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Old 11th August 2012, 05:35 PM   #51
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Originally Posted by badman View Post
I think the correct descriptor for this device is simple- it's a method for making phaseplugs with various wavefronts.

Pretty slick stuff but no getting around the challenges of that 180 degree turn in the real world. It could be mitigated by softening the transition and using some damping a la geddes, but then you're getting into a more complex construction and deeper assembly. But 2" of depth added to allow this transition to happen in a smooth fashion may be a very worthwhile tradeoff for sound quality apps.
IMHO, you are half right. This *is* a phaseplug. It's a device designed to equalize the pathlengths of wavefront.

Where we disagree is whether the bends are a problem. IMHO, high order modes, diffraction, and reflections cannot form below 18khz. (assuming a maximum height of 1/4".)

The wavefronts simply cannot form; the dimensions will not allow it. For instance, if you took a block of aluminum, then drilled a hole through the aluminum that was one micron wide, you would not be able to see through the aluminum. The hole is simply too small. You wouldn't be able to see through that hole until it's diameter was sufficiently large to allow light to pass.

That's what's going on in the Paraline. There is air in the Paraline, but there is not sound, at least not below 18khz. The sound does not form until the duct is large enough to allow it to form. Until the duct is large enough, it's just a bunch of air molecules squirting through the device with no waves formed.


As for Geddes, he wrote a patent on optimum phase plugs. Yet on this very forum he's stated that he did not pursue it because the differences were not audible. Perhaps there's a connection?

(Note to self - I should probably be careful about misquoting Geddes, he could make me look really dumb in a hurry, since he knows way more about this stuff than I do )

Here's Geddes patent on optimum phase plugs:

Patent US20040156519 - Phase plug with optimum aperture shapes - Google Patents
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Old 11th August 2012, 05:45 PM   #52
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Originally Posted by Don Hills View Post
To anyone trying this: Remember to keep the space at the junction of the driver(s) with the horn as small as possible. If the space is significantly larger than the width of the eye cavity, it will form a throat chamber which will act as a low pass filter. The ideal configuration for a tweeter/midrange pair would be a "naked" dome tweeter / compression driver with the dome protruding into the cavity formed by the cone of the midrange driver.

I do worry a little about possible interaction between the two drivers placed so close together, with the horn load increasing the interaction.
Don, this is a good point.

Click the image to open in full size.
Here's a measurement of two of the Goldwood cone tweeters in a Squaraline. (The Squaraline is the first one that I built, which has a video here: http://www.youtube.com/watch?v=z1ECVZ0LPOc&feature=plcp) The measurement is kinda crummy, but I've been too busy building Paralines to do proper measurements. This iPad program is about 80% as good as the results that I get from Arta, so it's better than nothing.

What we notice with the Goldwood tweeters are a few things:
  • The high frequency response of the Goldwood tweeters in a Paraline is almost more extended than on a flat baffle. If it wasn't for the peak at 4khz, I'll bet you could run the Squaraline to 10khz using cone tweeters. I believe that the peak at 4khz is due to geometry. I've found that you must have the tweeter centered in the Paraline to get flat response at high frequency. It's simply cannot be off-center.
  • As I understand it, the driver that we want for a Paraline is flat, to match with the surface of the line. If not flat, it should produce a flat wavefront at the driver exit, like a compression driver. Also, this is still a horn, and horns are impedance transformers. Therefore, we should watch out for drivers with a high VAS, as they'll need a larger air load. I'd imagine that if you used a driver with a large VAS, it would be peaky, just as it is in a horn. This doesn't necessarily mean that you need a driver with a large motor. But if it *doesn't* have a large motor, you probably want one with a light cone. It's a bit of a catch 22, because drivers with a large motor kinda defeat the purpose of a Paraline, which is it's incredibly shallow depth.

