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Old 22nd June 2017, 01:00 AM   #941
gedlee is offline gedlee  United States
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Quote:
Originally Posted by charlie2 View Post
Is diffraction frequency dependent?
I would say that it always is, except that I can imagine cases where it wasn't. For example HOMs cannot propagate below a certain frequency, then they propagate with maximum efficiency, and then die down. Each mode does this at a different frequency. In a very good waveguide (the kind I like!) there could be as few as one or two and those, and then well damped. Compare that with the Bose design and its orders of magnitude less.

How would this manifest itself? Mostly in imaging and the ability of the speaker to disappear. The frequency response could be quite smooth with lots of diffraction - kind of like the "mean value theorem".
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Old 13th June 2018, 08:53 PM   #942
Patrick Bateman is offline Patrick Bateman  United States
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Originally Posted by Tom Danley View Post
Hi Patrick
I have to say you have to be one of the most inquisitive, creative and ambitious fellows I have run across. I don’t know what you do now for a living but I hope it challenges you like acoustics does and if you get tired of it, get a hold of me we are growing still at work.

A couple thoughts;
Lengthwise modes. Consider a straight exhaust pipe or inlet tube, it can be “tuned” to maximize performance. What happens applies to horns. When the exhaust (or intake) valve opens, there is a pressure wave (or vacuum wave) travels down the pipe at the speed of sound (for that temperature) and at the end is figuratively nothing, nothing to slow down the flow and so, the mass of gas overshoots produceing a vacuum at the end of the exhaust pipe. That vacuum wave rushes back towards the valve and when everything is right, meets the new out flowing gas helping to “suck it out”
A simple exponential horn has an expansion rate which is doubles every x inches etc for frequency Y.
If it is about a wavelength in circumference at the mouth it has reached the plateau on the radiation impedance curve and there is no acoustic gain making it larger. By reaching the plateu at the low cutoff, that means that at all frequency above that, the acoustic terminating impedance is constant and the “active” part of the impedance transformation moves up closer to the throat.
Now, you have a resistance at both ends and in the ideal horn, those resistances are appropriate to swamp any evidence of this being an axial mode device. Make it smaller and there isn’t enough mouth end resistance and modes start to show up.
From the standpoint of the wave progression, the Paraline is a short horn with a small mouth, but is always connected to a final flare which provides the termination.
Also if one had a perfect homogeneous ribbon source the same size as a paraline (planar wave) aperture and you measured the polar patterns, you would also see things related to the geometry of the aperture.
Lastly, I suggest you rig up a simple experiment where the only thing you’re testing is how sound goes around corners up high. Don’t have it be part of anything else, only test one thing, say the thickness of the air passage. Fwiw I went with path length corrections because up to a high frequency they are not frequency dependant while a reflector is changing constantly up to a very high frequency.
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Hey you might get a kick out of this from testing last Friday, something new, at 100 meters.
Dropbox - 20170428113326.mts

I hate to sound like such a fanboy, but I had a funny realization about the Paraline today:

It may be the only practical solution for the tweeters in a CBT array.

Here's what I mean by this:

In an array, the maximum output at high frequency is dictated by the output of a single unit. For instance, the CBT36 has something like 144 tweeters, but the maximum output above 10khz is something like 90dB!

The reason that this happens is that the wavelengths are smaller than the tweeters themselves.

Click the image to open in full size.

For instance, in JBL's top of the line CBT, the tweeters are spaced at around 35mm. This means that when the wavelengths are shorter than 35mm, an interference pattern will emerge. The comb filtering means that the tweeters are no longer combining constructively, and output drops.

Click the image to open in full size.
A solution to the problem is to use a ribbon. This is because the ribbon generates a flat wavefront. The flat wavefront means that the array of ribbons can approximate a line source.

But...

There's still a problem:

We really want to curve that array. Every curved and shaded array that I've heard outperforms a straight unshaded array. I've never heard a straight unshaded line that didn't sound "off", particularly in the treble.

Click the image to open in full size.

Once we shade it and curve it, we get something like this.

So far, so good, right?

No. We're back to square one; our array has no 'sparkle' because the high frequencies are limited by the spacing.

