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Multi-Way Conventional loudspeakers with crossovers

Help 8" coax project.
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Old 18th November 2016, 10:27 AM   #1
Drummer 35 is offline Drummer 35  Spain
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Join Date: Oct 2016
Location: Madrid, Spain
Default Help 8" coax project.

Hi!

I want to build a pair of speakers for my room.

Bought a pair of 8" cones but had problems trying to find tweeters that could match that high SPL...Long story short. I returned them and got a pair of 8cx300ND drivers and a pair of fd250 crossovers.

DSC_2624.jpg
DSC_2628.jpg

And here, the specs.

Beyma Coaxial Speakers - Beyma 8CX300Nd/N coaxial speakers - Beyma 8CX300Nd/N 340 watt 8" coaxial speakers for all 2-way applications. Beyma 8CX300Nd/N coxial speaker and other Beyma 10" coaxial speakers here.

Beyma Crossovers - Beyma FD-250 crossover - Beyma FD-250 2-way passive 2.0kHz crossover. The Beyma FD-250 crossover is rated at over 500 watts RMS. Beyma crossovers here.



Those drivers are used in the Dynamikks 8.2 and 8.3 monitors and I hope I can get a good sound using them.

Need your help to design the cabinets.

I need they have a certain angle, like Sonus Fabers. Something like that:

new speaker.jpg

And they will be positioned like this.

new table-speakers.jpg

I think you get the idea.

With the dimensions you see in the photos the cabinets are around 30 liters volume.

Id like to know if Ill have reflex problems or distortion for using that shape for the cabinet and how would you tune that box for those drivers. I mean, what is the lowest freq I could achieve without damaging the speakers.


Thanks a lot for your help.

David

Last edited by Drummer 35; 18th November 2016 at 10:40 AM.
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Old 18th November 2016, 04:21 PM   #2
Drummer 35 is offline Drummer 35  Spain
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No ideas? I wanted to start cutting wood this weekend...
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Old 18th November 2016, 04:44 PM   #3
brig001 is offline brig001  United Kingdom
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Really you want to try a program like WinISD. It will model your speaker in the cabinet to see the frequency response. It can pick an optimum cabinet volume and reflex port and you can tweak them to see how the frequency response changes.

The cabinet shape looks good - you need to work out how you are going to brace it to make it rigid and how you are going to line or stuff it to help with internal reflections.

Brian
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Old 18th November 2016, 04:45 PM   #4
PeteMcK is offline PeteMcK
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they're fairly low Q drivers, to get lows out of them you'll need to use an EBS alignment, how big a cabinet can you put up with?

the Dynamikks 8.2 you refer to also have a helper woofer for the low end.... are you going to be using subs?
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency

Last edited by PeteMcK; 18th November 2016 at 04:48 PM.
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Old 18th November 2016, 04:57 PM   #5
Charles Darwin is offline Charles Darwin  United Kingdom
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Why didn't you buy the crossover Beyma makes specifically for your driver (FD-2CX)?

As for the box you can make almost any shape as long as you stick to the required volume. In your case I'd say 20-30L tuned to driver Fs (61Hz).

That'll give you the EBS alignment Pee mentioned. Standard alignment gives you a tiny box (<10L) and no bass to speak of.

Last edited by Charles Darwin; 18th November 2016 at 05:00 PM.
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Old 18th November 2016, 05:16 PM   #6
PeteMcK is offline PeteMcK
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Your cabs at ~45L need to be tuned in the low 40s to get good bass, but will still be ~6db down in the lower region, will need some tone control boost
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 18th November 2016, 05:22 PM   #7
Drummer 35 is offline Drummer 35  Spain
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Brian, I tried winISD and for that cabinet volume, if I want to get 45-50 hz. The graph shows a big db drop at lower freqs. Dont know what im doing wrong or how to improve that.

Regarding internal reflections, how can I avoid them? Some foam inside the cabinet? But, doing that wont reduce the internal volume? Ebs aligment? I have to study that. Sorry, but dont know what is it.

Pete, those 8.2 dont have subwoofer, so Id like to build something like that.

I bought that crossover because the Fd2cx has a 2,6khz xo freq and the seller told me is too much for an 8". Do you think is right or should I use other xo model.

Thanks a lot for your answers.

Last edited by Drummer 35; 18th November 2016 at 05:28 PM.
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Old 18th November 2016, 06:11 PM   #8
Charles Darwin is offline Charles Darwin  United Kingdom
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Not sure that forcing a 45Hz response out of a driver with an Fs of 61Hz is a good idea.
In fact I am almost certain that is not.

As for the EBS can have a visual of that if you use winISD or ISDonline.
Type in the required T/S parameters and click 'optimum' and 'draw it'.
This will give you a tiny box tuned to 90Hz or so. Below 90Hz the response drops at 24dB/oct.

Now change the tuning to 61Hz and increase the volume to 20-30L and click 'draw it'.
You'll get a response tha drops 3dB or so at 120Hz and then remains at that level down to 60Hz before dropping at 24dB/oct.


I would have thought it'd be a better idea to buy the xover Beyma designed specifically for this driver and use for their measurements rather than a generic one. It also includes the required level matching between tweeter and woofer.

Definitely use foam on the inside of the cab. Everywhere except the baffle and probably double behind the driver.
If anything foam will increase the apparent volume to the driver but only by a minimal amount.
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Old 18th November 2016, 06:50 PM   #9
PeteMcK is offline PeteMcK
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'Dont know what im doing wrong' - nothing, it's just the nature of that driver, (low Qts), my preference would be to use them in a 3 way...
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 18th November 2016, 07:12 PM   #10
Drummer 35 is offline Drummer 35  Spain
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Thanks Charles, will try that on winISD when I get home.

Will take your advice and wont go as low.

The specific xover (Fd2cx) is designed for the 5,6 and 8 inches models. It is 2,6khz 24db slope. So I bought a 2khz 12db slope. It has jumpers to select -3db at 3,5k and -3,-6 or -7,5db for hf. I thought I could tune better that one, but now you make me doubt about it.

Do I keep the Fd250 or return it, thats the question.

Thanks!
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