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Cabinet volume calculation


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Hi guys, I've got a project in mind for a hybrid electrostatic using ER Audio 505 panels and an 8" woofer.

I want the bass extension to be as low as possible without using ports or transmission line. I've done a lot of reading up on Thiele Small parameters etc but can't seem to find a definitive answer on the formula needed to work out the ideal volume of the woofer enclosure for a given set of driver parameters and desired f3. Can anyone help?

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Perhaps  stating  the T & S parameters of a given speaker that you would like to use  or  the cut-off frequency (F3) that you would like to aim for,an acceptable box volume etc.; would lead to suggestions of how to proceed with your intended project. 

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Perhaps  stating  the T & S parameters of a given speaker that you would like to use  or  the cut-off frequency (F3) that you would like to aim for,an acceptable box volume etc.; would lead to suggestions of how to proceed with your intended project. 

 

For this particular project I'm thinking an 8" woofer in a 30l container with an f3 of 30Hz, probably using some Linkwitz Transform goodness along the way. But really I'd like to know the equation so I know how to proceed for future projects with different variables. Any help on this front greatly appreciated. Teach a man to fish!

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It all appears impractical,especially if you require a seamless performance with electrostatic panels.Using D.B. Keeles' methods which incorporate normal  losses, an ideal C2  alignment would require a speaker with a Qt = 0.308,Fs = 12.99 Hz and a Vas = 150 l. This would yield a Qtc of 0.754 .I do not know of any commercially available 8'' speakers with these parameters.At this stage it might be better to contemplate using a 6.5'' speaker in a vented box if the enclosure size is a limiting factor.

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I don't suppose you have a link for details on this formula?

Ports are a no for me sadly. They are literally the antithesis of what ESL panels have to offer in terms of timing and phase response.

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Down load Winisd then enter your driver parameters.

It will give you box size for either sealed or ported .

Also it will graph the drivers response as you alter the volume.

Give it a crack and have some fun with it.

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If you enter D.B. Keele in your search engine,you will have free access to many of the  papers that he has generously made available.Sadly you are wrong in your general assumption of enclosure types. It is the performance area in the crossover region that is the primary concern in achieving a seamless transistion between the speakers. Providing a vented enclosure is correctly implemented it will certainly outperform a similar cost/size sealed box.I do not care for transmission lines and their related species as the  long delayed bass response is an inherent design flaw .

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Not all 8" drivers will be physically capable of a particular (low) F3, it depends on the driver's characteristics.

If you are set on sealed, and space is a concern, I'd suggest an isobaric arrangement, or better yet, an EQ'd enclosure, such as your previously suggested LT or similar.

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Not all 8" drivers will be physically capable of a particular (low) F3, it depends on the driver's characteristics.

If you are set on sealed, and space is a concern, I'd suggest an isobaric arrangement, or better yet, an EQ'd enclosure, such as your previously suggested LT or similar.

Yeah my understanding is that in order for the LT to work effectively the driver needs a big xmax and basically to be designed for small enclosures.

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If you enter D.B. Keele in your search engine,you will have free access to many of the papers that he has generously made available.Sadly you are wrong in your general assumption of enclosure types. It is the performance area in the crossover region that is the primary concern in achieving a seamless transistion between the speakers. Providing a vented enclosure is correctly implemented it will certainly outperform a similar cost/size sealed box.I do not care for transmission lines and their related species as the long delayed bass response is an inherent design flaw .

Yeah I agree which is why I was always surprised that Sanders ESLs use them. While less of an issue, there is still an inherent delay and hangover with ports that I want to avoid.

There will be some DSP going on that will delay the ESLs a bit to align them with the dynamic woofer.

I certainly don't have all the answers yet, but have simply found nothing on the market that suits my needs and space constraints, apart from John Watkinson's Legends but they're on the expensive side!

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Yeah my understanding is that in order for the LT to work effectively the driver needs a big xmax and basically to be designed for small enclosures.

 

It isn't specific to the LT .... it is just simply that if you want a direct radiating cone to play low frequencies at reasonable SPL, the cone needs to move a lot.

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Funnily enough I actually looked at these: https://www.madisoundspeakerstore.com/approx-9-subwoofers/sb-acoustics-sb23mfcl45-8-8-polypropylene-cone-sub-woofer-8-ohm/

 

I am wondering whether or not I can justify these though as they seem perfect: http://www.speakerbits.com/index.php?page=shop.product_details&flypage=child_special.tpl&product_id=3888&category_id=1208&keyword=scan+speak+8+inch&option=com_virtuemart

 

Talk me through the pros of going OB with the woofer, John. It's not something I've looked into much. Cheers

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Hi again, well, neither of those seems particularly well suited to your goal of 30 liters sealed. I mean, they will work, but I'd be inclined to look at those as suited for a ported cabinet. You can run the models on them yourself though. I don't think the former is available in AU yet although I suppose you could call always WES components and ask.

 

Maybe I shouldn't have mentioned OB, but the main advantage is that the radiation pattern will match the panel. Disadvantages are that you need significant volume displacement and a judicious combination of baffle size and EQ, as well as probably giving up your bass extension goal. With that said, with those panels it could work well in a "nearfield" situation. I used 1.2m when I had the earlier version of the panel.

