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Home » Audio » Speaker » cd horns: conic, os, quadratic...........
| cd horns: conic, os, quadratic........... [message #19473] |
Tue, 01 April 2008 12:35  |
Zeno
Messages: 122
Registered: May 2009
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Master |
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what differences are between pure conic horns, os horns and quadratic throat horns? conical horns have straight walls directly from the throat hole. os and quadratic horns are radiused from throat to match side walls. am i right?
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| Constant directivity tweeter horns and waveguides [message #19474 is a reply to message #19473] |
Wed, 02 April 2008 01:11  |
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Wayne Parham
Messages: 18786
Registered: January 2001
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Illuminati (33rd Degree) |
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Quadratic throat waveguides have conical flares with throats gradually curved to match the entry angle. Oblate spheroidal (OS) waveguides do too. So do prolate spheroidal (PS) waveguides, for that matter. Right now, it seems like more people talk about OS waveguides than do about PS waveguides or quadratic throats. Personally, I find the PS horn to be more useful than OS because OS is axisymmetric but PS can be made to radiate other patterns. In other words, you can have a 90x40 PS horn. It's the same principle though - a conical flare with throat curved to match entry angle. Each of these are nice shapes, in my opinion.
These days, most people talking about the OS shape with regards to waveguides are talking about Geddes waveguides. He has a couple other features that are important to his implementation, one that is easily realized and one that is not. The most important part - which unfortunately is also the one hardest to implement - is making the phase plug match the horn flare. This one isn't done yet, because he hasn't begun to machine throats or phase plugs for compression drivers. That makes the second feature more important to him. He puts open cell foam in the horn to partially absorb sound.
The reason these extra features are important to Geddes is they cut down on what he calls (HOM) high-order modes. Most sound traveling down the horn progresses along its axis. Wavefront propogation is mostly this way, with waves of equal pressure across the horn's cross-section. If the sound source was a perfect radiating sphere of acoustically small dimensions and the throat was conical and acoustically small, I think all wavefront progogation through the horn would be this way. Or if it generated perfect planar waves and the exit was a tube, either way. But real-world horns are not driven by point sources that radiate perfectly spherical wavefronts or planar waves nor are all their features acoustically small. The diaphragm and throat is acoustically small at low frequencies, but becomes large at high frequencies. That's why phase plugs are used, to try and correct this to some degree. Because of these things, some sound can be skewed within the horn, reflecting off the walls rather than propogating along its axis. This is something Geddes has focused his attention on. That's why he adds the foam. The idea is sound traveling along the horn axis will travel through less absorbent foam than sound bouncing off the walls, so the reflected sounds will be absorbed more.
To be honest, I'm not sure how audible HOM are. In fact, I'm not sure they've been measured yet, although it does stand to reason that they exist. Geddes claims night and day difference between a horn treated with foam to reduce HOM and one without, but I didn't hear any qualitative difference. I do like the horn shape, mostly for its absense of discontinuities from abrupt flare changes and for its constant directivity. I could do without the foam.
When you bolt a 1" compression driver onto a quadratic, oblate spheroidal, prolate spheroidal or pure conical flare, you get reasonably constant directivity within a band determined by the dimensions of the waveguide. At some low frequency point, it will lose pattern control. At high frequency, the 1" entrance becomes acoustically large, so the features within the compression driver are shaping wavefront propogation, not the waveguide. The waveguide has basically no influence at high frequencies if the throat is a gradually expanding shape. So the pattern narrows drastically in the top octave, since the exit flare angle of most 1" compression drivers is 5° to 10°. This abrupt beaming boosts on-axis sound in the top octave (right where some drivers exhibit breakup peaks) and may be more reason to put absorbent foam in a waveguide like this than anything else.
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| Re: Constant directivity tweeter horns and waveguides [message #19476 is a reply to message #19475] |
Wed, 02 April 2008 13:46 |
Randy
Messages: 6
Registered: May 2009
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Esquire |
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"If the sound source was a perfect radiating sphere of acoustically small dimensions and the throat was conical and acoustically small, I think all wavefront progogation through the horn would be this way. Or if it generated perfect planar waves and the exit was a tube, either way." Compression drivers use of the phase plug is to make wavefronts diverge to approximate a planar wave AFAIK. However, cones and domes make omnidirectional spherical waves. From this I would think a pure conical horn would be the best way to load a dome or cone driver. Your thoughts? Randy
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| Re: Constant directivity tweeter horns and waveguides [message #19480 is a reply to message #19479] |
Thu, 03 April 2008 12:42 |
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Wayne Parham
Messages: 18786
Registered: January 2001
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Illuminati (33rd Degree) |
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You're right, the H290 transitions smoothly from the throat expanding to the mouth without a diffraction slot. I like this horn, but it does beam in the top octave. No way around it in a horn like this without a diffraction slot.
