Damping Material In Subs. Why Bother? (2 Viewers)

[Danny Richie]

I thought that was your final post.

 

[Patrick Sun]

Taken out of context, that statement is funny.

 

[SteveCallas]

So you're calling yourself a woman? What's your opinion on living room drapes?

 

[Danny Richie]

Yea, yea, that's funny too. As a loudspeaker designer I always have to consider what women think about the look of a design.

You'd be surprised how many men have told me that they really wanted to build a line source speaker but the wife said no way.

 

[Mark--M]

I think i'll try stuffing it then. Cant hurt anyway.

So just to help my understanding... Can I expect more output in low frequencys with the same amount of power just like a larger box? Or does the stuffing simply drop the upper output to give you a similer response curve to a larger box?

 

[Danny Richie]

Technically yes, but not much. Maybe half of a db at best. Also, it may give a little at 20Hz but you may also loose a little at 50Hz as the bottom end starts to roll off a little sooner.

 

[ThomasW]

Not really, the primary benefit is that the bass quality will be tighter/more defined. The efficiency will drop slightly as the amount of damping increases.

The way to do this is by taking a CD or DVD with a good bass track you're very familar with. Add the main stuffing, then give it a good listen. Then add the 1/4 lb increments, listening to the track after each addition.

 

[Travis_G]

Tom Nousaines has a couple rules of thumb he published in March/April Car Stereo Review 1995. The article was called, "Make a small box act like a larger one with polyester fiberfill."



Travis

 

[ThomasW]

Nousaine prefers poly not because it's acoustically superior to fiberglass, but because it's less obnoxious to work with.

Look at any of the charts that list sound absorption coefficients, you'll see that fiberglass is much better than poly for attenuating low frequencies.

That's perhaps why it's also used to build bass traps.

For noncritical use (aka car subs) using damping to fake a larger box is fine. For high quality home audio it's better to actually build the box sized to the desired tuning, then fine tune the performance with damping.

 

[Travis_G]

Travis

 

[ThomasW]

Are you going to tell us how one successfully educates the damping material so it's able to differentiate between these two functions? ...... http://www.htguide.com/forum/images/smilies/icon_roflmao.gif

 

[Travis_G]

Thomas,

Damping material just does what it does. It does not need to be "educated."

Where wavelengths are comparable to the thickness of the material, frictional loss is prevalent. In other words, sound passing through the fibers causes them to vibrate and friction turns sound energy into heat.

When an enclosure is stuffed, and the wavelengths are long in comparison to the thickness of the material, a different effect is at work. When air in the enclosure is compressed, it heats up making it stiffer. The fibers absorb the heat.

Nousaine described it like this:



However, I am told that this description is incorrect. , a member of the AVSforum special member Noah Katz (I believe you know him Thomas ), described it to me like this. The fibers absorb the heat. In this case the fibers do not need to vibrate at all in order to be effective. The high surface area of the fibers helps them to do this.

His description makes more sense to me but you decide.

Travis

 

[Travis_G]

Mark Seaton, AVS special member described it like this Thomas:



regards,

Travis

 

[ThomasW]

Travis,

Obviously you're big on verbiage, and consentual validation.

Me, I'm big on building and experimentation.

So you have fun debating nuance, I'll go build something.

Oh, do I know Noah Katz?

No....

I know there's a person named Noah Katz that posts on AVS. He's computer analyzed stresses on an piece of 3/4" plywood. His computer studies tell him he can built a 32 cu ft sub, housing 2 Avalanche 18"s using only 3/4" material with no added bracing. And that the sub will function with no interference from wall flex.

Personally I find that claim laughable, so anything he posts is suspect in my book......

Wow imagine that I got something right..... Unfortunately Mark's been out of the loop too long, 3M took MiraFlex off the market years ago.

BTW you seem to be impressed with the term 'special member' on AVS. If you get your post count up you'll automatically become a 'special member' as well...

 

[Travis_G]

Thomas, I respect your right do disagree with me. I hope that we can disagree without being disagreeable.

BTW I believe what Noah stated was that the output from the walls flexing would be 20 dB from the drivers output, and would be out-of-phase with the drivers output. So worst case it would just reduce the output from the loudspeaker.

Dr. Linkwitz seems to disagree, but it is an interesting discussion if anyone would like to read.

 

[Mark--M]

Just in case anyone was wondering... I tried stuffing my sub this weekend. Not a huge change but seems to sound a little nicer. Not much difference in the response or output level. It was worth doing I guess.

 

[ThomasW]

Correct, the idea is to "fine-tune" the performance, not effect things by orders of magnitude.

The technique I use is to stuff per the 0.5- 1.5 lb/cu ft. Then adjust by adding 1/4 lb increments. Keep adding the 1/4 lbs until the last amount added no longer tightens the bass and the only effect is to lower the overall output. Remove the last amount added and you're done.