How to construct a elliptical waveguide for a tweeter? - diyAudio
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Old 5th November 2008, 03:54 PM   #1
Aoxomox is offline Aoxomox  Germany
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Default How to construct a elliptical waveguide for a tweeter?

Hello,

I want to use a Visaton tweeter (KE 25 SC) in a three way speaker.
To optimize the directivity I would like to mount the tweeter from the inside an use the case material on the front as a waveguide.
This should transfer from the 25mm round diameter of the tweeter to an elliptical outlet of the "horn". The depth of the horn should be arround 19 - 40mm.

1. Where can I find some formulas to calculate the growth function of such a horn?
2. Is there a cheap simulation tool available that can calculate the transfer functions?

Plz don't worry about phase shift or additional delays.
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Old 5th November 2008, 07:43 PM   #2
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I can't help you with the elliptical proposal, but I did do some studies of the results of various concentric, shallow waveguides with a rear mounted tweeter. The tweeter was the Peerless HDS, with the flange removed. Perhaps they will be of some value to you. The results were reported here:

DIY Waveguide experiments
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Old 5th November 2008, 11:55 PM   #3
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Its of interest to me as well; if you like, I can take some pics/measurements (both acoustic and mechanical) of the EOS waveguide that was part of the JBL bookshelf series (26 S2) a few years back.
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Old 6th November 2008, 06:38 PM   #4
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Hello Dan,

thanks for the interesting report on your concentric WGs.
How did you measure the frequency response? Soundcard?

I am interested in a simulation of the "horn" (waveguide). A former collegue wrote his phd in offline simulating a driver and a horn independently - so he can estimate with a transferfunction of a horn and a driver the total transferfunction.
But mainly I am interested in optimizing the directivity a bit. But just guessing the right geometry will lead to an undefined (worse) design I fear.

Any recomendations?
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Old 6th November 2008, 07:50 PM   #5
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My measurements are taken with SoundEasy. My approach was the only one I was capable of doing myself, which was to test what I can relatively easily construct with the tools I have. In fact, that was part of the idea. I wanted to test what an average DIY guy could quite reasonably build with typical tools and materials he would have for building a loudspeaker.

As far as theroizing what different shapes, sizes, etc. before hand, I would not be of much help.
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Old 7th November 2008, 12:43 AM   #6
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Short waveguides like this on dome tweeters will have only marginal effects as has been shown. To truely control the directivty the waveguide needs to be much bigger and it should be feed with a flat source rather than a round one. Then you can get some very highly controlled polar patterns. These shallow waveguides look a lot better than they perform.
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Old 7th November 2008, 01:53 AM   #7
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I'd say judging the performance of shallow waveguides depends entiely upon your goals. In my case I wanted them to do a couple things. On was to counter the naturally rising frequency of the Peerless HDS. That was accomplished by the waveguides as shallow as 1/2" deep. A second possiblility was to provide a boost a the low end so that when equalized out in the passive crossover, the low end of the tweeters performance was improved, including potentially reduced distortion levels. This was accomplished to small degree with the shallow 1/2" waveguides, but the boost was only a couple decibels. the third thing was some possible improvement in off axis directivity, which is visiby quite obvious when compared to the flush mounted tweeter, especially in the 3/4" deep waveguides. The 3/4" deep WG's provided more boost as well and consequently enhanced low end performance. To my eye, with the 3/4" waveguides, the off axis response and directivity is better with the rounded profile than with the two I did with flat sections, though I'm sure this is not true as the get deeper and wider.

I realize these don't offer the same level of improvements that your deeper, wider waveguides do, but they also don't have some of the physical limitations and they do provide generally positive improvements over the typical flush mounted dome tweeter. IMO, it's not everyone's cup of tea to have a 10"-15" waveguide in all their speakers. How they look is also important to some folks. In addition, these canb be constructd by virtually any DIYer. To each his own.
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Old 7th November 2008, 02:11 AM   #8
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Quote:
Originally posted by dlneubec
How they look is also important to some folks. To each his own.
I guess. To me the only thing that matters is the sound. I have sculpture in my living room to look at. I listen to my speakers.
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Old 7th November 2008, 02:52 AM   #9
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You can have both great sound and great aesthetics, IMO. I'm sure some don't care what their car looks like, since they only drive it, or what their home looks like since they only live in it, etc. Personally, I certainly care how what I create looks like, including speakers. I care what my speakers sound like even more, but I prefere not to settle aesthetically. YMMV.

For those who didn't look at the link, here is a graph comparing the results of the flush mount Peerless HDS tweeter at 0-15-30-45-60 off axis with the same tweeter mounted in a 3/4" deep waveguide with a 3/4" roundover. The latter was laid over the flush mount HDS results, but 15db higher. It is easy to see the boost provided and the directivity gained by this easy to construct waveguide, which takes nothing more than a 1-1/4" forstner or spade bit in a drill and a router with a 3/4" roundover bit, which most of us speaker builders already own, so it might not cost you a dime to implement. The differences are quite obvious. You can easily see how the rising response of the HDS was countered, how the low end was boosted by about 5db at 4khz and how off axis directivity was improved. When equalized to a flat responce in the crossover the benefits are not insignificant.

Click the image to open in full size.
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Old 7th November 2008, 05:17 PM   #10
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What is difficult for is judging the effects of different types of horns just from description.

Using a horn adapts/transforms the acoustic impedance of the speaker from motion into sound presure. In your case the idea was to reach a lower corner frequency (while the driver performs lower magnitude), to optimize frequency response on axis and reduce distortions a bit.

The loud-speakers case plays also a big role in directivity. A small speaker cabinet will bent the acousic waves differently than a big cabinet. This also influences directivity.

I know the e.g. Klein + Hummel is jusing waveguides for their studio monitors to avoid reflections from the mixer rack in sound studios.


So my main goal is shaping the directivity - less acoustic energy for the room to reduce hall effects and get a rather neutral behavier while increasing a homogeneous directivity in horizontal space.
Reducing harmonics through less motion of the driver is fine for me as well. Lower corner frequency is nice to have.
What is not critical for me is a smooth frequency response as I can correct that.

What approach can you recommend?
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