Remember that we're only talking about the top octave here. Below 10kHz, the horn sets the radiation pattern. But above that, the exit features of a 1" compression driver set the pattern. That's why the pattern narrows, and why on-axis SPL increases. It does so at the expense of off-axis energy, which falls off.
Earlier I said, "this abrupt beaming boosts on-axis sound in the top octave", which is true for all horns and waveguides that don't have a diffraction slot. It's usually so high in frequency that nobody really notices, it just gives a bit more "air." But it is definitely a measureable increase above 10kHz on-axis, and you can also see the absense off-axis. For that reason, most speakers having constant directivity without a diffraction slot in the tweeter sound better to me 10-20° off-axis than they do straight on.
If you're familiar with my crossover, there is an optional component C1a. A small value capacitor there can be used to decrease output in the top octave. Capacitor C1 bypasses the padding to increase HF output, but if you're using a compression driver/horn combo that becomes too strident in the top octave, you can install C1a.
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| Re: Constant directivity tweeter horns and waveguides [message #19481 is a reply to message #19478] |
Thu, 03 April 2008 13:06 |
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Wayne Parham
Messages: 18786
Registered: January 2001
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Illuminati (33rd Degree) |
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Kind words, Martin, thanks. Means a lot coming from you.My thoughts are just one man's opinion. I'm always learning and making incremental improvements as I do. I'm really interested in your thoughts. At low frequency, a horn can easily (and accurately, I think) be modeled with simple 1P formulas. But at higher frequency, where the diaphragm doesn't move as a piston, where horn features become acoustically large, etc, that's where an empirical approach has always been required. Any ideas on some of those thorny issues? I'm not sure we can make a closed form solution for them, seems like they can't be solved without complex modeing techniques. I guess that really takes us back to the FEA / BEM thread, but these topics are sort of related.
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| Re: Constant directivity tweeter horns and waveguides [message #19482 is a reply to message #19481] |
Thu, 03 April 2008 13:57 |
Martin
Messages: 220
Registered: May 2009
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Master |
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Wayne, You are way ahead of me when it comes to understanding and designing horns. But I am working on a couple of horn related modeling efforts. I'll list them below in no particular order. 1) I want to change my 1D modeling tool to get away from flat plane wave assumptions. I intend to model the waves in the horn curved such that they are normal to the side walls. This will mean more reflections when the geometry changes. I am not sure how significant this will be. 2) Modeling what goes on at the mouth is always interesting. If you follow a textbook derivation that is based on Webster's equation, then the mouth acoustic impedance is assumed to be modeled as a piston in an infinite baffle. In other words the volume velocity and pressure are constant everywhere on the surface that is defiend as the mouth. In the past week or so I have calculated the pressure profile, assuming a constant velocity, on the surface of the mouth. So I now have a position dependent acoustic impedance of the mouth. I think this will eventually lead me to velocities at the mouth in multiple directions, axial and across the horn's mouth (out of plane motions). 3) As part of 2), I have also started to include the edge diffraction effects of a baffle or just the edge of the mouth itself when calculating the mouth's acosutic impedance. I got this working last night and will continue to explore this over the weekend. I can also see this becoming more sophisticated as I look at a diffraction due to a discontinuity as the mouth transitions into the room and the wave shape "distorts" from a smooth curved profile. (Man, that sounds like a snake oil saleman's techno-babble.) 4) I also want to add some plots to my documentation to show the sound waves in the mouth as a function of frequency, above and below the cut-off frequencies for a few different horn profiles solved with Webster's equation modeling. 5) I think numerical solutions are the key, I am not smart enough to do a closed form solution. I will probably start with brute force approaches that calculate forever and then start to simplify and take shortcuts. 6) I also think there will be a lot of value in experimenting and measuring some of the compression driver horns. So I am putting togther a list of compression drivers and horn profiles I want to buy, model, and test (w/ Praxis) to help calibrate myself. I will also be very interested to substitute these into my home system to assess the pro's and con's of them compared to my direct radiator drivers. I guess I am keeping busy and horns have the bulk of my attention right now, but I have said that before and it has been short lived. Keeping busy, Martin
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