I think that one is pretty easy to explain.
If you apply a layer of felt to the front baffle it seems that the primary change is an attenuation of high frequency energy travelling along the baffle surface to reach the baffle edge where it would try to diffract.
The diffraction introduces baffle diffraction ripples in the front hemisphere response of the speaker through destructive/constructive interference etc... reduce the amplitude of the wave before it reaches the first sharp edge - in this case the baffle edge, less baffle diffraction occurs, so there is less disturbance in the response - less ripples due to diffraction. Nothing new there.
However if we line the side of the cabinet with felt instead of the front there will be no practical change for a couple of reasons. The first is that the horse has already bolted, so to speak - the high frequency energy has already travelled unimpeded across the front baffle and reached a sharp edge so diffraction into the forward hemisphere has already occurred before the wave even reaches the felt.
Even if there was a significant thickness of felt applied to the side - say 10mm, it will make little difference to the diffraction from the front corners because the total distance the wave travels as it rounds the corner (only half of which has any felt on it) is still very short compared to the total distance from the driver to the baffle edge that would otherwise have had felt.
The total felt absorption path length between driver and completing "rounding the corner" is simply too short. (How far around the corner counts as having rounded the corner probably depends on the frequency - the higher the frequency the shorter the distance past the corner that can be considered having "completed" going around a sharp edge ?)
What the felt on the side would be doing is absorbing the energy that had already diffracted around the front corners and was now heading towards the back corners to diffract from there. We're unlikely to hear that because (a) we won't have line of sight to the rear corners of the speakers and (b) the diffraction at the front corners is going to low pass filter the response of the wave travelling down the sides so that it doesn't have much high frequency content for the felt to absorb, and what low frequency content there is won't be effectively absorbed by the felt anyway. So the net effect of the felt on the sides - at least in the front hemisphere response is going to be next to nothing.
Yes, if you add felt around a tweeter on an infinite baffle you're not going to gain anything, and because the felt is not perfectly absorbent it will introduce a small amount of diffraction that wasn't there before, thus the deterioration.
However the exact same felt in the same place with a normal rectangular narrow baffle should be a net improvement - there will be the same small amount of diffraction from the felt itself, but a big reduction in the much larger diffraction from the baffle edge.
Makes sense.
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Even with felt, most of the wave is radiated into the forward hemisphere outside of the felt with only a small percentage passing through the felt.
I understand this with two comments. First is that I've found that it is not absolutely necessary to have a long length of felt for significant attenuation. I've recently found that as long as the felt is close, but not too close to a tweeter for instance, narrow strips placed around it are almost as good as full face applications. What is more important is depth. Halfway between the dome and edge can be as good, it's case dependent.
Two, when sound diffracts, it does "warp around" of course and should travel through the side felt, but it's apparently not as much as I had originally thought it would be. That's what surprised me, the nearly zero impact it had on-axis. I expected more than near-zero.
Dave
Ok, I hadn't read your article before posting my previous reply - now that I have I see you're using quite thick (10mm ?) pieces of wool felt.
I was thinking more along the lines of a fairly thin (1-2mm) layer of felt that was covering most of the baffle surface.
I think a distinction needs to be drawn between the effects of thin and thick felt - a thin wide layer would provide gradual progressive attenuation as the wave travelled along the surface of the baffle, while the narrow but fairly thick pieces you're using would have more of a "shadowing" effect where the outer baffle surface (including the baffle edge, hopefully) would be occluded from the view of the tweeter - leaving a dead area beyond the felt where there is no high frequency radiation on and just above the baffle surface.
Because the felt absorbs so well you won't get any significant diffraction around the felt blocks that would lead to high frequencies being redirected into the "shadowed" area including the baffle edge. That's why the narrow but thick strips are so effective, provided that they themselves don't diffract significantly.
The felt on the sides of the cabinet is physically occluded from line of sight to the tweeter, and the higher the frequency, the more "shadowed" the felt on the sides would be. If you look at a wave simulator the amount of wave diffracted at a sharp angle around the corner drops dramatically at high frequencies, so the wave travelling along the surface of the sides will be missing a lot of high frequencies compared to what travels along the front panel.
Low frequencies would wrap around tightly without difficulty but wouldn't be absorbed particularly well by the felt, meanwhile the high frequencies that the felt would have a good chance of absorbing will not wrap tightly around the edge and so will largely miss the felt, and by then the main source of diffraction has already occurred, so the on axis response change would be negligible.
