Stuffing sealed cabinet, and duvet filling?

[Kev06]

My 3-way design has separate sealed enclosures for bass and midrange. The bass will be about 27L (1 cubic foot), working up to 270Hz. The midrange will be about 8L (0.3 cubic foot) working at 270Hz-2.5kHz, and has a rear baffle angled at 40 degrees (rather than parallel with the front baffle). I don't currently plan to add any wall dampening, but the 1" plywood walls will be well braced.

I was intending to fully stuff both these enclosures with hollow polyester fibre at a rate of about 1lb per cubic foot, although with some testing to tweak the final amount. The main intention for stuffing the mid range is to reduce any standing waves and reflections. Similar for the bass cabinet, however I would additionally like it to increase the apparent box volume.

I have (as always!) a few questions:

1. Does that seem reasonable?
2. I've a redundant polyester-filled duvet - is this likely to be the same (or similarly effective) stuffing material?
3. I believe stuffing is less effective at suppressing lower frequency waves/reflections; should I do anything else for the bass cabinet or is it likely to be okay?

Thanks for the ongoing help!
Kev

EDIT: in case its relevant, here is the part-constructed cabinet, seen from the back - braces and rear wall still to be added:

 

[speakerdoctor]

I assume the mid enclosure is sealed off from the woofer enclosure (if the mid is open back).

With the cabinet fully sealed, stray fibers are not a problem. So, I strongly suggest you consider fiberglass stuffed at the same rate as your PET. If you decide to stay with PET, get crimped PET in the finest denier you can. Recycled blue jean material is also getting favorable reviews on speaker forums.

 

[Kev06]

Hi, thanks for the reply. Yes the mid driver is open backed, so its enclosure is sealed off from the bass enclosure.

Hmm, looks like I'd be going too cheap using the old duvet then! I was hoping to avoid the itchiness of fibreglass, but if its better than polyester I'd be willing - its certainly cheap enough. Maybe it could be put it in a baggy pillowcase or something to contain the fibres each time I remove drivers for tinkering.

I noticed someone mention the blue jeans stuff in another thread, not seen it around for sale here but then I've not really looked; some research needed, I guess.

Thanks again,
Kev

 

[Kev06]

Been looking around, I can't find the recycled denim insulation locally, just with expensive postage and in larger quantity. I also understand now that all poly fibres aren't equal and my duvet idea or the stuff from the local sewing shops isn't the best performing wrt increasing the apparent volume.

So I'll probably go for fibreglass or rockwool, as kindly suggested. I see there are heavier density cavity slabs available, but it would be easier to prod the more flexible loft insulation type around the braces - presumably that would be ok if packed in reasonably well?

Cheers
Kev

 

[speakerdoctor]

Been looking around, I can't find the recycled denim insulation locally, just with expensive postage and in larger quantity. I also understand now that all poly fibres aren't equal and my duvet idea or the stuff from the local sewing shops isn't the best performing wrt increasing the apparent volume.

So I'll probably go for fibreglass or rockwool, as kindly suggested. I see there are heavier density cavity slabs available, but it would be easier to prod the more flexible loft insulation type around the braces - presumably that would be ok if packed in reasonably well?

Cheers
Kev

yes

 

[sreten]

Hi,

Don't be too precious about the stuffing material, they all work well.

For optimum stuffing of the bass, max volume increase in the lowest
measured Fb via the impedance peak. The midrange you can relatively
overstuff compared to the bass density if you want to.

rgds, sreten.

 

[Kev06]

Thanks both. I wouldn't be going to any particular cost or effort to use fibreglass, in fact could probably pinch a bit from the loft, so it looks like (depending on POV) it should be roughly as good or marginally better than any other choice (once itchiness is sealed in the cabinet anyway). For once there doesn't seem to be any major conflicts of interest!

Thanks for the measurement idea too, Sreten. I have a means of measuring the driver impedance peak so that approach would work very well for me. I'm going to use EQ for the bass, but it seems worth optimising the mechanical side of things to begin with. I'm not too bothered about apparent volume for the mid-range as I'm crossing that over above where it has great effect (within reason), so perhaps overstuffing may be worthwhile in further reducing waves and reflections.

Excellent stuff, many thanks,
Kev

 

[PLB]

Hi Kev,

If handling FG is a problem for you, try this. It's something like 70% sheep's wool. I tried it and it worked well for me.

B&Q Homeeco Loft Insulation, (L)3m (W)370mm (T)200mm | Departments | DIY at B&Q

Peter

 

[andy19191]

I don't currently plan to add any wall dampening, but the 1" plywood walls will be well braced.

