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Offline dcathro  
#1 Posted : 17 October 2009 07:40:58(UTC)
dcathro


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Joined: 18/09/2008(UTC)
Posts: 517
Location: Melbourne

For the last 3 years, I have been using a pair of very good small 2 way speakers - Orelle Orators, designed by Chris Bryant and Martin.

I used to use a pair of large floor standers in my studio in London, but since moving to Australia I have been very content with the Orators.

As it is my 50th Birthday in a few months, and people want to spend money on me, I have decided to build a pair of speakers. I was going to cheat and buy a pair of second hand speakers that I like and just rebuild/modify them, but have now decided to go the whole hog.

Although this will mean a lot more work, and take a lot longer to complete, I hope that I will actually get a better result from this than I would have from modifying a commercial speaker.

I was (for about 7 years) using a pair of reworked Alon Phalanxs, and did really like the openness that the open baffle gave to the midrange and top end. The things that I was less enthusiastic about in these speakers were the complexity of the 4 way design and getting the drivers to integrate, the metal dome tweeter, and the relative inefficiency. I have been listening a lot to light paper cone speakers (Lowthers and Coral Beta 8s) in other setups, and have been very impressed with the life and dynamics.

To that end, I have decided to do a high efficiency semi open baffle speaker, with a light paper cone midrange, and tweeter mounted in a panel, and a bass in a vented box underneath. This will be a 3 way design with an average efficiency of 94db.

Obviously there will be an inductor on the base to roll it off somewhere aroound 200, depending on what th open baffle does to the bottom end of the W8

Suggestions as to what to use for the crossover parts would be most welcome. I have heard that the ClarityCaps are good, but would not know which ones to go for. I am going to try and match the efficiency of the drivers, and hpe not to need and padding resistors.

The Drivers

The Tweeter and Midrange will be the Seas Exotic T35 and W8

Seas Exotic T35 Tweeter

Seas Exotic W8 Woofer

Here are a couple of pictures:

UserPostedImage

UserPostedImage


I intend to match these with a simple first order crossover at 1.5 or 2 KHz with a not filter on the tweeter resonance (600Hz). High power is not one of my design criteria. Unfortunately the tweeter cannot be made open baffle, but the simple matching to the woofer is a bigger priority. Also as I have stated previously I like Alnico drivers.

The W8 has an Fs of 36Hz, but in an open baffle, I will get little bass under 100Hz. However the driver has an Xmax of 22mm, so I am hoping to run them without any crossover going into the bass driver.

To get high efficiency and bass extension is very difficult without having a very very large box. I am going for a cabinet between 170 and 200L, and will still be only getting a -3db of around 40Hz. To do this requires a high efficiency 15" driver, and I have taken a real punt by buying a pair of these:

Kilimanjaro Field Coil 150VWLs

UserPostedImage

UserPostedImage

These were not cheap (thankfully the high aussie $ is helping a lot at the moment), but I think these drivers might just give me the spead and quality that I am going to need to match up with the Exotics. The tunability of the field coil will also help with mating the bass. As a back up, I have also bought a used pair of more traditional JBL 2215a alnico 15' foam surround drive units for about AU$1K (landed). I looked at buying other new bass drive units, but JBL's current 15" alnico driver, the 1500AL, cost a cool AU$6000 per pair!



The Cabinet


As stated above the cabinet has to be big to get extension, but I do not want a pair of enormous and heavy monsters in my lounge. Also, I want a speaker that is very fast and lively. Therefore, I am going to take a different approach to the box.

Th cabinet will be constructed with a 2x2 inch pine frame at 480mm wide, 600mm deep and 750mm high, following the golden ratio of 0.8:1:1.25. This frome will then be covered by the cabinet walls of hooppine (local equivalent of birch) marine ply. The front baffle will be only 15mm thick, but with a sandwich construction of ply and aluminium (can anyone suggest a better metal?), whilst the walls will be only 3 mm thick. The cabinet will also have no or little damping!

