the Lowthers it's difficult to comment. My personal feeling is that horn loading is the best choice for drivers like the Lowthers. Note that it's a good idea to have 4X the radiating area for a dipole as compared to a conventional enclosure. This is especially true for dipole woofer/subwoofers.

                I'll be using chip-amps,based on LM1875/3875's and 3886's.
                Oh no, say this isn't true




                theAudioWorx
                Klone-Audio

                IB subwoofer FAQ page


                "Complicated equipment and light reflectors and various other items of hardware are enough, to my mind, to prevent the birdie from coming out." ...... Henri Cartier-Bresson

                Comment

                • Hank
                  Super Senior Member
                  • Jul 2002
                  • 1345

                  #9
                  Oh no, say this isn't true
                  You're bad, Thomas :P

                  Start a collection so we can buy him a real amp and he won't have to use "chip" amps

                  Comment

                  • BigSky
                    Junior Member
                    • Mar 2003
                    • 8

                    #10
                    Hi,

                    I sense an in joke about the amps.I haven't been around long enough to know if you rate them,or not,though.I guess you probably do,since Linkwitzlabs recommends them.

                    Anyway,thanks for the responses.

                    The only drawback is that the B&G RD-50's from Parts express won't be cheap
                    I'll have a look,but I think it'll be beyond my budget.

                    Regarding dipoles,in general,

                    Does the reduction in acoustic energy put into the room,as compared to other speakers - only 1/3 of the energy I believe - lead to loudish music,especially bass,barely being noticed in adjoining rooms.I hate having sounds booming through the house when I listening in my room,so most of my listening tends to be with headphones at the moment.

                    I remember reading something to this effect,several years ago,in a review of Audio Artistry speakers.It's an important issue,to me,due to illness in the family,but I've never been able to confirm it.

                    I'd be very grateful to anyone who could address this point from experience.

                    I wonder,do digital eq's/room correction help in this regard?

                    Thanks again,everyone.

                    BigSky.

                    Comment

                    • sfdoddsy
                      Senior Member
                      • Sep 2000
                      • 496

                      #11
                      This is one of the reasons I started relooking at dipoles. I live in an apartment now and was getting complaints from Philistinic neighbours.

                      Dipoles appear to excite the room less for a given loudness at my listening position.

                      EQ also helps since the most annoying sound leakage point is usually the highest room peak. However, EQ may make things worse since it really only applies to the listening position.

                      Steve




                      Steve's DIY Dipoles
                      Steve's OB Journey

                      Comment

                      • BigSky
                        Junior Member
                        • Mar 2003
                        • 8

                        #12
                        Dipoles appear to excite the room less for a given loudness at my listening position.
                        What's it like in an adjacent,or downstairs,room when your system's playing? I'm not worried about disturbing neighbours.It's family members in the downstairs rooms.(My system's in an upstairs room).

                        Thanks again for the answers.

                        BigSky.

                        Comment

                        • JonMarsh
                          Mad Max Moderator
                          • Aug 2000
                          • 15260

                          #13
                          Low frequencies will be attenuated anywhere to the sides, top, and bottom, so this should help you out with your situation. Behind the speaker, response will be just as strong as in the front, so be careful where you point the back of the speakers! :W

                          -Jon




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                          Comment

                          • BigSky
                            Junior Member
                            • Mar 2003
                            • 8

                            #14
                            Jon,

                            Thanks.Reading through some more of the stuff on Linkwitzlabs I came across some more conversation from Mr. L about this subject that I'd missed or forgotten about.So,everyone seems to agree that the dipoles generate a lot less troublesome sound.Excellent.

                            I may sell the Lowthers after all,and go for more suitable,if less ambitious drive units.

                            Cheers,

                            Bigsky.

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