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In Reply to: RE: Adding a tweeter to MMG posted by Satie on February 21, 2017 at 16:08:39
My purpose as you have intimated is to come up with an inexpensive mid/tweeter section for the GR-Research Open Baffle Subs, which I call a woofer section.
I am building the woofer section now, and am playing around conceptually with a dipole/open baffle mid tweeter section. One concept is the MMG, another is an open baffle line array, another is an open baffle MTM.
I may have line on an old pair of SMGs for cheap, I have ordered a pair of BG Neo 8 clones for playing around and I have a some other drivers from another project I never built. But unfortunately I am out of town on business until Friday, so I won't be building/buying anything until the weekend.
Follow Ups:
SMGs are too short to be a real line source, I suggest you drop that option.
The Neo8 clones from China have a serious primary resonance issue in the midrange around 300 hz which is rather minor with the original BG drivers. The second issue is that you need a multiplicity of Neo8 drivers to cancel out the very prominent 12khz cavity resonance - about 6 per side. But if these clones are well made they will give you an idea of the clarity you can get with this kind of driver.
Your idea for the MMG is solid but a 1.6 + a later addition of a ribbon tweeter will do much better. The 1.7 is already a 3 way and has a pretty good tweeter so it would be a waste to pay the premium for that when you intend to apply a ribbon.
I intend to xover the Neo 8 at a minimum of 1khz so 300 hz will be out the bandpass. I was wondering if the 12khz hump;
a) existed in the clone.
b) could be resolved with the notch filter outlined for the original Neo 8.
Yes, the 12khz cavity resonance is present in the clones, appears to be worse than in the original, at least initially, later measurements posted at madisound did not appear as bad as the earlier clones.
You can fit a notch filter just as you would for a single Neo8. Personally, I quickly moved to a line array once I heard the individual drivers since I did not want the phase shift from the filter and was aiming for a line array and for a crossover below the midrange, which I knew cancels the resonance at the far field. I did not want to lose the imaging in the vertical dimension that the line source provides.
I ran them to 300hz initially, and used them up to where their dispersion falls off at 5khz. Then I started running them further up and finally stopped low passing them in favor of letting them run and adding in the ribbon as a supertweeter as Apogee and Eminent Tech did.
Satie, Has anyone tried a mechanical solution to the problem? Something like damping the frame ala the "Razor" technique for Maggies.Or are you aware of the specific cause of this 12khz artifact?
Edits: 02/26/17
There is no way for damping to make a significant dent in the cavity resonance. It is formed by the physical spacing between the diaphragm and the magnet board. But it has one saving grace in that it does not have a syncronized on axis component. It is entirely random noise on axis so it cancels out as you increase the number of drivers. It is nearly gone at 6 drivers per side and entirely gone at 8 drivers per side. Waveguides can also be used to get cancellation of the cavity resonance peak with fewer drivers. Measurements I saw show it disappearing with 4 driver arrays in an 8" or so waveguide or constant directivity horn.
Satie, what is the shape of the wave guide, or do you have link to further info on this?
Here is the reference. With the CD horn/waveguide, you need to apply EQ to correct for falling response due to the horn geometry of 3db/octave starting about 3khz. CD EQ is a feature of many old electronic crossovers and all digital crossovers.Report with EQ is flt to 20khz and 3db higher sensitivity which brings you to the mid 90s db.
Edits: 02/28/17
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