anatech said:Hi soongsc,
I guess that is all up to the thread starter and participants here. If the will is there, I can't see any problems.
-Chris
I have repeatedly asked that this thread get back on track. Its not about microphones and the room acoustics question was admitted to be a mistake and moved.
The thread was started by Variac, ask him. I think that he started it at my request.
Hi Earl,
Well, I didn't feel it necessary to investigate the entire thing. My point was that the direction of a thread is determined by the participants. A thread is like a living thing. It needs tending, people being what they are.
Anyway, the moderating team simply does it's best. We can not guide a thread with an iron hand, and I don't think the general membership would accept that either. All we can ask is that everyone behave themselves and show each other some respect. Part of showing respect is to stay on topic in a thread.
All threads meander some. Especially as they get longer. It's very much like a discussion on the street, rather than a lecture at a university.
So, it appears to be back on track I guess. I don't really follow this thread so much. I normally live in B4 land, or a double chamber with forward firing woofer. Even sealed some times.
-Chris
Well, I didn't feel it necessary to investigate the entire thing. My point was that the direction of a thread is determined by the participants. A thread is like a living thing. It needs tending, people being what they are.
Anyway, the moderating team simply does it's best. We can not guide a thread with an iron hand, and I don't think the general membership would accept that either. All we can ask is that everyone behave themselves and show each other some respect. Part of showing respect is to stay on topic in a thread.
All threads meander some. Especially as they get longer. It's very much like a discussion on the street, rather than a lecture at a university.
So, it appears to be back on track I guess. I don't really follow this thread so much. I normally live in B4 land, or a double chamber with forward firing woofer. Even sealed some times.
-Chris
Hello Soongsc,
The picture shown by Tinitus is not related to a waveguide and comes from:
http://www.azurahorn.com/azurahorn_horns.html
Best regards from Paris, France
Jean-Michel Le Cléac'h
The picture shown by Tinitus is not related to a waveguide and comes from:
http://www.azurahorn.com/azurahorn_horns.html
Best regards from Paris, France
Jean-Michel Le Cléac'h
soongsc said:
but thanks fore correcting...its a true horn then, thats ok tooHOM killing alternatives
So, actually on topic:
Something I've been wondering about since I first read about the Summa: since the path length of the higher order modes propagation is longer than the direct sound, Dr. Gedde's foam plug design attenuates these longer paths more than the direct sound, providing an improvement.
But, since the reticulated foam is rather expensive and difficult to work with, what about merely covering the surface of the horn with absorbing material? Would this not also at least partially reduce the higher order modes?
So, actually on topic:
Something I've been wondering about since I first read about the Summa: since the path length of the higher order modes propagation is longer than the direct sound, Dr. Gedde's foam plug design attenuates these longer paths more than the direct sound, providing an improvement.
But, since the reticulated foam is rather expensive and difficult to work with, what about merely covering the surface of the horn with absorbing material? Would this not also at least partially reduce the higher order modes?
I would imagine that any material you did this with would create more problems than it fixed. If you think about the waves traveling along the waveguide fairly straight, with some bouncing around. What happens when you place something like foam or felt on the surface. If it's not applied smoothly and evenly, and I can't see how it could be, I would think that it would cause issues. If you think about the best way to deal with this, it really seems like the foam plus is the best and most ingenious way to do it, while also being very simple, and relatively inexpensive. It's expensive for a Diyer, and I'm not denying that the foam is expensive, but compared with other methods to fix such problems, this seems like a simple and low cost way to go.
Also, I could be wrong about my understanding of how the plug works. But my understanding is that, because the wave is bouncing around, it travels inside the foam longer, and thus is more attenuated than the direct waves. This surface treatment would have no absorption in that middle area, only in the area where there is material, thus massively reducing the effect, I would think, to the point of not even being worthwhile. It seems like a somewhat absorptive plug is key.
Also, I could be wrong about my understanding of how the plug works. But my understanding is that, because the wave is bouncing around, it travels inside the foam longer, and thus is more attenuated than the direct waves. This surface treatment would have no absorption in that middle area, only in the area where there is material, thus massively reducing the effect, I would think, to the point of not even being worthwhile. It seems like a somewhat absorptive plug is key.
@ppjones: yes, I don't have the mathematical sophistication to follow Dr. Geddes' analysis, but my intuition is certainly that the foam plug would be more effective.
What I'm curious about is the tradeoff between a plug of relatively low absorption material vs a thin boundary dampening layer with higher absorption.
What I'm curious about is the tradeoff between a plug of relatively low absorption material vs a thin boundary dampening layer with higher absorption.
What is suggested may have merit, but absorption on a wall tends to not be very effective. In my experimentation I have found that the more material there is in the waveguide the better. So I fill it as much as possible. There is a patent on a waveguide made completely of sound absorbing material, which would work as better than anything that you could put on the surface. This idea never came to fruition, although that is never a reason that it isn't effective.
I don't think that all the options have been tried or fully developed, I just use the one that has worked so well for me.
I don't think that all the options have been tried or fully developed, I just use the one that has worked so well for me.
