Waveguides

[ralphs99]

For anyone that's interested in waveguides, I've added a new page on my site that looks in detail at their performance.

Lots of experimental results and discussion. I think you'll find it interesting. Please check it out!

http://www.aeronet.com.au/waveguide.htm

Cheers, Ralph

[JoshK]

Excellent synopsis! Thanks for the hard work.

[mazurek]

I am seeing two tweeters suitable for experimentation, from pictures it looks like the Seas Millenium has a conical faceplate cutout that would couple well to a waveguide, can owners confirm? The other tweeter would be the Vifa XT25. Also, it looks like Scan-speak has some 20mm tweeters that look like they may have been designed for this. Thanks again for your research, I'm really hoping my friend can make me some waveguides to play with.

[mgoedeke]

Excel/TDFC and probably others using that diaphragm don't really perform that well on waveguides (and I have tried a couple, including ones starting at the surround and with slight compression over the surround). The velocity profile above 6khz is seemingly just too variable to get results one could really call good.

[mazurek]

One last question for you helpful people, how about a shallow, wide angle waveguide over a dome tweeter. For example, a 120 degree dispersion waveguide with only 0.5" to 0.75" depth. Would this shallow, wide dispersion waveguide be less likely to have horn coupling problems?

I am asking because I require minimal crossover modification if I use the same drivers (I left a space in the active crossover board for horn equalization), but would have to wait quite a while to experiment with ring radiators because of the cost of reprinting the crossover or buying a dcx.

[Pano]

Ralph - good work! Nicely presented on your site, too.

I was doing a "horn survey" recently to see how the given cutoff frequency of known commercial horns compares to the width of the mouth opening. There seems to be a lot of variation, even among horns of the same brand.

My Altec 811 horns are said to be usable down to 800Hz - about 1 wavelength/mouth width. Just as you found with your waveguides. A lot of horns and wavegiudes give a much lower usable frequency. Wonder why the difference?

I suppose you have looked at the waveguides over on the DDS Site?

[ralphs99]

Thanks for the support everyone!

-Mazurek,

The depth of a waveguide is determined by the other design parameters. The low frequency limit of pattern control sets the approximate mouth size, the driver diaphragm sets the mouth size, and the included angle determines the angle of radiation. The depth simply falls out of the geometric calculations. In gerneral waveguides are often very shallow, 15-30mm is common.

The issues due to throat termination persist no matter what the depth of the waveguide. Whether its 10mm deep or 50mm makes little or no difference at the throat.

Waveguide equalisation isn't difficult. The general characteristic of a waveguide loaded tweeter that has a flat response on a plain baffle is that the power response drops at approximately 6dB/oct over its range of waveguide operation. Correction is pretty well approximated with a single pole HPF network. Have a look at the paragraph called 'equalisation'.


-Panomaniac

I think there is a difference between cut-off frequency and the low frequency limit of pattern control. Horns are peculiar in that below their cut-off frequency no sound comes out! Waveguides on the other hand don't seem to take part in any acoustic impedance matching like a horn. Below its 'cut-off frequency' a waveguide ceases to exist, and the driver simply sees the baffle.
So in my statement of the limit of low frequency operation I meant the limit of pattern control.

Looking at the plot on my wavguide page of the exponential horn coupled to a compression driver, I would call the cutoff frequency to be 1.5kHz, but the limit of pattern control to be 3kHz. Conventional wisdom says that a horn shouldn't be used below one octave above its cut-off frequency, so two limits seem to be co-incident - at least in this case.

Cheers, Ralph

[Pano]

Originally posted by ralphs99
Below its 'cut-off frequency' a waveguide ceases to exist, and the driver simply sees the baffle.
So in my statement of the limit of low frequency operation I meant the limit of pattern control.

Makes sense to me! Seems that with waveguides, just like horns, "cut-off" and "lowest usable" are not the same thing. But as you say, the waveguide does not have the L.F. drop-off of the horn, it simply stops "guiding."

Do you plan any work on mid or bass waveguides?

[Ed LaFontaine]

FYI, Troels has put together an mtm using Vifa mids and a Monacor DT 300 tweeter. Of interest is the made-to fit waveguide Monacor provides for the DT 300.

http://www.troelsgravesen.dk/C17_II.htm

http://www.audiophonics.fr/monacor-t...9762615273291f

I'm not affiliated, just very interested.

[ralphs99]

Hi Ed,

Thanks for the links! Those Monacor waveguides look perfect.
I'll have to get myself a pair to test.

BTW, How is your waveguide research going?

Cheers, Ralph