    At this point, arguably the best results in the midrange have been found with cone tweeters and with planar magnetics. (Goldwood GT-25 and BG Neo 8, respectively.) The best results at high frequency have been compression drivers. Admittedly, I've only tried one ribbon and no domes. So still lots of testing to be done.

Last edited by Patrick Bateman; 11th August 2012 at 05:49 PM.
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Old 12th August 2012, 12:46 AM   #53
smf is offline smf  United Kingdom
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Originally Posted by Patrick Bateman View Post
IMHO, you are half right. This *is* a phaseplug. It's a device designed to equalize the pathlengths of wavefront.

Where we disagree is whether the bends are a problem. IMHO, high order modes, diffraction, and reflections cannot form below 18khz. (assuming a maximum height of 1/4".)

The wavefronts simply cannot form; the dimensions will not allow it. For instance, if you took a block of aluminum, then drilled a hole through the aluminum that was one micron wide, you would not be able to see through the aluminum. The hole is simply too small. You wouldn't be able to see through that hole until it's diameter was sufficiently large to allow light to pass.

That's what's going on in the Paraline. There is air in the Paraline, but there is not sound, at least not below 18khz. The sound does not form until the duct is large enough to allow it to form. Until the duct is large enough, it's just a bunch of air molecules squirting through the device with no waves formed.


As for Geddes, he wrote a patent on optimum phase plugs. Yet on this very forum he's stated that he did not pursue it because the differences were not audible. Perhaps there's a connection?

(Note to self - I should probably be careful about misquoting Geddes, he could make me look really dumb in a hurry, since he knows way more about this stuff than I do )

Here's Geddes patent on optimum phase plugs:

Patent US20040156519 - Phase plug with optimum aperture shapes - Google Patents

Patrick - sorry, I'm slightly confused by some of the above. Given that sound is just a series of compression/rarefactions in air and this must be happening inside the paraline here because otherwise we wouldn't get any sound out, there must be sound 'inside' the paraline? What I thought Tom Danley had said was that if you kept the slit narrow then you didn't get waves bouncing off the parallel walls of the paraline, because there wasn't enough space for a wave below 18khz to form as between the walls. That doesn't mean a wave isn't formed in the direction of travel out along the radial path. However, the distance out to the first bend is more than a few mm, and so don't we get problems arising at that point with reflections back from the edge of the 'eye' back to the driver? But as I previously posted, maybe it's not enough to be a problem. Does anyone know where I might read some more on this please?

Thanks
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Old 12th August 2012, 01:53 AM   #54
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Originally Posted by smf View Post
... However, the distance out to the first bend is more than a few mm, and so don't we get problems arising at that point with reflections back from the edge of the 'eye' back to the driver? But as I previously posted, maybe it's not enough to be a problem. Does anyone know where I might read some more on this please?

Thanks
There is no reflection problem, because the width of the passage is much smaller than the wavelength of the sound. The sound "flows like a fluid", rather than bouncing off the walls. It's the same reason why you don't have reflections at the folds in a tapped horn within its designed operating bandwidth. Tom Danley has pointed this out in several posts. The best explanation I have seen is in his patent application - Fig 13 and its description.
Patent US20090323997 - Horn-loaded acoustic line source - Google Patents
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Old 12th August 2012, 02:32 AM   #55
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Originally Posted by smf View Post
Patrick - sorry, I'm slightly confused by some of the above. Given that sound is just a series of compression/rarefactions in air and this must be happening inside the paraline here because otherwise we wouldn't get any sound out, there must be sound 'inside' the paraline?

Thanks
There's no sound inside the Paraline, because the duct is too small for the soundwave to fit inside of it.

This applies to all speakers - horns, transmission lines, ribbons, domes, whatever.

What happens in this situation is that the waveform is formed when the duct is large enough for the wave to form.


Here's a hypothetical example:

Get a piece of PVC pipe that's a mile long and 2" in diameter. Now put a speaker on one end of the pipe and play a sound that's 10khz. If you put your ear on the other end of the pipe, it will sound like the sound is a mile away, because 10khz is just 1.35" in diameter. So 10khz fits in the pipe easily. You'll also notice that the sound is hollow, due to a mile's worth of reflections.

