• Re: Yes, I think that concept's full of holes ... :-)) - lne937s 15:04:14 04/26/07 (6)
  In Reply to: Yes, I think that concept's full of holes ... :-)) posted by andyr on April 25, 2007 at 20:55:36:

  I think there are a couple of things at play here.

  One is rigidity. The more rigid a structure is, the less it will vibrate. When it does vibrate, more rigid structures will vibrate at a higher frequency. If this frequency is within the audible spectrum and pronounced, a more rigid structure may not necessarily sound better. There are two main ways to increase the rigidity of a solid flat structure: use a more rigid material or increase thickness- getting exponentially more rigid the thicker you get. For example, steal is stronger than aluminum, but aluminum's lower density allows it be used at much thicker gauges per unit of weight, allowing the structure to be lighter and more rigid when made from aluminum. If you plied on an additional layer of MDF, it would become more rigid. However, on a flat structure, the majority of the strength is on the outside surfaces. By reinforcing the outside surface, you can make something much stronger than the underlying core (e.g., carbon fiber with an aluminum honeycomb core). By applying a fiberglass and resin skin (or, if you have the $$$ Kevlar/carbon fiber) - the strength and rigidity of the panel will increase greatly with minimal impact on thickness or weight.

  The next is mass. The more mass that needs to be moved by the vibration, the more energy is required. The heavier a rigid panel is, the less it will noticeably vibrate. You could add mass to the panel (lead, etc).

  In terms of dampening, vibrations tend to travel along directional structures (strings of a guitar, grain of wood, etc). Typically, materials with a random structure (felt, asphalt dampening pads, etc.) dampen vibrations better- (the sound travel a short distance along any directional fibers and then hits a dead end, transfer of energy has to occur to reach the next directional segment of a different length, which converts a portion of that energy into heat, repeating itself over and over again within an inch of material). MDF, having a fairly random fiber structure, is actually fairly well damped. Is MDF the most dense or rigid material... no. Do the resonances in MDF that do exist sound "natural" or pleasing... maybe not- probably depends on the application. Beyond the material itself, by adding dampening material to the surface (felt, foam, cork, etc.) the vibrations of the structure will be absorbed, along with vibrations transmitted from the surrounding air.

  Overall, a mineral filled resin frame (like a thick Corian countertop or preferably one of the other brands that offer more mineral content) wrapped with a structural skin and then a dampening material may offer the stiffest, heaviest, and most damped frame. However...

  That does nothing about resonance in the driver- that may also need to be stiffened and dampened to eliminate any resonance...

  And then you get into personal theories. If the frame is too rigid, some people believe that no vibrations will be transferred and, therefore, no vibrations to be absorbed by the frame and there will be more audible resonance overall. Some people also believe that a rigidly coupled driver will sound better, some a decoupled driver. Some people also take the "musical instrument" perspective and believe that resonant frames/panels are actually a good thing- adding pleasing coloration, like the warmth of a tube amp....

  Making a more rigid/damped/heavier frame is relatively easy- the more subjective "make it sound better" question is the difficult one.

  
  
  
  

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  • Re: Yes, I think that concept's full of holes ... :-)) - andyr 05:49:43 04/27/07 (3)
    In Reply to: Re: Yes, I think that concept's full of holes ... :-)) posted by lne937s on April 26, 2007 at 15:04:14:

    Hi lne937s,

    Great post ... thanks! :-))

    I'm now at the stage with my IIIas that "improving" the frame is about all that's left to me! :-)) So I was very interested in your comments about vibration and how to deal with it.

    However, I think that the key point wrt vibration in Maggie frames is not whether this vibration is causing the frame to emit noise of some frequency (like vibrating box speaker sides can) but, rather, does the Maggie frame material help to damp the vibrations which are occurring in the panel driver assembly (and generated by the mylar flapping)?

    As you probably know, the IIIa (and its successors) have a bass/mid driver assembly which sits in a big rectangular rebated hole in the MDF frame and then a long ribbon "cage" which sits in a ribbon slot, between the mid side of the panel and the edge of the frame. Consequently, there is a thin strip of MDF between the ribbon "cage" and the bass/mid driver assembly.

    Now, re. MDF ... are you sure that MDF has "a fairly random fibre structure" and so is actually quite well damped? I thought MDF was mostly sawdust in a glue matrix ... so something made of particles/'chunks' of wood (like flooring-grade particle board) would be a much more damped material, due to the sizeable chunks of wood in the glue matrix??

    Or am I confused here?

    I suspect there are slightly different problems to be solved in a 2-way Maggie (which consists of a single sheet of mylar with two separate drivers on it, surrounded by the frame) and a 3-way Maggie which has 2 physically separate drivers and a narrow strip of frame between the bass/mid driver assembly and the ribbon "cage".

    In a 3-way Maggie, the important thing IMO is to reduce the vibrational effect which the bass/mid driver has on the ribbon "cage".

