Assuming that holding the basket mounting surface in a fixed position relative to the magnet position is a good thing, and further that the primary mode of magnet mass resonance against the basket involves an elongation of the basket in the Z-axis, if your brace is to act upon this resonance, redirected energy should be directed back along this primary mode or it may promote other basket resonances or higher modes of the first.
Ordinarily the magnet is dangling which may not be ideal in this regard, or significant for that matter but could also be the point of this excercise, or a point at least.
Stressing the back panel is going to modify the resonance, probably for the good. Notwithstanding this, the back panel mass could inadvertently be dragged into the magnet and basket resonance.
You suggest using a brace in your next design that could disconnect the panel resonance from the magnet, considering that it goes into the cabinet corners for example, maybe thereby acting as a grounding point, possibly softly but consistently mounted with regard to higher frequencies, and where the front panel could be referenced to this point as well, seems to have merit.
Ok, another (related) idea. There's doubt about whether this kind or degree of cabinet bracing is even needed, but the data I've seen about box vibration and audibility doesnt seem all that convincing in either direction. Seems like it always ended up as someones opinion, but no easy way to make my own educated opinion.
So, how about this: put an acceleroneter on the back of a woofer along with one of those transducers intended to make windows and blackboards act like speakers. Measure the response from voice coil to accelerometer. and then eq the response of the addex transducer to get the same response from it to accelerometer. Then drive the transducer with music (it vibrates the box) and compare the sound to driving the woofer (vibrates box and cone), see if the levels are loud enough to maybe matter.
Granted that this doesn't take masking into account, but if the level is low enough that wouldn't matter. It also doesn't take into account vibration of the air inside the box vibrating the box itself from the cone, but according to Geddes the mechanically coupled vibration would be more a concern than that from that coming throigh air. Worth setting up?
The idea being that if the level from mechanical coupled vibration seemsd possible to be an audible effect, then the setup could be used to see whether box damping/clamping/bracing/stiffening could compare in controlling it.
Id have to get another acceleroneter, of course.
So, how about this: put an acceleroneter on the back of a woofer along with one of those transducers intended to make windows and blackboards act like speakers. Measure the response from voice coil to accelerometer. and then eq the response of the addex transducer to get the same response from it to accelerometer. Then drive the transducer with music (it vibrates the box) and compare the sound to driving the woofer (vibrates box and cone), see if the levels are loud enough to maybe matter.
Granted that this doesn't take masking into account, but if the level is low enough that wouldn't matter. It also doesn't take into account vibration of the air inside the box vibrating the box itself from the cone, but according to Geddes the mechanically coupled vibration would be more a concern than that from that coming throigh air. Worth setting up?
The idea being that if the level from mechanical coupled vibration seemsd possible to be an audible effect, then the setup could be used to see whether box damping/clamping/bracing/stiffening could compare in controlling it.
Id have to get another acceleroneter, of course.
I'd rather use butyl in this day and age.
Those asphalt plates smell horribly. And they do not age well. Getting brittle and stiff over the years.
Most common CLD tiles for automotive use, damping door panels etc. has a butyl based layer. This used to be asphalt, some of them still use asphalt. Butyl stays more flexible long term.
It's also sold as butyl rope and/or tape, lots of automotive use for sealing various light assemblies and fender to car body seals etc.
It's very sticky and stays flexible. Excellent replacement for asphalt based damping panels. I've used both of the above mentioned butyl products in my line array build for different purposes.
Though Sorbothane might even be better for this particular job.
Attachments
Last edited:
I'm a bit unclear as to exactly what you mean here, Dave (?) I'm thinking it's a great idea to tie the magnet assembly to the back wall of said enclosure, and then mass load the very same back wall. Agree ?
(Enclosure supported on large SPIKES as well)
{{unless you are going to do reaction-forced driver mounting, but I do believe that is beyond the scope of this topic}}
Butylrubber is good stuff for damping and has low rebound. Those products shown, with a single adhesive layer, are effective when added to a surface of roughly the same thickness. In a constrained layer damping (CLD) it needs adhesive on both sides and can be "thin". As added material, on top of, say 3/4" MDF, they are much too thin though (= not CLD and is not as effective).