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Hi John,

 

I'm actually avoiding the use of ports on this project. If you pump the numbers in to UniBox with a 30l cabinet you get a system resonance of 45Hz. Using these details I calculated the Linkwitz Transform using the miniDSP Biquad calculator and it looks like it only needs to push the lower frequencies up by 8dB to achieve the 30Hz I'm after. I appreciate that this will result in a significant increase in movement of the cone at the bottom end, but I'm hoping that the 14mm xmax of the Scan-Speak driver will accommodate this. SPL is not a major concern for me as my room is small.

 

Do you have any photos of your project that used the 440?

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I'm actually avoiding the use of ports on this project. If you pump the numbers in to UniBox with a 30l cabinet you get a system resonance of 45Hz. Using these details I calculated the Linkwitz Transform using the miniDSP Biquad calculator and it looks like it only needs to push the lower frequencies up by 8dB to achieve the 30Hz I'm after. I appreciate that this will result in a significant increase in movement of the cone at the bottom end, but I'm hoping that the 14mm xmax of the Scan-Speak driver will accommodate this. SPL is not a major concern for me as my room is small.

 

The 14mm is the X-mech, which is the amount of movement before something mechanical prevents it from going further.    Tellingly, sometimes called "X-damage".    ie.   don't plan on using +/-14mm   ;)

 

How low you can push the response depends on how much maximum SPL you want.    Using (dangerously) +/-14mm as the limit, then a fairly simple model (winisd) shows this driver will max out at ~98dB@1m with a -6dB point of 20Hz and 24dB/octave ....   and ~104dB if the -6dB point is raised to 30Hz.

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The 14mm is the X-mech, which is the amount of movement before something mechanical prevents it from going further.    Tellingly, sometimes called "X-damage".    ie.   don't plan on using +/-14mm   ;)

 

How low you can push the response depends on how much maximum SPL you want.    Using (dangerously) +/-14mm as the limit, then a fairly simple model (winisd) shows this driver will max out at ~98dB@1m with a -6dB point of 20Hz and 24dB/octave ....   and ~104dB if the -6dB point is raised to 30Hz.

 

Thanks Dave. In all honesty 98dB would be frankly ridiculous in my room anyway! Furthermore I'm pretty sure the panels would max out before the sub. I'll have to check with Rob though. Thanks for the advice. 

 

Tom

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Hi John,

 

I'm actually avoiding the use of ports on this project.

 

I know... the point is not that you should use ports, but that the woofer is not the best tool for the job that you have... :) The linear Xmax is 8mm, if you set that in Unibox and go to the "Closed Box" tab you will see a red line - that is the max SPL you can get at a given frequency. What's more, that is at 1m half space, take off 6 dB for full space and then another few for being in room instead of 1m.

 

Only you can judge whether SPL suits you, but if I were paying that much for a woofer I'd be wanting to use it optimally :)

 

Do you have any photos of your project that used the 440?

 

No, but it was a Dipole12 woofer in a panel sitting on the floor and the panels mounted at ear height. It was an interesting experiment and worked out pretty well. I dismantled it because I found the whole setup to be inconvenient in my cramped workshop. Never got around to trying a desktop setup with them.

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Hi John, 

 

For some reason I just get a vertical pink line for piston range. Am I doing something wrong?

 

So would you say that I should be looking at 10" woofers instead of 8" ones? It seems that I'd be asking a lot of the 8" drivers 

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Hi Tom, the SB29NRX75 I suggested earlier is a 10". I just had a look around and even in 10s nothing much else really jumps out, given the constraints you have given.

 

I dunno, maybe relax the constraints a little? Have you considered using a subwoofer? That would be one way :) You could use the HDP-Express to make the whole thing active  :D

 

I'm not sure what you mean about the vertical pink line but here's the plot I get for the Scanspeak:

 

post-145332-0-14264300-1459697340_thumb.

 

And for the SB29NRX75:

 

post-145332-0-59715800-1459697483_thumb.

 

I've put the cursor at the excursion limit at 30 Hz. That's with the linear Xmax spec of the Scanspeak (sorry I had it wrong before, it's 9mm not 8mm), if you use the 14mm Xmech then the line is almost where the SB29 line is. Don't get me wrong, from what I know the Scanspeak is an excellent driver but it's over twice the price...

 

Anyway. Oh, did you see that Rob McKinley has an 8" woofer on the 505 panel page? You could just start with that (another option...) and upgrade later.

 

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Yeah I did see that actually. I've been speaking to Rob about it. I guess this all stems from briefly owning Electromotions that had 8" drivers and were ported and feeling there was almost no bottom end at all!

I suppose where my confusion lies is that my B&W sub is approx 30l and sealed with a 10" driver and that goes crazy low without even sweating. Is this because the drivers are built to only handle the first few octaves at the expense of being able to reproduce anything above say 200Hz?

I could continue to use this sub as it's bloody great (and I'm hardly a 4 on the floor kind of listener!), but I'd like to be able to address the issue of the low end extension with the speakers I build ideally. It would also free up $1000 to put into the project if I could sell the sub!

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