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Old 12th April 2007, 04:07 AM   #181
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Graham,

I don't fully follow you. You're right, that B200 version will sound terrible from the back. It's not a dipole. If you mean that you think the terminus of the long pathway should be to the front, I'd have to disagree. The whole point is to provide a longer path to the listening position for some of the lowest frequency content of the rear wave. With a rear exit, the extra distance to the front plane is a free ride, so I'd fight tooth and nail for it.

This thread has me wanting to revisit these designs, since I still haven't decided on a base or container for the woofer to augment my dipole WG's. When the B200 version didn't get down to Fs, I kind of abandoned it without trying adjustments.

The coax 15 version works quite well, but listening at the terminus has little output, although what comes out is only very very low in frequency, so it's difficult to gauge by ear. The B200 cab has more output out of the terminus, but higher in frequency. The distance differential isn't that much less, although the contained volume is. This has me thinking that I screwed up estimating the helmholtz slot needed, and that it's probably too large. I can even play around with that fairly easily. hmmm rekindled excitement over a forgotten design....fun.
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Old 12th April 2007, 04:23 AM   #182
jamikl is offline jamikl  Australia
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johninCr, at risk of flogging a dead, maybe not lying down yet, horse, have you tried a K slot on the end of your design. I remember you showing this idea quite some time back and was impressed by it.
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Old 12th April 2007, 04:34 AM   #183
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Default Progressive-Loss Mesh

JohninCR's split-path design is most interesting - a TQWT in acoustic parallel with a short open-back box. The backwave goes in two paths, with quite different delay times, in addition to the nominal back-to-front delay distance. Since the TQWT is folded, there's a bit of low-passing going on in that path.

It would be most interesting to have the spectra of the rearwave escaping from the back of the short box as well as the wave escaping from the exit of the TQWT. When I measured the spectra of the wave emerging from the exit of the Ariel TL, I was quite surprised to find it was flat (1 dB) from 30 Hz to 100 Hz, and dropped off quite rapidly above and below those frequencies. I surmise the complex labyrinth was doing some low-passing for the TL, since TL's are usually a bit more resonant than what I saw. The drivers (measured nearfield) dropped like a stone below 80 Hz, so the TL was definitely working as advertised.

As for the ideal performance of a simple flat-baffle dipole, yes, it's free of cabinet coloration, but most certainly not free of standing waves on the face of the baffle. When you measure a driver on a standard IEC baffle (80 by 115 cm), it is essential to mount the driver somewhat off-center, otherwise the measurement will be contaminated by standing waves on the baffle. This is audible and measurable, yet it occurs with the simplest possible flat baffle. So flat baffles are not free of standing-wave resonances.

Returning to what I think of as a Progressive-Loss Mesh, it can be applied to the full gamut of shapes, flat baffles, horns, and pipes, progressively dissipating the wavefront as it travels towards the edge of the surface. Unlike damping felt, the frequency characteristics and loss-with-distance can be controlled by the density (pitch) of the drilled holes, which are ideally much smaller than the smallest wavelength of the driver. Drilling hundreds or thousands of holes by hand would be extremely tedious, but this can be automated by a NC-controlled drill press - think of all the holes in a circuit board, for example.

As mentioned earlier, and what seems to escape most high-end speaker designers, it is the edge of the surface (regardless of shape) that creates (acoustic) standing waves. Edges, just as much as hard reflecting surfaces, reflect energy. That's how I look at any enclosure - AR3a, Altec A7, Bose 901, Wilson WATT, you name it - as a collection of edges. The more edges, the worse the sound. House of the Seven Gables might be a good way to sell a $500,000 house, or make a cuckoo clock, but it isn't the way to make a good-sounding speaker.

Just before I left Audionics I built a prototype ultralow diffraction speaker, which ended up looking like a giant vitamin pill supported by a very narrow wood stand behind the speaker. The prototype had lots of issues - resonances in the cardboard tube, mediocre performance from the Audax 2" mid dome - but it delivered on the low diffraction. When the lights in the room were out, the speakers disappeared so completely you could walk right into them while they playing - there was no "point-source" effect at any listening distance. The images simply hung in space, entirely free of the speakers, a most uncanny effect - and this was true of every recording, even old mono LPs.

The commercial MBL system comes closest to what those prototypes did back in 1979. As a result of that experiment, I've become sensitized to the sound of diffraction - which in the simplest terms is what creates the apparent source size when you walk towards the speaker (it is less noticeable when you don't move). When diffraction drops below the threshold value, it becomes very difficult to tell the size of the speaker as you walk towards it. The soundstage loses its hard edge on the left and right sides, and extends well outside the left and right speakers.

Now I want to create a low-diffraction dipole - and I suspect the Progressive-Loss Mesh is a good way to do it, provided the holes are small enough and there are enough of them (hundreds or thousands). The PLM can be applied both to the flat baffle and the length of the short-box/TL, dissipating energy along the length of the structure.

