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Old 3rd June 2013, 03:54 PM   #131
Face is offline Face  United States
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Originally Posted by AudioLapDance View Post
But the big one is interesting ... pushed back into the corner, it would extend the horn, provide lower loading and decrease crossover. Do you think it could make it to 200Hz?
Doubtful as the driver's FS is @ 300hz. I will be testing a SEOS-24 and BMS combo soon though. It will be mounted on top of a lower mid capable of a cardioid waveform from @ 180 to 500hz.
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Old 4th June 2013, 12:09 AM   #132
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Doubtful as the driver's FS is @ 300hz. I will be testing a SEOS-24 and BMS combo soon though. It will be mounted on top of a lower mid capable of a cardioid waveform from @ 180 to 500hz.
By cardioid lower mid, do you mean something like an h-frame or w-frame woofer, or something else? Can you post a link?

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Old 4th June 2013, 03:49 AM   #133
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A resistive type enclosure with multiple 2" holes in the side walls and stuffed with denim insulation. The woofer used is an AE TD-15M.
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Old 4th June 2013, 02:10 PM   #134
IG81 is offline IG81  Canada
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A resistive type enclosure with multiple 2" holes in the side walls and stuffed with denim insulation. The woofer used is an AE TD-15M.
That sounds quite ineteresting. I've worked with resistive vents in the past and would like to do so again sometime. I love my TD15M too, it's a great mid-woofer. Do you have pictures or a thread (here or AK - I believe you post there?) about this enclosure? I'm also quite fond of BMS drivers; been using 4550 on QSC waveguide over my TD15M for some time and am about to work with the tiny 4540ND on the small 6"x6" JBL PT.

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Old 4th June 2013, 09:24 PM   #135
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No, the controlled directivity is essential. But both waveguides in question have that, so there's no problem on that front. He's arguing that his waveguide has controlled directivity AND flatter response. His guide isn't as controlled at the bottom of the passband as the SEOS but that's the tradeoff he made for smoother response.
That's exactly right. The SEOS device produces some ripple, a result of its geometry. Here's some discussion about it, where Bill Waslo claims it to be audible, but suggests a way to correct for it in the crossover:
I personally think Bill's approach is appropriate for the SEOS - To do some sort of response shaping in the crossover to deal with that ripple. I think most people that design with those devices probably do something similar.

I just went about it a different way, making a waveguide that has inherently less ripple. It doesn't need notch filters.

We can discuss the merits of each approach. We can argue whether or not the crossover equalization approach is better, or whether it is better to do it acoustically with a different flare profile. We can talk about the optimizations and trade-offs, the secondary expansion to mitigate waistbanding, the depth and wall angle, etc. But we cannot even begin this discussion if we do not agree upon the data.

I think the two approaches are very similar, actually. Most speakers using SEOS horns are almost exactly like Pi Speakers, to be honest. I'm really glad to see all these new high-fidelity uniform-directivity waveguide speakers on the market. Fifteeen years ago, I felt isolated, surrounded by tractrix horn enthusiasts that saw constant directivity as nothing more than PA horns. Now days, the hifi scene has really embraced this paradigm, and it seems like all others are the outsiders. So I personally think that's excellent.

As an aside, Matt Grant is doing an exhaustive "Waveguide Shootout" right now. I think probably the SEOS/H290C comparison has been looked at pretty extensively, but some of the other popular devices haven't been. It will be interesting to see how they all stack up. I think the various approaches are all pretty well vetted out, like asymmerical versus axi-symmetrical, round/elliptical/rectangular, OS, PS, EC, conical or quadratic, amounts of secondary flare, etc. But Matt's measurements should prove very useful to show where each device is along this line of choices.
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Last edited by Wayne Parham; 4th June 2013 at 09:34 PM.
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Old 5th June 2013, 01:57 AM   #136
craigj is offline craigj  United States
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The SEOS device produces some ripple, a result of its geometry. Here's some discussion about it, where Bill Waslo claims it to be audible, but suggests a way to correct for it in the crossover:

But we cannot even begin this discussion if we do not agree upon the data.

Most speakers using SEOS horns are almost exactly like Pi Speakers, to be honest.
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Old 5th June 2013, 02:16 AM   #137
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I did jump from "what's different" straight away to "what's alike," didn't I? OK, so let's toast:

What I was trying to say was in spite of those implementation differences, the two design approaches are very similar. I think most people would agree with that statement.
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Old 11th June 2013, 10:32 PM   #138
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My working assumption is that speakers with constant beamwidth always sound better than speakers that don't, provided everything else is equal.

A second assumption is that speakers with non-constant beamwidth but uniform-directivity - those having just gradual change - sound better than speakers with directivity that shifts radically somewhere in the passband. This is especially true if the directivity shift happens in the peak of the Fletcher-Munson curve where we are most sensitive.

