Question on stuffing/insulation for 4x10 sealed bass cab

[GregS]

I'm about to start a build of a 4 x 10" speaker sealed bass cabinet - kind of like half an Ampeg 8 x 10 "fridge". My question is: Is it necessary to line/stuff the cabinet with either some kind of acoustic foam or fiberglass insulation?

Just about all of the information I find on the Web says that it is necessary to stuff a bass cabinet in order to prevent standing waves and boominess. Many say that their cabinet sounded better after they stuffed it. I'm wondering if this concept is more relevant to hi-fi applications and not so much to musical instrument applications? I have built guitar cabinets without stuffing and they're fine. Is it that different for bass?

Also, does anyone know if the classic bass amp cabs like vintage Ampeg 8 x 10's were stuffed or not? Anyone have experience with stuffed vs non-stuffed bass cabinets?

Any input much appreciated.

Thanks,
Greg

[eschertron]

The acoustic energy damping of any stuffing is *extremely* frequency-dependent. If you can get your hands on a copy of "The Audio Cyclopedia" or whatever it's called nowadays, you can find some tables that give you the damping factors. How much of the 'stuff' you put in the cabinet has as a large effect too, probably more than the variety of material used. I'd think stuffing a bass cab would reduce the highs and mids more than the bass, but may also affect the resonance freq of the cab, pushing it down a little.

[Mick Bailey]

I don't like fibreglass for speakers - too much of an irritant and the fibres get everywhere. See my response regarding dacron fibre;

http://music-electronics-forum.com/t33579/

This works very well for me as a material for stuffing. I haven't built a bass cab for years and they've been reflex ported so no stuffing required. Sometimes though I get bass cabs in where the cabinet sides resonate - usually when they're made of thinner material without internal bracing (generally Chinese made). Stuffing those eliminates cabinet resonance and improves the mechanical isolation of any crossover where fitted.

I've just been working on an Eden 2x15 and this sounded great - no stuffing. I don't think there's an automatic requirement to stuff a cabinet; to stuff or not to stuff probably depends on the enclosure design and engineering.

[ReadyTeddy]

Stuffing can often significantly improve the sound of a cabinet. As Mick noted above, ordinary polyester pillow/quilt stuffing works very well. Acoustically, it is usually preferred over fiberglass. Typically the stuffing density is 1/2 to 1 pound of stuffing per cubic foot of internal volume. It may seem like too much to you, but it isn't.

[GregS]

Thanks for the replies.

The way I see it, stuffing reduces the influence of the cabinet on the sound which may or may not be a good thing. By absorbing the sound waves, reflections that interact with the speakers are less intense. Thus, the frequency response is determined mostly by the speakers themselves. This makes sense for something like a PA application where the idea is to simply make the sound louder without changing it or adding anything to it. However, musical instrument amplifiers do colour the sound. It may colour it in a way that I like and it may not.

As for boominess, I understand that to be caused by cabinet flexing at frequencies where there are intense standing waves in the cabinet. My cabinets are ridiculously overbuilt so I don't anticipate that to be a problem.

The way I'll approach it is I'll try it without stuffing first and if I like it then great. If there are problems then I'll consider stuffing.

Thanks again for the insights.

Greg

[Earl Norton]

I once had a brown face Fender Bassman that had the speaker cab stuffed full of fiberglass insulation.....pink stuff. I'm pretty sure it was original as the speakers were the original oxfords. Parts express sells a non fiberglass stuffing for this purpose.

[Bill Moore]

I have used everything to dampen bass cabs, I used to like the fiberglass used for metal buildings that had a plastic backing, seemed to contain the glass better. I tried a few with the shredded rags pressed into a form used in automobile insulation, (jute?). I wonder why we couldn't use the bed liner stuff that is now availlable. I know several folks who have used it to quiet their vehicles painting it inside doors, and quarter panel sheet metal.

[Ken Moon]

Some quotes from various speaker design websites:

"From Altec - 1974
The normal method to check speaker damping is simple, but is easiest if you start out with too little fiberglass. We recommend the battery test method to check for damping. Attach a short length of speaker wire to the loudspeaker terminals in the normal manner, and bring the loose end around front – where you can easily listen to the loudspeaker sound while testing. Hold one of the speaker leads to either end of a standard 1.5 volt flashlight battery. Then touch the second lead to the other end of the battery and release. A properly damped loudspeaker will reproduce a clean “click” as contact is made and released.

The underdamped speaker will reproduce the battery contact as a “whomp” sound. Fiberglass may be added in a loose manner to the enclosure until the proper “click” is obtained, and then stapled into place according to above practices relating to the specific type of enclosure.

The overdamped speaker is more difficult to observe. It is a matter of degrees that becomes familiar with one or two experiments. Thus, the recommendation to start tests with too little, build up, and then go back down as required."

