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Old 27th November 2014, 06:48 PM   #1
ani101 is offline ani101  United States
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Question Open Baffle WTMW - design help

I am looking at doing a Open Baffle 3 way, WTMW.

The Woofers are going to be Alpha 15A, in U frame, crossed over at 200 hz
The TM are going to be Open Baffle. For now the T is Dayton AMT1, and the M is Dayton PS220. The PS200 crossed over to the 15A at 400 hz, and crossed over to the AMT1 at 5000hz.

I have a pair of each of the drivers on hand, and am planning to get another pair of the 15A

15A in Open Baffle has a sensitivity of around 90 db in U frame around 86 db, hence doubling on them, as after equalizing the T&M (heavily), would be around 90db, which would yield a system sensitivity of 90db and frequency response from 30 - 30,000 hz, or at least that is what I want to achieve.

One of the woofers is going to be close to the ground, so would be benefiting form floor bounce and radiating in 2pi space. The U baffle Alpha 15A sensitivity of 86 bd is after taking the floor bounce into consideration. So, the bottom woofer with floor bounce is 86db, what about the top woofer? Since it's center will be almost at 4 feet height, in a U frame it would be radiating in 4pi space??? So Floor / ceiling bounce can't be counted up - so it's sensitivity would be lower??? What would be the bass sensitivity?

Doable? Can be simulated and measured?
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Old 28th November 2014, 07:01 PM   #2
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Speaker surgery -- seems necessary to use the Dayton AMT1 and PS220 -- in a high quality dipole speaker.
AMT1. Removing the rear white absorption material would appear all that is necessary for decent dipole radiation patterns. Reversable surgery.
PS220. Carefully cutting off of the whizzer cone seems necessary for the "optimal" sounding dipole radiation pappern. Probably best to avoid this non-reversable surgery until after some testing with the whizzer attached is completed. Might still sound decent.

T-M-MB---WW 4-way topology as used by Linkwitz LX521 and JohnK NanoNote II, with driver diameters smaller than Fdipole @Xover, has substantial research and measurement data which prove sonic advantages over most other arrangements. A 4" midrange is often used above the 8" midbass.

A W-frame with counter-force 15" woofers would reduce both vibration and 2nd harmonic distortion. Two 15” stacked woofers require a tall 30” M-frame, or a very tall 36” H/U-frame.

The AMT tweeter has a limited vertical polar dispersion, and hence should be close to 38"-42" ear level. Putting a second dipole H-frame above this would both look awkard and create powerful vibrations which would need some nifty engineering solution.
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File Type: jpg T-M-MB---WW.jpg (92.4 KB, 311 views)
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Old 28th November 2014, 08:33 PM   #3
xrk971 is offline xrk971  United States
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This can be simulated easily enough in AkAbak except for the AMT as it is not a standard electrodynamic voice coil tweeter. They cross over easily though as they are flat impedance. I agree with LS that it is odd/awkward to put W on top.
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Old 29th November 2014, 12:46 AM   #4
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Dayton PS220 .... a few SPL/freq measurements with whizzer attached.
Dayton PM220 .... SPL/freq measurement -- similar design but no whizzer

P.S. A large H-frame cabinet above the MT will nasty up the dipole polar response. The dipole baffle on the LX521 example is narrow and clear above the tweeter to allow a good figure-8 radiation pattern, free from reflection-bounces.
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File Type: jpg Dayton PS220.jpg (111.2 KB, 275 views)
File Type: jpg Dayton PM220.jpg (91.7 KB, 274 views)
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Old 29th November 2014, 02:50 AM   #5
ani101 is offline ani101  United States
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So the PM220 is a better fit than the PS220? What are looking for? How does the whizzer cone affect the radiation pattern - i had selected the PS220 since it is a dull rangeand would be able to extend high enough to meet the AMT1.

What if the PS220 is used without the AMT1? Would that make any difference? The AMT1 was to be used without the felt backing.

Thanks for the replies, i will re-consider the design, especially the W on top.

Last edited by ani101; 29th November 2014 at 02:54 AM.
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Old 29th November 2014, 02:58 AM   #6
ani101 is offline ani101  United States
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Quote:
Originally Posted by LineSource View Post
Speaker surgery -- seems necessary to use the Dayton AMT1 and PS220 -- in a high quality dipole speaker.
AMT1. Removing the rear white absorption material would appear all that is necessary for decent dipole radiation patterns. Reversable surgery.
PS220. Carefully cutting off of the whizzer cone seems necessary for the "optimal" sounding dipole radiation pappern. Probably best to avoid this non-reversable surgery until after some testing with the whizzer attached is completed. Might still sound decent.

