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Multi-Way Conventional loudspeakers with crossovers

3-Way Build Project - Woofer help
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Old 18th September 2016, 04:44 PM   #151
jReave is offline jReave  Canada
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Hi spark,

I think I need to amend my last recommendation re your mid chamber. I think The strategy is still good but my suggested implementation leaves a little to be desired. Here's why.

One of the oft unmentioned advantages of a 3-way over a 2-way is that you can employ different panel resonance control measures for the mid and woofer chambers. In the case of the mid, you want to try to push the frequency of the panel resonances down below the xo point and in the case of the woofer, you want to push them above the xo point.

So here's how panel resonances work:

1 - increase panel stiffness and the natural resonance goes up in frequency.
2 - increase panel mass and the natural resonance goes down in frequency.
3 - if you increase panel thickness (by glueing 2 or more boards together for eg), you are increasing both mass and stiffness. However, panel resonance frequencies go up exponentially with increases to stiffness but go down only linearly with increases to mass so therefore stiffness wins out and the resonant frequency still goes up.

For the woofer chamber then, extensive bracing divides the panels into smaller sections which will then be stiffer and therefore have higher resonances. Which is good. If there is a chance that they are still fairly close to the xo point then you may want to add panel damping so that just in case the resonance is activated, it won't ring as loud or as long.

But with my mid chamber strategy, adding the extra layers of wood will increase both mass and stiffness and so the panel resonances are going to increase and are still likely to therefore be within the passband of the mid. Not so good. What you need to do instead is add mass without adding stiffness. So a material that is heavy but not totally stiff. Which is actually the definition of panel damping materials.

So your mid chamber would benefit from the shape I suggested but made out of layers of a damping material instead of wood. Hope I'm not too late with this update.

It's up to you what material you use. Bitumen pads are standard but get expensive quickly. I've seen others use vinyl tiles, roofing tiles or even recycled car tire rubber mats. Availability and budget are my usual determining factors.

For a little more info on panel damping, try this currently active thread (although the OP is asking re a 2-way not a 3-way):
http://www.diyaudio.com/forums/multi...-confused.html
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Old 19th September 2016, 05:20 AM   #152
eriksquires is offline eriksquires  United States
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Multi layer sonic barrier is the bomb. Much better imho than psa backed thin rubber sheets.
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Old 25th September 2016, 03:20 PM   #153
spark010 is offline spark010  United Kingdom
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Quote:
Originally Posted by jReave View Post
Hi spark,

I think I need to amend my last recommendation re your mid chamber. I think The strategy is still good but my suggested implementation leaves a little to be desired. Here's why.

One of the oft unmentioned advantages of a 3-way over a 2-way is that you can employ different panel resonance control measures for the mid and woofer chambers. In the case of the mid, you want to try to push the frequency of the panel resonances down below the xo point and in the case of the woofer, you want to push them above the xo point.

So here's how panel resonances work:

1 - increase panel stiffness and the natural resonance goes up in frequency.
2 - increase panel mass and the natural resonance goes down in frequency.
3 - if you increase panel thickness (by glueing 2 or more boards together for eg), you are increasing both mass and stiffness. However, panel resonance frequencies go up exponentially with increases to stiffness but go down only linearly with increases to mass so therefore stiffness wins out and the resonant frequency still goes up.

For the woofer chamber then, extensive bracing divides the panels into smaller sections which will then be stiffer and therefore have higher resonances. Which is good. If there is a chance that they are still fairly close to the xo point then you may want to add panel damping so that just in case the resonance is activated, it won't ring as loud or as long.

But with my mid chamber strategy, adding the extra layers of wood will increase both mass and stiffness and so the panel resonances are going to increase and are still likely to therefore be within the passband of the mid. Not so good. What you need to do instead is add mass without adding stiffness. So a material that is heavy but not totally stiff. Which is actually the definition of panel damping materials.

So your mid chamber would benefit from the shape I suggested but made out of layers of a damping material instead of wood. Hope I'm not too late with this update.

It's up to you what material you use. Bitumen pads are standard but get expensive quickly. I've seen others use vinyl tiles, roofing tiles or even recycled car tire rubber mats. Availability and budget are my usual determining factors.

For a little more info on panel damping, try this currently active thread (although the OP is asking re a 2-way not a 3-way):
http://www.diyaudio.com/forums/multi...-confused.html
Hi jReave,

I missed this reply but thankfully i'm not making any cuts until Thursday

The revised Mid chamber dampening makes sense and will mean a little less woodworking too I can see that self adhesive bitumen pads are a much more expensive option, 100-ish for enough to dampen 2 cambers.

Here's what i've got in mind. Do you think its suitable?
https://www.amazon.co.uk/iNSpira-Vib...ducing+Damping

Update: I've located a cheaper and I think better product?
https://www.polymax.co.uk/rubber-she...und-absorbtion

Last edited by spark010; 25th September 2016 at 03:43 PM.
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Old 25th September 2016, 05:31 PM   #154
jReave is offline jReave  Canada
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Either one of those look like they would work fine in the mid chamber. If you wanted to also use some in on the woofer chamber walls, then the 1st one looks better, more specifically designed for that purpose.

