Diffraction

[mondogenerator]

OK, so the foam is actually absorbing the sound wave as intended, since it is porous.

lol i agree. youre both right. what mechanism, or what change occurs in the absorption mechanism is of relativley little interest (to me).

What occurs to me though, is that used in this way at an incidence of near zero absorption still occurs. some math nut can calculate it if they like, but when a point source is radiating say, between 2" and 8" away from the absorbent, only a certain amount is actually at exact parallel to the absorbent/baffle. even so, it has even more depth to penetrate thus more absorbtion. Even so, SO WHAT if 50% is DIFFUSELY reflected. far better than the 'pond skipping' that occurs with no baffle absorbent....

Even at grazing incidence, the absorption/diffusion of energy is still affected by the foam. Ive tried it, and im sure others have. in a small experiment i tried, A small ring of 1" melamine foam placed around a dome tweeter 1-2" from the diaphragm, almost totally killed output althogether. Its at times like those I wish i had some measuring gear

 

[gedlee]

The problem is, most people are not interested enough in the absolute response. Aesthetics take priority for the majority, I think.

Dave

Thats really the problem isn't it. "Damn those theorists, I know what I want!"

 

[dirkwright]

Even at grazing incidence, the absorption/diffusion of energy is still affected by the foam. Ive tried it, and im sure others have. in a small experiment i tried, A small ring of 1" melamine foam placed around a dome tweeter 1-2" from the diaphragm, almost totally killed output althogether. Its at times like those I wish i had some measuring gear

That's interesting. Do you think that the foam changes the total SPL output of the driver when mounted on the baffle? I wonder if the total SPL output we experience is a combination of what the driver is producing and what the baffle reflects. So, if there's an L pad in the crossover for the tweeter, then that would have to be adjusted if foam were added. Is that correct? I can see for a horn tweeter the foam should force the directivity pattern to stay in the intended region, i.e. 60x40 degrees, or what ever.

The way I see it now, the wave actually still propagates along the hard baffle surface, but must go through the foam, thus the wave is absorbed to some extent as it travels to the baffle edge. How much is absorbed as it travels appears to be a function of the foam's NRC, the wavelength, and the distance to the baffle edge. It would be ideal I also assume for the longest wavelengths produced by the tweeter to be extinguished by the time they reach the baffle edge, thus no diffraction at the baffle edge will occur. So, there would be some calculation involved in determining the ideal baffle dimensions with a given crossover for the tweeter and for a given foam type. I don't know how to do that unfortunately. I also assume that the foam absorbs sound in a decaying exponential curve shape with some kind of coefficient, but it's been a long time since I was in university calculus!

 

[dirkwright]

While true, the existing simulators are helpful. The BDS, linked earlier, was possibly the most advanced software available to DIY for years. Along with Jeff's newer software, one can examine any axis and get a reasonable simulation. They do use generic driver geometry and while it does not model it in 3D, it is sufficient since, as you point out, measurements are needed afterwards. But since anyone doing any serious design is going to measure, it's a bit of a moot point.


Precisely.

I would never go through the effort to build a baffle and driver cutout with recess and rear chamfer before modeling the basic baffle response. My experience with the BDS and other diffraction modeling software is that it is reliable enough for most work. I would like to see software that could reasonably model nearby driver diffraction, I'm not aware of any doing this, but then I am one who will not construct a system without some amount of diffraction control for this, even with my current dipole system.

The problem with any diffraction of this sort, baffle and driver, is that even were one able to model to high degree of accuracy, one must still decide on how to handle it. If no diffraction control is used, in this case requiring felt or similar between drivers, what do you do? It's a global issue. Treat the source and it's largely controllable. Short of that, you get into the problem of polar response. What good is a model of inter-driver diffraction if no diffraction control is to be used? Conversely, what good is a model when the inter-driver diffraction can be largely controlled? The latter works on a polar basis. You build, add felt, measure and you're done.

