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Old 12th December 2003, 11:59 PM   #1
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Default Bipole/Dipole and edge diffraction

I have designed and am in the middle of building a tower system with three woofers, a mid-tweeter, and a tweeter. The first two woofers and the tweeter will be front-firing (MTM, or possibly XOed, qaisi "Articulated Array, like Polk), the third woofer rear firing in Bipole (in phase with the front), and the mid-tweeter rear-firing in Dipole (out of phase) XOed from the Bipole woofer at 300Hz, which is the baffle step F3 of my eight-inch baffle. I figure that the Bipole woofer will take care of the baffle step while the mid-tweeter should provide a nice WIDE sweet spot. Have I missed anything? Also, is there any point, other than aesthetics, in rounding the baffle edges? Won't the Bipole/Dipole compliment cancel out diffrations?
Please critique,
-fortyquid
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Old 13th December 2003, 03:34 AM   #2
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Is this post a bait?

Isn't using dissimilar drivers to cover overlapping frequencies in order to obtain a wide sweet spot like climbing up the down escalator?
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Old 13th December 2003, 04:01 AM   #3
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>I have designed and am in the middle of building a tower system with three woofers, a mid-tweeter, and a tweeter. The first two woofers and the tweeter will be front-firing (MTM, or possibly XOed, qaisi "Articulated Array, like Polk), the third woofer rear firing in Bipole (in phase with the front), and the mid-tweeter rear-firing in Dipole (out of phase) XOed from the Bipole woofer at 300Hz, which is the baffle step F3 of my eight-inch baffle.

====

OK, so the speaker is ~6.38" deep?

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> I figure that the Bipole woofer will take care of the baffle step while the mid-tweeter should provide a nice WIDE sweet spot. Have I missed anything?

====

Hmm, how will the woofers be wired, and are they all the same? If all the same, it will take two on the rear to compensate for two on the front.

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> Also, is there any point, other than aesthetics, in rounding the baffle edges?

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It takes large radii to be of real benefit, basically you begin curving the baffle at the edges of the drivers, but even a small 3/4" radius will reduce 'hot spots', and of course it looks nice.

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>Won't the Bipole/Dipole compliment cancel out diffrations?

====

No, they reduce/eliminate baffle step loss, not the standing waves and 'hot spots' that occur due to discontinuities at the baffle edges.

GM
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Old 15th December 2003, 01:52 AM   #4
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Woohoo! Thanks, GM. Exactly what I wanted to know.

====

OK, so the speaker is ~6.38" deep?

====
I calculated that wrong on a number of different levels....;-)
I was (mis)using the following formula from http://sound.westhost.com/bafflestep.htm

f3 = 380 / WB
(where WB is the baffle width in feet)

Should I add the cabinet depth to the WB to cross over to the Bipole woofer?
Getting ahead of myself....
====

Hmm, how will the woofers be wired, and are they all the same? If all the same, it will take two on the rear to compensate for two on the front.

====
Sorry, I thought I made that clear. OK, let's put it this way. What's your advice for me? I have six 830341 5.25" Peerless woofers. FR~ 60-5000Hz, vented, RMS SPL = 100db considering excursion peak @ 80Hz. Very smooth FR, +- 2db. Znom = 4Ohm. I was thinking of putting two of them on the front of the cabinet, one vented, the other sealed, to conserve space. They would be seperated (XOed) at the baffle step F3, as mentioned a little later. Now, I love the dipole sound that I get by sticking some little multimedia speakers out of phase behind my mains; it seeems to give a much broader sweet spot and certainly a more open, spacious sound. However, my little drivers cannot afford a 6db/oct EQ below the relatively high frequency which my necessarily small baffles would dictate if I were to leave them open, a la Linkwitz (I've a fairly small room already stuffed to bursting.) So I thought I would go Dipole down to the baffle F3 (with the rear-firing mid-tweeter @ ~ 200Hz, off the top of my head) then X-over to Bipole with the rear-firing 830341 to take care of the baffle step, and eliminate the need for cancelation EQ inherent to the Dipole configuration. I figure that it won't matter that the mid-tweeter has different tonal qualities from the woofer/tweeter complement on the front because all the sound that you hear from the back of the cabinet has actually reflected a b-zillion times by the time I hear it, rendering the the minor differences in timbre functionally irrelevant.
So, what do you think? Have I done something stupid anywhere?
Thanks for posting,
-fortyquid
P.S. No, this isn't a bait. I just have limited resources. The 830341s aren't sold anymore.
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Old 19th December 2003, 03:48 AM   #5
GM is offline GM  United States
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>Woohoo! Thanks, GM. Exactly what I wanted to know.
====
You're welcome!
====
>I calculated that wrong on a number of different levels....;-)
I was (mis)using the following formula from http://sound.westhost.com/bafflestep.htm

f3 = 380 / WB
(where WB is the baffle width in feet)
====
OK. FWIW, using 4316.282" (359.7ft) is a little more accurate since it uses a speed of sound that's closer to a typical in-room temperature/pressure.
====
>Should I add the cabinet depth to the WB to cross over to the Bipole woofer?
====
Yes, due to the front/rear radiation meeting in the middle, so to speak.
====
>>Hmm, how will the woofers be wired, and are they all the same? If all the same, it will take two on the rear to compensate for two on the front.

