gedlee said:
Can you name a few specific reasons? Isn't it the ideal to have no energy transfer from the driver to the cabinet?
ezkcdude said:
Ideal, but impossible. Isolation works only if the suspension is soft, like a car. If the suspension is soft then the frame moves back and forth lessening the actual cone displacement. While this in and of itself may not be a problem it does this in a very frequency dependent way that adds a whole new source of resonances, at very low frequencies. Hence, in your attempt to issolate the driver from the cabinet in order to not make it vibrate, you have created more resonances than you have avoided. The bottom line here is that the driver must be rigidly mounted and the cabinet has to be made to take up the energy source. Damping is by far and away the best way to do this. The lighter the enclosure material the better the damping employed works. Heavy enclosure materials all but defeat any damping treatment because the mass stores so much energy. Light and well damped is the way to do this. Been there and done that.
gedlee said:
Precisely! That is why two drivers mounted back-to-back - and rigidly coupled - is the answer to this longstanding problem! (At least, for the woofer.) The reaction forces cancel, thus eliminating the driver movement, and enabling a "soft" coupling between the driver frame and cabinet. Thoughts?
ezkcdude said:
Twice as many drivers and poor directional characteristics. It works as you say, but the resulting system now has more problems that the one you solved, which was not that big of a problem to begin with IMO. Enclosure radiation is just not that big a deal and its easy enough to make a well damped cabinent that doesn't exhibite any significant issues. But you can't do anything to the system that sacrifices directivity control - thats everything, or at least the single biggest problem. Solve that first and then nibble away at the remaining problem. You'll find that cabinet vibration and driver coupling are far enough down the list that you won't actually get arround to them.
ezkcdude said:
I'm afraid that this is not the case in any kind of sealed or semi closed box design. For Open back or Open baffle designs it may be more true but in the other designs the air coupled mass vibrations can be rather significant and excite the panels just as the driver frame can.
So we have this additional problem along with the one discussed by Gedlee.
There are a couple of manufacturers who have used your idea of back to back mounted drivers with success in the very low frequencies. But as far as I know the drivers are still rigidly mounted to the cabinet. As well as each other.
Few said:
Was this cabinet compared to another that was eqaully stiff and well damped but had less mass.
Once a driver is tightly coupled to the cabinet the vibrations produced by the driver need to be absrbed by the cabinet. This kinetic energy is best dissapated if it converted into another forum of energy (like heat). One way to do this is to loosely fill dry sand, silica, lead shot etc into voids in the cabinet walls or into the bracing.
ezkcdude said:
THe problem with trying to isolate the driver is that the softer (better) isolating materials are not strong enough so the optionsis to only partially isolate by using stiff sponge rubber rings (shore hardness 60+).
In the end it will be a mix of technologies (siff FRP/Carbon fiber cabient walls) with energy conversion fillings and some degree of isolation which will help most. A sort of "a bit of this and a bit that" approach.
gedlee said:
push-push drivers work best at bass frequencies so it may also be that one needs different approaches for differnt frequency ranges.
For example a push-push bass with a stiff, damped, heaviily braced cabinet mated to a widerange/midrange in a lighter (but equally stiff) tear drop shaped cabinet maed to a tweeter that has no cabinet.
I don't see how I'm mixing problems. Whether the sound is coupled from the driver, to the air inside the speaker enclosure, and then to the enclosure walls, or instead directly coupled from the driver to the enclosure walls, there's a source of vibrational energy that's coupled to the enclosure walls. Even if the walls are intended to reduce sound transmission from inside the enclosure, the walls have to vibrate in order for sound to be heard outside the enclosure. So in the two cases you described it still comes down to the walls' resistance to vibrational excitation. Are you arguing that heavy walls are more resistant to vibration when the vibrations are coupled by air, and light walls are more resistant when the vibrations are directly coupled from the driver?
Few said:
Admittedly, its more complex than I alluded to, but yes, for a free standing partion, mass is more effective at reducing transmission than stiffness. We say that a partition is stiffness controlled at LF and mass controlled at HF. But we all know that there is more LF transmission through a wall than HF, hence, mass is the dominate factor.
When the walls are in an enclosure it is a mixed problem and both actions are at work. But I think that the bigger point is that the greater the mass the less effective any damping will be, and its damping in an enclosure that is the most effective IMO. This means that a light stiff enclosure will damp the vibrations better than a heavy one. The mass acts as an energy storage device maintaining the vibrations once they get moving.
I have built the exact same enclosure with light weight composites; very stiff; very heavy corian type of massive walls; and even contrained layer damping. For all practical purposes they all worked the same. There simply was no measureable or audible difference. The conclusion that I came to was that cabinet vibration, once brought down to a reasonable level, was simply not an important factor is the sound quality.
gedlee said:
The last clause does not make sense as written (does not follow).
Ken
no disagreement with the general point though
kstrain said:
The transmission loss rises with frequency, does that make more sense? Hence, its the mass that matter most and a limp massive partion works best. Stiffness is only a factor at LF and then its most not a good thing. Remember this is loss that we are talking in so the SPL difference drops.
gedlee said:
Do you have any documentation on the carbon fiber box you built. Did you use a layup form and what did you use as a release agent?
FRP (in India) is not all that more expensive than MDF given that 6mm of FRP = 12mm of MDF in stiffness and the FRp is so much lighter (ater all we pay for the resin in weight and fiberglass mat is quite cheap). Both MDF and FRP can be messy.
navin said:
No documentaion, just experiments.
Of course there was a form and we simply used the classic wax release agent.
PVA PVA PVA!
I'm going to say it again for anyone who builds a mold or plug (form) and tries to get the fiberglass/epoxy/carbon/whatever off it:
PVA!!
Polyvinyl Alcohol is your friend!
I'm going to say it again for anyone who builds a mold or plug (form) and tries to get the fiberglass/epoxy/carbon/whatever off it:
PVA!!
Polyvinyl Alcohol is your friend!
gedlee said:
http://www.tapplastics.com/shop/product.php?pid=67
(I am getting ready to order some myself and the link was handy)
I haven't tried it....
but Marten Design uses monocoque construction in their Coltrane line of loudspeakers. I'm sure once the prototyping is over and the design kinks are worked out it is an easy way to build a cabinet....but getting there must be quite a journey.
Chris
but Marten Design uses monocoque construction in their Coltrane line of loudspeakers. I'm sure once the prototyping is over and the design kinks are worked out it is an easy way to build a cabinet....but getting there must be quite a journey.
Chris
gedlee said:
I've used the Tap Plastics version. It comes just as a liquid. I'm sure you can get a spray bottle and use it that way or you can use it in your spray gun (it's water soluble). Some places also sell misters.
The mistake I made once was using a "spray can" of mold release that wasn't PVA based. It might as well have been glue. I think it may have been silicone based.
If you type "PVA mold relase" or "PVA relase film" into google, you should have no trouble finding it.
Some sources:
www.tapplastics.com
www.aircraftspruce.com
www.fiberglast.com
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