3.5 way speaker design, should I put the woofers together? - diyAudio
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Old 14th June 2012, 03:26 PM   #1
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Default 3.5 way speaker design, should I put the woofers together?

I am designing a 3.5 way speaker system (at least I think that is the correct term haha ) consisting of a planar-magnetic tweeter, 2" dome midrange, 6.5" mid/woofer, and a second identical 6.5" driver but used only as a woofer.
It may sound like a strange combination, but having reviewed the characteristics of the drivers I plan to use it should work pretty well. I will be using a pair of the Foster planar/ribbon tweeters, crossed over at 6K to a HiVi DMB-A dome mid, then that will cross at 1K to a HiVi D6G woofer (yes I know this is not an ideal crossover point, but compromises need to made).

The reason behind this theory, is looking at the response graph for the woofer, there is a strong resonance between 1-2k then the response drops off very fast. I plan to use the mid/woofer with a low pass at 900 Hz or so, so hopefully combined with the resonant peak and the rolloff from the filter should enable it to pass smoothly to the dome mid. There will likely be lots of trial and error, but thats part of the fun isn't it?

This leads me to my actual question regarding this project (all of the above is to put everything in context), is that I plan to use a second 6.5" driver as a woofer only, operating up to around 200 Hz (once again, trial and error and listening tests to find the best xover point) to augment the bass response, hence it being a 3.5 way.

In short, both the 6.5" drivers will only have a low pass filter, but one will be set to around 1K and the other to around 200 Hz. Is it okay to put these into the same cabinet together and have them share the same airspace, or would it be beneficial to put a divider between them and run as two independent enclosures? I thought about running the upper one in a sealed cabinet and the lower one in a vented box, but towards the lower end may get some phase differences between them so if I use two separate enclosures they will probably both be vented.
Or, would it likely not make much difference? I am pretty sure that I could get away with putting them together in the same airspace, as the cone excursion required by frequencies >200Hz should be small enough that the back wave from the woofer shouldn't have much effect, because the top woofer will still be carrying all the same info as the lower woofer, just with an extended upper range.

I want to do this properly, and I value any relevant input from others who either have more knowledge than I or have undertaken similar projects and know what works. Sorry if my post is a bit too long, I try my best to explain the situation in full to give a clear view of my intentions and help you to visualise the scenario. Thanks you for your time, any help is appreciated as always
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Old 14th June 2012, 03:43 PM   #2
fpitas is offline fpitas  United States
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Is it okay to put these into the same cabinet together

If there's a problem, I'd think it would be from the midrange backwave getting out the woofer-only cone. A decent amount of cabinet stuffing (like Acousta-Stuf) would probably minimize that.

Last edited by fpitas; 14th June 2012 at 03:48 PM.
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Old 14th June 2012, 03:44 PM   #3
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by flyingtele View Post

In short......Is it okay to put these into the same cabinet together and have them share the same airspace
in short....yes, its ok they share the same box

its only the lowest frequencies that makes them move at all, meaning they will move the same anyway

now, if you would be using slightly differently specced woofers, which might be possible, then it 'could' be a different matter, depends
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Old 15th June 2012, 02:03 AM   #4
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Originally Posted by tinitus View Post
now, if you would be using slightly differently specced woofers, which might be possible, then it 'could' be a different matter, depends
Both woofers are identical, so that shouldn't be an issue.

Thanks for your input, it confirms what I had thought

And regarding the mids back wave being reflected through the woofer, I will be using internal baffles as well as stuffing to help break up and dissipate any standing waves, so that should take care of that problem.
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Old 15th June 2012, 02:14 AM   #5
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the crossover point for the '.5' woofer is best set at the baffle step frequency, = 115/ width in metres
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Old 15th June 2012, 02:23 AM   #6
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A single box is a poor effort, you should really isolate the woofer from the mid.

If not, you creating what is called a "passive radiator".
Passive radiator (speaker) - Wikipedia, the free encyclopedia

Adding stuffing will not be enough to dampen fluctuations in air pressure. Sound will travel from one cone to another in your existing design causing distortion.

In fact a true passive radiator would not suffer as much distortion as your design, as it only has one active element. In your design, you have two active elements working against, and interacting with each other.
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Old 15th June 2012, 07:49 AM   #7
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Originally Posted by Alvis View Post
A single box is a poor effort, you should really isolate the woofer from the mid.

If not, you creating what is called a "passive radiator".
Passive radiator (speaker) - Wikipedia, the free encyclopedia

Adding stuffing will not be enough to dampen fluctuations in air pressure. Sound will travel from one cone to another in your existing design causing distortion.

In fact a true passive radiator would not suffer as much distortion as your design, as it only has one active element. In your design, you have two active elements working against, and interacting with each other.
I don't see how I will be creating a passive radiator, the only difference between the signals the two 6.5" drivers will receive is the upper one will have more midrange, the bass content will be the same between them. And I doubt the midrange back-wave would have sufficient energy to modulate the cone of the lower woofer. In an actual passive radiator system, how much midrange information from the active driver comes out of the passive radiator?
Its not simply a midrange-only driver sharing the same cabinet with a woofer, that's just daft haha.

And to Pete McK, thank you for your suggestion, I will definitely consider that.
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Old 15th June 2012, 08:11 AM   #8
Boscoe is offline Boscoe  United Kingdom
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In all honesty I think this speaker will most probably turn into a mess, not because of your lack of skill or knowledge but because there is many crossover points and just dealing with one is hard enough. If your using passive crossovers (which are the work of the devil) the problem will be exaggerated.
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Old 15th June 2012, 08:36 AM   #9
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Would the two woofers be side by side or vertical?
Seems like if you want to port the lower woofer and seal the mid woofer you would want separate boxes anyway right? That would leave you with a separate box for the mid/tweeter and the tweeter is sealed so then you'd have only the volume to worry about on the mid.
I agree with the number of crossover points being tricky
It could seem a little redundant doing that but it may also be more efficient. You're sort of getting 12" worth of woofer. Like a 12" coaxial woofer idea...
Sounds like an idea to try though!
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Old 15th June 2012, 02:12 PM   #10
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Originally Posted by flyingtele View Post
I don't see how I will be creating a passive radiator, the only difference between the signals the two 6.5" drivers will receive is the upper one will have more midrange, the bass content will be the same between them. And I doubt the midrange back-wave would have sufficient energy to modulate the cone of the lower woofer. In an actual passive radiator system, how much midrange information from the active driver comes out of the passive radiator?
Its not simply a midrange-only driver sharing the same cabinet with a woofer, that's just daft haha.

And to Pete McK, thank you for your suggestion, I will definitely consider that.
I guarantee you will have midrange coming out the woofer despite the multiple crossovers.

A friend of mine owned a piar for years without even realizing it, until he went to recone them and said "Where's the magnet off these speakers??"

They have almost all the movement and sound of an active speaker.
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