Re: HOM killing alternatives
The foam is almost ideal because it's density is low, and it's consistent. That's why you don't want to use wool or poly; their density is inconsistent. In fact the density would vary with time due to gravity.
I bought a huge roll of the stuff for $80. Admittedly, using a solid "bun" of the foam is the way to go.
jason_watkins said:
The foam is almost ideal because it's density is low, and it's consistent. That's why you don't want to use wool or poly; their density is inconsistent. In fact the density would vary with time due to gravity.
I bought a huge roll of the stuff for $80. Admittedly, using a solid "bun" of the foam is the way to go.
If something has to be in the path of direct wave, it really should be a tradeoff between lost of detail and lowering HOM. This should also be done using a fully broken-in driver.
In a previous post of a wave guide I did using a direct radiating driver, diaphragm edge was generating HOMs. It seems the wave needs to be properly guided starting from the diaphragm.
In a previous post of a wave guide I did using a direct radiating driver, diaphragm edge was generating HOMs. It seems the wave needs to be properly guided starting from the diaphragm.
Re: Re: HOM killing alternatives
I use wadding (not certain which type it is as I've had it for many years, but probably just the usual polyester fibre though it feels different from the normal dacron stuff) in XT1086 and RCFH100 horns. I fill the horn with layers (from the throat out to beyond the mouth) cut to size and held in place by gentle tension on the outer layer (which buldges outward to approximate the desired shape). There is no sign of any change in a period of order a year.
I do not find it simple to measure the on HOMs, but (to address the above points):
it is quite easy to get consistent distribution at least across the width of the horn, on the scale of the shortest wavelength involved, provided the wadding is used at its natural density and not teased out too much.
(I don't think the distribition in the other dimension matters so much.)
I guess the absorption is more than with foam (where is the proof that the foam density is ideal? - I missed that), but I don't see that as a problem as I end up with a response (over angle) that is easy to equalise - even on "dreadful diffraction horns" (note, I say here what I've said elsewhere: I am certain Earl's waveguides are better, no argument about that).
If the HOMs are really cavity modes (i.e. involve reflection from the ends of the horn as well as the walls) more absorption should help. I still only get a few dB at the frequencies of the first few modes, and by calculation, that is barely enough.
So I'm not convinced the foam is a done deal over wadding until I see comparative measurements.
Ken
edit: parenthetical comments about layers added
Patrick Bateman said:
I use wadding (not certain which type it is as I've had it for many years, but probably just the usual polyester fibre though it feels different from the normal dacron stuff) in XT1086 and RCFH100 horns. I fill the horn with layers (from the throat out to beyond the mouth) cut to size and held in place by gentle tension on the outer layer (which buldges outward to approximate the desired shape). There is no sign of any change in a period of order a year.
I do not find it simple to measure the on HOMs, but (to address the above points):
it is quite easy to get consistent distribution at least across the width of the horn, on the scale of the shortest wavelength involved, provided the wadding is used at its natural density and not teased out too much.
(I don't think the distribition in the other dimension matters so much.)
I guess the absorption is more than with foam (where is the proof that the foam density is ideal? - I missed that), but I don't see that as a problem as I end up with a response (over angle) that is easy to equalise - even on "dreadful diffraction horns" (note, I say here what I've said elsewhere: I am certain Earl's waveguides are better, no argument about that).
If the HOMs are really cavity modes (i.e. involve reflection from the ends of the horn as well as the walls) more absorption should help. I still only get a few dB at the frequencies of the first few modes, and by calculation, that is barely enough.
So I'm not convinced the foam is a done deal over wadding until I see comparative measurements.
Ken
edit: parenthetical comments about layers added
Re: Re: Re: HOM killing alternatives
I spent about a year trying out different materials and densities, even using "batting" as you suggest. I did not post or present these results, I doubt that I ever will. But its not as if I didn't do a lot of work to find the best option available. What I use is the best that I found.
The results are always a tradeoff between absorption of the main wave at HF and absorption of the HOM. Too much HF absorption is a problem in a CD waveguide because the eficiency already falls at -6 dB/oct and one cannot afford too much more loss that that. In the Summa the output of the DE250 at 10kHz is exactly the same as that of the woofer. Any more loss from the foam would require the woofer to be lowered in efficiency. One could argue that in the Summa there is so much output to spare that this is not an issue, but thats another discussion.
kstrain said:
I spent about a year trying out different materials and densities, even using "batting" as you suggest. I did not post or present these results, I doubt that I ever will. But its not as if I didn't do a lot of work to find the best option available. What I use is the best that I found.
The results are always a tradeoff between absorption of the main wave at HF and absorption of the HOM. Too much HF absorption is a problem in a CD waveguide because the eficiency already falls at -6 dB/oct and one cannot afford too much more loss that that. In the Summa the output of the DE250 at 10kHz is exactly the same as that of the woofer. Any more loss from the foam would require the woofer to be lowered in efficiency. One could argue that in the Summa there is so much output to spare that this is not an issue, but thats another discussion.