To stop this thing going off like a cello, I am going to pinch an idea from the french speaker maker Ocellia and use torsion boxes to make the cabinet rigid without adding great mass:

UserPostedImage

UserPostedImage


Mounting the Open Baffle


The Alons had the panels attached to the bass bins via rubber isolation mounts that were tensioned to get the right isolation. Most other open baffle speakers including the new Nolas use a sigle large baffle. I would like some form of isolation between the bass and the midrange and trebble - isolating the tweeter from the mid might be too difficult (but suggestions welcom :)).

I was thinkin of having a contured ply (15mm metal reinforced) sloping (~10deg) panel with angled (60deg) metal support struts ftom the middle/top of the panel down to the end of a metal base (not terribly heavy) that was also fixed to the panel bottom (sorry no pictures yet :)). This would be a bit like an open frame cabinet. The idea then would be to have these sitting on an isolating ball bearing system (as recommended by Barry Diament), which I current have under the Orators. This will allow me to play around with the alingnment, and will also allow me to easily swap out the open baffle head unit and effectively create a different speaker. With swappable simple external crossovers and bass drivers tunable for efficiency, I could have a very versatile speaker system.

An Alternative Head Unit


An alternative open baffle head unit that could be interesting would be a pair of these:

UserPostedImage

UserPostedImage

UserPostedImage

These photos were borrowed from here:

Audio Circle

These are a new Field Coil Lowther that look very tempting. Nelson Pass is very keen on open baffle lowthers mated with high efficiency 15" bass drivers, and for all their issues, I have to say that I have never heard a driver sound as alive in the mids as a Lowther.

Sorry for the long and rambling post, suggestions and comments welcome.

Cheers

David


Offline frank23  
#2 Posted : 17 October 2009 10:17:19(UTC)
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wauw, isnt't the DIY section the greatest!

what a project!

I have about completed mt JBL project using the 2235 15" in a 150l cabinet, the 2123h 10" paper mid (100dB sensitivity) and a compression driver on top.

About JBL bass drivers, the 2235 is regarded as being the best classic JBL for low end duties as it has an Fs of 20Hz. You can make one by reconing a 2231, 2234, 2235 or 2225 15" chassis as they all share the same chassis and magnet and pole plates. The 2235 has double the xmax of that of the 2215: http://www.jblproservice.../Theile%20Parameters.pdf

Maybe you want to read the Linkwitz pages on his open baffle project. His bass is also open baffle, very difficult to achieve. He has added a 2nd tweeter facing rearwards now I believe: http://www.linkwitzlab.com/orion_us.htm

I would worry most about driving the mid full range in an open baffle without a lower crossover as the excursion will become large and create much more distortion in the mid than needed. My system is using an active crossover, but that would mean more amps and more complexity. Linkwitz has gone all active too.

Maybe an in between route is a good idea. Using an active crossover for bas > mid and then use a passive crossover from mid to tweeter.

Btw, there is just too much to think about in your posting! Great project.

Frank

Edited by user 17 October 2009 10:26:02(UTC)  | Reason: Not specified

User is suspended until 09/12/2283 19:47:11(UTC) zonepress  
#3 Posted : 17 October 2009 12:37:01(UTC)
zonepress

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This looks like an exciting project, David, lots of fresh ideas. But I would agree with frank23: go active and most of the compromises forced on you by convention will go away. You'll have a far freeer hand to choose drivers on their merits and integrate them optimally. Maybe darkmatter will chip in with some of his own thoughts on this Paper-speaker-that-never-was project of his.
BigGrin
Sumer is icumen in!
Offline Henry Cleo  
#4 Posted : 17 October 2009 13:32:35(UTC)
Henry Cleo


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There's a very interesting thread at diyaudio.com about an 18 inch Goldwood bass driver in an H-Frame used with a Jordan JX92S on an open baffle. Not your combination of drivers, but lots of ideas and discussion.
Offline Martin Colloms  
#5 Posted : 17 October 2009 14:56:56(UTC)
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Hi David

I could advise anything, but will try to stay on track!

For the big woofer all you need is enough volume for it to tune for Qt of about .35-0.4 in-box, and make the box as rigid as you can. Unless the baffle is huge the free field tonally balanced sensitivity of the mid is likely to be closer to 92dB. Will your woofer get there given that it its large and floor loaded and will have closer to a 2Pi sensitivity in your room?