I think it all simply comes down to absorbing the wave running parallel to the baffle before it reaches any edges of discontinuities to diffract off. If you can do that you've solved the problem.
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1/2" thick in the article except for the extra strips on the interior sides, 1/4", making that stepped. Stepped worked better, probably what Dunlavy found since most or all of his speakers used that. I've moved to preferring 1" thick (deep) felt strips rather than full baffle applications.
2mm would provide almost no effectiveness on typical baffles. Part of what's required is enough thickness to provide an initial "wall" so-to-speak. At least my testing indicates that.
The high frequency part is largely moot. The only frequencies that matter are those where the dimensions of the baffle are a significant percentage of the frequency wavelength. Narrow baffles affect higher frequencies, but at the same time smaller (less wide, not necessarily less deep) pieces of felt are still affective due to the felt effectiveness at higher frequencies.
True, but the lack of effectiveness is at all frequencies. The felt is not expected to work at higher frequencies since diffraction is not significant at higher frequencies. The effects I've strive to influence seldom go higher than 4-5KHz with the largest impact most often intended in the 2-3KHz area.
That's it in a nutshell. The issue is what geometry and felt properties work optimally.
Dave
The felt on the side of the cabinet sounds like a Catch 22 to me: The felt can't be seen unless there is significant diffraction at the corner. If there is significant diffraction then the frontal reflections will have already occured.
Thought of another way, the felt only has a chance to work on energy that has gone around the corner (heading into the rear hemisphere). Why would this have an effect on frontal energy?
LMS Cabinet Shop has diffraction modeling and you can turn it on and off. If you forget to turn it on and do a polar simulation then there is nothing when the mic swings past the baffle. This is to be expected but it is a bit of a shock the first time you see it.
David S.
Thought of another way, the felt only has a chance to work on energy that has gone around the corner (heading into the rear hemisphere). Why would this have an effect on frontal energy?
LMS Cabinet Shop has diffraction modeling and you can turn it on and off. If you forget to turn it on and do a polar simulation then there is nothing when the mic swings past the baffle. This is to be expected but it is a bit of a shock the first time you see it.
David S.
Because frontal energy is combination of direct (primary wavefront) and diffracted energy (time delayed secondary wavefront).
They key may be due to the fact that only a portion of the diffracted energy passes through the felt. This may even not be the case. As has been pointed out in other threads (I think it was DDF on the PE board), damping materials such as foam and felt (and stiff fiberglass, the topic at the time) have differing effectiveness based on angle of incidence. At more extreme angles, materials become more reflective, therefore the wave cannot enter it as effectively. This is what I think may make thin applications relatively ineffective and is probably why a layer on the side is nearly totally ineffective. Thick pieces on the front near to the driver have a much larger cross section that provides angle of incidence of close to 90 degrees. The closer it is to the driver, the larger the included angle of direct, high angle of incidence material presented.
Ignoring that, the reason why it's not a catch-22 is because diffraction measured on-axis is a result of the diffraction and that diffraction is not just a consequence of a wave expanding into 4-pi. That is only the initial positive going wave. Diffraction will be a continuous event, it's as cyclic as the main wave. This means that if the wave expansion could be absorbed to some degree in its positive going portion as it "turns", it will also be absorbed during it's other half cycle, thus reducing the total diffraction energy that is contributed to the front hemisphere.
The main step area cannot be significantly altered, the energy fully goes into 4-pi. The diffracted energy as measured is primarily a redirection of energy. If it's partially absorbed, there is less to redirect. Therefore a thick enough piece on the side should have, I thought, some beneficial effects. However, I found that it largely makes the baffle appear wider, almost the same as placing similar depths of mdf in the same position. This is an indication that the reason the felt is not effective is due to angle of incidence being nearly 90 degrees. The felt appears to be not much more than an extension of the baffle.
Dave
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Yeah, I was thinking that also. If you've got a fancy CNC cutter of some kind, get a chunk of foam and make it so!
Otherwise, buy a ready made plastic wave guide and line it with felt.
The more I consider it, the less I like the idea of such as waveguide. As Dr. Geddes has pointed out, the OS waveguide is probably the best form. One made of felt/foam will, I suspect, be less effective as much due to the angle of incidence issue discussed above. A wave passing over the long length of felt/foam will actually be turbulent near the surface, since the surface is not smooth. It won't necessarily be damping any of it.
Still, I'd like to see measurements of two waveguides, everything identical with the exception of the material.
