Applying damping material to cabinet interior surfaces

[Guest russwollman]

Has anyone applied dynamat or deflex panels or other damping materials inside the cabinets of your Advents? Any advantage?

Additional internal cabinet bracing is probably a good thing, too, but I haven't tried that yet. What would you use, plywood?

I like the sound I have now but I'm in a mood to tinker a bit.

Best wishes to everyone here.

[Doug G.]

Hi Russ,

I have never done any of those things but remember that any change you make to the cabinet will result in a change in sound.

Whether or not a change will be audible can only be determined through listening (or measuring if one is fortunate enough to have the instruments) but there will be a change as the cabinet resonance will be changed.

I guess it all comes down to whether or not you wish to retain the original sound as Kloss intended or want to experiment.

Doug

[Guest russwollman]

I guess from the dearth of replies that this modification isn't a popular choice, so I'll leave them alone.

I just bought a pair of Smaller Advents to massage, so that'll satisfy my tinkering urge.

Thanks, Doug.

[stan461]

Has anyone applied dynamat or deflex panels or other damping materials inside the cabinets of your Advents? Any advantage?

Additional internal cabinet bracing is probably a good thing, too, but I haven't tried that yet. What would you use, plywood?

I like the sound I have now but I'm in a mood to tinker a bit.

Best wishes to everyone here.

Glad to see you are back Russ.

I am also curious, so I'll look into what is available for damping cabinet interiors. I'd like to try that.

But first, I'm going to replace the foam blocks in my Large Walnut Advents will Acousta-Stuf and listen for any differences that may make.

[speaker dave]

Has anyone applied dynamat or deflex panels or other damping materials inside the cabinets of your Advents? Any advantage?

When I was at Snell we investigated cabinet damping quite a bit. We found very little effect from applying any surface materials. What did work well is a technique called constrained layer damping. This entails a damping layer between two rigid outter layers. It seems that damping material works better when the outer layers force it to receive vibration "in shear".

There was a Swedish goo that we used that was specifically formulated for its damping properties. I can't remember its name but I think they ended up selling it retail.

You could apply a layer between the cabinet and a sheet of masonite, or maybe flat porcelain floor tiles.

This was the only technique we found that really changed the sound on the "knuckle rap" test.

David

[Guest russwollman]

When I was at Snell we investigated cabinet damping quite a bit. We found very little effect from applying any surface materials. What did work well is a technique called constrained layer damping. This entails a damping layer between two rigid outter layers. It seems that damping material works better when the outer layers force it to receive vibration "in shear".

There was a Swedish goo that we used that was specifically formulated for its damping properties. I can't remember its name but I think they ended up selling it retail.

You could apply a layer between the cabinet and a sheet of masonite, or maybe flat porcelain floor tiles.

This was the only technique we found that really changed the sound on the "knuckle rap" test.

David

I wonder how significant the knuckle rap test is. I liken it to kicking the tires on car. That's what got me started on all this. In any case, thanks for your reply, David.

[speaker dave]

I wonder how significant the knuckle rap test is.

We actually did accelerometer tests for the various techniques, but we found the results to be mixed. I expected to see a similar resonance profile with reduced Q, but there was no clear indicator that one approach was superior to the next. (Each method we tried made differences, however the accelerometer tests didn't point to a clear winner.)

The constrained layer method did truly sound different with the knuckle rap test, and so we went with that. You would think that if it sounded different, and it clearly did, that the measurements would easily reveal that. In this case they didn't.

Whats wrong with kicking tires?

Regards,

David

[Carlspeak]

Hi Russ,

I have never done any of those things but remember that any change you make to the cabinet will result in a change in sound.

.................

I guess it all comes down to whether or not you wish to retain the original sound as Kloss intended or want to experiment.

Doug

Doug is absolutely right with his last statement. However, did HK really intend to have that 'boxy' sound or was it a compromise? Guess we'll never know. My tinkering has shown the knuckle rap test on the front BB can be improved significantly with a front to back brace installed just above the woofer hole. I made mine from a piece of scrap 2X3 lumber.

I've done response tests with a mic attached to speaker BB's and found most cab resonances in average size classic speakers occur in the 100-400 hz range.

With regard to Deflex, it seems to marketed in different ways. The Deflex home page (UK site) says it dampens cabinet resonances. Other sites that sell Deflex write that it addresses midrange and woofer back waves. That seems to me to be a completely different sonic problem from cabinet structurally induced resonances.

[Carlspeak]

I believe the problem here is a given: an Advent enclosure. I don't think the OP is trying to design a new one. Thus he is faced with a box of rectangular shape having parallel sides and wants to damp it so the 'knock' test is improved.

[speaker dave]

Any squarish enclosure will be trying to go from square to round when faced with pressure from within and a cyllinder-shaped enclosure, even with the material being thin (the Hsu and SVS enclosures are made out of fiber-board molds for concrete posts), automatically eliminates wall flex. Of course, the flat top and bottom can still flex, but usually they are made of thick wood and the driver is occupying the entire top or bottom area and the port or ports are at the other end and prevent a lot of flex.

Howard Ferstler

I think your assumption is that a round interior will receive pressure from the woofer at all surfaces at once and so not break into any vibrational modes. That isn't my understanding of how it works.

First, the greater driving mechanism is from the vibration of the woofer chassis. The coil drives the cone mass forward and an opposite reaction force is applied to the chassis/magnet mass. If you decouple the driver from the cabinet, as done by KEF, you can filter the energy above a certain frequency. We used rubber grommets and foam tape for 8 inch woofers and rubber motor mounts on the 12 inch trochoidal woofer of the reference 105. The 104.2 used a pair of woofers that worked directionally out of phase (acoustically in phase, of course) with an aluminum rod between. They also floated on foam and grommets but the greater improvement was from the vibration force of one canceling out the other (similar to balance shafts for internal combusion engines.) Either approach dramatically reduced cabinet wall vibration by reducing the amount of stimulating energy.

Audiophiles don't like the notion of soft mounting drivers but it works as well as anything I have seen.

Note that although the mechanical drive to the cabinet wall was reduced there was still a secondary effect of pressure drive, but at a much reduced level.

I've seen some cabinets that, like your Waveform, had that hard solid sound when rapped. I assume they had some pretty rigid material and lots of wall thickness, and certainly worked very well. The egg shape would give an extremely low diffraction cabinet. Great for better on and off axis response but no improvement to the power response (if that matters to anyone ).

My only point was that surface damping is difficult for normal cabinets. Relatively heavy mdf (oxymoron?) has a high mechanical impedance and most surface damping compound isn't seen by it. The same damping materials may work perfectly well on car door panels due to the lower mechanical impedance of thin steel. I had some big BBC monitors (old LS5/1s) that sounded very dead because they had relatively light plywood cabinets with heavy felt lining, well adhered. Lighter walls, heavier damping layer: effective damping. They gave a dull "thunk" sound rather than the solid rock Maple crack of a lot of high end cabinets.

I think that Carl is right that a front to back brace is a useful practical step. Since any brace is really stiff along its length, you can get away with something quite small, such as a 5/8" or so dowel rod.

Just my $.02

David

[speaker dave]

There was a Swedish goo that we used that was specifically formulated for its damping properties. I can't remember its name but I think they ended up selling it retail.

It was Noise Killer Yellow by Vibratec of Sweden.

David