Baffle edge diffraction is always a problem and there are several ways to combat it. Rounding edge, placing felt on the baffle, and so on. From what I have read, felt on the baffle is the most effective method but also the ugliest.
This had me thinking, why not make a speaker grille with some sort of felt or foam material integrated into the grille that sits flush with the baffle? Surely this shouldn't be too difficult to pull off. Just make a frame, wrap some cloth around it, and glue the damping material of choice inside the frame and account for the distance between the frame and the baffle because of the mounting method used.
Here's an example of what my speakers measure like, taken from my measurements when I made new crossovers for my speakers https://www.diyaudio.com/forums/multi-way/315184-crossover-current-speakers-6.html#post5272658
In my case I would aim to cover these areas with the material
There's not a lot of space between the drivers and above the top woofer so that will be a challenge, but otherwise I see no problems. As for mouting the frame to the speaker, I don't know what method to use. A lot of people use magnets under the veneer but I have no veneer, just birch ply.
Mounting the magnets in holes on the back side of the baffle is another way but hard to do on finished cabinets. But since this is intended to be something that improves the sound (and protects the drivers) the grilles will stay on all the time so using traditional speaker grille pegs wouldn't be too much of a problem for me I think.
I'm thinking a 12mm MDF frame (need at least 10mm for the grille pegs), and either a 10mm "Plastazote" closed cell PE-foam
What do you think? Is this actually a good idea or is it just waste of time and money?
This had me thinking, why not make a speaker grille with some sort of felt or foam material integrated into the grille that sits flush with the baffle? Surely this shouldn't be too difficult to pull off. Just make a frame, wrap some cloth around it, and glue the damping material of choice inside the frame and account for the distance between the frame and the baffle because of the mounting method used.
Here's an example of what my speakers measure like, taken from my measurements when I made new crossovers for my speakers https://www.diyaudio.com/forums/multi-way/315184-crossover-current-speakers-6.html#post5272658
In my case I would aim to cover these areas with the material
An externally hosted image should be here but it was not working when we last tested it.
There's not a lot of space between the drivers and above the top woofer so that will be a challenge, but otherwise I see no problems. As for mouting the frame to the speaker, I don't know what method to use. A lot of people use magnets under the veneer but I have no veneer, just birch ply.
Mounting the magnets in holes on the back side of the baffle is another way but hard to do on finished cabinets. But since this is intended to be something that improves the sound (and protects the drivers) the grilles will stay on all the time so using traditional speaker grille pegs wouldn't be too much of a problem for me I think.
I'm thinking a 12mm MDF frame (need at least 10mm for the grille pegs), and either a 10mm "Plastazote" closed cell PE-foam
An externally hosted image should be here but it was not working when we last tested it.
or a 12mm "speaker front foam" 
An externally hosted image should be here but it was not working when we last tested it.
and then wrapping it in stretchy fabric. 
What do you think? Is this actually a good idea or is it just waste of time and money?
Well, since that is precisely what Avalon have been doing (well, a variation thereof) with their grill designs for the best part of two decades -draw your own conclusions.
If you don't want to, short version: it can work very well when implemented carefully.
If you don't want to, short version: it can work very well when implemented carefully.
Oh, didn't know they did that. I've seen (and heard) Avalon speakers at expos but never had a closer look at their grilles. I will definitely look into how they are doing it.
No mystery about what they're doing: the grill is largely backed with damping material. Most Avalon speakers are (or were) designed to be listened to with the grills on for that reason. The specifics vary from speaker to speaker of course depending on the drivers, baffle shape &c.
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I tried felt on the baffle when I developed my PVC anti-diffraction frame. I didn't find felt effective. The frame works very well.
Diffraction Mitigation with PVC
One of the characteristics of diffraction-induced frequency response anomalies is that the on-axis and off-axis curves will cross: there will be frequencies at which the off-axis response is actually higher than the on-axis response (at the same distance). Response issues that are not caused by diffraction will not be solved by diffraction mitigation, so you may want to look for that.
Whatever you do, please take on and off-axis measurements, and let us know what works for you. Good luck.
Diffraction Mitigation with PVC
One of the characteristics of diffraction-induced frequency response anomalies is that the on-axis and off-axis curves will cross: there will be frequencies at which the off-axis response is actually higher than the on-axis response (at the same distance). Response issues that are not caused by diffraction will not be solved by diffraction mitigation, so you may want to look for that.
Whatever you do, please take on and off-axis measurements, and let us know what works for you. Good luck.
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