For tweeter sensitivity matching I love auto transformers ( on a laminated air gapped audio transformer stack) using the shunt crossover inductor as the step element. For prototyping make a pair oversize with lots of taps, so you can experiment to your hearts content for level and crossover point.

You will need a basic sound measuring station, even third octave is a great help.

You may not be right to aim for a flat response but you still have to see what you are doing .

I have had good experiences with Mundorf caps , the rectangular mid price ones, while the top symmetrical designs are pricey but reliably good.

Your biggest problem is the 100Hz low pass crossover for the bass unit.

In practice, bite the bullet and go for full motional impedance compensation for such a low frequency,and due to diffraction the bass inductor may need to be 30 -70 % larger than the book suggests to shut the big cone up before it sticks its craw in the mid range.

Another favoured route is to drive the upper system direct and have a second amp to electronically low pass the bass driver. Then you can have any bass alignment you want and any effective sensitivity, making for easier system alignment.

My choice would be the latter despite the additional complexity. But I would not put the bass amp in the speaker, just leave it with the main electronic stack.

Martin C
Offline Simon Briggs  
#6 Posted : 17 October 2009 18:45:44(UTC)
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Great project and will follow with interest BigGrin
Offline dcathro  
#7 Posted : 17 October 2009 23:30:20(UTC)
dcathro


Rank: HIFI Veteran

Joined: 18/09/2008(UTC)
Posts: 517
Location: Melbourne

frank23 wrote:
wauw, isnt't the DIY section the greatest!

what a project!

I have about completed mt JBL project using the 2235 15" in a 150l cabinet, the 2123h 10" paper mid (100dB sensitivity) and a compression driver on top.

About JBL bass drivers, the 2235 is regarded as being the best classic JBL for low end duties as it has an Fs of 20Hz. You can make one by reconing a 2231, 2234, 2235 or 2225 15" chassis as they all share the same chassis and magnet and pole plates. The 2235 has double the xmax of that of the 2215: http://www.jblproservice.../Theile%20Parameters.pdf

Maybe you want to read the Linkwitz pages on his open baffle project. His bass is also open baffle, very difficult to achieve. He has added a 2nd tweeter facing rearwards now I believe: http://www.linkwitzlab.com/orion_us.htm

I would worry most about driving the mid full range in an open baffle without a lower crossover as the excursion will become large and create much more distortion in the mid than needed. My system is using an active crossover, but that would mean more amps and more complexity. Linkwitz has gone all active too.

Maybe an in between route is a good idea. Using an active crossover for bas > mid and then use a passive crossover from mid to tweeter.

Btw, there is just too much to think about in your posting! Great project.

Frank


Hi Frank,

I am aware of the 2235, and infact, the 2215a drivers that I bought were sold to me on the basis that they had 2235 cones in them. I then found out that 2215s cannot take 2235 cones, and the seller was mistaken. A bit difficult to cancel after they have shipped them from America.

I am also aware of the Linkwitz speakers and have heard the Plutos. I have never heard an open baffle bass that I liked. I think it is easier to do the bass in a box, however you can swap over to the OB in the low 100s to get thet fantastic open quality, free from enclosure colouration.

The ability of the W8 to run full out at the bottom is something I am going to have to test - really don't want to have to put a cap on it if I can avoid it, I want to keep this as simple as I possible can. Active drive is a possibility down the track, but I will run them passive to begin with.

Found out those Lowther drivers are going to be about US$5K a pair - ouch, but what a simple system that would be, and very tuneable with both drivers being field coils. Anyway that is something for the future, when the AU$ rules the world BigGrin .

Thanks for your thoughts!

Best Regards

David
Offline dcathro  
#8 Posted : 17 October 2009 23:32:46(UTC)
dcathro


Rank: HIFI Veteran

Joined: 18/09/2008(UTC)
Posts: 517
Location: Melbourne

zonepress wrote:
This looks like an exciting project, David, lots of fresh ideas. But I would agree with frank23: go active and most of the compromises forced on you by convention will go away. You'll have a far freeer hand to choose drivers on their merits and integrate them optimally. Maybe darkmatter will chip in with some of his own thoughts on this Paper-speaker-that-never-was project of his.
BigGrin


Thanks ZP,

Active is in the plan, but not initially. I am just too much of an amateur to do an active project from the start.

