Internal Sound Deadening Treatments for Cabinets

[Loren42]

I thought I would see what ideas some of you might have for deadening my DIY cabinets.

Right now they are 3/4" ( 19 mm) thick MDF with internal bracing (no one panel is equally divided).

I am about to do the last tidbits to these cabinets before applying veneer and this would be a good time to make any last minute improvements.

The design can be seen at Pyramids.

I'll put up some better CAD drawings soon of the internal bracing.

Just wondering about any other treatments that I should consider.

 

[annunaki]

So long as proper bracing techniques are employed mineral wool sheets or "rockwool" work excellent for taming/absorbing unwanted internal resonances.

For truly inert cabinets, constrained layer damping should be employed combined with isolating the drivers from the enclosure(s). Then using rockwool/mineral wool sheets will prevent any reflections from being audible.

One must be somewhat careful with vented applications as over dampening can cause reduced low frequency output.

 

[lsiberian]

I thought I would see what ideas some of you might have for deadening my DIY cabinets.

Right now they are 3/4" ( 19 mm) thick MDF with internal bracing (no one panel is equally divided).

I am about to do the last tidbits to these cabinets before applying veneer and this would be a good time to make any last minute improvements.

The design can be seen at Pyramids.

I'll put up some better CAD drawings soon of the internal bracing.

Just wondering about any other treatments that I should consider.

It's my belief the best way to treat a ported cabinet is by trial listening. But this requires patience and a lot of screwing.

50% coverage of the walls is where to go if you want to do a 1 shot deal. Still I suggest your ears be your guide.

 

[Loren42]

It's my belief the best way to treat a ported cabinet is by trial listening. But this requires patience and a lot of screwing.

50% coverage of the walls is where to go if you want to do a 1 shot deal. Still I suggest your ears be your guide.

At this point you feel that 3/4 MDF is enough?

Woofer is a 15" JBL 2235H. There are two ports (only the holes are shown) on the front. All braces are mounted asymmetrically on the face of each panel.

Here is the rendering for the cabinet:

.

The base (at its widest) is 24" by 24" and tapers down to 20" by 20" at its top. It sits about 34" - 35" tall.

Here is an internal view from the back of the braces:

Comments welcome.

 

[WmAx]

At this point you feel that 3/4 MDF is enough?

Comments welcome.

That is one massive resonant box. You can reduce resonance substantially, and I can tell you exactly how, but it will require a far more complex structure. You can still have the same external appearance; but internal design needs to radically re-designed, and you will need to add some size to compensate for the internal volume that will be occupied by the new structure, if you want the same effective volume available to the woofer in the end.

What frequency are you crossing the woofer to midrange? (an important question relevant to the structure suggestion)

-Chris

 

[Loren42]

That is one massive resonant box. You can reduce resonance substantially, and I can tell you exactly how, but it will require a far more complex structure. You can still have the same external appearance; but internal design needs to radically re-designed, and you will need to add some size to compensate for the internal volume that will be occupied by the new structure, if you want the same effective volume available to the woofer in the end.

What frequency are you crossing the woofer to midrange? (an important question relevant to the structure suggestion)

-Chris

Currently set to 400 Hz.

I am using a 3-way active crossover(Ashly XR2001).

 

[WmAx]

Currently set to 400 Hz.

I am using a 3-way active crossover(Ashly XR2001).

Since you are using active crossover, why not use a mid-range driver that can work well down to about 150Hz? This would allow you to address the resonance more practically. You never specified if you wanted some explanation of how to address resonance. Do you?

-Chris

 

[Loren42]

Since you are using active crossover, why not use a mid-range driver that can work well down to about 150Hz? This would allow you to address the resonance more practically. You never specified if you wanted some explanation of how to address resonance. Do you?

-Chris

I am open for suggestions. The midrange panel is 8" wide. I am using a 6.5" Eminence LAR-MR. Not many midranges offer a very wide frequency range down to 150 Hz.

Driving the crossover range downward to the 200 Hz range would also help with another problem; diffraction baffle step.

The lower woofer baffle is about 22" - 23" wide horizontally across the voice coil. F = 4650 / 23" = 202 Hz.

The actual step will be a little more complex than that because the panel is asymmetrically shaped.

Sure, tell me more about cabinet resonance. I am trying to do more research on the subject.

 

[WmAx]

I am open for suggestions. The midrange panel is 8" wide. I am using a 6.5" Eminence LAR-MR. Not many midranges offer a very wide frequency range down to 150 Hz.

There are some fine pro mids that will work fine to 140-150Hz. I recommend scratching the one you were going to use, because this will be a huge factor in sound quality.

Sure, tell me more about cabinet resonance. I am trying to do more research on the subject.

