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Old 14th May 2013, 07:33 PM   #8751
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ra7,
I have also mounted dome tweeters on horn lenses and have had some success with this approach. The differences with a compression driver are many though. For one you can not load the dome tweeter as you can with a compression driver as this will cause many problems with a typical soft dome as they can not handle any of the typical loading of the diaphragm as a true compression driver is made to handle. So the efficiency will never approach a CD and that is one of the real limitations with this approach. Another factor is that while you can get increased output in the lower registry of the dome tweeter as you have found you will then cause a real decreasing output with increasing frequency and this will require much eq which is something of a vicious circle of attempting to get back to what you had before adding the horn or waveguide. Typically you will see very short waveguides used in this type of application as you are using a shallow waveguide only for directivity and not for any gain. But even a short waveguide will cause some tilting of the response curve and this will again require eq to correct. Since the dome tweeter does not create a waveform in the same manner as a compression driver you will create some unusual interactions between the dome tweeters normal directivity and any horn or waveguide. It really is a complex situation and this is one of the reasons this type of application is usually only seen in powered monitors that have much eq to correct some of the problems created by the mismatch between the two disparate devices. Don't get me wrong, it can be made to work but it is not a simple substitution of a CD for a dome tweeter, it is much more complicated than that in reality.
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Old 14th May 2013, 08:04 PM   #8752
ra7 is offline ra7  United States
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Thanks Kindhornman. The EQ is indeed hard to master. I haven't observed what's happening off-axis in the ring-radiator and SEOS-12 combo, but it will be interesting.

The most important thing though is whether it results in a smoother response compared to CDs. I will be looking into it, but was wondering if anyone else has seen it before.

Smooth means the small wiggles and resonances are absent. These are only visible in un-smoothed high resolution measurements.
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Old 14th May 2013, 08:22 PM   #8753
wesayso is offline wesayso  Netherlands
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Here's the XT25 in a WG-300 waveguide: Test Vifa XT-300/K4 (XT25TG-30/04) am Waveguide WG-300
Some data for you to compare it to the SEOS waveguide...

Last edited by wesayso; 14th May 2013 at 08:26 PM.
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Old 14th May 2013, 08:37 PM   #8754
Face is offline Face  United States
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My experience with domes loaded in waveguides is not nearly as complex as yours Kindhornman. As you mention, energy decreases with frequency. This usually allows the use of a simple one cap network, or a single cap plus contour circuit, which is generally as simple as, or even simpler than your average dome tweeter network.

Here are some examples of well designed dome waveguides: acoustic waveguides Measurements: https://plus.google.com/photos/10163...ums?banner=pwa
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Old 14th May 2013, 08:49 PM   #8755
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Quote:
Originally Posted by ra7 View Post
I keep hearing from the experts about how conventional dome tweeters are smoother than compression drivers and better in the home environment. So, I started experimenting with the Vifa XT25 ring radiator in a SEOS-12 waveguide. Results are promising. No problems with distortion or perceived dynamics, as long as you watch the low end on the tweeter. And you get that last octave in the top end, which is missing in most 1" CDs, and is non-existent in 2" CDs. It really makes a difference in terms of the ambiance of the recording.

The EQ required to flatten the response is substantial though, and I haven't gotten the overall balance right yet. But it does sound better defined than a CD. Instruments have distinct signatures, and notes have better precision/definition.
I wonder about matched termination to the driver. The LeCleac'h (and most likely, other horns) are sensitive to the exit angle of the compression driver matching the entrance angle of the horn. If they don't match, you get reflections, which shows up as ripples in the FR and in the time-domain response.

Dome tweeters are intended to be accurately flush-mounted on a flat surface; even mismatches of 1mm are measurable, and edge diffraction from the boundaries of the front baffle create substantial reflections that are measurable and audible. In the development of the Ariel, there were measurable and audible differences between 0.75" and 1.5" radii on the cabinet edges, with the larger radius obviously superior in terms of image depth and freedom from coloration on vocals.

Mounting a dome tweeter in a waveguide or horn is nowhere close to what the original designer intended, so it's not surprising that a lot of equalization is required ... if the dome is designed to be flat-response radiating into a hemisphere, it certainly won't be flat radiating into a waveguide or horn.

EQ is all well and good, but what about time response? That's what I'd be concerned about. What kind of reflections are created when the air-load, as seen from the diaphragm, is completely different than the intended design condition?

At an intuitive level, I'd expect the LF response of the dome tweeter mounted in a waveguide or horn to be substantially boosted, with an increase in power-handling. I'd also expect to see a series of ripples in the FR and time domain, as a result of mismatch, that are not present when the dome-tweeter is mounted on a large flat baffle.

Ra7, you mentioned a need for equalization when a dome tweeter is mounted in a waveguide. Is this HF boost equalization, or do ripples need to be notched out as well?

Last edited by Lynn Olson; 14th May 2013 at 08:54 PM.
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Old 14th May 2013, 09:16 PM   #8756
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Are JBL the only ones using an elliptical entrance to the waveguide?
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Old 14th May 2013, 09:17 PM   #8757
Face is offline Face  United States
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When experimenting with a SEOS HA-25 waveguide in front of an RAAL 70-20XR(OEM), I noticed that an airtight seal was needed between the WG and ribbon face place, or there were ripples and other nasties present. AutoTech HA-25 - AutoTech Waveguides - Waveguides DIY Sound Group
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Old 14th May 2013, 11:42 PM   #8758
ra7 is offline ra7  United States
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Thanks Wesayo, Face, Lynn. Face, I hadn't seen those waveguides before. Thanks. Will order some to play with.

Lynn, the low end of the XT25 gets a huge boost, about 10db in the 1-4 kHz range. Pretty useful. Further up, the response evens out. But as you mention, there are many dips and peaks, apart from the low end boost. These, I'm guessing, are reflections near the mouth. The radiating area of the XT25 is really quite large, definitely larger than the 1" entry of the SEOS-12.

Apart from real measurements, I guess I was posing a more fundamental question about whether given the right matching between dome/ring radiator and waveguide, can it be superior to a CD/horn combo? Is the response of a traditional tweeter inherently smoother than that from a CD? If all we are craving is directivity, then it might be a worthwhile solution.

I doubt any tweeter can compete with the low end of a 2" CD, or even a 1.4" CD, but the XT25/SEOS-12 combo easily does 1.2 kHz, LR4.
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Old 15th May 2013, 12:55 AM   #8759
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Are JBL the only ones using an elliptical entrance to the waveguide?
That's an elliptical mouth; the throat is round, I think you'll find.
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Old 15th May 2013, 02:20 PM   #8760
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The hole that opens to the dome is elliptical. I have them in front of me. Edit: whether the waveguide is elliptical at that point and it's a straight cut through it or the cut has a curve to it to make the hole elliptical, I can't eyeball.

Last edited by dumptruck; 15th May 2013 at 02:25 PM.
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