Although it might look like it a 1st glance, I don't think there's much savings with the 2nd one for the same amount of volume of material. Plus I'm not 100% sure but it looks like the 2nd one may also require a separate glue purchase.

Both are still expensive in my opinion. I haven't done the calculations, so I'm not sure how much material you will need. Something that might also work is to combine layers of damping sheets with some layers of wood for the mid chamber, making sure that they are all glued together into one homogeneous mass. So say, 3 layers of 5mm sheets, then 1 layer of about 15mm wood, 3 more damping layers, etc. Or something like that depending on thicknesses of materials.

But then again, for the mid chamber, this strategy uses so much material, that both of the above listed damping compounds are overkill. Nothing wrong with that mind you. I'm a fan of overkill. But a simple diy remedy with some cheap heavy flexible vinyl tiles all glued together would be cheaper in the end. I think.

Your speakers. Your budget. Totally your call.
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Old 25th September 2016, 07:11 PM   #155
spark010 is offline spark010  United Kingdom
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Overkill yes but I like overkill too With regards to the wood layer option. I'm not sure if you have MDF in mind but wouldn't that increase stiffness as opposed to mass? I could create a homogeneous mass by combining thick felt, if you think that might work too?
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Old 25th September 2016, 10:43 PM   #156
jReave is offline jReave  Canada
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I don't think thick felt has enough mass for very effective damping. Better at absorbing internal waves and many people use it as such.

With layers of wood + damping material + wood + damping material, the wood layers are not combined directly, so there is no increase to each wood panel's stiffness. Instead it's like separate panels of wood (almost like separate boxes inside each other actually) that are heavily damped on both sides that are all glued together so all the resonances are lowered and heavily damped. Not quite as good as using all damping material but it should cut costs just a little bit.
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Old 26th September 2016, 06:17 PM   #157
spark010 is offline spark010  United Kingdom
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Quote:
Originally Posted by jReave View Post
I don't think thick felt has enough mass for very effective damping. Better at absorbing internal waves and many people use it as such.

With layers of wood + damping material + wood + damping material, the wood layers are not combined directly, so there is no increase to each wood panel's stiffness. Instead it's like separate panels of wood (almost like separate boxes inside each other actually) that are heavily damped on both sides that are all glued together so all the resonances are lowered and heavily damped. Not quite as good as using all damping material but it should cut costs just a little bit.
That's all makes sense. I'm going to go with all dampening material rather than layering wood and bitumen. I've located a distributor of the self-adhesive pads and i'm waiting for costs. Fingers cross, it may come in cheaper than the Amazon purchase

http://www.acoustic.co.uk/userfiles/...eet-BIT(2).pdf
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Old 26th September 2016, 07:59 PM   #158
spark010 is offline spark010  United Kingdom
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For reference, if anyone is looking for a UK suppler of DS 10 Acoustic Damping Sheet (5mm bitumen sheets with adhesive backing), most of the product for sale online seem to be manufactured by CMS Danskin UK
Type DS Acoustic Damping Sheet - CMS Danskin Acoustics

I've ordered 2 x DS 10 sheets from Insulation Express | Specialists in Insulation and Building products

jReave: My calcs for how much dampening material i'll need per cab come out 0.9 square metres. That's 5 x 5mm sheets layered, giving a total depth of 25mm per side of the mid range enclosure.

We have talked/you've already advised that the rear panel will need a different material. However one thing i'm unsure of is; the back of the baffle panel. Should I use the same material as the back panel or layer bitumen sheets?
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Old 26th September 2016, 11:24 PM   #159
jReave is offline jReave  Canada
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Sorry, I don't seem to recall mentioning anything in particular about the back panel. More cabinet depth equals more room for insulation equals more absorption of the back wave, so I wouldn't increase thickness very much at the back at all. Maybe add a single 5mm sheet of damping and that's it. Catch me up on what you are thinking.

Front baffle panel? I was thinking about this the other day. Again, to keep the resonance frequency low for the mid baffle, maybe it's better to just double up the thickness of the woofer front baffle panel and keep the mid baffle 1 layer of wood only and then yes, add some damping to the back of it instead. That should keep it pretty quiet. Make sure that there is also about a 45 degree chamfer around the backside of the mid in both the wood and the damping pads so that it has plenty of room to breathe back there.

Might be a good idea to draw up a little diagram so I can have a look at it before starting in on the work?
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Old 27th September 2016, 04:37 PM   #160
spark010 is offline spark010  United Kingdom
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I may have misunderstood but Roxul fibre board was a suggestion for the back panel? Its easier for me to use the bitumen sheet, so i'm good to go with that.

I'm mocked up two images for the internal dampening layout. Vertical is present in the cab but not included in the mock ups.

1. No chamfer
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2.
With a 45 degree chamfer. Is this what you mean?
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I've had to order the DS 10 bitumen sheets from amazon, the supplier I located had a minimum order of 10 sheets. There not much difference in cost at all.
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