To lessen vibrations transmitted from the basket/magnet, I used machine screws (not overtigthened) in slighly oversized cabinet holes with viton o-rings + washer and nut on the inside of the cabinet and a large viton o-ring between the basket and MDF. Viton rubber has very low rebound compared to most rubber materials, meaning vibrations are to a larger degree converted into heat instead of been given back to the system (the cabinet) after a short time interval. The magnet itself rests in a kind of "cradle" with a viton strip under the magnet. A good NASA report on damping qualities for various kinds of rubber materials, page 160 and onwards: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810017924.pdf (Solid silicone, often mentioned on forums, is not a top notch material)
To lessen vibrations transmitted from the basket/magnet, I used machine screws (not overtigthened) in slighly oversized cabinet holes with viton o-rings + washer and nut on the inside of the cabinet and a large viton o-ring between the basket and MDF. Viton rubber has very low rebound compared to most rubber materials, meaning vibrations are to a larger degree converted into heat instead of been given back to the system (the cabinet) after a short time interval. The magnet itself rests in a kind of "cradle" with a viton strip under the magnet. A good NASA report on damping qualities for various kinds of rubber materials, page 160 and onwards: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810017924.pdf (Solid silicone, often mentioned on forums, is not a top notch material)
My concern with using a C-clamp would be that the tolerance of the screw to thread is often not very tight, and if it loosens, it may actually end up rattling. If cranked tightly, you may deform your driver basket. I would think a machine screw with a few drops of LOCKTITE would work better. If you would like to try rubber instead of felt, ACE Hardware usually stocks small amounts of sheet rubber. I have purchased small amounts at Parkrose Hardware on Sandy Blvd in PDX.
To CLD the box itself, Green Glue squished between two layers of plywood (/your choice of box building material) should work well. I imagine this is what Geddes uses, or a similar product. Green Glue stays viscous over time. Beware that some of competing products will actually run over time. Could be a big mess in the box.
My understanding of CLD is that it always contains a viscous or soft material between two rigid layers. If that definition is correct, does standard dynamat actually qualify as CLD? I wouldn't think so.
To CLD the box itself, Green Glue squished between two layers of plywood (/your choice of box building material) should work well. I imagine this is what Geddes uses, or a similar product. Green Glue stays viscous over time. Beware that some of competing products will actually run over time. Could be a big mess in the box.
My understanding of CLD is that it always contains a viscous or soft material between two rigid layers. If that definition is correct, does standard dynamat actually qualify as CLD? I wouldn't think so.
The idea of the C-clamp was because the end of the screw has a ball and socket that wouldn't require that whatever it contacts rotate with the screw. But, yea it might go loose. I came on a more convincing scientific reason, though, and one completely conforming with theory: the metal of the "C" part looks like a right booger to cut off and I'm demonstrably lazy about that kind of thing.
I believe that at one time Geddes was using melamine glue (Grainger carries it) as the viscous layer for CLD, though a thread linked to in an earlier post here seemed to indicate that he by then didn't think much of anything other than some cross-struts in a box was necessary. I think at one time (before that?) he switched from melamine glue to some other substance, but don't remember what it was, might have been something proprietary.
I believe that at one time Geddes was using melamine glue (Grainger carries it) as the viscous layer for CLD, though a thread linked to in an earlier post here seemed to indicate that he by then didn't think much of anything other than some cross-struts in a box was necessary. I think at one time (before that?) he switched from melamine glue to some other substance, but don't remember what it was, might have been something proprietary.
Depending on the material you use, I am not sure friction should present much of a problem. You will not be able to crank too hard on the material without threatening the driver basket, unless the material is very soft like silicone caulk. If using rubber or a similar material, a dab of liquid dish soap will relieve the friction.
Last edited:
If the magnet, ie motor assembly is seen as the 'centre of the universe', ideally the cabinet should move with it (or vice-versa depending on your point of view). Would this be best done by grabbing the back panel in the middle or creating a more overall encapsulating brace to work from? I'm not saying that either wouldn't work, there are too many pros and cons to make a blanket determination. Damping is usually good thing, and IMO simplifying the nature of the modes.
IIRC it was two part polyurethane mixed with glass microspheres.
The breathing mode of the cabinet is significant for a number of reasons. It has the greatest movement magnitude, it is monopole in phase, and it is excited by a wide band and has the potential to resonate as a 'one note bass'.
For it's purpose, when stuck to sheet metal, Dynamat is an excellent CLD.
Stick it to a thicker piece of wood? It probably doesn't count as CLD anymore. And if you use it to add weight to the walls mass loaded vinyl would be a better option.
I merely tried to explain what the butyl product was. Hence the name Dynamat came up, as a cushion between the clamp and speaker butyl tape would be a viable option compared to using an asphalt based product.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.