View Full Version : i came back for a minute to ask a question

PV Audio
11-28-2005, 11:19 PM
only because i have had no success elsewhere, and i'll be gone again after this.

When talking of damping materials, you have your basic dacron, polyester fill, egg crate foam, fiberglass and variations of those. i was wondering, is there a line in the middle that says how dense hte material can be before it loses its effectiveness and begins to take up Vb? say you have a sheet of egg crate foam. condense that, and make several layers of the condensed version very thick one of top of each other. would that foam have the same damping characteristics as a single sheet? when does it begin to affect the vent air velocity (polyester fill i mean)? if the density DOES matter, is it better to have a lighter material that damps less and takes up less air, or denser material that damps more, but changes the characteristics of the enclosure? thanks.


11-28-2005, 11:35 PM
Ok, i don't have a clue to your question, but heres one... Why you leavin?

PV Audio
11-28-2005, 11:38 PM
hiatus, not leaving.

11-29-2005, 12:33 AM
Oh ok, good.

11-29-2005, 12:39 PM
This may not answer your question, but look here;

Absorption coefficients of different materials. People look at these charts for
HT theater construction. Materials are tested to certain standards. Essentially, each material has a
NRC rating {Noise Reduction Coefficient } for different frequency ranges.

Look at the 703 material, 1" compressed fiberglass. At 125Hz and below it's useless for sound proofing or sound conditioning. Use a thicker 703 and it performs better at lower frequency but doesn't give you alot more at higher frequency.

For subwoofer boxes, you really don't need to add any 'loose fill' to the box
unless your subwoofer is also playing higher frequencies but you shouldn't design a subwoofer to do that.

For the majority of tweeters you don't need fill.

That leaves you will midbass and midrange. You should use fill. For sealed box midrange many people like to oversize the box to let the standing waves travel through the fill to get absorbed/diffused to reduce any sound coloration that otherwise might occur. So, if you oversized your sealed box for midrange and over fill it, then it's all good. Most people that do this experiement anyways to find a sweet spot in sound they like.

For ported boxes using midranges the way you stuff it becomes more critical, you want sound conditioning but don't want to block the ports. I suggest making some 'pillows' on the walls to absorb the waves but leave the center of the box
'free air' without obstruction to let the port 'breath'.

I guess you need to look at what the materials really do before passing judgement. ie, I can't find NRC data for 'Acousta-Stuf'
polyfill so we really don't know how it performs until we see test data. We don't know really how much is needed if we don't know
what it really does. Most people experiment. I made some sound conditioning pillows and installed them in the box, do the simple
echo test and I notice a difference, it worked, but I have no idea on how much is worked but it was better than a bare box.
Fiberglass is way better than Walmart polyfil or Acousta-Stuf but who wants to work with loose fiberglass?

PV Audio
11-29-2005, 02:44 PM
great info as usual thy, but what i really mean is how do you KNOW it will work? if you take a 1.5" sheet of damping foam, compress it into a centimeter high, does it still work? i'm just not really understanding HOW damping materials work, versus WHY they work, which is similar, but completely different at the same time. does the air within the "cells" of the foam what really affects the damping qualities? what if you lined a TL with VERY dense foam, instead of just packing it heavily? i asked elsewhere, but no one could tell me why, so i figured i'd try here even tho i don't expect any different.

11-29-2005, 05:00 PM
Again, let me sidetrack; [warning, I'm not a guru on this topic, I'm using the common sense card in my back pocket]

Read this;

5 points of isolation.

1. Mass
2. Mechanical de-coupling, or mechanical isolation.
3. Absorption
4. Resonance
5. Conduction

This is in reference to HT construction, but some applies to speaker box building.

1. Mass
You need mass for woofers otherwise imagine making boxes out of thin cardboard, won't work well. You don't want the walls of your speaker box to

2. Mechanical de-coupling, or mechanical isolation.
This is where I'm not so convinced in terms of speakers. Some esoteric builders
like to place some material between two [or more] sheets of wood to make
an isolation layer, could be rubber, etc. I've tried this and it does decouple one
piece of wood from aonther but you have a similar resonant frequency as the single layer of wood. In other words, for sound proofing it's cool, but for speaker box building are we sound proofing or trying to make the box have less resonance by adding more mass? Not having that isolation between wood will
lower resonance.

3. Absorption
This is what you want for those [open back] midwoofer/midrange drivers, the sound from the speaker inside the box to be absorbed so there is no standing wave coloration.

4. Resonance
You really don't want your box to resonate badly as this sound add
coloration. You can add mass to lower resonance.

5. Conduction
I don't know how this would apply to speakers ATM.

So.. we have mass and aborption as probably the bigger factors [there is stiffness too]. We get mass
by using heavy materials like MDF over plywood unless you want to use plywood
that is 'thicker'. If you made a speaker box out of thin sheetmetal then you may have
less mass and obviously, you can have a higher resonance. So we use dynamat
inside this metal box to dampen it, to lower resonance, but it doesn't add much mass -> how heavy is dynamat in relation to a hunk of wood? not heavy at all.

On the other hand, there is thicker mass loaded vinyl 1/8" that is 1 pound per sq. ft. 1/4" thick is 2 pounds per sq. ft., dynamat is typically 30 mils thick, 4x thinner than 1/8" MLV. some of the uber dynmat is still 1/2 the thickness of 1/8" MLV.
Since MLV isn't cheap, we still opt to make our boxes out of wood because it's
cheap mass for the money, ~ 3/4" MDF is about 3 pounds per sq. ft.

But, three layers of 1/4" MLV to equal 3/4" MDF has 2X more mass than
3/4" MDF, ~ 6 pounds per sq. ft. But we can't make our boxes out of this because
it's not stiff like wood.

After we figured out mass for your speaker project you have to figure out the
absorption, those waves that bounce around those walls that have high mass.
You need 'space' for these waves to get 'buried' into to dissipate into heat.
If you were to take 3" thick fiberglass and compress it into a paper thin material,
you no longer have the same absorption characteristics, rather you have paper
thin material with mass. You changed your material from absortion type of
material to a mass only material. In other words, I don't think you can cheat
by compressing the material so much that it doesn't function anymore as intended.

I don't have a scientific explaination on how it works but I guess you can visualize
'quicksand' as a simple analogy. You walk on the ground which has mass, then
you step in quicksand and fall into it, can't get out {fiberglass}. If you remove
the water from the quicksand, you have only sand, you won't easily fall into it.

When you want to replace your fiberglass filled TL box with a denser 'white foam',
the sound waves will see sand, not quicksand :crazy: and not function properly.

PV Audio
12-01-2005, 06:33 PM

12-01-2005, 06:36 PM
good post ^^ thnks