Can you elaborate on that? It seems contradictory to your previous explanation of how HOMs occur.
You know, I gradually discovered this when I started playing around with the Jordan aluminum coned drivers. I'm a technically oriented person, and I seldom try anything different unless I know technically it should resonably deliver the qualities advertised. However, as many audiophiles know, metal coned drivers just have that metallic sound. That was something I did not quite like either, and it becomes more annying as the volume is turned up; that is why I set forth to see if the problem can be improved on. This journey also led me to discover the importance of looking at CSD data. I have demonstrated this using a JX92S driver in the EnABL thread. Not that I agree that EnABL works, but to investigate how it effects performance, and its limitations.
Do you think live heavy metal needs to be turned down after 30-60 minutes, or needs blankets over guitar stacks, etc?
Or carried to the extreme; orchestras should install foam in the saxes, cellos etc?
Its not contradictory, maybe you didn't understand.
HOM, means Higher Order Mode. They are proagation modes other than the zeroith mode - the primary mode. The primary mode is noted by the fact that the wavefront is always normal to the boundaries. No HOM has this characteristic - they all reflect off of the bondaries. Now there is nothing in that definition that says that these modes have to be created by the waveguide, they can exist at the throat. If an HOM exists at the throat then it will propagate as an HOM. The HOM are easy to define at the throat because it is a boundary and its definition actually determines the solution set. The zeroith mode, at the throat, is a flat wavefront - constant phase and amplitude across its entire extent, v(r)=1.0 . Any deviation from this IS an HOM and will propagate down the waveguide as an HOM. THIS IS WHY I USE FOAM! Compression drivers do not have a perfect wavefront, so there are always HOMs from the get-go. The foam attenuates these.
A dome does not have a perfectly flat wavefront, its not flat itself, so it too will generate HOM right at the throat.
It is also possible to generate HOM within the waveguide itself as we have discussed many times.
No contradictions here.
I'd still expect that quite a lot could be done with edge treatments.
I'll be doing some with/without measures soonish. Like the miniature foam plug?
Nothing graphical, but I measured 1.2 and 1.5dB of ripple reduction at 1400 and 2800 respectively
All measures are approximate, here, this is RTA based, but is accurate. I'm happy with the results. My crossover is also coming along, I got it to +/- 2dB for a decade 1250-12500, and most of that is ripple around the frequencies listed above- where loading falls apart. Between 4 and 10k it's better than +/- .75dB.
Have you tried putting the same thickness of foam in front of a direct radiating driver?Its not contradictory, maybe you didn't understand.
HOM, means Higher Order Mode. They are proagation modes other than the zeroith mode - the primary mode. The primary mode is noted by the fact that the wavefront is always normal to the boundaries. No HOM has this characteristic - they all reflect off of the bondaries. Now there is nothing in that definition that says that these modes have to be created by the waveguide, they can exist at the throat. If an HOM exists at the throat then it will propagate as an HOM. The HOM are easy to define at the throat because it is a boundary and its definition actually determines the solution set. The zeroith mode, at the throat, is a flat wavefront - constant phase and amplitude across its entire extent, v(r)=1.0 . Any deviation from this IS an HOM and will propagate down the waveguide as an HOM. THIS IS WHY I USE FOAM! Compression drivers do not have a perfect wavefront, so there are always HOMs from the get-go. The foam attenuates these.
A dome does not have a perfectly flat wavefront, its not flat itself, so it too will generate HOM right at the throat.
It is also possible to generate HOM within the waveguide itself as we have discussed many times.
No contradictions here.
I doubt it would give the same effect as when used in the waveguide. A direct radiating driver has a throat? Sounds like you are reinventing the terminology to suite a purpose.
Getting a little bit absurd.
Then I suffer from fatigue even listening to live rock performances. If so shouldn't I also suffer the same way listening to it's recording?
Soongsc - sounds like your just being argumentative. If you don't belive in waveguides and foam and you don't accept what I say, then why come to a thread called "Geddes on Waveguides"? Do you think that you are going to change my thinking to yours?
Having spend most of my audio experience at the "audiophile level", I came here to learn. But not to only accept all that what is said. This spirit should be very common in the US that I know. I do not claim to be right, nor do I claim to be an authority. Up to now, I have posted variety of real data that shows there are different ways of doing things, which is a journey. What I would have expected is to receive some pointers what problems I would expect if I went down a particular route. But all I received was the "you are wrong" "you bad boy" kind of attitude. I would love to have known more about your journey, but it seems lots of the detail data of experiments are not available. I would expect the foam to also change the loading on the driver, but this aspect was never addressed.
As you can see, I take caution to stay away from your commercial related threads. It would be more appropriate that your product specific comments be in those threads. Remember how the EnABL thread was split?
I hardly ever talk about my products here.
My impression of you is not that you are here to learn as you virtually never listen to what I have to say. You seem to want to tell the rest of us what reality is with little or no supporting evidence - best case is "I heard it, therefor it is". Very few here are as forth coming with their experience as I am and very few are as obstinet and opinionated as you are. I go out of my way to be tollerant of your comments, but sometimes they just get to be too much. When you directly attack what I do, with nothing more to support your position than your "beliefs" then you simply must expect me to respond, in kind.
