Best Stuffing for Speakers Tested - Unexpected Results

"Best" is a relative term. All three work, but if you prefer to use polyfil it'll get the job done. You just need to use more of it than the other two.

And I'm only looking at the ability of the stuffing to damp the interior of the box from the perspective of that one panel. The real difference between these is probably low enough that it really doesn't matter.
With that said, when compared by weight, fiberglass and rockwool nose out the polyfil by a small margin.

My preference for fiberglass mostly came from almost always having some on hand. I like to use what I have and tend to be realistic about the differences between materials. Even if there is an audible difference, a difference doesn't necessarily mean one is better than the other.

Moving on to direct panel damping, rubber damping was almost useless and doubling the panel thickness is effective but mostly down low. Double layer CLD is effective across the tested frequency range. I would consider that to be something worth doing if you are looking for peak performance. Like I said in the video, it can be combined with stuffing to make it even more effective.

Important to put all of this in perspective. These differences are relatively insignificant when compared with other much more important design considerations when build a speaker. Driver selection, box design (volume, alignment) and crossover design are orders of magnitude more important to how the speaker will sound.
I do these tests to demonstrate just how small the difference actually is between these and how little they will affect the sound.

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[IBuildIt]

"Best" is a relative term. All three work, but if you prefer to use polyfil it'll get the job done. You just need to use more of it than the other two.

And I'm only looking at the ability of the stuffing to damp the interior of the box from the perspective of that one panel. The real difference between these is probably low enough that it really doesn't matter.
With that said, when compared by weight, fiberglass and rockwool nose out the polyfil by a small margin.

My preference for fiberglass mostly came from almost always having some on hand. I like to use what I have and tend to be realistic about the differences between materials. Even if there is an audible difference, a difference doesn't necessarily mean one is better than the other.

Moving on to direct panel damping, rubber damping was almost useless and doubling the panel thickness is effective but mostly down low. Double layer CLD is effective across the tested frequency range. I would consider that to be something worth doing if you are looking for peak performance. Like I said in the video, it can be combined with stuffing to make it even more effective.

Important to put all of this in perspective. These differences are relatively insignificant when compared with other much more important design considerations when build a speaker. Driver selection, box design (volume, alignment) and crossover design are orders of magnitude more important to how the speaker will sound.
I do these tests to demonstrate just how small the difference actually is between these and how little they will affect the sound.

[ChonkTek]

I did some tests 25 years ago with an SPL meter, two 12" subwoofers in a sealed enclosure (trunk tire well enclosure) and various amounts of polyfill. I did individual frequency plots on a graph, 1Hz intervals. I found that adding polyfill decreased peaks and extended lows. There was a threshold where overall I started to lose some amplitude on all frequencies if I stuffed the enclosure too much. It was a great experiment to prove to myself that spikey response in a subwoofer can be tamed some.

[chadridsdale]

I just found your Channel last night. My goodness do I appreciate what / how you are testing. I have a few projects that im procrastinating on and now im glad I did. Thank you

[trevorbartram]

Two great videos, thanks! The preferred CLD method in UK DIY years ago was bituminous felt panels (a thick roofing product) covering all box internal surfaces. I built the Wireless World Tabor DIY design using Audax drivers using this technique in the late 70s. I had many comments regarding their excellent sound quality. When it comes to bracing, the matrix method first introduced by B&W and, later, used by Paradigm is beyond the capabilities of most DIYers. It is used in my present speakers Paradigm Studio Monitor 30s purchased twenty years ago that I have never felt the need to replace. However, I must admit I'm tempted by the KEF LS50 Metas with their use of a sculptured disc to absorb high frequency energy within the box, truly revolutionary and a tribute to British engineering.

[davej]

I build my encloser from 3/4 inch (19mm) Burch ply. It had a slight buzzy ring.

So, cut the front baffle off, put a heaver mid brace inside. Then used 'Flex-Seal' brush on rubber on all surfaces inside and out. Then I lined the inside back and sides with a scrap piece of "Mohawk" Berber carpet but no other filling.

So much better. I do not have all the 'Wiz-Bang' test gear but to these damaged Tinnitus ears, it sounds so much better. I have ear damage that shows up as a 20 dB White Noise all day and night.

[AurasphereAcoustics]

I really appreciate all you contribute to the community John. With this video, there are some real issues
1. You have the accel on a planar face that is not decoupled from the emitter
2. This means that the majority of the energy is transmitting usiing the mechanical box
3. As you increase the damping, it is an internal airspace mod in terms of the standing waves but in your case the transmission change is almost negligible because its still using the box as a wave guide of sorts to the top of the box...being a membrane.

I dont use any planar facings in any of the enclosures we do and they are proprietary masonry thats is extremely engergy absorbent (200mm can stop a 50 cal) and generally cast in a single piece.

BUT

Its quite amazing for tuning the boxes if there are critical issues

You can also do progressive damping by using acoustically inert diaphragms, tuning them to required freq and suspending them in the internal fill material which creates a type of hemholz if you really have issues esp in larger boxes. There is a noticeable choking that happens when I load a single driver sealed right up.

Great vids and always challenging!

[noahnovotny]

To me, using fill is never something I do to “fix” cabinet wall resonances. It’s only to help reduce standing waves or port resonances within the enclosures. I have tried many types: polyfill, pink insulation, Dacron, rock wool, demin, etc. The way I test is with impedance sweeps. I have found that lining the walls with 1.5” fiberglass is the best in midbass speakers. I put cheese cloth type material over it in ported boxes for piece of mind. Putting filling in the cabinet like you did dampens the airflow to the port which reduces output. I think that’s why people say overstuffing boxes will get weak bass.