Regards

David

Edited by user 17 October 2009 23:34:35(UTC)  | Reason: Not specified

Offline dcathro  
#9 Posted : 17 October 2009 23:34:11(UTC)
dcathro


Rank: HIFI Veteran

Joined: 18/09/2008(UTC)
Posts: 517
Location: Melbourne

Henry Cleo wrote:
There's a very interesting thread at diyaudio.com about an 18 inch Goldwood bass driver in an H-Frame used with a Jordan JX92S on an open baffle. Not your combination of drivers, but lots of ideas and discussion.


Thanks Henry,

I will have a look at that. There are so many great DIY threads and ideas on the web, it is fantastic.

Regards

David
Offline dcathro  
#10 Posted : 18 October 2009 00:02:03(UTC)
dcathro


Rank: HIFI Veteran

Joined: 18/09/2008(UTC)
Posts: 517
Location: Melbourne

Martin Colloms wrote:
Hi David

I could advise anything, but will try to stay on track!


More the problem is that you go over my head, I am a real novice at speaker building!

Quote:

For the big woofer all you need is enough volume for it to tune for Qt of about .35-0.4 in-box, and make the box as rigid as you can. Unless the baffle is huge the free field tonally balanced sensitivity of the mid is likely to be closer to 92dB. Will your woofer get there given that it its large and floor loaded and will have closer to a 2Pi sensitivity in your room?


Are you saying that the bass is likely to be too efficient for the mids? That I am likely to have an area of depression in the low mids do to the baffle being too small? The JBL 150-4C on which the bass cone is based was designed to cross over at about 500Hz. The driver is naturally more sensitive in the upper bass and mids, and I am going to have to work to keep that area down and achieve low bass extension and sensitivity.

Quote:

For tweeter sensitivity matching I love auto transformers ( on a laminated air gapped audio transformer stack) using the shunt crossover inductor as the step element. For prototyping make a pair oversize with lots of taps, so you can experiment to your hearts content for level and crossover point.


Looking at the Seas Exotic graphs for the tweeter and mid, I would have thought that these things would mate together very well without the need for sensitivity adjustment - that is what I am hoping anyway, but I am probably being very niave! Laugh

Quote:

You will need a basic sound measuring station, even third octave is a great help.


Any Suggestions? Something cheap!

Quote:

You may not be right to aim for a flat response but you still have to see what you are doing .


Flat response would be nice, but not the primary aim.

Quote:

I have had good experiences with Mundorf caps , the rectangular mid price ones, while the top symmetrical designs are pricey but reliably good.


Thanks, I can try a few different ones.

Quote:

Your biggest problem is the 100Hz low pass crossover for the bass unit.

In practice, bite the bullet and go for full motional impedance compensation for such a low frequency,and due to diffraction the bass inductor may need to be 30 -70 % larger than the book suggests to shut the big cone up before it sticks its craw in the mid range.


Yes the crossover here is going to be the most challengeing, especially if I don't cap the W8.

Quote:

Another favoured route is to drive the upper system direct and have a second amp to electronically low pass the bass driver. Then you can have any bass alignment you want and any effective sensitivity, making for easier system alignment.

My choice would be the latter despite the additional complexity. But I would not put the bass amp in the speaker, just leave it with the main electronic stack.


Yes, agreed that this is the preffered approach. Unfortunately, I am already way out of my depth in attempting to build these things even passive. For example, I need to find someone who can help me make the cabinets, as I have no skills or equipment to do this.

Thankfully someone else (who really knows their stuff BigGrin) is working on an active project using the Seas Exotics. I will looking to adopt and modify their solution to work here down the track.

Thanks for your thought Martin.