    The inside edge of bass/mid driver assembly vibrates ... this causes the thin strip of frame to vibrate and this causes the ribbon cage to vibrate. Which is not good ... if I sit and listen in the almost-dark, I can see the ribbon "shimmer" when loud bass transients occur.

    In other words, it's not the "sound" which this vibrating MDF frame makes which is important (you commented about whether the MDF vibrations "sounded" natural?) ... the issue is, is the frame transferring vibrations from one driver to another!!?? :-))

    The only way I can see to minimise this transfer of vibration from the bass/mid driver assembly to the ribbon cage is to make a long vertical sawcut along the inside edge of the inner ribbon cage flange ... to physically separate most of the length of MDF which supports the ribbon cage, from the MDF which surrounds the bass/mid driver assembly. This IMO *must* be more effective than changing the frame material?

    Do you agree ... or again, am I confused? :-))

    However, wrt the frame material, can you comment on what is likely to be the more vibrationally inert material?

    1. MDF vs. flooring-grade chipboard (aka "particle board") ... due to the sizeable particles of wood which are in the flooring, compared to the "dust" in MDF?

    2. MDF (or flooring-grade chipboard ... whichever is the one you specify in 1. above) with a CF skin bonded either side vs. the same core but having CF on one side with lead sheet on the other side? This adds weight but sacrifies stiffness!

    Regards,

    Andy
    
    
    
    

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    • Re: Yes, I think that concept's full of holes ... :-)) - lne937s 12:14:27 04/27/07 (2)
      In Reply to: Re: Yes, I think that concept's full of holes ... :-)) posted by andyr on April 27, 2007 at 05:49:43:

      I just want to put forth a disclaimer and say that a lot of this is just based on physics and is purely theoretical. And even if you make the stiffest/most dense/most damped frame, there is no guarantee that it will subjectively improve the sound.

      Regarding MDF, there is a degree of relativity in the statements about dampening. MDF (which is essentially a thick sheet of paper) is a bunch of wood fibers pressed together with no specific order. The random fiber structure applies in comparison with solid wood or plywood- where fibers are aligned in a linear fashion (grain of the wood). However, hardwood is stronger/stiffer, so there is a trade-off. The comparison would be similar between woven fabric and felt. Either way, the thicker you make the material, the less resonant, stronger, and heavier it will be.

      Is MDF the most damped material? No. Many other readily available materials have much better dampening (like the asphalt-based fiber boards that are often used behind drywall to dampen vibrations). However, they also tend to be less rigid and less dense. MDF is often seen in speaker construction because of a combination of dampening, uniformity, mass, stiffness... and price, of course. It is a compromise material. Depending on the application, and whether mass, stiffness, or dampening is needed, other materials may work better. Chipboard (I assume you mean particle board and not OSB) would probably dampen vibrations pretty well, but it is not very strong/rigid- you would need a very thick sheet.

      Perhaps the best approach would be to laminate on different layers, with the stronger ones on the outside (where they make the most difference to the overall stiffness) and the more dead layers in the middle (aka: constrained layer dampening). Any time you transfer energy from one substance to another, there are mechanical losses- the more transfers, the more absorption of vibration. You might consider a lower density particle board, asphalt board, rigid foam, or some other substance that does a great job of dampening vibrations but is not strong enough to make an entire panel from. Also, using multiple layers of different structure/thickness will fight the resonant frequencies of each individual layer... Then I would add another layer to the very outside surface of some even less dense dampening/absorption material (eg, foam, felt- I've been fascinated cork), which would not only further dampen the panel, but also minimize sound bouncing/diffracting off the surface.

      Now, Carbon Fiber would make it stronger than just about anything else, per unit of weight... but it is by no means necessary and the directional structure of woven fabric would not be the most acoustically damped. Fiberglass mat would probable do the best job here for the money to add skin strength... but carbon fiber looks really neat.

      In terms of mass-- If starting from scratch and using a thick enough frame, I do not think the lead would be necessary. If you wanted to add something to an existing frame- mass would be something good to add (along with stiffening and dampening).

      For the 3 series- I think this would be an ideal time to decouple the ribbon from the woofer panel, as you suggested. No matter how dead, heavy and stiff the frame, it will still transmit vibrations. Some high-end dynamic speakers have decoupled drivers that sit on top of each other... Magnepans (being line sources) could have two completely separate frames that do not need to touch- just be side by side. That would also allow you to make frames specific to each application- The ribbon would need to be well damped for high frequencies, but would not need the mass or bracing of the planar magnetic's frame.

      Anyway those are some thoughts...
      
      
      

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      • Re: Yes, I think that concept's full of holes ... :-)) - andyr 02:19:11 05/09/07 (0)
        In Reply to: Re: Yes, I think that concept's full of holes ... :-)) posted by lne937s on April 27, 2007 at 12:14:27:

        Hi lne937s,

        Tried to PM you but it bounced ... have you specified your email address correctly on MUG?

