Waveguides for cones/domes

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Recording monitors have used waveguides for quite a while for the upper/mid ranges with conventional cones/domes in 3/4 way systems. For the price of a bit of size and complexity they seem to offer improved directivity control, crossover integration and a degree of distortion reduction compared to examples without waveguides. The use of compression drivers seems rare.

I suspect designing and constructing such waveguides would be a nontrivial DIY task but if it could be achieved would one expect the results to be a significant improvement for home use over unloaded conventional cone/domes and/or compression drivers with waveguides?

PS I was unsure whether or not to add this to the Geddes on Waveguides thread. Please move it if I have not followed the local convention.
 
Thanks for the link which has provided some useful information to ponder but I still lack a quantitative feel for the potential performance benefits of a deep mid+tweeter cone/dome compared to, say, a compression driver solution. Clearly it is a more difficult DIY task (and more interesting) but do the potential benefits make it worth looking at?
 
I am not familiar with JBL products. They do not seem to use deep waveguides on their recording monitors but have a couple of expensive mid/high compression driver speakers which cross at 9-10 kHz. I have had an unsuccessful quick browse for measurements or technical information. Is it possible to successfully integrate mid/high waveguides at this sort of frequency?
 
An article I wrote about the practical construction of wave guides appears here..

http://sound.westhost.com/articles/waveguides1.htm

This article has extensive references but does not go deeply into a lot of theory.

The major feature of the cone/dome driven devices is that they are shallow and operate in what is known as the acoustic near field, except at the highest frequencies.
There is not a lot of data available on the net about shallow waveguides as such and it must be gleaned from various sources most of which have some quite formidable mathematics.

In classical horn theory simplifications such as plane waves and resistive impedances above a cut off frequency are made, but these are of little use in understanding shallow waveguides as the wavefronts are highly curved and have complex impedances because of it.

I don’t want to start a which is better the compression driver or cone/dome debate again as these appear in other threads. Suffice to say that Genelec a very respected maker of shallow wave guides only use a compression driver in a very large speaker system capable of high output.
rcw
 
andy19191 said:
I am not familiar with JBL products. They do not seem to use deep waveguides on their recording monitors but have a couple of expensive mid/high compression driver speakers which cross at 9-10 kHz. I have had an unsuccessful quick browse for measurements or technical information. Is it possible to successfully integrate mid/high waveguides at this sort of frequency?


JBL crossed in UHF drivers above mid/high horns in that range. Midrange waveguides playing down into the 250 - 400 Hz are relatively recent, and their general purpose mid/high compression driver/waveguide combos now reach 18 kHz easily, so "supertweeters" basically only appear TOTL consumer gear anymore:

http://www.jblpro.com/catalog/general/ProductFamily.aspx?FId=25&MID=2

http://www.jblproservice.com/pdf/AE Series/AM6200,95-WH.pdf

http://www.jblpro.com/catalog/support/getfile.aspx?docid=270&doctype=3

http://www.jblpro.com/catalog/support/getfile.aspx?docid=277&doctype=3
 
andy19191 said:
Recording monitors have used waveguides for quite a while for the upper/mid ranges with conventional cones/domes in 3/4 way systems. For the price of a bit of size and complexity they seem to offer improved directivity control, crossover integration and a degree of distortion reduction compared to examples without waveguides. The use of compression drivers seems rare.

I suspect designing and constructing such waveguides would be a nontrivial DIY task but if it could be achieved would one expect the results to be a significant improvement for home use over unloaded conventional cone/domes and/or compression drivers with waveguides?

PS I was unsure whether or not to add this to the Geddes on Waveguides thread. Please move it if I have not followed the local convention.

A waveguide is almost always going to be an improvement in the areas that you suggest. In the case of Genelec, they do use an OS contour (they told me so) but a shallow waveguide is going to be difficult to impossible to analytically determine its effect. Basically Genelec just did a "cut and try" approach to find what they liked. The approach that I take would fail for such a large angle device that is so shallow for a number of reasons. One has to use BEM or the like, which IMO is a cut and try approach, albeit the "cutting" is done in a file on a computer.

So the answer is that a shallow waveguide like this is going to be trival to design, as long as you are happy with the results. If not, then try again.

But as the coverage angle gets narrower and narrower,and the waveguide deeper, the dome approach will begin to be problematic and the results will not only not be very predictable but likely not very good. AT this point you really have to use something with an inverted dome and/or a phase plug. A flat piston will work well no matter how narrow the waveguide gets, but a dome has problems at angles less than about 120 degrees total coverage.