I've literally been wracking my brain all week, trying to figure out a way to fix this. A line of tweeters won't do it, the spacing is too great. A curved, shaded array with ribbons WILL fix it, but yowza that is going to be one heck of a carpentry project. I can barely build a six sided box, I can't even comprehend making a curved array with ribbons. I don't own a CNC machine, and in the city where I live, it would cost me about $500 per month just for the real estate required to park a CNC machine on my property. (I live in San Diego, where $6000 is considered a reasonable price for a home that's eight feet away from the next home over and has no parking.)

So I kept wracking my brain, and pondering, and then it hit me:

A Paraline. Because you can curve the wavefront IN THE PARALINE and that means that you can make a box that looks like this:

Click the image to open in full size.

Click the image to open in full size.
Instead of this.
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Old 17th June 2018, 01:09 PM   #943
zobsky is offline zobsky  India
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Re. CNC, . maybe you need one of these or its brethen Shaper | Origin
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Old 17th June 2018, 03:20 PM   #944
nc535 is offline nc535
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If you have mastered the paraline - ie. are able to build one with a smooth frequency response - then you don't need to curve the wavefront. You can build a continuous line source, except for small gaps between paraline segments, as tall as you want. Being quasi-continuous, it won't exhibit the combing that motivates curving a discrete line array.

And if you use a small full range driver on the paralines, it will be affordable.

If you do build a curved wavefront paraline then could use it in a home-scale Jericho horn, which would also be "interesting"
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Old 17th June 2018, 08:30 PM   #945
Patrick Bateman is offline Patrick Bateman  United States
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So I decided to try and build some Paralines again. Here's why:

Click the image to open in full size.

Click the image to open in full size.

When I started building Paralines, about five years ago, I didn't comprehend that the Paraline is a diffraction slot.

Click the image to open in full size.
Fifty years ago, engineers figured out that there's a couple things you want to avoid with diffraction slots. First, you want to keep them short. Second, you want a smooth transition from the slot to the rest of the horn or waveguide.

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That's why modern diffraction horns look like this. A 2" deep diffraction slot that's smoothly blended into the rest of the horn.

My Paralines violated these rules in a huge way; they were way too big. And they weren't even terminated! An unterminated horn is just a terrible idea; you get massive diffraction at the mouth.

Click the image to open in full size.
I've been building horns for cars for well over ten years, and they're a really great example of this. If you measure one of these things in "free air", the performance is ABYSMAL. But you put one of these things up against a boundary (like a dash) and suddenly they work. And similar to diffraction slots, a lot of people that put these in their cars find that they sound better when they're shallow, not deep.

Click the image to open in full size.
Here's some polars of a "VDOSC" style device I built a year ago. No, it's not as smooth as an OS waveguide, but it's smoother than most of the DIY paralines I've seen. Which begs the question: Is the VDOSC fundamentally superior? Or was the improvement due to the termination of the horn, and the reduction in the size of the diffraction slot?

Click the image to open in full size.
In this pic, you can see that the diffraction slot portion of this horn is much smaller than the Paralines I built five years ago.

Click the image to open in full size.

Here's the 'guts' of the device I'm working on. It's a fairly straightforward Paraline, but with the following improvements:

1) The pathlength of the Paraline is much shorter. Instead of 15cm, it's 5cm.

2) I'm building it in pieces. This isn't the first time I've 3D printed a Paraline or VDOSC, but all the other ones I've built were one piece units. By doing it in pieces, I can seal off every surface. I've found that loudspeakers in 3D printed enclosures must be sealed off, because the speaker can 'push' the air right through the gaps between layers. This is a complex topic BTW; I've noticed that PLA is way more sensitive to this than PETG, and the temperature of your print makes a difference too. Basically if you use a low temperature plastic like PLA, the layers will harden before the next layer is applied. And this leads to leaks in the speaker. Long story short, the walls of this Paraline will be sealed off using epoxy.

3) This Paraline will be terminated with a horn, as it should be.
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Old 17th June 2018, 09:03 PM   #946
Patrick Bateman is offline Patrick Bateman  United States
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If this thing works, here's a few DIY ideas I had:

Click the image to open in full size.
This is the Danley SBH 10. My wife isn't too keen on me putting big Synergy Horns in the living room, but I could probably talk her into this.

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Apex Jr sells these nifty Vifa coaxials, and I think they'd be great for something like an SBH. There are tons and tons of car audio coaxials out there, but car audio coaxials aren't well suited for horns. They typically use heavy poly cones and their tweeters are too large for a proper Paraline. This Vifa uses a 3/4" tweeter and a paper cone.