Best Regards

David
Offline dcathro  
#11 Posted : 18 October 2009 00:03:14(UTC)
dcathro


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darkmatter wrote:
Great project and will follow with interest BigGrin


Suggestions would be most welcome Simon BigGrin

Regards

David
User is suspended until 09/12/2283 19:47:11(UTC) zonepress  
#12 Posted : 18 October 2009 00:53:06(UTC)
zonepress

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The problem with shallow crossovering is that it exposes tweeters to hazardous LF and also lets cone breakup be heard unattenuated. My assumption is that you'll be shooting for a series capacitor at the tweeter, a series inductor at the woofer and at least one low-pass-something at the mid, to keep all this peaky breakup subdued somewhat. The excursion demanded of a driver goes up with decreasing frequency at a 12 dB/oct rate. An inverse 12 dB/octave will merely keep this demand constant with decreasing frequency, and a 6 dB/octave slope will just cut its rate of ascent by half. It's a good thing you don't list high power handling as a priority. What Martin mentioned about the big coil needing to be a lot bigger than theoretical is related both to the woofer's rising inductance, and it's increasing directivity coupled with cone breakup as frequency goes up. Maybe you should look at Linkwitz's cascaded filter topology:
UserPostedImageUserPostedImage
It was conceived with higher slopes in mind, so in the case of 1st order filters it may be amenable to some simplification - or not!
If you go the SEAS way, your biggest issue is the W8's behaviour above 2 kHz. You may need to cross nominally way lower than this, and once this is nice and well-behaved, decide how to handle the tweeter. And I'd stick to series elements only, if possible. Loops inside crossovers make me antsy. To paraphrase Bob Dylan (of all people!), I know something is happening inside loops, but I don't know what it is. Do you?
Laugh
Sumer is icumen in!
Offline frank23  
#13 Posted : 18 October 2009 16:14:59(UTC)
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For 1/3rd octave analysis there is quite a simple to use system that I have bought myself, the Behringer Ultracurve DEQ2496 with a Behringer ECM 8000 measurement microphone. The combi only costs about 350 euro and combined with speaker building experience and common sense can be put to very good use.

It has many different functions, most too pro to use in the home, but the ones for a home speakerbuilder are:
- pink noise generator
- RTA (real time analyser) with 31 1/3rd octave bands and a great "average" setting which averages the last 5secs for every band so adjustments are easily made for analysing the pink noise output from your speakers using the microphone
- built in phantom power for the ecm 8000 measurement microphone
- graphic and parametric eq, even with auto eq, just push the button, but I like to do my own eq with the parametric equalizer

There is much more besides this, but of no real uses to a speaker builder. You can feed in analogue or digital (spdif optical or AES) and the output also does both. I use mine in the digital domain only as I find the DAC section itself isn't very good.

It is I think by far the cheapest and easiest system for a home speakerbuilder. The only thing I miss is an impuls measurement. And of course it doesn't show phase, but those are for when you want to invest in Clio...
Offline ashleym  
#14 Posted : 18 October 2009 20:52:03(UTC)
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We use

http://supermegaultragro...om/products/FuzzMeasure/

but this is for Macs.

Have you thought about the LAT subwoofers? They claim in the specs they are suitable for open baffle but dont give any guidance, perhaps an email to them might help? Their form factor, sorry -shape (I've been out at a design show with mrs a today), would add an alternative option to the finished speaker. The Qt is 0.55 and Fs 24Hz. I'm sure I've seen somewhere that for an open baffle woofer you need as high as possible Qt

http://www.tymphany.com/...ducts/pdf/LAT700-001.pdf

Edit- just remembered to check the sensitivity and they are 89dB/W.....

Edited by user 18 October 2009 20:57:03(UTC)  | Reason: Not specified

Offline mat  
#15 Posted : 19 October 2009 10:37:47(UTC)
mat


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Hi David,

Project looks great, plenty to keep you busy.

My 2 cents for what it worth. Go second order on the LP crossovers, and the HP to the tweeter. The drive unit links don't seem to work for me so I am making some guesses.

Quite difficult to shut up the bass driver and stop it invading the mid range, with just series inductance, because it response will typically be rising, you won't get a 6dB/oct rolloff with an inductor alone especially when cone break up is factored in. ( the cone might be self damping and be flat in baffle if so then ignore me.) If the cone behaves pistonic-ish it may need a shunt cap of 60mic ish to get rid of the break-up and achieve a 6dB rolloff.