Oh yes - before I forget - Bud's EnABL pattern could also be realized as a set of cutouts, perhaps as very small cutouts closest to the driver, getting bigger towards the edge of the cabinet (highest frequencies are dissipated first). This would be another interesting variation of the PLM.
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Old 12th April 2007, 05:35 AM   #184
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I'm a newbie, and this is all way, way over my head. But, I'm going out on a limb here to throw out an idea for everyone to take or leave.

Porous panels are available that might work as a Progressive-Loss Mesh. Imagine very hard foam rubber, or better yet, a membrane filter that is up to one inch thick, where the pore volume can range from 30% to 90% (in 10% increments). You can get some information on these at http://www.porex.com/porous/index.htm. (And no, I am in no way connected to them). One of their products is a duffuser for the powder coating industry, so the price may not be completely out of the question.

I hope this helps.

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Old 12th April 2007, 05:37 AM   #185
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Jamikl,

I actually have a K-slot type cutout on the shortest panel of the short tube on top. The intention was to create an early pressure release that is frequency dependent in an attempt to eliminate/reduce resonances. Unlike a typical K resonant enclosure, my goal is to avoid resonance or changes in the rear wave in any way. I just want a dual pathway with only low frequency content going the longer route.


Lynn,

I really appreciate the interest, I presented the idea back when I first built them over a year ago, but was unable to interest anyone willing to offer meaningful input other that to request an impedance plot.

I'm pretty sure that the slot + possibly the volume beyond it is what acts as the lowpass filter, keeping the higher frequency content out of the long pathway avoiding resonance.

Can I ask a question about the standing waves on a flat baffle? Is that the result of the secondary source coming from the edge (diffraction artifacts)? If I have it wrong please explain in visual terms.

For me it took listening to a magnet mounted driver with no baffle at all to start hearing edge diffraction from baffles. I'm sure many of the technical will chuckle, but now I can often hear the outline of the baffle from diffraction occuring at the edges.

I hope you're onto something with the idea of a gradual pressure release to eliminate diffraction. The problem I keep running into is, if I address diffraction on the front side, diffraction on the back becomes more difficult to cure. I hoped my dipole waveguides would solve the dilema, but based on vibrations in my roundovers, I doubt it. That means horns, especially round ones, must have tremendous amounts of edge diffraction. Maybe I'll have to go with a front and rear driver dipole as the only solution.
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Old 12th April 2007, 05:52 AM   #186
BudP is offline BudP  United States
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Hi Lynn,

Thanks for the intro.....

The EnABL pattern could indeed be used as a set of cutouts. Lot of work though. I usually just apply the pattern blocks at every edge termination on the front and back of a driver, it's mounting bezel, the surface it mounts on and the surfaces connected to and incident to that mounting surface. Also a lot of work, but easy and tedious to perform. Not even particularly exacting work either, as the pattern allows quite a bit of flexibility in size and position mistakes.

However, the results are worth it. Invisible speakers. Absolutely no artifacts emitted from the surfaces that are not part of the information packet describing the sonic event being portrayed. And all this with no reduction in SPL or transient color, or micro dynamic detail. It is not a damping process. Just a one way gate, at every edge, that leaves the energy in question with ony complete emission into the air as a remaining path.

You have seen my speakers, they are not simple in shape nor in distribution of masses. I have worked to reduce the number of edges on the outside of the box but for every mounting plate terminus, every driver emitting surface and for the periphery of the front face, there is a full, two row pattern of blocks. I also have a ring of pattern blocks midway back, around the outer cabinet surface. This rings location controls where the front of the sonic recreation occurs, in real space, according to the correlator, which is quite expert at these calculations.

All of the blocks are applied with a pen and some acrylic paint, thickness of about 3 mils max when dry. You can paint over them, shellac over them or leave them exposed, without any alteration in their performance of the task of eliminating standing waves, caused by surfaces with an edge. Anyone on this forum can accomplish this with their speakers, a little time and some easily learned skills.

Truly you do have the correct, important point, singled out here. Once the standing waves caused by transient signal and terminus edges have been controlled, everything else falls into place very quickly and near perfect speakers become not only possible but actually easy to build. Just a lot of tedious handwork.

I have been sent a pair of Lowther's to treat. I could make a training guide, with pictures as I proceed, but, Lowther's are very touchy beasts due to the whizzer cone and a mistake could easily turn them into ultra clear ear wax removers. I will do this in another thread, rather than pollute this one, but I am more than willing to help anyone here learn how to free their speaker designs from a limitation that almost no one realizes is present and addressable, because they have never experienced a speaker without edge defined sonic corruptions. The only place you get that is right next to the orchestra, playing in a field, far from buildings. Usually, they are also marching.
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Old 12th April 2007, 06:42 AM   #187
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Quote:
Originally posted by johninCR

Can I ask a question about the standing waves on a flat baffle? Is that the result of the secondary source coming from the edge (diffraction artifacts)? If I have it wrong please explain in visual terms.

For me it took listening to a magnet mounted driver with no baffle at all to start hearing edge diffraction from baffles. I'm sure many of the technical will chuckle, but now I can often hear the outline of the baffle from diffraction occuring at the edges.

