Waveguides for cones/domes

[andy19191]

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.

[dlneubec]

You might look at this thread.

I posted a link there to some studies I did on shallow DIY waveguides and also some of the results were posted there:

DIY waveguide thread

[andy19191]

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?

[ZilchLab]

JBL Consumer has been building "tweeters" this way for many years; find some at your local thrift to study:

http://manuals.harman.com/JBL/HOM/Te...LS810%20ts.pdf

Yeah, they've been doing elliptical oblate spheroids for nearly 20 years, as well. They're not quite that cheap, but ARE available to the resourceful DIYer.

I just posted a JBL PT waveguide midrange solution here:

http://www.diyaudio.com/forums/showt...00#post1866400

[andy19191]

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?

[rcw]

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

[ZilchLab]

Originally posted by andy19191
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/genera...x?FId=25&MID=2

http://www.jblproservice.com/pdf/AE%...6200,95-WH.pdf

http://www.jblpro.com/catalog/suppor...=270&doctype=3

http://www.jblpro.com/catalog/suppor...=277&doctype=3

[gedlee]

Originally posted by andy19191
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.

[andy19191]

> 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).

[gedlee]

Originally posted by andy19191

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.