Geddes on Waveguides

[Brett]

Originally posted by Nordic
Now if only I could find affordable compression drivers, they all seem to be rather big ticket items.

CD's aren't that expensive. Look at the stooopid prices charged for some 'phile domes.

[gedlee]

I don't know about your location, but from here a compression driver is only a little more expensive than a good 1" tweeter. But a whole lot more robust.

[Patrick Bateman]

Did everyone see this waveguide article?

I haven't had a chance to digest it, but seems very well thought out.

http://sound.westhost.com/articles/w...des1.htm#intro

[bzfcocon]

Very interesting and useful practical info - although a similar idea (but without the computations and stuff) is used by Zaph, which btw does not seem to get a citation (or maybe I missed it).

What i was wondering is whether one can make a similar waveguide but asymmetric, that is, with elliptical or section (the aspect ratio being something like 1:2 ). The reason in my case would be the difficulty to accomodate a circular waveguide on an existing baffle.

I guess there will be negative consequences (besides the difficulty of construction), but will it still be usable ?

[gedlee]

Originally posted by Patrick Bateman
Did everyone see this waveguide article?

Well done, but with a fatal flaw - "The theory behind the waveguides to be described is that a dome driver produces what is fair approximation of a spherical wave over its piston range, so if we put one of these in the end of a conical horn, the wave will propagate down the horn in much the same way as it would from a theoretical monopole point source"

This statement is incorrect (and attributed to me!!). A dome source moves axially NOT radially, and does not produce a spherical wavefront particularly at the higher frequencies. This is clearly pointed out in my book - which was not referenced. (I show this problem in detail in the appendices on my website.)

It is of note that a dome does not represent a source for which an "ideal" waveguide can be constructed, however, in theory a compression driver does - the OS.

I think that the first part of the statement:

"I can conclude that the method outlined can produce a constant directivity device that has lower distortion and better power handling than the original driver alone, and there are no measurable results that would indicate the presence of "horn sound"."

is probably true, but the the last part may not be. There is clearly something happening at just under 10 KHz, which I would suspect as being a result of the non-radial wavefront from the dome creating a HOM. This could be found to yield some residual "horn sound".

Originally posted by bzfcocon

What i was wondering is whether one can make a similar waveguide but asymmetric, that is, with elliptical section.

I guess there will be negative consequences (besides the difficulty of construction), but will it still be usable ?

This can be done, but the common method of doing it - maintaining the same area as an OS waveguide - is incorrect. One wants to maintain the boundary shape, not the area. They are certainly more difficult to make however.

[Patrick Bateman]

Originally posted by gedlee

This can be done, but the common method of doing it - maintaining the same area as an OS waveguide - is incorrect. One wants to maintain the boundary shape, not the area. They are certainly more difficult to make however.

The necessity to maintain the boundary shape is because the OS waveguide dimensions define the coverage angle, correct?
So for instance, let's say the driver spacing on your baffle dictated that the waveguide could be no taller than eight inches. In this hypothetical situation, that height and the depth of the waveguide would yield a fixed vertical coverage angle.

This gets back to the idea that we don't have a "cutoff frequency", like we do with a horn. For a waveguide the relevant parameter is the "coverage angle."

Am I paraphrasing that correctly?

[gedlee]

Originally posted by Patrick Bateman


The necessity to maintain the boundary shape is because the OS waveguide dimensions define the coverage angle, correct?
So for instance, let's say the driver spacing on your baffle dictated that the waveguide could be no taller than eight inches. In this hypothetical situation, that height and the depth of the waveguide would yield a fixed vertical coverage angle.

This gets back to the idea that we don't have a "cutoff frequency", like we do with a horn. For a waveguide the relevant parameter is the "coverage angle."

Am I paraphrasing that correctly?

Patrick (or John actually isn't it) - mostly correct.

The concept of "cutoff frequency" and "loading" don't really have a meaning to a waveguide, in a waveguide its all about pattern control and minimizing diffraction. That said, these concepts are quite overblown in horn theory too. Different horn contours only have different loading and cutoff "in theory" because in a real device, its basically only the throat and mouth areas that matter. For a given throat and mouth area virtually any horn contour in a device of finite length will have a "loading" that is within about 1 dB at all frequencies.

The need to maintain the contour shape and not the area came from a hureristic argument. I was interseted in how one would make a non-symmetric device. I, like most, had become locked into 'horn" thinking in that it is the area change that matters (Websters Equation). But as I looked arround at all of the "correct" waveguide shapes I noticed that they all had exactly the same boundary shape but NOT the same area change with propagation. Then it dawned on me that - of course! - this is what is different about a waveguide and a horn. In a waveguide its the boundary rate of change that matters (all waveguides have catenoid boundaries) and not, as horn theory says, the area change. This was a epiphany for me - although it seems lost on most that I tell it to.

[audiostar]

Earl,

you are one serious player in the world of audio. check out his web
site Gedlee and his resume. He has written two books and is working
on another. also he has written articles for the journal of audio
engineering society. somebody please buy his speaker and post a
review.

[angeloitacare]

This can be done, but the common method of doing it - maintaining the same area as an OS waveguide - is incorrect. One wants to maintain the boundary shape, not the area. They are certainly more difficult to make however.

hi Ged

there is a austrian company, haigner horns, which makes elliptical wave guides.

http://www.theaudioeagle.com/columns/column05.html

They use CNC , quit difficult horns to be done. I had some email exchange with the owner. He sweares that his wave guide is much better than the spherical horns. What is your opinion in this regard ?

Angelo

[gedlee]

Originally posted by angeloitacare
there is a austrian company, haigner horns, which makes elliptical wave guides. They use CNC ,

Well we all use CNC!!

He sweares that his wave guide is much better than the spherical horns. What is your opinion in this regard ?

And who doesn't "swear" to that!? Does he show supporting data, or just the typical meaningless claims of "increased dynamics and detail" and "It sounds cleaner, smoother, more extended and lower distorted"?

I will compare my performance data with anyone on the planet. I won't deal in superlatives with no meaning.