>Sorry, I thought I made that clear. OK, let's put it this way. What's your advice for me? I have six 830341 5.25" Peerless woofers. FR~ 60-5000Hz, vented, RMS SPL = 100db considering excursion peak @ 80Hz. Very smooth FR, +- 2db. Znom = 4Ohm. I was thinking of putting two of them on the front of the cabinet, one vented, the other sealed, to conserve space. They would be seperated (XOed) at the baffle step F3, as mentioned a little later.
====
OK, so two are either in series for 8 ohms or in parallel for 2 ohms. Few consumer amps can handle 2 ohms, can yours? If not, then driver variances won't average out, and since Le sums, this must be taken into account when designing impedance compensation network.

So now we probably have an 8 ohm LF, 4 ohm lower mid, and no baffle step compensation since the 3dB higher voltage sensitivity of the single 4 ohm cancels out the bipole's acoustic gain, so you'll have to resort to an electrical BSC if your amp can't handle a nominal 2 ohm load.
====
>So I thought I would go Dipole down to the baffle F3 (with the rear-firing mid-tweeter @ ~ 200Hz, off the top of my head) then X-over to Bipole with the rear-firing 830341 to take care of the baffle step, and eliminate the need for cancelation EQ inherent to the Dipole configuration. I figure that it won't matter that the mid-tweeter has different tonal qualities from the woofer/tweeter complement on the front because all the sound that you hear from the back of the cabinet has actually reflected a b-zillion times by the time I hear it, rendering the the minor differences in timbre functionally irrelevant.
So, what do you think? Have I done something stupid anywhere?
====
Assuming I'm invisioning it correctly, probably not, except for my comments on the woofer situation.
====
>Thanks for posting,
-fortyquid
P.S. No, this isn't a bait. I just have limited resources. The 830341s aren't sold anymore.
====
You're welcome! OK, understood.

GM
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Old 20th December 2003, 04:32 AM   #6
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"since Le sums, this must be taken into account when designing impedance compensation network. "
Can't I figure the impedence of the dualMW complement by merely doubling the impedence as shown by my sim. for the single driver, as they are wired in series? (Your observation about impedence capabilities applies.)

I'm begining to thing I might prefer to XO to my woofers higher, (above the excursion dropoff) for lower distortion and I'm wondering if the baffle step will be exhibited at a) the 8"WB frequency, b) the 23.5"WB frequency (front + half depth), or c) not at all? SL said something about baffle step not being an issue, since the rear-firing woofer "contributed to the total output" at that frequency or something like that. Doesn't make sense to me; I would think that, the output being out of phase, it would contibute negatively, i.e. drop an additional 6db.... Of course, I might have read it wrong anyway. So what gives?
Thanks for all the extremely helpful advice so far!
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Old 21st December 2003, 04:17 AM   #7
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>Can't I figure the impedence of the dualMW complement by merely doubling the impedence as shown by my sim. for the single driver, as they are wired in series?

====

If it allows you to load Le to plot a fairly accurate one.

====

>I'm begining to thing I might prefer to XO to my woofers higher, (above the excursion dropoff) for lower distortion and I'm wondering if the baffle step will be exhibited at a) the 8"WB frequency, b) the 23.5"WB frequency (front + half depth), or c) not at all? SL said something about baffle step not being an issue, since the rear-firing woofer "contributed to the total output" at that frequency or something like that. Doesn't make sense to me; I would think that, the output being out of phase, it would contibute negatively, i.e. drop an additional 6db.... Of course, I might have read it wrong anyway. So what gives?

====

Sorry, but I didn't follow any of this.

====

>Thanks for all the extremely helpful advice so far!

====

You're welcome!

GM
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Old 21st December 2003, 06:50 PM   #8
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I just want to know where to expect the first-order HP rolloff, characteristic to the Dipole design, to start. SL has the calculations for open baffle, but not for dual driver/dual chamber setup. The main difference, I suppose, is the cabinet depth. I figure that I will get shelved HP for the transition from 2pi to 4pi dependant on the {baffle width} (a) dimension, then an additional constant 1st order HP when the front wave meets the rear wave out of phase, dependant on the {baffle width+cabinet depth} (b) dimension.
Is this correct?
How do I figure the (b) frequency?
Thanks again for the help.
-fortyquid
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Old 3rd April 2008, 09:15 PM   #9
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Quote:
Originally posted by fortyquid
...The 830341s aren't sold anymore.
In case you needed to get some spares, or just wanted to add some more, or maybe if you wanted to make some surrounds with the same drivers ~ these woofers are on clearance at Madisound:

http://madisound.secureserverdot.com...oducts_id=1757

Only $10, looks like a good deal...
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Old 4th April 2008, 05:37 PM   #10
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Thumbs down Your design sounds terrible mate

Sorry, but a rear firing midtweeter.... no mid firing front.... you're trying to do way too many different things. Ideally, one would want to do a single design style for a speaker, to keep the power response and whatnot consistent.


Sorry to be harsh, but really, it'd take years to make something like that even sound decent.
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