Speaker acoustic treatment

[NateTTU]

I've done lots of research in the past for this and had decided to go the cheaper yet arguably one of the best ways to fill kill your speakers back waves, fiberglass. However, I do have some health concerns and wanted to get some feedback. I know many of you have used the common pink fiberglass and some, if your lucky to find it, have used the rigid type. My question really is how much fiberglass really gets blown around in the room? I do have bad allergies so I hate to stir something up everytime I want to watch a movie. I know fiberglass in general isn't the best for your health, but not sure if there is much exposure with it from being just used in speakers. In all I'm wanting to stay away from fiberglass, but don't really care to shell out about $100 for acoustic foam to place in my speakers. However, I feel it might just be the price I have to pay to get the job done.

[Hdale85]

Ports don't work like that. There is no risk of fiberglass getting blown around the room don't worry bout it

[ThomasW]

The biggest problem is the amount of fibers in the air when lining the cabinet. After that it's not much of a problem. If you're still concerned attach a single layer of a sheer grill cloth to the inside opening of the port.

[Amphiprion]

Not really any at all gets out, but there is the chance. I've always used standard Wal-Mart polyester stuffing and found it to be good enough. It's not like most people are measuring impedance curves to determine the right amount of stuffing anyway, so why worry if fiberglass is x % better than polyester?

[Dennis H]

I'm usually a big fan of fiberglass but I don't know if I'd risk it in a ported box if you have bad allergies. I'd spend the extra money and go with the (patent pending ) JonMarsh method as described in the Modula threads. Buy some Sonic Barrier from our friendly sponsor PE (similar to the Whispermat Jon uses) and line all the walls with that. Also buy some polyester quilt batting and fold several layers to make a small wad to help kill midrange reflections right behind the mid drivers. The quilting stuff holds together better than the loose pillow stuffing. Don't block the path between the driver and the port.

[Jim Holtz]

I'm usually a big fan of fiberglass but I don't know if I'd risk it in a ported box if you have bad allergies. I'd spend the extra money and go with the (patent pending ) JonMarsh method as described in the Modula threads. Buy some Sonic Barrier from our friendly sponsor PE (similar to the Whispermat Jon uses) and line all the walls with that. Also buy some polyester quilt batting and fold several layers to make a small wad to help kill midrange reflections right behind the mid drivers. The quilting stuff holds together better than the loose pillow stuffing. Don't block the path between the driver and the port.

Hi Dennis,

I believe Nate is building Statements so he should definitely NOT put a wad of stuffing behind the mains. Lining the walls with 1" foam as specified works great.

Jim

[Jim Holtz]

I've done lots of research in the past for this and had decided to go the cheaper yet arguably one of the best ways to fill kill your speakers back waves, fiberglass. However, I do have some health concerns and wanted to get some feedback. I know many of you have used the common pink fiberglass and some, if your lucky to find it, have used the rigid type. My question really is how much fiberglass really gets blown around in the room? I do have bad allergies so I hate to stir something up everytime I want to watch a movie. I know fiberglass in general isn't the best for your health, but not sure if there is much exposure with it from being just used in speakers. In all I'm wanting to stay away from fiberglass, but don't really care to shell out about $100 for acoustic foam to place in my speakers. However, I feel it might just be the price I have to pay to get the job done.

Hi Nate,

Fiberglass will work fine. I prefer to use foam simply because it's so much easier to work with. I used 2" wedge foam from Foam by Mail. The smallest quantity of 12 square feet should easily do the Statements, center and Monitors if you're going to build them. It costs $15.99+ shipping. It works great.

Jim

[Amphiprion]

Also remember from our friend Zaph that the right thickness counts just as much as the material.

[Dennis H]

Hi Dennis,

I believe Nate is building Statements so he should definitely NOT put a wad of stuffing behind the mains. Lining the walls with 1" foam as specified works great.

Jim

Ooooops.... :P

[kingpin]

Dont forget about felt.

[NateTTU]

Hi Nate,

Fiberglass will work fine. I prefer to use foam simply because it's so much easier to work with. I used 2" wedge foam from Foam by Mail. The smallest quantity of 12 square feet should easily do the Statements, center and Monitors if you're going to build them. It costs $15.99+ shipping. It works great.

Jim

Jim,

I have already purchased some foam from foam by mail for the mid tunnel sections. I purchased the 1'' wedge stuff and its pretty nice. I did think about buying the 2'' stuff to do the rest of the lineup (inside), but thought it would certainly take more than 12 sq ft? Did I read your "statement" wrong or can it indeed be done with such a small amount? If this is the case then I probably will take the plunge with foam instead because I had planned on buying about two of those 48 sq ft. packages (@ $60 a bundle)!

