Sandwich MDF/plywood in speaker construction ?

I'd just go with plywood. MDF is heavy and bendy, which means the resonances will be pulled down in frequency, which in turn means they're more likely to be excited.

Plywood (especially the good stuff - PM me for a timber merchant that stocks birch ply and can cut to specs) is light and strong, which pushes the resonances up in frequency - more difficult to excite, and therefore less likely to be audible.

Chris
 

turk 182

Member
2012-10-26 3:03 pm
can you ensure a void free bond between the two? and glue messes from excess glue ooozing out can be a PITA?

if it's a touring box the weight is a bad idea...a permanent install that could be different...

why 1" plywood? when the virtues of 3/4" void free baltic birch's virtues have been talk about at length around here...ooops just noticed chris post!
 

andy19191

Member
2005-04-17 11:42 am
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Are there any benefits in gluing 1/2" MDF sheet and 1/2" plywood together to get 1" sheet material to use in constructing some floor standing speakers ?

I'm wondering if there's any sonic benefits of doing this v's just buying some 1" plywood ?
There are unlikely to be any significant benefits if the glue is strong. Depending on the details of what you are doing 3/4" birch plywood may well be preferable.

If the cabinet handles midrange frequencies rather than just the low frequencies then the drivers banging about on the baffle will be driving resonances in the cabinet some of which are likely to be audible if not damped. A millimetre or two of effective damping material between the two sheets could do this in a constrained layer damping arrangement. Note that only one sheet will carry load so there will be a drop in cabinet stiffness compared to both sheets carrying load. Optimum performance will occur when both sheets are same thickness but this is a bit of a waste of material and so the constraining sheet is often thinner. Thinner and stiffer can shift things back towards optimum.
 
The speakers are going to be three way active, the bass driver is going to be a 10" Visaton TIW 250 XS, Satori MR13P-4 and the tweeter is undecided at the moment.

I was going to hard glue the layers with the hope of using or combining the best properties of both materials in one sheet but i was mistaken judging responses on here. I'd heard that some commercial speakers were doing some thing similar ?
 

turk 182

Member
2012-10-26 3:03 pm
what's the old saying...only believe half of what you hear and none of what you read in promotional literature...


your idea/thinking has been the subject of many a thread...constrained layer damping for one...in most cases it becomes, " is the increased cost and complexity justified in terms of measurable performance gains?"
 
I avoid MDF (althou it makes good vent spacers, and bases).

Use of your laminate will likely make it easier to finish (MDF on the outside), and it will have the benefit of 2 distinctly different layers, but i suspect that your heavy 1” thick material will not give as gooda sonic presentation as well braced quality ply.

I have yet to design a speaker that needs more than 18mm quality plywood. Using bracing can be more effective than making the box walls thicker. Most can be made using 15mm (i did an experiemnt using 15mm BB to illustrate how effective mounting the woofers push-push can be at significantly reducing the need of the box to be stout. I would suggest using 2 x 8” push-push instead of 1 10”. Or choosing 2x10” that work in a similar size box to your existing choice.

From my perspective, 2 x 8” in a 15mm thick (properly braced) box will outperform 1 10” in your 1” compositr material (even if well-braced).

How big a box are you building?

dave
 

andy19191

Member
2005-04-17 11:42 am
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How about damping between the drivers and baffle?
Damping? A soft spring to isolate the driver from the baffle can work well if the reaction force from the moving cone on the body of the driver is sufficiently small not to move the body significantly. This follows from the ratio of the masses. This tends to mean yes for a tweeter, perhaps for a midrange but likely no for woofers which tend to benefit from the added mass of the cabinet.

Damping is needed to get rid of energy when it is in the cabinet in significant quantities at resonant frequencies. Away from resonances the small damping force will be negligible compared to the larger ones associated with mass and stiffness. It is only at resonance where the two larger forces cancel each other that damping has any significant influence.

A stiff baffle also keeps energy out of the cabinet which can be seen in the baffles of some of the pro companies.
 

andy19191

Member
2005-04-17 11:42 am
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I was going to hard glue the layers with the hope of using or combining the best properties of both materials in one sheet but i was mistaken judging responses on here. I'd heard that some commercial speakers were doing some thing similar ?
What best properties?

Boutique audiophile companies do all sorts of technically doubtful things for marketing purposes. The objective is to come up with something that seems plausible and attractive to the unenthusiastic but uninformed which is their target market. If you want to see technically competent designs look more to the larger established pro companies. They tend to have a house style but within that the designs tend to give significantly more weight to technical performance for cost considerations.