If the midrange enclosure is not passively isolated from the woofer enclosure the midrange will drive the low order cabinet resonances. The peaks of the low order resonance are likely to be the loudest radiation coming the cabinet. Adding bracing will do nothing effective for these resonances but adding effective damping will. Like stuffing, the effectiveness of some of the extensional damping pads is open to question.

 

[Kev06]

Thanks Peter, I'd not seen that wool/poly insulation before, looks a lot nicer to handle than FG. Having done some research it seems hard to beat the FG for performance, certainly at a similar price, but the improvement in pleasantness could well outweigh the slight performance difference; I'm almost certainly going to have the drivers in and out multiple times whilst fiddling and tweaking things.

Thanks also, Andy. The mid range enclosure isn't passively isolated, TBH I hadn't fully appreciated its contribution to the low order resonances (I'd assumed the bass driver affecting the mid-range would be the main issue). So perhaps I should look more seriously at wall dampening then, and also learn a bit more - I had believed until this point that bracing the panels reduced resonances with a similar end result to damping them, but obviously not. Such a learning curve!

Thanks again,
Kev

 

[Kev06]

Okay, I 'think' I understand the reason for wall dampening better, now. Braces don't actually stop resonance, rather they divide up the unsupported/unconstrained wall area into smaller sections, which therefore resonate at a higher frequency instead - i.e. the resonance is raised in frequency rather than abolished. It seems that about 4" between braces is one rule of thumb for ply walls, and happily that is roughly what I have. However, wall dampening materials (such as layers of roofing felt) work differently by trying to reduce the magnitude of these resonances, hence why they can help in addition to bracing. In fact its easier for them to suppress higher frequency resonances so bracing and wall dampening compliment each other for that reason, too.

(obviously the stuffing I've been discussing isn't intended to reduce wall resonances. It may help prevent the effects of these propogating through the internal volume, but of course it can do little to prevent any wall resonances radiating sound externally).

Apologies if this is all very basic but I'm just at the learning stage - does it seem like I've understood it?

Thanks
Kev

 

[andy19191]

Apologies if this is all very basic but I'm just at the learning stage - does it seem like I've understood it?

To some extent. Stiffness and damping (and mass) do not really complement each other. Engineers will tend to talk about mass-controlled, stiffness-controlled and the like to indicate what force is dominant and hence what to change to most efficiently effect an increase or decrease.

At a resonant frequency the forces due to mass are exactly cancelled by the forces due to stiffness leaving only the relatively weak forces due to damping to reduce the cabinet motion. Stiffness will reduce cabinet motion below the low order resonances and to a degree at frequencies between the low order resonances but it has increasingly little effect at higher frequencies where there are lots of resonances and the forces due to mass are dominant.

For equal loudness a panel deflection at a higher frequency will usually be smaller although it is complicated by how things cancel. For example, consider the motion of a woofer and tweeter of equal loudness. So the increased stiffness needs to beat this effect. In addition, over the frequency range where the lowest resonances typically lie the ear can perceive resonances below the signal more easily at the higher frequencies.

 

[keilau]

Hi, thanks for the reply. Yes the mid driver is open backed, so its enclosure is sealed off from the bass enclosure.

Hmm, looks like I'd be going too cheap using the old duvet then! I was hoping to avoid the itchiness of fibreglass, but if its better than polyester I'd be willing - its certainly cheap enough. Maybe it could be put it in a baggy pillowcase or something to contain the fibres each time I remove drivers for tinkering.

I noticed someone mention the blue jeans stuff in another thread, not seen it around for sale here but then I've not really looked; some research needed, I guess.

Thanks again,
Kev

The polyster fiber, particularly the pillow stuffing variety, has shown to be ineffective in speaker stuffing to create a larger effective volume. Fiber glass is much better, but with its handling problem.

Ken Kantor's Auralization blog

Some specially formulated polyster is supposed to be good wanding material. Such as the Acousta-Stuf, is supposed to be effective. But I cannot find any technical information to back it up.

Monacor MDM-3 Acoustic Damping Pads is 3/4 wool and a good performance. But the small package that it comes in making it quite expensive. In the US, I can buy cleaned, carded pure wool at 1/3 of the price. I expect wool will be easier to find from woolen mills in UK. (Get cleaned, carded wool. Not the raw wool.) Both Monacor and wool are excellent wanding material, but slightly more expensive than fiber glass.

The measurement of the impedance curve change will be a good way to evaluate the effectiveness of your stuffing material. If you end up trying more than one, come back and show us your results.

