Geddes on Waveguides

[Paul W]

We at Ai may actually be the first to make drivers with this new phase plug concept.

Count me in

Earl,
The constant directivity waveguides I've been experimenting with over the past few months sometimes sound overly bright when voiced flat.

This is almost certainly a problem with the source material rather than the speaker. With good sources, for example AIX 24/96 DVD-A and many new SACD recordings, they sound so good they are simply stunningly. However, mediocre material sounds too "hot"...the speaker is clearly revealing flaws in the recording process. Unfortunately, mediocre sources are far too common. Do you strike a compromise when voicing CD (and if so, what is it) or do you let the sources stand on their own merit?
Paul

[catapult]

I modified my simple oblate spheroid spreadsheed so you can set the initial throat angle using the Goal Seek function in Excel. No effort was made to make it "pretty" but it works.

http://www.crestviewcable.com/~catapult/Geddes.xls

[JoshK]

Originally posted by gedlee
Ed

The math that you sent is great for the baffle radius, but neglects the finese at the throat. Your waveguide has a slope of zero at the throat, but the exit of the compression driver has a slope of 6.5 degrees. This mismatch in slopes will cause more HOM. You want to match the input of the waveguide radius and slope to the same on the exit of the driver, otherwise there is a mismatch. This math is tricky, but it can be done with a cut and try approach.

Preliminary data of ours suggests that the throat is very sensitive to small perturbations of the wavefront and that the transition from the driver to the waveguide must be as smooth as possible. We actually fill in the small cracks here with clay so that the transition is seamless. No real hard data on this but the "soft" data suggests that smoother is measurably better.

I am the guilty for the math part. I didn't take into account the slop of the exit of the CD because..well I didn't know I should have. Makes sense, but I didn't think of it.

I changed your formula in that spreadsheet a bit so that the function gave a roundover at the mouth (going asymptotic) while not affecting (too much) the profile at the throat and conical section.

[gedlee]

Originally posted by Paul W

Count me in

Earl,
The constant directivity waveguides I've been experimenting with over the past few months sometimes sound overly bright when voiced flat.

This is almost certainly a problem with the source material rather than the speaker. With good sources, for example AIX 24/96 DVD-A and many new SACD recordings, they sound so good they are simply stunningly. However, mediocre material sounds too "hot"...the speaker is clearly revealing flaws in the recording process. Unfortunately, mediocre sources are far too common. Do you strike a compromise when voicing CD (and if so, what is it) or do you let the sources stand on their own merit?
Paul

This is a big issue - one that we wrestle with all the time. We are so used to directional highs with very low power responses that the speakers can certainly sound too bright when voiced to flat. But exactly as you say, a good recording does not suffer ths problem. SO what is one to do? It is a delemma. We at Ai do voice down the highs but just a bit. We look to getting a +-2 dB sound field but we always shoot for the low end of the 2 dB. I have found that if the HF polar response field does not rise in power response - and believe me it can - then it is probably OK. But we almost always find that this requires a subtle rolloff of the response right on axis.

Remember I said that I do the EQ and crossover for the best polar and power response NOT the best axial response. A flat axial response will almost certainly sound too bright. We are just not used to hearing a flat power response to HF.

As to your comments about the waveguides sounding "stunning" - this has been my experience too. Dynamics that no small tweeter can even come close too and yet as clean and uncolored a response as I have ever heard from even the best tweeters.

Oh, by the way. This impression holds up very well even when the speakers are blaring sound at ear splitting levels in a disco. We have some disco installations and it is simply amazing how well these speakers hold up under extreme sound output levels. The music sounds like crap, but thats the way its supposed to sound!!

[pooge]

Earl,

I think you would do well to rename the foam phase plug to something like "HOM absorber foam" or the like to distinguish from the phase plug in the compression driver. You are welcome to use my trademark "HOM absorber" if you like.

[Ed LaFontaine]

Josh

As a representative of the village idiots, Mongo says this is just way too much fun!

I'll bet a function can be added to the worksheet. One that will set the origin of the profile equal to a point on the curve that is tangent to a selected angle.

I haven't forgotten my commitment to you....just looking for the right profile.

[Hartono]

I vote for HOME

HOM ELI-MINATOR (patent pending)

[catapult]

Ed and Josh,

My spreadsheet linked above shows how to do it. Basically, just add a 'back offset' input that shaves a bit off the backside until the initial throat angle is right. It doesn't take much. You can use Goal Seek to find the proper amount to shave off.

[JoshK]

Originally posted by Ed LaFontaine
Josh

As a representative of the village idiots, Mongo says this is just way too much fun!

I'll bet a function can be added to the worksheet. One that will set the origin of the profile equal to a point on the curve that is tangent to a selected angle.

I haven't forgotten my commitment to you....just looking for the right profile.

It shouldn't be hard at all...as soon as I get a few moments I'll play with adding that. I can also look at how catapult did it, but knowing me, I'll want to derive it from scratch and then look at his solution.

[Ed LaFontaine]

Well done, Dennis