Another:

"Loudspeaker fill serves a number of purposes, but the more surface area that is included, the better the outcome in terms of loudspeaker performance. Polyester will inherently have less surface area than most glass fiber materials. Thus, I would probably come out on the side that glass fiber is usually the better way to go. Foam material with a surface area similar to or higher than glass fiber will also perform as well as or better.

"Surface area in this context is the composite surface area formed by all the interstices in any porous material. It can also be thought of in terms of density. The denser the better as long as the material is significantly porous. Polyester is generally less dense than other porous materials, such as glass fiber and open-cell polyethylene / polyurethane foams.

The one caveat would be related to the actual surface area / density of polyester. For a more thorough analysis, I would recommend obtaining information on the specific fill materials being used. A simple comparison of their (ASTM) flow resistivity values should be sufficient. The hard part would be obtaining that sort of information from loudspeaker manufacturers."

Link to a PhD thesis that has some stuffing info:

http://grputland.com/files/thes.pdf

Another option to stuffing:

Acoustic Foam 2-1/2" x 24" x 18" UL 94 260-515

[Steve Conner]

An unstuffed cabinet will have a series of peaks and dips in its frequency response through the lower midrange, as the sound reflected from the rear wall goes in and out of phase with that coming directly from the cone, and the cabinet goes through its own standing wave modes.

For a hi-fi speaker this is unacceptable, but you might feel that it improves the sound of a musical instrument speaker. You could probably stuff it with old pillows, blankets or couch cushions for a temporary test.

Damping sound waves inside the cabinet is a different function to damping vibrations of the cabinet walls. Some hi-fi speakers use extensive bracing to shift the resonances of the walls, and/or bitumen pads glued to the insides of the walls to damp them, but I've never seen or heard of anything like this in a musical instrument application, unless you count the Mitchell "Sand-Amp" or the little post that functions as a token brace to the rear panel in a 4x12.

[bob p]

I have a friend of a friend who designs his own hifi speakers. He's an old-timer engineer who has been doing this for decades. He's on something like his 10th revision of his DSP amp / speaker combination. The speaker cabinets include an inner cabinet and and outer cabinet, and a sand layer in between to damp the cabinet. It works.

[GregS]

Thanks for the addtional replies.

I've finished the cabinet so I thought I'd update the thread with the results. Like I said earlier, I tried it unstuffed first. I've played it by itself as well as in a full band rehearsal. These are the results:

The cabinet sounds better to me than anything I've ever played through by a large margin. I believe this is mostly due to the speakers I chose for it. There is a great depth to the sound, it has good bottom end, and also very good clarity in the upper frequency ranges. It also sits very well in a full band mix. There is no "boxiness" to the sound or "boominess" at any frequency. This is without any stuffing. Since its performance leaves nothing to be desired I'm not going to change it.

My own conclusions after this are that it is not an absolute necessity to stuff a bass cabinet. It is not a Hi Fi application so colouration of the sound from the cabinet is not necessarily a bad thing. "Boxiness" is not a problem as long as the cabinet volume is sufficient for the speaker(s) used. If the cabinet is built solidly enough "boominess" is also not a problem.

Greg

[big_teee]

Thanks for the addtional replies.

I've finished the cabinet so I thought I'd update the thread with the results. Like I said earlier, I tried it unstuffed first. I've played it by itself as well as in a full band rehearsal. These are the results:

The cabinet sounds better to me than anything I've ever played through by a large margin. I believe this is mostly due to the speakers I chose for it. There is a great depth to the sound, it has good bottom end, and also very good clarity in the upper frequency ranges. It also sits very well in a full band mix. There is no "boxiness" to the sound or "boominess" at any frequency. This is without any stuffing. Since its performance leaves nothing to be desired I'm not going to change it.

My own conclusions after this are that it is not an absolute necessity to stuff a bass cabinet. It is not a Hi Fi application so colouration of the sound from the cabinet is not necessarily a bad thing. "Boxiness" is not a problem as long as the cabinet volume is sufficient for the speaker(s) used. If the cabinet is built solidly enough "boominess" is also not a problem.

Greg

Can you tell us what speakers you chose.
If you said already, I missed it.
I'm thinking of building a bass cab myself!
Also can you give the dimensions of your Cab?
T

[GregS]

The speakers are Eminence Legend B810. I believe they are the replacement speakers for SVT 8 x 10 cabinets.

The outer dimensions of the cabinet are:
Height: 25.5"
Width: 25.5"
Depth: 16"
I don't have my plans in front of me at the moment so these dimensions are according to memory. I'll update the thread if any are off. Using 3/4" plywood this allows a baffle of roughly 24" x 24".

Greg