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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: Re: Duke...opinions please! posted by Duke on March 6, 2007 at 14:57:51:
Well you asked me to give my opinion on the downside of using a Cheney-esque slotted waveguide. In playing devil's advocate here, hopefully I won't totally rain on your parade - in the end I think it's worth a shot. Okay, here are the downsides I see:Thanks, Duke, you won't hurt my feelings...and I appreciate the fact that you took the time to respond.
1. As you widen the radiation pattern with the diffraction slot, you'll be spreading the energy thinner so to speak. And most of that panel area's output probably won't even make it through the slot. So the SPL in the sweet spot from that section of the panel will probably go down. Whether or not this will produce wierd effects, I don't know.Cheney says that he measures a 1.5 dB loss in power output overall due to the slot, since, being fixed at the perimeter, the center of the diaphragm makes the greatest excursions and is unmasked. He also says to eq the top end to compensate for the CD effect. I am hoping that I won't need to do this, as I will still have roughly 90% of the original surface area propagating as originally designed.
2. A diffraction slot may introduce coloration, but that coloration probably won't be audible at low or medium output levels. I definitely think the diffraction slot was an improvement on the VMPS speakers, so this probably isn't a big concern.I have not had the opportunity to hear the VMPS ribbons in either guise, but they seem to be well thought of. My listening is mostly at low to medium level.
3. I don't know the best way to deal with that portion of the output that's occluded by the plywood - whether to absorb it, or try to channel it out the sides, or what. I think Brian Cheney had plenty of extra efficiency in the planar driver he uses his diffraction slot waveguide on, so he could get away with lots of absorption.I think that the panels will probably damp the overlayed part of diaphragm to the point that only a little absorption will be necessary. Whatever energy remains will be either reflected back through the diaphragm, or continue through the masks, greatly attenuated and of low frequency.
4. Line source propagation holds up for something like four line lengths' distance. So if your slot is two feet tall, it should give you line source propagation out to about 8 feet. [In a case where the line source extends from floor to ceiling, the effecive line length is tripled because of the reflections off the floor and ceiling - thus also tripling the distance to which line source propagation holds up.I think that you are essentially correct, with the refinement that effective line source behavior is also frequency dependent.
5. The upper and lower unoccluded sections may act as two separate radiators, which can result in some vertical comb filter effects.This aspect is the most worrisome to me, as the whole point of this exercise is to get rid of combing in the horizontal plane...I will just have to see if the trade-off is worth it. What I might try is a mask that has a gradual transition from the full 3+3 panel width down to a narrow slot at listening height...I am thinking of an hourglass shape...Scarlet O'Hara in the corset scene with Mammy comes to mind. Sorry for the digression.
Now after all this negativity on my part, let me say that often acoustics and ears do not work the way the brain reasons out they should. So I'd say go ahead and give it a shot! The cost of failure is small in this case, and the possible benefits are large.Thanks for the encouragement, Duke. And as you say, the cost is low and the modification will be completely reversible.
If you do forge ahead and try it, let us know how it works!I certainly will.
Duke
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Follow Ups
- Re: Duke...opinions please! - David Yost 06:26:26 03/07/07 (0)
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