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In Reply to: RE: There are at least two answers. posted by John Marks on May 13, 2015 at 10:53:36
@Marks.
How are they getting around baffle refraction with all those edges ....
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They have baffle diffraction, of course when the sound wave hits the edges. But so do almost all speakers. Even if the baffle is flat without raised edges there is diffraction when the sound wave hits the sharp edges of a baffle unless the baffle is properly designed and slowly curves around to the sides. This is expensive so it's usually only done on the super costly speakers that often cost like a car or worse.
Err not so quick, raised edges is about as bad as it gets for refraction and no , good baffle design is not super expense, but it does seems baffle edge refraction is, well expensive ..
What you see is a gap between the front baffle and the side walls. In all the Harbeth models I've seen in person, the side wall edges are flush with the front baffle, not raised. I'm not sure why the gap is there, other than to serve as a mount for the grille.
Anyway, the shape of the baffle edge only makes a difference at frequencies where the size of the geometrical features are a significant fraction of the wavelength. My guesstimate would be ~1/4 wavelength. The gap on Harbeth speakers looks to be around 1/4" or 0.6cm, so the presence of the gap shouldn't really have much effect below 10KHz. And even then, the presence of the gap is just going to change the pattern of ripples not their magnitude. Also, the tweeter's radiation pattern is somewhat directional at those frequencies, so the level of diffracted sound is already well below the direct output. I suspect it's sonically invisible.
In order to achieve a meaningful reduction in diffraction effects by rounding the edges, the edge radius has to be pretty large. Large enough to make wood and MDF enclosures impractical, and certainly impossible if you're following the BBC design philosophy of lossy cabinet walls decoupled from the baffle. A compromise for MDF construction is flat beveled edges like Joseph Audio uses. Note the bevel depth is quite significant; it needs to be in order to make a difference. IMHO all the DIY speaker builders who are using a router bit to put a 1/2" radius on the baffle are just wasting their time. If these guys really wanted or needed to suppress edge diffraction in their designs they would use felt.
Another thing to consider is that it might be better to factor diffraction effects into your design than try to suppress them entirely. A rectangular baffle with square edges is easy to model and there are spreadsheet calculators available. You can adjust the baffle dimensions and tweeter placement until the response is optimized, or maybe even use diffraction to help compensate for the tweeter's response. Designing for a complex curved or beveled shape requires CAD software and something like finite difference time domain or finite element frequency domain simulations. Without that you're just taking shots in the dark.
Hello Dave,Well , Yes ,
The pics i have seen of some of their models have shown raised edges, if i'm wrong then my apologies to Harbeth. As to baffle diffraction, i do agree you need large radius, we used to use at least an 3 inch radius and yes the issue is above 10K where the high frequency smearing takes place. A little baffle smearing is not bad, as it helps coherency between drive units and while felt does clean up the diffraction and high frequency detail , too much kills coherency and make the tweeter "I'm here" sound that some may hate.
As to designing with complex shape or curves , remember prototyping , yep, before and even after software, we actually had to do it...PS: I did take another look and yes it's a gap and then flushed, appreciate the correction, this particular model is 5 db down at 10K thou, so the refraction issues I'm assuming is not shown due to the lack of high frequency energy. I'm sure the tilt down is to give the speaker some balance .
Regards...
Edits: 05/15/15 05/15/15
Perhaps you were thinking of those. But they also had felt surrounding the tweeter.
And I agree that adding felt isn't always a good thing. But I've only tried it as an addition to speakers that weren't originally designed with it.
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