T-M-MB---WW 4-way topology as used by Linkwitz LX521 and JohnK NanoNote II, with driver diameters smaller than Fdipole @Xover, has substantial research and measurement data which prove sonic advantages over most other arrangements. A 4" midrange is often used above the 8" midbass.

A W-frame with counter-force 15" woofers would reduce both vibration and 2nd harmonic distortion. Two 15” stacked woofers require a tall 30” M-frame, or a very tall 36” H/U-frame.

The AMT tweeter has a limited vertical polar dispersion, and hence should be close to 38"-42" ear level. Putting a second dipole H-frame above this would both look awkard and create powerful vibrations which would need some nifty engineering solution.
Thanks, i will read up more on these two designs. Is the third image in the diagram a recommendation?

Regards
Ani
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Old 29th November 2014, 04:08 AM   #7
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I PROMISE..... that if you read JohnK's tech paper on "Dipoles and Open Baffle Design Considerations"

Dipoles and Open Baffles

you will decide to build a 4-way dipole speaker similar in structure to the design in my first picture.
---------
I PROMISE.... THAT IF YOU ALSO READ:
NaO Note II RS Details

you will realize that years of genius engineering was invested to get the SCARY GOOD measured data, and decide to use this build to learn from the best.

CROSSOVERS will be the challenge. Do you plan to use passive vs. active(digital vs. analog) crossover?

If you want to save money on drivers... you can start with the 15"ers and PS220 and AMT1, and just purchase a pair of modest cost 4" midrange.

If you do not have measurement equipment, you may decide to copy the 3-speaker top from the LX521 or NanoNoteII with their MiniDSP Xover file.
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Old 3rd December 2014, 05:16 PM   #8
ani101 is offline ani101  United States
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Quote:
Originally Posted by LineSource View Post
I PROMISE..... that if you read JohnK's tech paper on "Dipoles and Open Baffle Design Considerations"

Dipoles and Open Baffles

you will decide to build a 4-way dipole speaker similar in structure to the design in my first picture.
---------
I PROMISE.... THAT IF YOU ALSO READ:
NaO Note II RS Details

you will realize that years of genius engineering was invested to get the SCARY GOOD measured data, and decide to use this build to learn from the best.

CROSSOVERS will be the challenge. Do you plan to use passive vs. active(digital vs. analog) crossover?

If you want to save money on drivers... you can start with the 15"ers and PS220 and AMT1, and just purchase a pair of modest cost 4" midrange.

If you do not have measurement equipment, you may decide to copy the 3-speaker top from the LX521 or NanoNoteII with their MiniDSP Xover file.

The more i read, the more more i am leaning towards multi driver setup. Do you have any thought / opinions on driver beaming and directivity?

Would using a small 4" provide more benefits over the 8"'s whizzer cone? In this case, i might just substitute a normal 8" driver instead of the Full/Wide range PS220, as now with the 4" it would have to cover such a large FR.
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Old 3rd December 2014, 08:14 PM   #9
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A typical 8" speaker starts to beam(narrow directivity) at about 500Hz, where the cone width is about 1/4 the sound wavelength. The Dayton PS220 Voice Coil SPL/freq data shows this. The 8Polars measurements shows this.
At about 1/2 wavelength(1000Hz), the beaming can easily be heard by ear, and most full range speakers are angled-in 15-30 degrees so the listener hears the flatter off-axis SPL sound. A whizzer is a funky way to extend frequency response(compare PS220 vs. PM220 Voice Coil plots), but every point on the 8" cone still vibrates and can create resonant nulls = beaming.

The ideal dipole speaker has identical front-radiation and rear-radiation patterns. This generates the optimum figure-8 polar pattern with 4.77db more direct sound to the listener than indirect sound to the side-walls. Rear motors and front whizzers change front/rear radiation patterns... not good.

--CROSSOVERS will be the challenge. Do you plan to use passive vs. active(digital vs. analog) crossover?
--Do you want learn by cloning the 3-way top from LX521 or NanoNoteII?
--Do you want to select drivers and build, then measure... tune... measure... tune?
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File Type: jpg 8Polars.jpg (108.9 KB, 181 views)
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Old 8th December 2014, 01:41 AM   #10
ani101 is offline ani101  United States
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Thanks for the quick explanation. The graphic is very informative.

I am going to start out with active amplification.
Cloning looks like an attractive way to start, but i would also like to measure and tune. Active or Passive both would require measuring and fine tuning.

do you have any tips for measuring for OB vs Boxed speakers.

For Boxed speakers, the driver axis is about 4ft off the ground and then measured gated at 300Hz, indoors. Measure each driver without the crossover on axis nearfield (close to dust cap) and then plugin the crossover and take system measurement on tweeter axis at around 24 inches far. Do similar measurement technique apply for OB or do we need to do something different?
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