 

[Kev06]

Ah, many thanks again, Andy - my understanding is/was still too simplistic, then. I begin to see now why (for instance) people can argue about heavy vs stiff panels.

This is my first serious speaker project so I've relied heavily on rules of thumb and the experience and knowledge of others, but even where I struggle to grasp the complexities its still very useful to know what the general issues are. Theres clearly scope for improvement in almost every aspect of the project; I'm keen to get something half decent up and running, but also looking forward to learning and experimenting with it.

(which perhaps again indicates a stuffing material thats pleasant to work with).

Cheers
Kev

 

[Kev06]

Thanks also, keilau. Useful run-down and links for me to digest. I'd noticed the monacor material on sale here, but as you say it would be quite expensive for a fully stuffed sealed cabinet, so probably something like generic wool or the 70% wool insulation Peter used would be a reasonable compromise for this initial iteration of the cabinet. Maybe when I've refined the design and tinkering has subsided, some future and potentially final cabinet would warrant itchier or more costly filling.

Cheers
Kev

 

[keilau]

Thanks also, keilau. Useful run-down and links for me to digest. I'd noticed the monacor material on sale here, but as you say it would be quite expensive for a fully stuffed sealed cabinet, so probably something like generic wool or the 70% wool insulation Peter used would be a reasonable compromise for this initial iteration of the cabinet. Maybe when I've refined the design and tinkering has subsided, some future and potentially final cabinet would warrant itchier or more costly filling.

Cheers
Kev

In the US, I can buy cleaned, carded pure wool for US$18 per pound. I expect that to be cheaper in UK where wool is more popular and plentiful.

Nature's Comfort Wool Batting ( Twin )-Quilting Weight - The St. Peter Woolen Mill

Pure wool is very effective as speaker cabinet stuffing material and the cost is reasonable. (I gave up on fiber glass a long time ago even though it is still cheaper, but not worth the itchy problem.) The B&Q 70% wool insulation Peter used is £18 per package. It may be quite expensive depending on how much you get by weight. Fluffy wool is very light by weight, usually less than a pound per cubic meter.

I must mention that typical wool batting for quilting and comforter is NOT the long fiber wool recommended for the Bailey transmission line cabinet stuffing. Some of the woolen mill can provide the long fiber wool by special order. The long fiber wool is coarser, stiffer and mostly used for industrial applications. Domestic use wool is finer and feels smoother. For closed box stuffing, you want to use the finer wool.

 

[Kev06]

Thanks very much for the info, I hadn't considered there would be such variation within wool! I'll go have a look at the B&Q stuff tomorrow, see if I can judge its weight and coarseness. We can certainly get carded wool here too, though it may require postage unless I can find a local source.

If I can find something cheaper here than over in the states that would be an event in itself!

Cheers
Kev

 

[Kev06]

Just a small update; went to see the B&Q wool/poly insulation this morning. It looked slightly coarser than the carded lambswool I've seen used in sewing etc. but not bad at all, and not even vaguely unpleasant to handle. Unfortunately though, the only local store that stocks it had kept theirs badly - the bundles were all soaked with water so I've no idea about the true weight.

I'm spending too long faffing about with this, so I'll just bung in some fibreglass for now. Its readily available everywhere without extra postage charges, and there seems less variation in fibre sizes to track down or to cause doubt over relative performance.

Cheers
Kev

 

[PLB]

Hi Kev,

Sorry for the late response, but I have some data that might be of interest to you as attached. I have a 2/3 to 3/4 full package of that insulation material from B&Q and it weighed in at greater than 4lb, so I'd guess a full package would be somewhere around 5lb or more. Hope this helps. Rotten luck at your B&Q having their stock water logged.

Peter

 

[Kev06]

Thanks very much Peter - both for the data and the info on that B&Q insulation!

I like graphs, they help me visualise things; it shows what Sreten mentioned, that all the stuffing materials do quite a good job, and also demonstrates why within the options speaker doctor suggested fibreglass over poly stuffing. To me, the wool looks like a good compromise between performance and handling pleasantness, assuming the former can be compensated for with a slightly larger box.

Hmm, perhaps I'm about to change my mind yet again! I was set to use fibreglass, but I recently read a suggestion that the hard/brittle fibres (that cause itching) can get into the voice coil gap of some drivers and cause problems over time. Now that you've kindly estimated the dry weight of that B&Q stuff I've been able to do a few calculations and it looks great, if I dried it out anyway. Without postage charges and delays it seems like a good option.

Cheers
Kev