Whenever I post anything, I welcome a good discussion. We cannot decide what other can or cannot hear, but if we share this experience, it should be a positive contribution to the whole discussion. I even took the time to examine my ears to see how different the response would be because I did notice a difference. Do you think it's bad to share such experience? You are certainly knowledgeable in the area of audio, but it doesn not mean you are always right. I would expect the normal thing to do is to do more research, which is what I am doing. If data shows results supporting or contradicting to what anyone has said, then I show it.
Up to date, I have probably posted more data than anyone else in this thread. The variety of configurations is probably also the most. I have asked how HOMs are seen in the measurements, and nobody ever responded directly; when I provided my interpretations on how to identify presence of HOMs, all you did was "disagree". So I certainly don't know what else to do. Up to now, you have posted data on what the foam does to the impulse response, but it does not seem distinguishable from a filter. We have never had any answers to that. I'm sorry if you feel offended when you cannot answer questions.
BTW, "only the teacher knows most", is the old Chinese culture.
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Dr Geddes, over the weekend I did some experiments to reduce the size of a waveguide. During the process I discovered that manipulation of the loudspeaker's roundover can improve directivity control.
And then I noticed that the graphs that you've published correlate with this.
For instance, the Summa uses a 15" waveguide, and based on that diameter, should have directivity control down to 900hz. (speed of sound/waveguide diameters.)
Yet your own graphs show directivity control down to 400hz. (The cyan trace in the graph above is 45 degree off axis I believe, and the -6dB point is at 400hz.)
So I'm curious - is this true? Do your speakers perform better than advertised?
Here's the thread: http://www.diyaudio.com/forums/multi-way/158741-reducing-waveguide-size.html
And then I noticed that the graphs that you've published correlate with this.
For instance, the Summa uses a 15" waveguide, and based on that diameter, should have directivity control down to 900hz. (speed of sound/waveguide diameters.)
Yet your own graphs show directivity control down to 400hz. (The cyan trace in the graph above is 45 degree off axis I believe, and the -6dB point is at 400hz.)
So I'm curious - is this true? Do your speakers perform better than advertised?
Here's the thread: http://www.diyaudio.com/forums/multi-way/158741-reducing-waveguide-size.html
I'll post the measurements in the next few minutes. The results are promising - it seems that we can dramatically reduce the depth of a waveguide if we're willing to increase the cabinet's width. And I don't mean by an inch or two; I'm talking a HUGE roundover, like 100% of the waveguide's diameter. Taken to the extreme I guess you'd just use a spherical or elliptical enclosure.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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John
Most of the width of the directivity at 600-700 Hz is probably due to the woofer. A larger woofer, in a sense, extends the directivity downward in frequency, because its getting narrow but starting wider than the waveguide. At some point they are the same and this goes down with woofer size.
The large radius works as long as there is enough "conical type" transition. I have some experience here. I tried a waveguide that was just a radius. That would make it a donut (torus) or half a donut with the driver in the hole. I have suggested this several times, mostly in regards to dipoles with minimum diffraction. It works "OK" but it does not have as constant and as narrow a pattern as a good waveguide, but better than a free piston or piston in a baffle. If you extend a tangent line out to form a cone, then this is the Peavy Quadratic Throat waveguide. Better is to use a cross section of OS at the throat, to a large radius (part of a torus in 3D) connected at the point of tangency. Different values of radius make this a Kind of range of devices of different size and angle.
Most of the width of the directivity at 600-700 Hz is probably due to the woofer. A larger woofer, in a sense, extends the directivity downward in frequency, because its getting narrow but starting wider than the waveguide. At some point they are the same and this goes down with woofer size.
The large radius works as long as there is enough "conical type" transition. I have some experience here. I tried a waveguide that was just a radius. That would make it a donut (torus) or half a donut with the driver in the hole. I have suggested this several times, mostly in regards to dipoles with minimum diffraction. It works "OK" but it does not have as constant and as narrow a pattern as a good waveguide, but better than a free piston or piston in a baffle. If you extend a tangent line out to form a cone, then this is the Peavy Quadratic Throat waveguide. Better is to use a cross section of OS at the throat, to a large radius (part of a torus in 3D) connected at the point of tangency. Different values of radius make this a Kind of range of devices of different size and angle.
That's by far the best looking picture of the Summa that I've ever seen.I'll post the measurements in the next few minutes. The results are promising - it seems that we can dramatically reduce the depth of a waveguide if we're willing to increase the cabinet's width. And I don't mean by an inch or two; I'm talking a HUGE roundover, like 100% of the waveguide's diameter. Taken to the extreme I guess you'd just use a spherical or elliptical enclosure.
Picture the monolith from 2001, but with a big fat roundover at the edges. Or an elliptical front. Or both.
It could also explain why I found that the Klein & Hummel speakers sounded very good, but couldn't "disappear" the way that the Summa does.An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Big fat roundover would be like Kugellwellen expansion?
That's by far the best looking picture of the Summa that I've ever seen.
Big fat roundover would be like Kugellwellen expansion?
The Red Summas are what John has undernieth his blue paint. Nobody seemed to like the red, but I loved it. What I don't like, and this has now changed, is the screws on the woofer.