The compression driver has a real advantage over any direct radiator in terms of thermal modulation, since it has a much larger voice coil than the smaller dome will have.
 
> Suffice to say that Genelec a very respected maker of shallow wave guides only
> use a compression driver in a very large speaker system capable of high
> output.

Indeed but they have used their waveguides for a long time and it has become one of their distinguishing characteristics in the market. Some of their competitors like K&H also use waveguides but many do not. This suggests that the pros may not necessarily outweigh the cons in real world designs. I lack the knowledge about those pros and cons to judge hence the questions.

> so "supertweeters" basically only appear TOTL consumer gear anymore:

Consumer gear? or, perhaps, audiophile? I can appreciate the marketing difficulties of trying to sell a very expensive audiophile speaker that specs-out at 15 kHz but having two sources crossing over at 10 kHz and that far apart on the face of it looks like a recipe for more harm than good acoustically. I have not considered the problem at any length and so perhaps there is a workable solution hence my question.

> One has to use BEM or the like, which IMO is a cut and try approach, albeit
> the "cutting" is done in a file on a computer.

For linear acoustics it would be straightforward to use inverse design methods to get the computer to generate an optimum profile subject to specifying what one wants to optimise. Including nonlinear effects in and around the driver would make the task more challenging but still viable for those with the relevant sort of background. I am considering starting with the former for cones/domes and possibly moving on to the latter if things work out. But first I am trying to determine if the project has much real world relevance (not that this matters much for a hobby project that probably won't get started anyway).
 
andy19191 said:

For linear acoustics it would be straightforward to use inverse design methods to get the computer to generate an optimum profile subject to specifying what one wants to optimise. Including nonlinear effects in and around the driver would make the task more challenging but still viable for those with the relevant sort of background. I am considering starting with the former for cones/domes and possibly moving on to the latter if things work out. But first I am trying to determine if the project has much real world relevance (not that this matters much for a hobby project that probably won't get started anyway).

You can assume that the acoustics is linear, thats not a problem. But "inverse design" from all that I have seen is anything but "straightforward". How would you define "optimum"? If you are talking about a computer just trying a bunch of contours and picking the best, well, yes, this is straightforward, but time consuming and not guaranteed to find the global optimum. But if you are talking about a computer iterating directly to the optimal contour for a given problem, that's not so easy. Something like this was done as a PhD. thesis down in Aus. Might be worth looking at that. Lynn Olson posted that work.

As to the relavence, yes, if it can be done it would be great. To take a driver with a given polar response and design a waveguide that could create an "optimum" alternate polar response would be a major feat and well worth doing.
 
andy19191 said:
Recording monitors have used waveguides for quite a while for the upper/mid ranges with conventional cones/domes in 3/4 way systems. For the price of a bit of size and complexity they seem to offer improved directivity control, crossover integration and a degree of distortion reduction compared to examples without waveguides. The use of compression drivers seems rare.

I suspect designing and constructing such waveguides would be a nontrivial DIY task but if it could be achieved would one expect the results to be a significant improvement for home use over unloaded conventional cone/domes and/or compression drivers with waveguides?

PS I was unsure whether or not to add this to the Geddes on Waveguides thread. Please move it if I have not followed the local convention.

I posted some measurements of a Vifa ring radiator on a oblate spheroidal waveguide designed for a compression driver.

I'm 90% sure I posted them in this thread:

http://www.diymobileaudio.com/forum/diy-mobile-audio/60146-creating-perfect-soundstage.html

if they're not there, post a reply in the thread, and I'll upload them.

Long story short - the compression driver worked a lot better. Yes, it costs four times as much, but it's money well spent.
 
Come on now. Please don't be a troll Zilch.

Just because Dr. Geddes has ruled out dome tweeters for his own designs on the grounds of power compression, doesn't mean that his input and participation isn't highly desirable here.

I, for one, would be interested in leaning about BEM for this sort of thing. Mark Dodd at KEF has gotten some pretty excellent results and, though I've read a paper of his on the subject, I'd love to get some hands on with it.
 
cbrunhaver said:
Please don't be a troll Zilch.