Now have that speaker play 100hz. The sound will seem to emanate from the mouth of the pipe. This is because 100hz is 135" long. So it can't fit in the pipe at all. When your speaker plays 100hz, all it's doing is pushing the air molecules through the pipe, like they're a liquid. YES, it's still 100hz, and it will be 100hz when it exits the pipe, but inside of the pipe it's just air being pumped at 100 cycles in a second... there is no wavefront and the wave does not exist until those air molecues exit the pipe, where they enter a duct that's big enough to allow the wave to form.


If you imagine sound as light it's easier to grok; make a hole big enough and the light will exit, but as long as the hole is too small for the light to exit, it won't.
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Old 12th August 2012, 05:55 PM   #56
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Quote:
Originally Posted by Patrick Bateman View Post
There's no sound inside the Paraline, because the duct is too small for the soundwave to fit inside of it.

This applies to all speakers - horns, transmission lines, ribbons, domes, whatever.

What happens in this situation is that the waveform is formed when the duct is large enough for the wave to form.
Patrick,

There is sound inside of a Paraline or any horn, and the sound pressure varies inversely with the distance from the driver. Sound waves are made as soon as the diaphragm or cone moves.

For example, here are the dB (A scale) measurements of a conical horn crossed at 1250 Hz, using pink noise:
110 2 meters from mouth
115.2 1 meter
119.7 .5 meter (19.5 inches)
122.5 .25 meter (9.75 inches)
124.2 .125 meter (4.875 inches)
126.3 at horn mouth, 26 inches from driver throat screen
131.7 13 inches from screen
136.8 6.5 inches from screen
141.0 3.25 inches from screen
143.7 at screen (144+ dB SPL on impulse, flat)
149.1 at screen, xover output turned up to +11 (+6.5 dB over last test)

The pink noise had basically the same frequency response at each distance.
If you were to use a very small microphone in a Paraline you would find the same sort of inverse distance increase in level.


Have you compared your "Squarealine" output to the same speaker with no horn?

Other than a more narrow pattern at upper frequencies boosting on axis response, is the sensitivity increased broadband?

Art

Last edited by weltersys; 12th August 2012 at 05:57 PM.
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Old 13th August 2012, 05:55 PM   #57
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Originally Posted by weltersys View Post
Patrick,

There is sound inside of a Paraline or any horn, and the sound pressure varies inversely with the distance from the driver. Sound waves are made as soon as the diaphragm or cone moves.

For example, here are the dB (A scale) measurements of a conical horn crossed at 1250 Hz, using pink noise:
110 2 meters from mouth
115.2 1 meter
119.7 .5 meter (19.5 inches)
122.5 .25 meter (9.75 inches)
124.2 .125 meter (4.875 inches)
126.3 at horn mouth, 26 inches from driver throat screen
131.7 13 inches from screen
136.8 6.5 inches from screen
141.0 3.25 inches from screen
143.7 at screen (144+ dB SPL on impulse, flat)
149.1 at screen, xover output turned up to +11 (+6.5 dB over last test)

The pink noise had basically the same frequency response at each distance.
If you were to use a very small microphone in a Paraline you would find the same sort of inverse distance increase in level.


Have you compared your "Squarealine" output to the same speaker with no horn?

Other than a more narrow pattern at upper frequencies boosting on axis response, is the sensitivity increased broadband?

Art
I wish I had time to make some illustrations of why I do not believe that a wavefront can form in the Paraline. (Well, a wave can form, but not below 18000hz.)

