MDF or plywood? - Page 5 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 01-11-09, 08:01 PM
Senior Shackster
Fred

Join Date: May 2008
Posts: 373
Re: MDF or plywood?

Ask enough questions and sooner or later you're bound to ask a good one.

Some of this stuff is not as strait forward as it would seem at a glance. Given the size and weight of some of the subs built here, it would be nice to know what has maximum effect for minumum weight penalty. I wish I had taken more math and physics in school. It would have been useful. Confusing, but useful.
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Old 01-11-09, 08:48 PM
Senior Shackster
Fred

Join Date: May 2008
Posts: 373
Re: MDF or plywood?

Quote:
The more I've been thinking about this the more I think that a single point brace could be nearly as effective as one running the full length
If I understand damping properly, it is the process of converting energy stored as momentum to energy stored as heat through friction.

In order for the single point brace to be effective, enough energy being applied to the mass, the side of the box, has to be transfered to the single point brace rather than vibrating the mass (which transfers energy to the surrounding air as sound waves). Something tells me that dosn't happen.
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Old 01-11-09, 09:55 PM
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thxgoon

Join Date: Feb 2007
Posts: 1,506
Re: MDF or plywood?

Quote:
fredk wrote: View Post
In order for the single point brace to be effective, enough energy being applied to the mass, the side of the box, has to be transfered to the single point brace rather than vibrating the mass (which transfers energy to the surrounding air as sound waves).
The brace serves as a point on the surface which is constrained and cannot move. The material could still flex between the brace and the next point of constraint.
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Old 01-11-09, 10:15 PM
Senior Shackster
Fred

Join Date: May 2008
Posts: 373
Re: MDF or plywood?

Quote:
The material could still flex between the brace and the next point of constraint.
That is exactly why I don't think it will be as effective as you think. Unfortunately the math/physics is beyond me.
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Old 01-11-09, 10:39 PM
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thxgoon

Join Date: Feb 2007
Posts: 1,506
Re: MDF or plywood?

Quote:
fredk wrote: View Post
That is exactly why I don't think it will be as effective as you think. Unfortunately the math/physics is beyond me.
Ya, it would be interesting to test.
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Old 01-11-09, 10:51 PM

Andrew

Join Date: Jul 2007
Location: College Station, TX
Posts: 1,131
Re: MDF or plywood?

It seems one important aspect of resonance is being forgotten, quality factor (Q). Audibility of resonances has been established based on Q, frequency, source material and acoustic environment all related to transducer output level.

One example where understanding this situation is vital would be a tower speaker which has each set of drivers divided into their own sub chambers using typical bracing techniques. Such a technique would seem to be superior in terms of minimizing resonances due to added bracing, but as the panels are divided out the panel Q is likely lowered causing more easily excited resonances that will be more audible over a larger range.

Andrew
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Old 01-11-09, 11:52 PM
Senior Shackster
Fred

Join Date: May 2008
Posts: 373
Re: MDF or plywood?

Its easy to ignore what you don't understand. I have seen the term used in several places/contexts, but I don't remember ever reading a good explanation of exatly what it is. Sort of reminds me of Q from StarTrek.

Alright, a quick search turned up the following:
Quote:
Q can be thought of as an overall system response, accounting for resonance of the system. In general, systems with a low Q can recover from resonance quickly
Sounds like damping [factor] or something related to me.

Andrew. Is there a good write-up somewhere on Q as it relates to cabinet construction?
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Old 01-12-09, 12:01 AM

Andrew

Join Date: Jul 2007
Location: College Station, TX
Posts: 1,131
Re: MDF or plywood?

I am not aware of a write-up on Q as it relates to cabinet construction. My knowledge comes from studying physics and working with various materials as well as discussion with others.

This is an example of three different quality factor levels of resonances. In general the higher Q values are less audible all other factors held equal.

Andrew
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Old 01-12-09, 12:32 AM
Elite Shackster

thxgoon

Join Date: Feb 2007
Posts: 1,506
Re: MDF or plywood?

Q is directly related to damping, and is another useful way of expressing it.

Quote:
Such a technique would seem to be superior in terms of minimizing resonances due to added bracing, but as the panels are divided out the panel Q is likely lowered
I'm curious on this as it seems counter intuitive. How's it work?
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Old 01-12-09, 12:40 AM

Andrew

Join Date: Jul 2007
Location: College Station, TX
Posts: 1,131
Re: MDF or plywood?

Quote:
thxgoon wrote: View Post
I'm curious on this as it seems counter intuitive. How's it work?
As the panels modal bands are spread out by chamber separation there is a far wider energy spread. This causes a larger range of resonances which are more easily excited.

In other words the Q is lowered due to the energy dispersal which makes the resonances more audible as shown in peer reviewed research. Additionally, this lower Q means the resonances are spread across a larger range meaning they are more likely to be excited.

Andrew
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