I'm not sure that is possible.

cbrunhaver said:
I, for one, would be interested in leaning about BEM for this sort of thing. [/B]

Let me give you my take on BEM and FEA etc. I first got interested in FEA back in 1978 or so when the Japanese were doing such interesting things with it for transducers. I went back to school to get my PhD with the sole intent of learning FEA because "it was the future of loudspeakers". I did actually learn it and my PhD used FEA for a problem in room acoustics, the first time that this was ever done (to my knowledge). I had to write my own code because all of the FEA at the time was structural. When I graduated I contiinued to use it, but then it struck me what the limitations were.

I was not learning anything about HOW to design a better loudspeaker because FEA could only analyze a given design, it did not tell me what to do to make the speaker better, only how a given speaker would work. I began to look at more generic analytical methods which were very limited in applicability to real problems, but because they were analytic they could be used in ways that FEA could not. Analytic solutions can give insights into the nature of things that FEA cannot.

I have not used FEA in almost 20 years - except recently in magnetics because it is so effective at that problem. All of my work in sound radiation, waveguides, perception, litterly everything for which I have made a contribution came from an analytic approach, not a numerical one.

Where the numerical approach, like BEM and FEA, work great is for refining a design down to the last significant figure. It works absolutely great in automotive where they need to squeeze out the last penny from the design or the last gram from the frame. But almost no design breakthroughs have ever come from the numerical route.

This is changing a little these days of massive computer power where the computer can iterate billions of calculations, but this is a recent thing.
 
Having asked specialists in the field of modelling I have been told that if you want to determine the higher order mode production in a waveguide of arbitrary geometry, (and not just directivity), then it is necessary to calculate eigenvalue solutions to the wave equation, and for these to be accurate enough to be worthwhile you need to use a combined boundary and finite element model.

In inverse problem modelling you tend to end up with a solution set rather than a solution, and since you are going to use a device such as a cone or dome that produces a wave front that is pre determined, (unless you put some sort of a phase plug in front of it, and then you do not get exactly the wavefront you want), you are restricted as to the shape of the input waveform anyway so you get more bang for your buck, and much less grief, solving the forward problem.
rcw
 
rcw said:
Having asked specialists in the field of modelling I have been told that if you want to determine the higher order mode production in a waveguide of arbitrary geometry, (and not just directivity), then it is necessary to calculate eigenvalue solutions to the wave equation, and for these to be accurate enough to be worthwhile you need to use a combined boundary and finite element model.

rcw

This is true, if it is even possible that way, I mean, no one has actually done it! So why not just use contours where we can calculate, exactly, the HOM problem and use those! DA! Or is that just too simple? I fail to understand the facination with "arbitrary geometry". Is it because everybody wants to add there own little "twist" to the design, like a twig a basil to the sauce. I COULD do the arbitrary geometry problem, I have the knowledge, but I don't see the point.

I will admit that I have come to believe that the use of Tensor analysis and Lie geometry in manifolds could do an almost arbitrary geometery. Certainly it could do more than just the few geometries that the wave equation allows. The seperable wave equations assume Euclidian Geomtery, but Tensors don't, so they could find solutions that went beyond what the normal approach can find. Anyways, nice thought, beyond my time horizon at this point.
 
Analytic solutions can give insights into the nature of things that FEA cannot...
...almost no design breakthroughs have ever come from the numerical route.

Thanks for the insight. Unfortunately, I don't have the (math) background for that kind of work.

I was reading an interview in Stereophile with Richard Sequerra where he said "Ninety percent of the people in audio are gifted amateurs, and now they're up against a wall. Now, unless you work with the fundamentals of science, you can't make progress."

I'm serviceable in magnetic FEA (using FEMM) and was hoping there was a BEM package out there like it that could be used to better understand horns via simulation of different contours etc.

I'm content on standing on the shoulders of giants for the moment but do want to get a as deep a conceptual understanding as I can manage. I'm just not in a position to go back to school at the moment.
 
cbrunhaver said:


Thanks for the insight. Unfortunately, I don't have the (math) background for that kind of work.

I was reading an interview in Stereophile with Richard Sequerra where he said "Ninety percent of the people in audio are gifted amateurs, and now they're up against a wall. Now, unless you work with the fundamentals of science, you can't make progress."

IAWTC, but it doesn't mean you have to go back to school.

For instance, the designer of the Vandersteen line of loudspeakers attributed it's sound to the shape of the loudspeaker basket. Yet I always thought that was a fundamental misunderstanding. IMHO it's unique sound isn't due to the basket, it's due to the dipole radiation pattern. Yet when I first encountered his speakers I knew nothing about loudspeaker radiation, and took his word for fact.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.