If anyone is interested in reading the stuff that's brought me to this hypothesis, please review these two posts. I know Art's reviewed the first one, because Art's project inspired my project, and the following post is a response from Tom to Art

ProSoundWeb Community - Index

"hen the wavelength is short enough (frequency high enough) the sound that went to the pointy end, bounces back and arrives out of phase with the driver pressure.
One finds (as you raise the frequency) that you eventually have a BIG cancellation notch when that “driver to dead end” distance is about 1/4 wl.
The two low pass filter effects strongly attenuate above band energy from the cone drivers and helps make the distortion especially low.
It was that notch, or pondering that notch that made me wonder about and then try what became the Tapped horn.
I thought what happens if I substitute a source of the opposite phase for that reflection? (a source which was present in the back side of the radiator), then they add instead of cancel. Some considerable fiddling in the computer eventually resulted in boxes that work better than similar sized normal bass horns using this new principal.

Anyway, up to now, the only function a phase plug has is to occupy an excess air volume that would have other wise made the acoustic “low pass filter” too low in frequency.

Once one is dealing with a radiator who’s dimensions are approaching the wavelength size, then the other function of a phase plug comes in handy.
The speed of sound governs how a pressure disturbance radiates away from its source.
If one has a radiator that is “large” acoustically and also has a single exit point, one finds that just like in the Synergy and Unity horns, one gets a deep cancellation notch when the difference in the two paths is 1 / 2 wl. The range of coherent summation is limited to the frequencies BELOW the region where cancellation begins.
This is like a pile of subwoofers, when the array is less than about 1/4 wl across, they all add together and feel “mutual radiation pressure” while a significantly larger spacing produces directivity and then lobes.

Here, a phase plug can be shaped so that the acoustic passages all have the same length or have a length appropriate to the desired exit wave front shape.
One big difference in the sound of compression drivers (IMO) after being eq’d flat is that many have a phase plug that produces a converging wavefront at the summation, while what one needs at the throat of a horn is a diverging wavefront. That “clash” can cause diffraction or interference, which produces Higher Order Modes that Earl Geddes describes.

So far as the Paraline as used in the VTC array and GH-60, this is an acoustic device which can be shaped to provide an exit wavefront that can be flat, a line source or diverge or converge, an astigmatic point source with positive or negative focal point..
It works by allowing the sound to expand radially between two plate that are too close together to support any reflected modes between them so only radial expansion takes place.

A correction slot who’s shape defines the exit wave front shape and who’s dimensions are small enough to allow the sound to bend around the corners, is placed in the radial path. The sound passes through the slot and what emerges on the other side is a wave that travels to the center from each side, bends around a corner and exist at a center slot having entered at a center hole at the rear. The VTC site had a nice graphic of the one they are using.

It probably sounds weird to suggest that you can bend sound without ill effect but you can when the acoustic dimensions are small enough. The difference as it is in the examples above is that keeping the difference in path lengths less than about 1/4 wl at the highest frequency of interest..
I used to work with 21KHz levitation sound sources and needed to place a microphone in the levitation furnace to monitor the source sound level.
Well, very very few things are “happy” at 1500 degrees C but I found that a Zirconium / Alumina tube with a 1/16 inch bore passed 20KHz sound out of the furnace to an external microphone with no problem.

Funny, the external microphone’s heatsink wasn’t large enough on the first one and the microphone melted.
Later fooling around (research) showed that a 3 foot long, 1/16 inch bore copper tube could be wound around a small coffee cup and not effect the sound passing to the microphone.

A constantly re-occurring theme in much of what I do is that many things depend on how large X is compared to the wavelength.
Anyway, I hope that makes some sense.
"


aaaand this one:

Plane Wave Tube Construction

"rho = density of air
c = speed of sound
d = diameter
Upper limit = 1,22 * c / d
lower imit = c / 4 / length of tube