First order to the midrange is usually fine. The LP crossover on the mid, needs to be far enough away from the HP crossover to avoid humping the output. Again may need to be second order to ditch the cone break up depending on how lively the cone is. Often crossing over at 2-3k with a two element filter acoustic roll off is more like third order, in which case a third order HP to the tweeter will give a better integration and power response, although a second order filter to tweeter could still work.

1st order crossovers often sound 'nice' but it is very difficult to get low distortion, tonally balanced sound without being in control of all the variables. Often to make first order work you need to design your own drive units with the set goals in mind, ie aim for exceptional out of band behaviour.

for the rest of us 2nd order is a minimum when it comes to LP filters.

Having said that, there is no doubt that for every passive component you add, some fidelity is lost, you have to make the blend between max detail retrieval and correct tonal balance. oh and avoid bloody resistors.

Best of luck david, please keep us updated!

mat.


Offline frank23  
#16 Posted : 19 October 2009 10:41:07(UTC)
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ashleym wrote:


this looks nice, it is 150$ so not too expensive, but what do you need for add-ons? a measurement microphone and phantom power unit / mic pre amp?

ok, more info here: http://www.supermegaultr...uzzMeasure/gs/index.html

Edited by user 19 October 2009 12:21:17(UTC)  | Reason: Not specified

Offline Simon Briggs  
#17 Posted : 19 October 2009 12:35:10(UTC)
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dcathro wrote:
darkmatter wrote:
Great project and will follow with interest BigGrin


Suggestions would be most welcome Simon BigGrin

Regards

David


Will do, and like this project and have thought about this kind of speaker many times myself BigGrin
Offline dcathro  
#18 Posted : 24 October 2009 08:23:44(UTC)
dcathro


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I went to a cabinet makers today - nice guy by the name of Adam.

They were not convinced about the torsion box and are trying to talk me out of it. They think that it will be prone to vibration, and will cost too much to build.

I have asked them to estimate the job as asked, and also for just plain 19mm plain marine ply walls - original recomendation from the the Bass driver designer.

This second option would have the wall screwed and glued to a 2" * 2" pine frame, with the back panel remavable. This option would simplify the whole thing.

What do people think?

Cheers

David

Offline frank23  
#19 Posted : 24 October 2009 12:21:09(UTC)
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I currently have the JBL 2235 15" in a standard JBL 4507 150 litre cabinet which is 19mm MDF with a 25mm MDF baffle. It is braced in a simple way and damped with some wooly stuff on the panels. Crossover is 4th order at 200Hz.

The cabinet is rather lively I think. I have the mid and high on top and you can clearly feel the low cabinet vibration through the mid and high cabinet.

But I don't really know what to do about it. For now I like the sound as it is. My biggest worry is that when putting in more bracing, resonance frequencies will move up and distort the midrange as the panel surface is really large (a few square meters), which is not what you'd want. Maybe just putting bitumen on all the panels is a good solution.

In your case, I'd think I'd go for the marine ply cabinet, add some bracing, glue diagonal strips of hardwood on the panels (for distributing resonances, I think I remember Tannoy used to do that in their large studio monitors) and put bitumen on the space left on the panels.

Edited by user 24 October 2009 12:24:51(UTC)  | Reason: Not specified

Offline mat  
#20 Posted : 26 October 2009 09:31:25(UTC)
mat


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a flat sided box is quite easy to damp.

bracing is good, need to keep the unsupported areas quite small, then add high mass bituminous type material, the automotive industry has many good examples of rubber based damping sheets, buy them and stick them everywhere. Overkill is usually the best approach. Woolly stuff will absorb some sound energy from the air, but is useless for panel vibrations. Woolly stuff should really be in the middle of the airspace where velocity is highest to be most effective.


Worry about the box modes, if they area problem they will clump and act at one frequency, that will fire straight back out through the cone, they won't be removed by woolly stuff, you need to break up the box volume with small baffles, it can be a bit trial and error.
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