In subwoofers I use 3 inch pink insulation because it does the best job of “increasing” box size. Again, I only line the panels and adhere it with spray adhesive or hot glue. It works best with slot ports because you can get large even coverage. Getting a 30% increase in theoretical box size is common in common shaped enclosures. Whether or not that box volume gives an output increase or just changes the port tuning is still up for debate in my lab.

To fix cabinet resonances, bracing is the only thing I’ve found that truly makes a significant enough difference for me do it. I know your measurements show that it doesn’t help at higher frequencies, but my cabinets really never have any resonances that high other than ports. Honestly I’m not sure why your getting such high peaks on yours. Maybe I just don’t notice them as much as the low 100-400hz ones.

[myturkishlife]

I have always been a fan of the "CLD" technique, have glued thicker MDF panels with silicone inside factory speaker enclosures many times always given good solid results.
Great video👍

[TheRealWindlePoons]

I once built a pair of speakers based on the CLD principle. I used MDF lined with ceramic tiles, sheet glass and aluminium sheet bonded in place with rubber based adhesive. Back corners had panels giving triangular vertical columns which were filled with sand. Stuffing was traditional long-haired wool. The resulting floor-standing enclosure was exceptionally "dead" but each box weight 100kg.

[kennethnielsen]

Thanks for sharing your thoughts and ideas.

[Subsonic-gn]

I would have put the stuffing right behind the woofer. Also, try lining the inside with No-Rez dampening material as a comparison to an empty box - I would, bet that this alone would make a huge difference. The rubber lining you placed was way too thin to make a difference. The CLD method sounds interesting by sandwiching silicone in between the wood, but you have also effectively doubled the thickness of the wood. I would be interested to see if glueing the two pieces of wood was just as effective than the silicone. Interesting tests, thank you for posting!

[SuperMcgenius]

When you brace in the middle of the box you up with two panels of the same resonant frequency, this is why asymmetrical bracing and trapezoid boxes will generally give you a better performance. Mass does work but is best with Multiple techniques of dissimilar Materials that will not exasperate resident modes. Enjoy the hobby. 😀

[BischBaschBosch]

Interesting series mate. Used to work as a Q&R for Bowers & Wilkins many moons ago in one of my brief career deviations. Materials, material density, isolation and insulation is something that can be talked about without end. As is shape and compartmentalisation. Whether you want a speaker characteristic or a flat response etc. All about marginal gains at a certain level. For DIY'ers, I think your info is sound. Good stiff box, braced and plenty of fixings as you've done. Also driver sensitivity matching as best you can which I think you've touched on. Acoustic loose fill stuffing (looks like cotton wool) is effective and cheap alternative to the stuffing options you've shown. Particularly good around internal bracing.

[VioletGiraffe]

Thank you for these tests, very interesting. I had the same idea as CLD, but I want to use vibration dampening materials from the car audio industry, they come in sheets with adhesive pre-applied, quite convenient to use and specifically designed for this task.

[derekleclair]

Great experiment. Gives great examples as to why to brace and double baffling with cld!

[paulthetubew]

Very interesting! what software were you using for the test? The damping material used is usually to control the resonance and other box parameters without regard to panel vibrations. Speaker enclosures can be viewed as filter networks, particularly for vented enclosures (such as Butterworth alignments.) However, controlling resonances will indirectly control the amount of energy available at any frequency to flex panels. You might consider using a measurement microphone to measure the acoustic radiation from a panel as opposed to the contact sensor on the panel. You might consider a box made of mdf instead of plywood. Mdf is a good material for do it yourself enthusiasts has high density and greater acoustic losses. Internal bracing is also effective in reducing any panel vibrations. Of course if you really want to eliminate you can build enclosures out of concrete, which I did for a customer in the mid 1970s. Wharfedale had speakers, which you poured sand into a sandwich construction enclosure in the 1960s. A speaker box has 6 surfaces, the most important is the baffle which is aimed at the listener, any radiation from top, sides, and rear are radiating into the room and contributing to reflected and not direct sound. The bottom of the speaker if sitting directly on a floor is coupled to that radiating 'panel', which changes the sound. That can be modified in a number of ways, a layer of material such as Vibrathane, a large mass (concrete), speaker spikes, or a combination of these.Keep up the good work! - Paul - former speaker designer.

[tweakerman]

Natural lambs wool, I find is the best, because its a natural & not man-made, like fiberglass & polyester dacron, that colour the sound, nice video👍

[yvesboutin]

I like your videos a lot because I don't have the software to take some measurements, but I have to agree with some of the comments posted that stated damping is used mostly to avoid sound from the bass driver to bounce back in the cabinet and coming back with a delay and smearing the response. The fact that some materials are damping the cabinet walls is just an added bonus, so that material should be placed in the area around the mid-bass driver to absorb or slow down those pesky resonnances. My preference goes to acoustical foam because it is a very efficient sound absorbant and its elastic properties damp slightly the cabinet walls and stay where you put it. Beside what I have said before, I still appreciate the efforts you put on those videos, they make our knowledge about acoustic advance by leap and bounds. Thanks!

[patrickthunnus]

I found it odd that the various stuffings weren't directly behind the woofer. I always thought the role of any stuffing was to absorb/diffuse the rearward wave of the cone and flatten the impedance curve, kinda like how the vintage dynaco speakers did with their aperiodic design. Every acoustic suspension speaker I've ever opened up had the stuffing directly behind the woofer.

[krishanthadissanayake]

An idea of fillings inside the box is to HIDE some air inside the box(inside the cavities in the fillings)...so speaker cone is free to move, fiberglass whole is the best i find, you get better dynamics, Bass become more pleasurable