Hi Cats Squirrel some of this is above my head. My dilemna is i have the opportunity to use this mahogany (quite cheap source - budget is forever a massive factor in my case) . There is enough to double or even treble the thickness (40-120mm) of the plinth. Supposing i am using this material, in your opinion ar there any other building techniques (i'm thinking sandwiching with something else perhaps) that you can advise on.
cats squirrel wrote:
mouse wrote:Hi peeps
i have to decide on a material for a plinth for my 401. I have access to a lovely piece of mahogany 40mm thick however it is only 14" deep so it will mean sticking two pieces together - my joiner ensures me he uses resin rather than a standard wood glue so to all intents and purposes would be as solid as a 1 piece block. any thoughts out there? Also is mahogany a good option (sapele - i think is the spelling)?
have a look here, hopefully you will be able to sort the facts from the beliefs.http://www.vinylengine.com/phpBB2/viewt ... sc&start=0
sorry for the tardy reply, but I'm on holiday, and can't always get to a computer. If you have decided on mahogany, then I can suggest how to make the best of it. And contrary to JD's beliefs, it is all about science, physics, material science and acoustics (yep, another science!).
As mentioned before, the plinth can be thought of as having three frequency regions, controlled by stiffness, mass and damping.
As you have chosen mahogany, lets have a look at it. It isn't stiff, but it may be stiff enough. The mass depends on its dimensions and density, so having decided on its breadth and width (as near in ratio to 1:1.61 for best results), its mass then depends on its thickness. It will vibrate most at certain frequencies (if excited) depending on the things I've mentioned before. These are the modal frequencies, or resonances. Now comes the down side.
Mahogany has a damping factor around 0.04, not a damping material, but about the same as plywood, and double that of mdf. As, at resonance, damping factor is 1/Q, the resonance peaks will have a Q of about 25, not a good thing. Only damping will bring these down. Then there is the critical frequency, which, once you have decided on the material, will depend solely on the material's thickness. I don't have data to hand for mahogany, but for plywood, it is 16kHz/mm, meaning a 100mm thick plinth will have a critical frequency of 160Hz. At this frequency, the plinth will be transparent to the sound, and above the critical frequency, it is controlled by damping, which it doesn't have! And they say glulam (glued laminate) plinths are good! If you make your plinth less thick, you lose mass. Bummer!
Having two or more spaced layers of solid mahogany will help, as long as they are of different thicknesses, to spread the resonance and critical frequencies. Of course, you could add damping! If not, your mahogany deck would sound, shall we say,'enhanced' say strong boomy bass, lively midrange and sparkling top end. If that floats your boat, you're onto a winner!
Good luck, <any questions, just ask>
ps connect the deck directly to the plinth, what you are trying to accomplish is to transfer the vibrations from the deck to the plinth, although, unless it has damping qualities, it isn't going to help much. Having a separate arm pod is a good thing, as mounting an arm directly onto a vibrating plinth isn't going to win any votes in the 'clarity and focus' department.