[Jim Holtz]

Jim,

I have already purchased some foam from foam by mail for the mid tunnel sections. I purchased the 1'' wedge stuff and its pretty nice. I did think about buying the 2'' stuff to do the rest of the lineup (inside), but thought it would certainly take more than 12 sq ft? Did I read your "statement" wrong or can it indeed be done with such a small amount? If this is the case then I probably will take the plunge with foam instead because I had planned on buying about two of those 48 sq ft. packages (@ $60 a bundle)!

Hi Nate,

I'm guesstamating that the 12 square feet will do it but I could be off a bit. It could be short as I'm now running it through my head. Perhaps two bundles of 12 and you'll be sure to have enough. Fiberglass is available just about anywhere and is reasonable. One roll would be plenty. I think it's around $20 or so but I haven't bought any for a long time.

Jim

[Carl V]

If ya like the foam, by all means use it.

Box construction & lining always opens a can of worms.

I've always had good luck with lining walls with a whispermat
type of materail & then glueing on a layer of 1" or 1/2 " Felt
ordered from McCaster. This helps the resonance & the suprerssion of back wave noise
(the boxy sound).
Interior reflection & interior resonance are 2 different issues.
Stuffing in general has preferentially been
Wool...either shearling or Long fiber wool ( I just rec'vd a supply-Thanks!)
I've used compressed Fiberglass in the past on the back wall area.

Loose fiberglass I've always stuffed into panty hose. This allows me
to glue the fiberglass/pantyhose "pillows" onto the wool liners or onto
the compressed fiberglass lining.

There is a post from Ken Kantor while he was at NHT, where stuffing
material was studied. Google or HTG search engine surely will bring it up.
It originally was on the bass list....gawd how I miss the good ol' days
when the bass list was rich with advice & knowledge.

[ThomasW]

There is a post from Ken Kantor while he was at NHT, where stuffing
material was studied. Google or HTG search engine surely will bring it up.
It originally was on the bass list....gawd how I miss the good ol' days
when the bass list was rich with advice & knowledge.

How does damping work?

In a sealed enclosure the driver moves in and out some distance. Multiplying the displacement distance by the surface area allows us to determine the volume of air pushed. While the front side of the woofer pushes air molecules which fill our listening areas with sound, the air in a sealed enclosure is compressed and expanded with the inward and outward motion of the driver. To more simply describe the situation in the box, we see that the quantity of air molecules in the box is fixed (it's sealed), while the motion of the driver's cone changes the volume of the sealed box.

Going back to the ideal gas laws, when you compress a volume of air, the temperature will increase. Similarly, when you increase the enclosure volume of a sealed enclosure, the temperature will reduce.

We generally stuff low frequency enclosures with insulating materials, most commonly fiberglass. The idea is that the fibers or other material helps to absorb some percent of the heat generated by the compression of the air in the enclosure. This heat is then released during the expansion half of the cycle. This is known as isothermal operation (temperature stays the same). As stated earlier, all heat is not absorbed, but the degree to which this occurs is referred to as isothermal operation.

With some percentage of heat absorbed the trapped air does not change in pressure as much as an unstuffed volume. In fact, it behaves like a slightly larger volume. In theory, this phenomenon could provide an apparent increase in volume of ~40%. As noted by Vance Dickason in the Loudspeaker Design Cookbook, practical limits with real materials tend to max out at about a 20% apparent gain (going from memory here-number may be slightly diff). With a sealed system this results in a lower Fb and lower Qb.

When stuffing is placed in a sealed box there is also resistive damping to varying degrees depending on the material. In a very old message to the DIY Bass List by NHT founder/designer Ken Kantor he noted his observations that in a real box it is common to see roughly equal parts resistive damping and isothermal operation. (check the bottom of this page for an archive of one of these posts)

The following is an E-Mail addressed to the bass list on stuffing an enclosure. Ken Kantor is the former CEO & co-founder of NHT.

From: Ken Kantor
Date: 04 Apr 95 03:41:37 EDT
Subject: Stuffing Stuff

"In light of recent discussions, let me share some thoughts regarding cabinet stuffing. I'll do this from a practical point of view, partly because the physics side has been well articulated by Doug. The other reason I'll stay away from theory in that, in the matter of cabinet fill, theory has proven over the years to be of only limited help in real-world speaker design. I'll also confine most of my comments to issues related to sealed systems. Vented systems do share a
few of these same issues, but really the goals and the physics of stuffing a vented box are different.

