I'm sure I must have missed this somehow, but to me, a very tangible effect of the foam might be on polar response rather than the elusive HOMs. Much like JBLs acoustic lens perhaps, but a more elegant solution. Is there a post comparing horizontals with and without the foam that I missed? Thanks.
I highly doubt the sound is physically affected by going from a matte to a glossy finish.
speed of sound is about a Million times slower than light
very good reflectors like telescope mirrors are finished to ~1/10 wavelength
for sound 1/10 wave in air @ 20 kHz ~ 1/16", 1.5 mm
intentional sound diffuser surfaces are lumpy on human fingers, handspan to arm length size scale
in the phase plug and throat 1/16" may not be to good - but thats more about the relative tolerance, smoothness of the impedance of the narrow passages
for the walls of the horn itself I don't think even 1/16" stepping/roughness would be a practical problem, but diffraction grating effects may occur with very uniform period of the steps as in 3D printer layer thickness or a CNC program path stepping
but very little finishing effort is needed in filling, sanding, solvent "etching", ect. to get below 1/100 wave at 20 kHz
still a coarse grain finish by eye or touch
diffraction grating math for efficiency/power in the modes for fractional wave modulation period, grazing incidence looks non trivial but I suspect it is a non issue at audio
very good reflectors like telescope mirrors are finished to ~1/10 wavelength
for sound 1/10 wave in air @ 20 kHz ~ 1/16", 1.5 mm
intentional sound diffuser surfaces are lumpy on human fingers, handspan to arm length size scale
in the phase plug and throat 1/16" may not be to good - but thats more about the relative tolerance, smoothness of the impedance of the narrow passages
for the walls of the horn itself I don't think even 1/16" stepping/roughness would be a practical problem, but diffraction grating effects may occur with very uniform period of the steps as in 3D printer layer thickness or a CNC program path stepping
but very little finishing effort is needed in filling, sanding, solvent "etching", ect. to get below 1/100 wave at 20 kHz
still a coarse grain finish by eye or touch
diffraction grating math for efficiency/power in the modes for fractional wave modulation period, grazing incidence looks non trivial but I suspect it is a non issue at audio
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I have no experience in this area, but the description of the process and listening impression seemed convincing to me. I wish I could find that site.
I meant glossy. At least what I read and the pictures I saw were of various material, and the conclusion was if made glossy, the material did not matter so much.
In the tests that I performed there was no effect on the polar response due to the foam.
Sorry, I missed your post at first.
A waveguide is not purely conical. It is a gradual transition from the drivers exit angle to the final waveguide angle. It is also not a simple radius either. The equations are well known and posted in this thread somewhere.
As the angle of the waveguide gets larger and larger the amount of HOMs will increase and move lower in frequency, so narrower angles appear to be better. But the mouth radius determines the lowest frequency at which the waveguide will control the polar response. This means that narrower angles need to be longer. This makes wider angles appear attractive, but they don't work as well, so it is a tradeoff. 90 degrees - a 45 degree waveguide - appears to be a sort of sweet spot.
The mouth radius should be as large as possible, but again this makes the device larger. All of this is why smaller waveguides tend to not work as well as larger ones.
With the foam used in your wave guides, it will not matter, this is for sure.
A straight cone into a compression driver creates a sharp edge which would be a significant source of diffraction. This edge needs to be "smoothed out". A simple radius is easiest to imagine, but it is not quite the ideal. The OS curve is the theoretical ideal.
As a rule of thumb that's not bad, but its also not exact. The HOMs depend also on the angle of the waveguide as well as the throat radius. They go lower as the angle increases and would start about 3.4 kHz for a 60 degree wall angle (120 degree included angle). For 45 degrees they cut-in at about 6 kHz.
Those numbers are for the axisymmetric HOMs. The non axisymmetric modes cut-in at much lower frequencies - but, in theory, there should not be any non axisymmetric excitation to excite these modes. However, in practice we know that this is not the case, but to what extent is not known.
Those numbers are for the axisymmetric HOMs. The non axisymmetric modes cut-in at much lower frequencies - but, in theory, there should not be any non axisymmetric excitation to excite these modes. However, in practice we know that this is not the case, but to what extent is not known.
Interesting, yet it should be working that way. If you add up all of that surface area around those pores do you not have a very significant source of diffraction? Less down the center more to the edge? I wonder how much surface area all of those bubbles have if they were one plane.
Pete
In theory, yes, the foam has a delay but it is pretty much a constant across the plug (its curved you know) so there isn't any refraction, like in a lens, just a constant delay. Interestingly enough, I started playing with foam to see if I could get some effect on directivity, like a lens, but that came up blank. I noted some real changes in sound quality, if not polar response. So I pursued that course.
As to "diffraction" we are talking about pores that are pretty small to have any effect in the audio band. I hope that you are not confusing "diffraction" with "refraction" - that would cause a serious misunderstanding.
In theory, yes, the foam has a delay but it is pretty much a constant across the plug (its curved you know) so there isn't any refraction, like in a lens, just a constant delay. Interestingly enough, I started playing with foam to see if I could get some effect on directivity, like a lens, but that came up blank. I noted some real changes in sound quality, if not polar response. So I pursued that course.
As to "diffraction" we are talking about pores that are pretty small to have any effect in the audio band. I hope that you are not confusing "diffraction" with "refraction" - that would cause a serious misunderstanding.
Ok I see. I have 4333s hanging around here too, and was thinking the foam might be trying do the same trick, but better. I was looking at the old lenses and wondered if that was where the foam actually stated life. Makes sense since people didnt like how they looked even though they worked pretty well. My 4333s are about to become subs.
Foam and the slant plate lenses are not in the same ballpark acoustically.
The 2307 exponential horn without the 2308 lens (properly equalized) sounds great on axis, though only covers about 10-15 degrees up above 10 kHz, and the small mouth makes it go too wide too high for proper transition to a 15".
With the 2308 diffraction lens, it is wide and diffuse, but lacking detail and "snap", transients seem blurred.
To quote Earl:
"The HOMs depend also on the angle of the waveguide as well as the throat radius. They go lower as the angle increases and would start about 3.4 kHz for a 60 degree wall angle (120 degree included angle). For 45 degrees they cut-in at about 6 kHz."
The 2307 has only around 30 degree included angle at the mouth, the HOMs may be above audibility.
The same would be true for my narrow (13 degree) Maltese conical horn...
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