Now let's talk about horns driven by compression drivers. One of the handy pieces of information that used to be included with compression drivers (but is not always now) is the plane wave tube response. A plane wave tube is a tube the same size as the exit of the driver (or smaller or larger and including an adaptor to have a smooth transition from the driver's throat to the size of the tube). There is absorbing material in the tube which is designed to provide a certain load to the driver - a load of rho * c * diameter of the tube, where rho is the density of air and c is the speed of sound. This is usually called a "rho c" load; rho * c is the specific impedance of air. This load is supposed to be constant at all frequencies and allow easy comparison of different drivers, but in reality the size of the tube determines the frequency range measurements are useful in. The upper limit of the tube is set by the diameter. 1.22 * c / d is usually given as the upper limit. There will typically be a notch at this frequency and other notches above it. The lower limit is c / 4 / length of the tube. So only the response between these frequencies should really be looked at. With high frequency horns, you can typically build them large enough to obtain a rho * c * some constant input impedance that is relatively flat above the cutoff frequency (depending on the type of horn). This means in theory the frequency response on the horn will be the same as on the plane wave tube (although the sensitivity will be different - compression drivers' sensitivity on a PWT is very high). The only caveat here is that many high frequency horns have a polar response that varies with frequency. In other words, at low frequencies the horn is not big enough to the control the sound radiated by it, so the sound spreads out over a wide angle. At high frequencies, the horn becomes acoustically large compared to the sound waves being radiated, and the sound is confined into a narrower angle. This means that at higher frequencies more power is concentrated over a given area, so the sound pressure level is higher in that area (but lower outside it). This is effectively an acoustic equalizer, so this effect needs to be added to the plane wave tube response to come up with the on-axis response of the horn / driver combination. This also explains why typical horn design programs do not predict the on axis response of high frequency horns very well - they don't typically include this factor.
"
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Old 13th August 2012, 06:04 PM   #58
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Dipoles are all the rage here right?

Well how about a dipole paraline?

The extremely shallow depth of the BG drivers makes this do-able. I made some sloppy measurements with RTA Lite, and found that the response of the NEO 8 was one of the flattest of all the drivers I tried. On the downside, the horn filters out a lot of the high frequencies.

I'd have to run some polars, but I'm guessing that the paraline reduces high frequency energy similar to the way that a constant directivity horn does. Basically that the high frequencies are spread across a larger coverage angle, so the overall SPL level is lower than if you listened to a planar on-axis.

Last edited by Patrick Bateman; 13th August 2012 at 06:06 PM.
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Old 13th August 2012, 06:38 PM   #59
badman is offline badman  United States
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Quote:
Originally Posted by Don Hills View Post
There is no reflection problem, because the width of the passage is much smaller than the wavelength of the sound. The sound "flows like a fluid", rather than bouncing off the walls. It's the same reason why you don't have reflections at the folds in a tapped horn within its designed operating bandwidth. Tom Danley has pointed this out in several posts. The best explanation I have seen is in his patent application - Fig 13 and its description.
Patent US20090323997 - Horn-loaded acoustic line source - Google Patents
This would only hold if the pathlength were small relative to the wavelength. The parallel surfaces aren't the problem- the abrupt bend that is on the order of 1/4 wave at 1kHz, is a meaningful concern. Any one dimension that is meaningful in size is sufficient to create disruption.

In a tapped horn, the waves are long relative to the length of the segments with the bends.

That's not true with a several inch pathlength producing treble.
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Old 13th August 2012, 06:50 PM   #60
JLH is offline JLH  United States
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Originally Posted by badman View Post
This would only hold if the pathlength were small relative to the wavelength. The parallel surfaces aren't the problem- the abrupt bend that is on the order of 1/4 wave at 1kHz, is a meaningful concern. Any one dimension that is meaningful in size is sufficient to create disruption.

In a tapped horn, the waves are long relative to the length of the segments with the bends.

That's not true with a several inch pathlength producing treble.
It doesn't matter how long the wave has to travel to get to a bend, it is the dimensions of the bend itself that matter. In a "properly" designed paraline that bend is acoustically invisible to the wave front. The acoustical image size the bend presents is too small to effect the wave front traveling around it. However, almost everything you have stated holds true for frequencies above 18KHz in a 1/4" sized paraline.
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