Most professional designers would agree that practical experience, combined with trial and error, is best way to find the optimum stuffing material, quantity and method for a given design.
This is why good designers routinely experiment with fill in the development of a new system, ala Vance's data cited here. This particular information is a valid data point, but it is important
not to over-generalize. If you are designing a system that differs substantially in shape or volume or source impedance (passive crossover) from a known you will need to iterate for best
performance.

In my practice, adjusting the filling is the last step in getting the bass right, and is used mostly to fine-tune the system Qtc and resonance. As increasing amounts of polyester are added to a
sealed box, the resonance and Q gradually go down. This can be shown mathematically to be due in roughly equal parts to the effects of simple resistive damping and isothermal conversion. At some point, a minimum is reached, and further material simply reverses the trend by taking up volume. During the filling process the impedance curve is constantly monitored, and
convergence to optimum usually takes only a short time. Filling also has the important effect of reducing internal reflections, to reduce standing waves and comb filtering. However, the amount of filling has comparatively little effect on its efficacy in this regard.

[Side Note- it is a common misconception, I believe, that professional designers rely heavily on LEAP and SPICE and CALSOD to define their designs a priori. On the contrary, professional designers use these modeling tools mostly to guide and optimize revisions. Unlike DIY designs, a typical commercial 2-way will go through perhaps 3 revs of each driver, 2 to 4 box trials, and easily a dozen+ crossover changes.]

Lining the walls of a vented enclosure to reduce internal reflections, or filling a transmission line to absorb the back wave, highly absorptive wool or fiberglass are ideal. However, these materials will not generally provide the desired results in a sealed system. It is true that they will provide more reflection absorption than polyester, but the later is quite good in this regard in the critical midrange. In a sealed system you don't want absorption at lower frequencies anyway; you want damping and isothermal conversion. I have tried "all-out" efforts using fiberglass lining and
polyester fill to achieve the best of both worlds. I found the results to offer little practical benefit over polyester alone, but its worth looking into.

All NHT systems now use polyester fill, of one variety or another. We used to use fiberglass in our vented designs, but found a Danish polyester that mimicked the properties of
fiberglass very closely. I don't know if this kind of polyester is available to hobbyists. Excluding this special poly, there are essentially two kinds of fiber available: pillow stuffing,
and audio-spec polyester. The later type allegedly has hollow core fibers, but I have been unable to verify this with my keen eyesight! Sorry, but forget the pillow type. Sure, it's easy
to get. If you use enough, it will damp the midrange, and that's better than an empty box (by a lot). But it will have little effect on the lower frequencies.

Well, that's pretty much all I know about stuffing speakers."

_________________________________________

In another message Ken Kantor added (excerpted):
"Exact enclosure volume is not critical, and stuffing can be added or subtracted to fine tune the response. I recommend adjusting the stuffing by monitoring the impedance versus frequency of the sealed box system. Add stuffing to lower the frequency where the impedance is highest. When that impedance peak starts to rise in frequency, you have added too much. The NHT/SW3p uses 820g of acoustic polyester stuffing with the 1259, but your enclosure may do better with slightly different amount."

[NateTTU]

The biggest problem is the amount of fibers in the air when lining the cabinet. After that it's not much of a problem. If you're still concerned attach a single layer of a sheer grill cloth to the inside opening of the port.

I haven't made a final decision as of yet, just because I like how well fiberglass works. This grill cloth suggestion is a very good idea. The pantyhose idea is interesting to say the least but would require a lot more work. I'm assuming this type of grill cloth will work:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=260-335

[Hdale85]

any cloth that won't restrict air flow will work. You can go to your local fabric store and find something just blow through it if you feel resistance don't use it. Just suggesting this because you don't need 70" of the stuff

[Winter]

Nate,

I apply a layer of polyester quilt batting over the semi-rigid fiberglass. As Dennis H stated above, the polyester is in thin sheets rather than loose batting. It comes in different thicknesses, and is available at Walmart.

[NateTTU]

Thanks for all the suggestions guys, again the people on this forum are very helpful and friendly. The grill cloth over the port seems like the easiest solution for me right now as I already had purchased the fiberglass a while back. However, I have to ask two more questions real quick though.
1) Since I never have purchased the PE grill cloth, are the holes in the cloth very small or large? I guess the whole point is to keep out the fiberglass fibers which are very small so I'm guessing if it was brought up as a solution that it indeed has a relatively tight weave.
2) The price to obtain the minimum order at PE is pretty cheap, about $14 shipped. However, Dennis is right and that there is no way I need this much. I'm curious if anyone out there may have some extra scraps they weren't planning on using. I'm guessing all I would need is 5 8''x8'' squares. I'm thinking this would be enough to cover these ports.

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=268-322

I'll pay whatever you think is fair, plus shipping of course.

Thanks.