Click the image to open in full size.
Another idea I had, was something like the Sound Physics Labs "Runt." Basically putting dual Paralines at the apex, so that you can make the speaker a two-way instead of a three way. In all of my Unity horn projects, the most difficult part for me is getting the xover correct. Going to a two-way simplifies things.

Click the image to open in full size.

For the past few weeks I've been listening to a speaker that I built that's very similar to Follgott's project (Line array prototype (with waveguide and CBT shading)) and I REALLY like the way it sounds. But it lacks 'sparkle' on the high end, so I'm hoping that something like a Paraline might be able to produce that 'sparkle.'

Here's some videos of SBHs and Jerichos, and you can hear that they have that 'sparkle" that my line array is lacking. The last video is a real treat, a dude in the UK built a Jericho in a VAN(!)

YouTube
Danley SBH 10


Danley J6-42

demo of eveness of the pattern control... - Harwood Acoustics
A DIY Jericho, in a VAN, out of the UK
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Old 17th June 2018, 09:21 PM   #947
Patrick Bateman is offline Patrick Bateman  United States
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Now I hate to muddy the waters further, but I noticed something interesting in the Danley Jericho J6-42. If I'm not mistaken, this is the newest Jericho horn.

Click the image to open in full size.
Click the image to open in full size.
Here's a pic of the high frequency units. There are eight.

Click the image to open in full size.
Click the image to open in full size.
Here's a pic of the high frequency units from a Bose DeltaQ. There are four.

Those two look REAL similar to me. Obviously, it's hard to be 100% sure without opening them up. But one of the ideas I had, when pondering Paralines, was to simply use a diffraction slot that's mounted at a 45 degree angle. The reason that this can work, is because the dimensions are so small. For instance, if you keep the exit of the slot down to around 19mm, the wavelengths below 20khz won't form until they exit the slot. (This is also the reason that the layers in the "old school" Paraline are so short.)

It's possible that this solution is superior, because there are WAY fewer reflectors. In an "old school" Paraline there are as many as four bends:

1) There's a right angle bend inside the BMS compression driver

2) There's a right angle bend where the compression driver meets the Paraline

3) There's a 180 bend in the center of the Paraline (ouch!)

4) There's a right angle bend at the exit of the Paraline into the mouth of the waveguide

Last edited by Patrick Bateman; 17th June 2018 at 09:24 PM.
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Old 17th June 2018, 11:19 PM   #948
nc535 is offline nc535
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so the VDOSish solutions avoid the bends of the paraline but don't give you the flexibility of generating arbitrary wavefronts like the Danley paralaines; they do enable line arrays without combing issues

but there is still the cost issue of so many compression drivers. OTOH, I'm afraid that with full range drivers used in place of CDs, one can't avoid forming bandpass chambers that limit the HF. OTOH again, it should be possible with 3D printing to make a phase/volume plug that follows the cone profile arbitrarily closely.
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Old 18th June 2018, 02:18 AM   #949
weltersys is offline weltersys  United States
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Quote:
Originally Posted by nc535 View Post
so the VDOSish solutions avoid the bends of the paraline but don't give you the flexibility of generating arbitrary wavefronts like the Danley paralaines; they do enable line arrays without combing issues

but there is still the cost issue of so many compression drivers. OTOH, I'm afraid that with full range drivers used in place of CDs, one can't avoid forming bandpass chambers that limit the HF. OTOH again, it should be possible with 3D printing to make a phase/volume plug that follows the cone profile arbitrarily closely.
V-DOSCŪ type horn throats can be designed to create "wavefronts" that are a cylyndrical or section of a vertical arc just as the Paraline, though since the bends are far less severe, more cabinet depth is sacrificed.

For home use, large quantities of compression drivers are not required, a single driver could drive a flat front horn throat of any desired vertical dispersion arc, though the taller the throat, the greater the depth.

Although it is possible to make a phase/volume plug that follows a cone profile closely, most cone or dome tweeters TS parameters don't lend themselves to becoming great compression drivers.
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Old 18th June 2018, 02:35 PM   #950
nc535 is offline nc535
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Comprehension dawns; wish I had read the VDOSC patent long ago.
It seems the basic principle for vdosc, paraline and layered combiner are the same

"passages are all of lengths which are
practically identical from the input orifice to the output
orifice of the conduit"
to make a planar wavefront and tapered lengths to produce a curved wavefront
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