        Thanks for your input re. good materials for Maggie frames. I wanted to follow through on a couple of ideas from your second post ...

        Firstly MDF vs. what I called "particle board". What I meant by the latter was the sheet flooring which builders use ... which seems to me to be strong and of similar rigidity to MDF? From a theoretical point of view, it seemed to me that the chips of "real" wood in this material should act to damp vibrations better than the wood fibres (or sawdust?) in MDF?

        However it doesn't finish well compared to MDF - which I suggest is the main reason why it's not used for speaker cabinets.

        I believe you can get stuff called "HDF" - which is more dense ... this means it is heavier than MDF but would it be stiffer? If so, then this sounds like it should also be a better material than MDF?

        I'm guess I'm confused here between what seem to me to be two different concepts:
        * making something thick/heavy/stiff so that it becomes less able to be vibrationally excited in the first place, and
        * making something "internally gooey" so that it can absorb any vibration you apply to it! :-))

        Also, as I understand it, a CLD construction is implemented for shelves to attenuate vibrations applied to, say, the underside of the the shelf from travelling through the shelf to excite a component resting on the top surface. IE. CLD is good for minimising the transmission of vibrations through the thickness of a shelf.

        However, with Maggie frames - particularly for 3-way Maggies - what we want is to minimise the *sideways* transmission of vibrations ... not the front-to-back transmission!

        For instance, if you tap the top a CLD shelf, the vibration will be
        attenuated by the time it gets to the bottom surface. However, if you
        tapped one *end* of a CLD shelf, would you be able to feel the vibrations at the other end of the shelf?

        IE. does the vibration attenuation caused by CLD construction work parallel with the layers as much as it does across the layers?

        By all means respond directly to my email address:
        redwood dot andrew at gmail dot com,
        rather than using a MUG PM.

        Regards,

        Andy
        
        
        
        

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      • Re: Yes, I think that concept's full of holes ... :-)) - Dawnrazor 17:17:46 04/27/07 (0)
        In Reply to: Re: Yes, I think that concept's full of holes ... :-)) posted by lne937s on April 27, 2007 at 12:14:27:

        I am planning on building some new frames for my MMGs, and was considering a mixed materials approach, planning on making a 3 layer sandwich around the driver.

        This discussion has further solidified the route I was taking, except that I will be using none of the materials that were mentioned.

        It sounds like PG attaches his frames by drilling through the metal frame of the driver. How much reduction in vibration does just doing that contribute when compared to the staple approach Magnepan uses on the 2 way models like the mmgs. Or said another way, is that necessary?

        Thanks for the post.
        
        

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  • Thanks... - Jim Treanor 16:00:00 04/26/07 (1)
    In Reply to: Re: Yes, I think that concept's full of holes ... :-)) posted by lne937s on April 26, 2007 at 15:04:14:

    for elucidating and illuminating the factors in play, including how the outside surface might be reinforced. Very helpful.

    I've often wondered how a Maggie would sound if its frame were made of a composite similar to what Mitch Kotter came up for turntables back in the late 70's. Of course it would too heavy to move around, given the size of the panel.

    In re-doing my MG-2A some years ago, I decoupled the driver (or at least modified the coupling characteristics) to some extent by applying a thin layer of silicon caulk where the driver joined the mounting panel...with, to my ears, good results.

    Thanks again. You've given me some very useful food for thought.

    
    Jim
    http://www.geocities.com/jimtranr/index.html
    
    

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    • Also, take a look at Gilmore - lne937s 16:39:18 04/26/07 (0)
      In Reply to: Thanks... posted by Jim Treanor on April 26, 2007 at 16:00:00:

      http://www.glacieraudio.com/new Glacier Audio Site/Products folder/Gilmore/Gilmoreaudio1.htm

      For a composite frame open-baffle speaker, look at Gilmore from Glacier Audio- I believe they were in a Stereophile article some time ago. They are made out of Corian (the Dupont countertop material)
      
      

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• Thanks, Andy. I have the shop and tools...and maybe the skills-- - Jim Treanor 09:37:47 04/26/07 (0)
  In Reply to: Yes, I think that concept's full of holes ... :-)) posted by andyr on April 25, 2007 at 20:55:36:

  I built new frames for my MG-2A's back in the 90's--but what I'm trying to determine is whether I can reasonably reduce the stock frame's vibration by altering its vibrational mode(s) via the Swiss-cheese filler method I outlined in my original post...either with hardwood or some other solid dowel material. So what I'm asking is, am I on the right track physics-wise?

  My thinking is that if the concept's workable, I'll integrate it with a wide-stance strutless clamp-stand design I can implement with materials on hand (wood again--no metalworking skills here)--and at minimal cost.

  
  Jim
  http://www.geocities.com/jimtranr/index.html
  
  

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