I will throw in my support for offsetting drivers. It's not the imaging issue some seem to think. It does not eliminate diffraction, of course, but it does aid in reducing it's impact if one designs the baffle and layout with care and intention. What is occurring is simply re-distributing its effects in time and thus also the frequency domains. The results can at times be such that across a reasonably window in the polar response, the magnitude is reduced. If in addition you use diffraction control such as reasonable roundovers or better (IMO), sufficient felt, the fact that the magnitude is reduced makes the combination of offset and felt extremely effective.

The problem is, most people are not interested enough in the absolute response. Aesthetics take priority for the majority, I think.

Dave

Does any of this software allow you to alter the absorption characteristics of the baffle?

 

[dlr]

Does any of this software allow you to alter the absorption characteristics of the baffle?

No. I had hoped to work with Paul (BDS author) to test felt damping on my large baffle for addition to the BDS, but never got enough time. Given the variability/consistency of some materials, especially felt, it would be an approximation, but useful to a degree. It would not help for inter-driver diffraction since the program would first have to include that option and none do to my knowledge.

But in my testing, damping of the wave as it propagates across the surface of any material such as felt/foam is minimal. The wave must pass through the material to be useful. This is why I often now only use strips of felt. Fully covering a baffle maximizes the effects, but with very rapidly diminishing returns.

Dave

 

[Vortex]

Great topic - just google'd some links around diffraction. If it has already been posted, sry, ignore.
Diffraction Doesn't Have to be a Problem - some tests with felt too..
Mr. Linkwitz' article about diffraction
Loudspeaker diffraction loss and compensation - by True Audio
A study-like math crazyness - by Tore Skogberg. The only one I haven't read in whole yet. Omg.

Now: confused than ever. But I'll try the play with the felt around mids and tweeters. Mic will show if a trick is working or not.

 

[DDF]

No. I had hoped to work with Paul (BDS author) to test felt damping on my large baffle for addition to the BDS, but never got enough time. Given the variability/consistency of some materials, especially felt, it would be an approximation, but useful to a degree. It would not help for inter-driver diffraction since the program would first have to include that option and none do to my knowledge.

But in my testing, damping of the wave as it propagates across the surface of any material such as felt/foam is minimal. The wave must pass through the material to be useful. This is why I often now only use strips of felt. Fully covering a baffle maximizes the effects, but with very rapidly diminishing returns.

Dave

Fully felt covering baffle and relatively small aperture to tweeter provides some strong off axis absorption down low to help control tweeter bloom. I think what we hear from felt is as much this as just diffraction control.
Attached a measure/experiment I made on one of my speakers with felt (this is the mcmaster carr stuff) covering the front baffle around tweeter, to illustrate.

 

[norman bates]

The music gets sharper with treatment.

I'm using foam, but look forward to felt.

Found f10 sheets (12" x 12" x 1/2") for $12.95 from grainger.
White though........

F10 Felt Sheets and Strips - Felt - Grainger Industrial Supply

 

[whgeiger]

Alternative Source

The music gets sharper with treatment.

I'm using foam, but look forward to felt.

Found f10 sheets (12" x 12" x 1/2") for $12.95 from grainger.
White though........

F10 Felt Sheets and Strips - Felt - Grainger Industrial Supply

Try
McMaster-Carr
Regards,
WHG

 

[norman bates]

Nice.
I see 12" x 12" x 1/2" for $22, but in grey.

Ty.

Got screws, now need to setup rendezvous with buddy to build tweet box, then foam the front of mine.

 

[Ron E]

But in my testing, damping of the wave as it propagates across the surface of any material such as felt/foam is minimal.

This is my experience as well. I tried it to see what felt would do for tweeter ripple caused by not flush mounting. I used about 4-5 layers of craft felt cut into star patterns and rotated between layers. There was surprisingly very little effect, as I recall. - only one type of felt was tried before I flush mounted it instead.