This will be easiest, and lowest weight, if you do this(and use the mid that can be crossed low): Build the cabinet in two parts and have the connect with a very soft/compliant material via a joint. A latex foam sheet is ideal and I can specify source for this material cut to specification. The woofer section and mid-range section must be decoupled. This is because the large woofer section is acting as a huge surface area to transmit mid-range band vibrations. You will build the mid-range/treble module with double walls; exterior layer then adhere in Dynamat or similar material to the inside. Clean the aluminum side of Dynamat, wipe down with solvent to clean it, and use a polyurethane adhesive to attach an inner wall/second sub cabinet inside. This will create two stiff layers with a damping core between then, resulting in massive conversion of vibration to thermal energy, by way of this trapped damping layer, in an effective shearing point between the two stiff walls. Still use lots of internal bracing; much more than you show in your picture. For the woofer module, use 3x the bracing you currently show, being sure braces attach to the walls with points no more than 5" from each other. This will ensure that the lower module has a 1st resonance point over 200Hz, well past the crossover point/band the woofer will operate. So, the woofer will effective operate in an inert cabinet, and since the mid-module is seperated via a soft layer that acts as a de-coupler, the mid-range frequencies can not transmit to the woofer section and excite it's walls. BTW, B&C is a superb driver manufacturer, and they have at least 2 mids that can be used down to 150Hz with no compromise.

My suggestions are meant to result in a cabinet that will not produce any coloration that is audible. This is a critical feature if you intend to have a reference-grade system for music reproduction.

-Chris

 

[Loren42]

There are some fine pro mids that will work fine to 140-150Hz. I recommend scratching the one you were going to use, because this will be a huge factor in sound quality.




This will be easiest, and lowest weight, if you do this(and use the mid that can be crossed low): Build the cabinet in two parts and have the connect with a very soft/compliant material via a joint. A latex foam sheet is ideal and I can specify source for this material cut to specification. The woofer section and mid-range section must be decoupled. This is because the large woofer section is acting as a huge surface area to transmit mid-range band vibrations. You will build the mid-range/treble module with double walls; exterior layer then adhere in Dynamat or similar material to the inside. Clean the aluminum side of Dynamat, wipe down with solvent to clean it, and use a polyurethane adhesive to attach an inner wall/second sub cabinet inside. This will create two stiff layers with a damping core between then, resulting in massive conversion of vibration to thermal energy, by way of this trapped damping layer, in an effective shearing point between the two stiff walls. Still use lots of internal bracing; much more than you show in your picture. For the woofer module, use 3x the bracing you currently show, being sure braces attach to the walls with points no more than 5" from each other. This will ensure that the lower module has a 1st resonance point over 200Hz, well past the crossover point/band the woofer will operate. So, the woofer will effective operate in an inert cabinet, and since the mid-module is seperated via a soft layer that acts as a de-coupler, the mid-range frequencies can not transmit to the woofer section and excite it's walls. BTW, B&C is a superb driver manufacturer, and they have at least 2 mids that can be used down to 150Hz with no compromise.

My suggestions are meant to result in a cabinet that will not produce any coloration that is audible. This is a critical feature if you intend to have a reference-grade system for music reproduction.

-Chris

Chris,

Thanks for the info. The double wall is sort of like the old Warfdale double wall cabinets with sand between them, only Damplifier acts as a more flexible absorbing agent. Pretty cool idea, I think, but that may be more than I can commit to with this design.

However, I might be able to add some additional braces and dampening around the midrange walls and woofer.

I am using all hardwoods for the braces now, but it is easy to add some more.

Can you give me some specific examples for the midranges you like?

 

[lsiberian]

Chris,

Thanks for the info. The double wall is sort of like the old Warfdale double wall cabinets with sand between them, only Damplifier acts as a more flexible absorbing agent. Pretty cool idea, I think, but that may be more than I can commit to with this design.

However, I might be able to add some additional braces and dampening around the midrange walls and woofer.

I am using all hardwoods for the braces now, but it is easy to add some more.

Can you give me some specific examples for the midranges you like?

First seal the upper drivers. Add more bracing and 2" rockwool on every wall, doubling it to 4" on the back wall.

Your goal is to push resonances to above the audible range. But even pushing them higher will help some.

I suggest adding 1/4" thick peel-n-seal to the exterior of the top module. Then covering it with another layer of MDF. Try to get the 1/2" MDF for a lower weight. Once you deal with that then worry about the bottom part.

which I would add more bracing too and then some rockwool.

 

[Loren42]

First seal the upper drivers. Add more bracing and 2" rockwool on every wall, doubling it to 4" on the back wall.

Your goal is to push resonances to above the audible range. But even pushing them higher will help some.

I suggest adding 1/4" thick peel-n-seal to the exterior of the top module. Then covering it with another layer of MDF. Try to get the 1/2" MDF for a lower weight. Once you deal with that then worry about the bottom part.

which I would add more bracing too and then some rockwool.

Thank you.

Upper drivers are already sealed. The cutaway back view shows a gray 3/4" thick MDF sheet and the dark green horizontal plate that make up the internal part of the cabinet.

Any good source for rockwool you can recommend?

 

[lsiberian]

Thank you.

Upper drivers are already sealed. The cutaway back view shows a gray 3/4" thick MDF sheet and the dark green horizontal plate that make up the internal part of the cabinet.

Any good source for rockwool you can recommend?

spi-co.com

but you can also get it online form ata acoustics.

 

[Loren42]

spi-co.com

but you can also get it online form ata acoustics.

The rock wool cited in ATS Acoustics has a density rating of 2.5 lbs/foot. Is that satisfactory?

 

[lsiberian]

The rock wool cited in ATS Acoustics has a density rating of 2.5 lbs/foot. Is that satisfactory?

It should work fine.

 

[Loren42]

It should work fine.

Thank you very much.

I'll add some more hardwood bracing and rock wool.