21" Maelstrom Plans - Thinking Out Loud

My comments:

1. You gonna give us a good shot of your Christmas socks this year or what? :bigsmile:

2. Those diffusers sure are purdy!!

3. Resonance

I had this discussion with another member here a while back and it is my current understanding that putting in braces to reduce panel size does not change the resonant frequency of the panel. Resonant frequency is determined by density and stiffness.

I have been told that resonance is described by mass spring theory. This seems to be backed up by formulas given for panel absorber design. You change the tuning of the panel not by changing the area of the front panel, but by changing the depth of the air cavity (the spring) or the thickness (or density) of the front panel (the mass).

All this leads me to wonder why cross bracing is any more effective than just increasing the thickness of the subs panels.

I'm looking forward to comments from those more knowledgeable than I on the topic.

fredk said:

My comments:

1. You gonna give us a good shot of your Christmas socks this year or what? :bigsmile:

2. Those diffusers sure are purdy!!

3. Resonance

I had this discussion with another member here a while back and it is my current understanding that putting in braces to reduce panel size does not change the resonant frequency of the panel. Resonant frequency is determined by density and stiffness.

I have been told that resonance is described by mass spring theory. This seems to be backed up by formulas given for panel absorber design. You change the tuning of the panel not by changing the area of the front panel, but by changing the depth of the air cavity (the spring) or the thickness (or density) of the front panel (the mass).

All this leads me to wonder why cross bracing is any more effective than just increasing the thickness of the subs panels.

I'm looking forward to comments from those more knowledgeable than I on the topic.

Click to expand...

In terms of physics, all mechanical systems have a natural resonance frequency. That is a frequency where the system will maximally absorb energy, when stimulated with that given frequency. If you look a simple system, like a transducer, it is reflected by the resonance peak you see in the response. It is easier to see if you look at the impedance curve, which shows a rising impedance at the resonance.

Panels in subwoofers are the same. There are multiple resonant frequencies, for the various parts of the enclosure based upon the physical properties of the panels. Those properties are length, mass, and stiffness of the panel. Maybe an easier analogy is that of a guitar string. The frequency a guitar string will play (its natural resonance frequency) is based upon the length of the string, the tension (stiffness), and its mass. A lighter, shorter or tighter string all translate to a higher natural frequency. All you have to do to convince yourself is play a guitar string, and tighten it, play the thinner (lower mass) strings and compare them to the thicker (higher mass) strings. If you play a string and put your finger down on part of the fret (making it shorter), you get a higher note.

All of this, directly translates to panels used in loudspeakers and subwoofers. You use braces, to shorten the panel, and raise the natural resonance of that panel. Why raise it? Because you don't want the device that you are designing, to play at a frequency that will stimulate the natural frequency of the materials that make it up. That has the effect of storing energy, and releasing it in ways that do not coincide with accuracy. So we add braces to INCREASE the resonant frequency of a given panel. What other things can we do to achieve the same thing? We can use lower mass, which is counter to what most people are thinking about when building a sub but lower mass materials, will have a higher natural resonance. We can also use stiffer panels, by choosing a different materials that is stiffer than MDF.

The one factor I've left out is damping. Damping is anything that reduces the amplitude of the oscillations in a system. It is the ability to stop the oscillations quicker. A mechanical system that most people understand and can relate to is the shock absorbers in your car. The system has a spring, and a shock absorber that moves a piston through a cylinder filled with oil to create a frictional loss. This system damps the spring oscillation. Take the shocks off a car, and it will bounce happily down the road in and uncontrolled manner. The damping nature of the shocks create a frictional loss, that damps the oscillation. That is the same thing that happens when you add damping to any system, acoustical or mechanical.

MDF has good damping properties, even though it has high mass. Adding thicker panels, actually lowers the resonance frequency, but it increases the damping. Using braces, to effectively shorten the panel raises the resonance frequency.

Make sense?

Kevin Haskins said:

Panels in subwoofers are the same. There are multiple resonant frequencies, for the various parts of the enclosure based upon the physical properties of the panels. Those properties are length, mass, and stiffness of the panel. Maybe an easier analogy is that of a guitar string. The frequency a guitar string will play (its natural resonance frequency) is based upon the length of the string, the tension (stiffness), and its mass. A lighter, shorter or tighter string all translate to a higher natural frequency. All you have to do to convince yourself is play a guitar string, and tighten it, play the thinner (lower mass) strings and compare them to the thicker (higher mass) strings. If you play a string and put your finger down on part of the fret (making it shorter), you get a higher note.

Click to expand...

Keven, this is exactly how I understood it, and I use the same analogy as well. So when you take a 36" panel, and put a brace in the middle, you are effectively splitting it into two 15" resonating panels...like fretting a guitar at the 12th fret...raising the pitch of how it resonates.

So, my question still holds, why can't I use 1/2" panels in place of 3/4" panels for the bracing?

All that said, this is academic for me right now, as I found a sale on 3/4" birch ply and bought two full panels of it for the cost of two 1/2" panels. And they weigh a good 40lbs less then equivalent MDF. I did however pick up several 1/8" and 1/4" MDF panels for future sound diffuser projects I'll be working on!

I have all next week off work, I took the week off for my birthday...so if the Maelstrom 21" shows up Friday, I can actually start the build process ASAP! :sn:

Kevin Haskins said:

You can use 1/2", but I saw a study once where it was shown not to be as effective. It was one study so it may not hold for all cases but I use 3/4" just to be safe. It doesn't take up that much more space.

Kevin Haskins
Exodus Audio

Click to expand...

I'm just trying to keep the weight down a bit. Using 1/2" and a good lattice style cutout pattern I figure would save several pounds over a 15cu box. But as I said, I got the 3/4 at a great price, so no worries there.

So, going foward, assuming Mal-X 21" with 3/4" birch plywood, somewhere between 12-15 cu feet.

1) I assume I would double up the material on the baffle?

2) Do I line the panels with anything?

3) Should I be stuffing the box with anything, and are those Deep Pockets pillows I see people using a good idea?

4) Anything else I'm forgetting to prep?

I've already started cleaning up the garage/shop and unloaded the wood into the shop. I have a bunch more cleaning to do before I start cutting, not to mention drawing out some plans, maybe picking up some foam or pillows...waiting for the driver and cutting and gluing!