Dave
Let me be a little pedantic about terminology here. Diffraction refers to the sound that bends around a solid object. It is the sound that bends around a cabinet edge and otherwise fills in the shadow region behind the plane of the baffle. Anything we perceive in the front half space, say in the axial response, is not diffraction but reflection due to the diffraction. When the sound wave sees and edge and starts to bend backwards to fill the void, it senses a different acoustic impedance and sends off a secondary waveform (out of phase) from the edge. Everyone may call the ripple we measure as diffraction but that isn't really correct.
My comment was that once the sound passes that edge, any energy that diffracts (bends off into the back half space) might or might not be abosrbed by felt around the corner. We would never know since it was bound for the half space behind.
As a more extreme example, if we had infinite space behind a cabinet, or alternatively, if we had a perfectly anechoic wall, we would not detect a difference. The diffracted image might be absorbed by the wall or it might be carrying on forever in the rearward direction. It would make no difference as long as the edge reflection was unchanged.
By your measurements the damping around the corner makes little or no difference, hence it is not changing the edge reflection. As to whether it is changing the diffracted energy, you would need to take measurements from behind the cabinet to know.
David S.
Yes, we are getting into the pedantic, but it's a good point to keep distinctions. The terms as we used them refer to the frame of reference being the front axis. Were one to reference a point beyond the baffle on the axis of the baffle, time delay between direct and diffracted/reflected would be essentially zero, so how would one refer to that? But I digress.
I think that my point still stands. If the felt were effective at damping when mounted on the side, it would absorb some of the "diffracted" energy which as a consequence should reduce the "reflected" energy, since diffracted energy as an expansion into 4-pi space still has an impact on the reflected energy since the two are inextricably related. The damping effect of the diffracted energy would, in effect, alter the rate of expansion into the 4-pi space, delaying it and damping some of it. This is going into a bit of the esoterica I suppose, we can't do much more than speculate.
But if it is correct that the angle of incidence is critical as I suspect it is, it's all moot in this particular instance.
Dave
It wasn't my suggestion, I simply stated the it would interest me to see it. But I can suggest whatever I care to, whether or not I care to act on it. What are you, the suggestion police? Sheesh, chill out.
Dave
In all the textbooks that I have read diffraction is that wavefront that is sent off from the edge, in this case. This would make the forward zone the direct plus diffraction field and the "shadow zone" the diffraction only. I think your distinction here between an edge reflection and an edge diffraction is symantic.
In all my writing and use of the term, the diffraction wave propagates in all directions not just into the shadow zone. And I believe that this is consistant with the physics literature on the sibject as well.
When a wave hits an object in space we refer to how the sound "diffracts" off of it. There is no distinction made between the sound that "reflects" off of it from the sound that diffracts off of it. It's all the same thing. In the equations we use two terms to dscribe the field everywhere - the direct field and the diffraction field. In the shadow zone there is only the diffraction field, everywhere else the field is the sum of the direct field and the diffraction field.
For points behind the plane of the baffle, if there were no diffraction there would be no sound. Behind the plane of the baffle there will be diffracted energy but no reflection, hence no comb filtering. Comb filtering is a phenomenon for the front half space only, and, as you say, diminished when approaching 90 degrees because the direct sound and edge reflected sounds are at essentially the same time.
No, I think these two cases are hard to separate. A sound wave going down a rigid surface can't expand sideways. It is only when it hits the corner that its energy is allowed to expand (diffract) around the bend. This act of expanding signals a change of acoustic impedance which causes the reflection as well. This is just like a signal going down a transmission line: when it hits a change of impedance (open, short, or just different) a reflection is caused.
To have a hard edge, where energy diffracts around the corner and then to replace it with a hard edge with flush absorption, appears to me to be the same case. Rather than continuing along a hard surface the wave is suddenly allowed to bend into an absorbing medium. To the extent that the absorber is a perfect absorber, the sound wave goes around the corner never to be seen (or heard from) again. No difference.
David S.
All the textbooks? All that I recall usually define diffraction as the bending of light around a solid object. I assume that some will discuss the secondary effects of reflection due to diffraction but the term generally refers to the bending around the object.
This is usually the dictionary definition as well.
Diffraction: "The bending and spreading of a wave, such as a light wave, around the edge of an object."
"All the textbooks that I have read" - let's not start misquoting me again.
"The bending of light"! That's hardly a phycicists deffinition since lght can only be "bent" by a gravitational field and that's not diffraction. The light is diffracted at an edge as a wave and appears to "bend", but that not what actually happens.
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