| Re: The right waveguide [message #72617 is a reply to message #72616] |
Tue, 08 May 2012 12:58  |
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Wayne Parham
Messages: 18726
Registered: January 2001
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Illuminati (33rd Degree) |
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I agree with Chano, surrounds speakers are best with wide patterns, and so as much as I like waveguides, I would not use them for surrounds.
Beyond that, the SEOS devices are prone to standing wave modes in the passband, which is not something I would want. The SEOS12 is only 3.5" long, so has resonant peaks at fairly high frequencies, some over 2kHz. All waveguides will have resonant modes, but ideally, they are below the passband or at least right at the bottom edge.
Some of that you can damp in the crossover, but some of it, you can't. Amplifiers with low damping - like tube amps - will tend to exacerbate these peaks because they act somthing like current sources. And it makes it vulnerable to parameter shifts, like running at high power levels, because they heat up the voice coil and increase resistance.
I actually considered using SEOS waveguides at one time. But that was before I realized how short they were. That told me they couldn't have a true oblate spheroidal flare profile. I assumed they SEOS horns were elliptical oblate spheroidal horns when first looking at them, but upon learning more about them, I realized they couldn't be that, because they're too short.
I have been making a pure oblate spheroidal waveguide from wood for about ten years now that offers excellent performance. It is fairly expensive though, so I decided to have a mold made, to provide an inexpensive plastic implementation, designed for baffle mounting. This new H290C waveguide culminates my experience with conical horns and waveguides over the past 30 years. I believe it is the best waveguide on the market for this application.
For more information on our H290C waveguide, see the link below:
And for a comparison of the two waveguides, see the following links. These charts show amplitide response and second and third harmonics. They were made under the same conditions - halfspace, anechoic (outdoors) - using the same measurement system. Each waveguide was mounted in the center of a 1 meter square baffle and the measurement microphone was placed 1 meter from the baffle, on-axis. The compression driver used was the same unit, a B&C DE250, serial number 03120711.
Note that the H290C is flat +/-1.5dB from 1.5kHz to 10kHz. It stays between 104.5dB and 107.5dB through this entire span, and there is no periodic ripple.
Now look at the SEOS12 over the same range. It's between 102.5dB and 107.5dB, about twice as much ripple. And the ripple is periodic, with peaks at 1kHz, 2kHz and 4kHz, which is an indication of strong internal standing waves.
The SEOS12 averages about 3dB less sensitive than the H290C too, so is needs twice as much power to reach the same SPL. This allows speakers with the H290C waveguide to generate less distortion than those having the SEOS12.
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