I hope you're onto something with the idea of a gradual pressure release to eliminate diffraction. The problem I keep running into is, if I address diffraction on the front side, diffraction on the back becomes more difficult to cure. I hoped my dipole waveguides would solve the dilema, but based on vibrations in my roundovers, I doubt it. That means horns, especially round ones, must have tremendous amounts of edge diffraction. Maybe I'll have to go with a front and rear driver dipole as the only solution.
I visualize the expanding wavefront meeting a characteristic impedance, the same as radio waves seeing the characteristic impedance of free space. This impedance stays the same as long as the wavefront experiences no change in the expansion characteristics, but changes abruptly when the wavefront (at the speed of sound) encounters the edge of a boundary surface.

The edge behaves similarly to a kink in an RF cable or a fracture in an optical surface - some of the energy continues to move in the same direction, some diffracts in all directions with associated frequency-dependent prismatic effects, and some even returns in the direction it came from.

All it takes is a discontinuity in the wave expansion surface - this is very well known in the RF field, where getting rid of reflections in cables is troublesome. (When I was at Tek they made a gizmo called a Time Domain Reflectometer, or TDS, or more simply, cable radar. Even hard-to-see kinks were plenty visible on the TDS display.)

In optics, light moving across a hard edge causes loss of resolution in a stopped-down camera lens, and also causes those interesting cross pattern in astrophotographs. (The diffraction is caused by the vertical and horizontal supports of the 2nd mirror in the telescope.) The star-like artifacts can be greatly reduced by using a variable-density filter to "soften" the hard edges of the lens aperture or the mirror supports.

Since the edges of the loudspeaker enclosures are much smaller than the wavelengths passing across them, the wavefront diffracts in all directions, and there are frequency-dependent prismatic effects as well. In addition, the reverse-reflected energy is free to travel all the way across the cabinet face and encounter the opposite edge. Since there are almost no losses moving across the face of the cabinet, this succession of reflections sets up a standing wave, very similar to the standing waves set up inside conventional box cabinets.

(Note: an acoustic standing wave is NOT the same as the walls of the cabinet flexing. You can have quite strong standing waves with a rigid cabinet - think of the long reverberation times of a concrete-block bathroom, for example.)

Yes, JohninCR, I can hear cabinet diffraction - with pink noise (the ideal stimulus) it is strongest at a 135-degree angle with respect to the front surface of a conventional box speaker. It actually sounds like a little bitty tweeter emitting right at the cabinet edge.

I don't see any reason why smoothing out the termination of the front wave should degrade the rear wave. If the treatment is symmetric (front and rear), both should improve at the same time. The null region should improve most of all, no small gain, considering the importance of having the early room reflections have the same spectra as the direct-arrival wave.

That's my big beef with horns - the far off-axis region can get pretty ugly, with severe time distortions thanks to strong horn-edge diffraction effects. That's where all those narrow "spikes" in the 5 to 10 kHz polar pattern come from - the frequency and time response within those spikes is going to be pretty bad.

Is this audible? Oh yes. The ear uses fine-grained information in the 5 to 10 kHz region for localization and depth perception - that's what stimulates the outer ear (the pinna), which is used for precision localization. And guess what, depth information is an area where horns are not that good compared to the best direct-radiators.

Hard reflecting surfaces and edges have destructive effects on image quality and timbre - especially when the dimensions involved are similar to head, shoulder, and pinna dimensions, which are essential for localization of sound. Vocal timbres in particular are susceptible to artificial-sounding colorations, partly because human beings have such acute discrimination between voices, and also because vocal-tract dimensions can be similar to dimensions of loudspeaker diffraction effects (a few inches).

I can't emphasize enough the first millisecond (14 inches long) is the most crucial thing the loudspeaker does - although the first 3 milliseconds are pretty important too. Note that I'm discussing reflections that are in the 0 to 42 inches long, kind of awkward considering our speakers are the same size! Thus the importance of (any) smoothing techniques to soften the acoustic edges so they re-radiate less energy - and have smoother polar patterns.

What makes this more difficult is the dynamic range of the ear. A 20 dB reduction of edge reflection energy might seem a huge triumph, until we consider we really need 60 to 100 dB of reduction to say it's actually gone for good. This where we hope that auditory masking will save the day, so the quieter reflections fall below the auditory threshold.
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Old 12th April 2007, 06:59 AM   #188
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Thanks Lynn. Bud, I'm quite interested in your strategy(ies). I look forward to more detail.
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Old 12th April 2007, 07:12 AM   #189
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JohninCR, I think your symmetric horns are really pretty slick. The only suggestion I have to smooth out the surface between the front and rear horns, instead of having two curved surfaces that face each other (this creates reflections). You could even fill in that area with cardboard or foam-core (the stuff that architects and art students use) to prototype the sonics. I think you'll like the results.
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Old 12th April 2007, 07:27 AM   #190
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Lynn,
I'll definitely try that. I do plan to bridge the gap with wood and fill the interior with sand to make the entire unit quite massive and dead. I just hope to avoid a convex curve, since that would add size. If I can get away with a concave shape, I think it would be better in terms of aesthetics, or maybe just slightly inset flat pieces.
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