When I say "directivity shift", I'm not talking about 20%, by the way. I'm talking about at least 50%. Beamwidth that stays constant within 20% is perfectly fine, certainly, at least for home theater or hifi. It's the shift from 90° to 180° in less than an octave that you'll hear, not the little bulge of 10° or 20° in a pattern that averages 80° or 90°. A 20% shift amounts to about 2dB at the very edge of the pattern, which is completely inaudible. But a 90° to 180° shift is 6dB at the edge of the pattern, which is most definitely audible. A shift like that screws up the spectral balance in the reverberent field.

So speakers with collapsing directivity (like DI-matched two-ways) can sound very nice provided the directivity change is smooth and gradual. The DI-matched two-way approach is a worthwhile compromise where constant directivity is impossible or impractical.

My third assumption has to do with the "provided everything else is equal" part. This assumption is where sound radiators are concerned, those with truer (flatter) amplitude response sound better than those with peaks and dips. This is true not only of direct radiators but also of horns.

What I'd like to see in this thread is a list of waveguides and corresponding response charts that are found to be smoother than constant-directivity horns.

This is what I was originally looking for, and also a discussion about the differences between systems with true constant directivity, say from 200Hz to 10kHz contrasted with those with collapsing directivity up to around 1kHz, e.g. the matched-directivity approach.

Waveguides offer the promise of smoother response than constant directivity horns, but at the expense of slightly less pattern control. For example, without a diffraction slot in the throat, they cannot maintain beamwidth in the top-octave, narrowing instead to the compression driver exit angle. They sometimes waistband a little at the bottom end of their range too, depending on the shape of the flare nearest the mouth. But in general, a waveguide provides constant directivity and also provides much smoother response than a constant directivity horn.

Waveguides are designed to provide smooth wavefront propogation. The wave, where it contacts the waveguide, is always perpendicular to the surface of the flare. This provides a nice, clean spherical section as the wavefront exits the mouth. It makes them act something like tractrix or LeCleach flares, but with nearly constant directivity. But different shapes and flare profiles offer different optimizations, and correspondingly different performance metrics. Some geometries provide smoother response than others.

An argument can be made that as long as response ripple is constant across all axes, then it can be equalized flat. The idea is that if directivity is constant, then the power response is the same shape as the on-axis response, so equalization in one plane is appropriate to all planes. I think there is merit in that argument, but I do not agree that just because a horn is equalized flat, it will sound as good.

There is a big difference between equalizing for mass-rolloff and using a series of tank circuits to tame response ripple. The conjugate filter for mass-rolloff is a simple single-pole high-pass, and is not a resonant condition. That is quite different than the conditions that cause ripple, and I have not found any cases where the underlying mechanisms that create this ripple come without additional penalty. Sound quality suffers.

You can always take a constant directivity horn and EQ out the ripple. We've seen several cases already in this thread of constant directivity horns with peaks and dips, the JBL 2370 and 2380 horns, for example. They not only exhibit mass-rolloff, but also have 5dB peaks in the passband. This can be equalized flat, but even so, those kinds of horns still sound harsh.

Consider that 5dB represents a 3x increase in power. Equalization requires a significant power shift - To remove a 5dB peak means the power is cut 3x at the peak, which also means that it must be raised in comparison by 3x everywhere else. This also means excursion is increased and everything else that goes with it. That is not the only issue, in fact, it may not even be the most significant issue. But whatever it is, there can be little doubt that a constant directivity horn is nowhere near as smooth sounding as a properly designed waveguide.

I have said many times before, I even prefer a good radial horn to many constant directivity horns, purely because of their sound quality. I can remember so many discussions over the years with tractrix horn guys, many that use a simple first-order capacitor and nothing else. They trade everything to get smooth response - out goes directivity, power response, excursion at the low end, etc. And when I say "out goes directivity" I don't just mean the horizontals, but even more so the verticals, because with a single cap, the forward lobe becomes a paper-thin strata. But still, they love the pure sound they get in that one pinpoint spot.

What I like about a good waveguide is we can achieve this kind of smoothness, and still provide nearly constant directivity. It really is a design approach that has one foot in the constant directivity world and the other in the audiophile response purity world. Of course, there is a continuum of optimizations one can choose, spanning between those two worlds. The waveguide can be more constant directivity or more smooth, or somewhere halfway in between.

Which brings me back to the suggestion that we make this thread list waveguides that provide response that is very smooth. I'd say a worthwhile criteria is no more than 3dB variance in an octave, i.e. +/-1.5dB. The bottom end can start anywhere from 1kHz to 2kHz or so, but above that point, we will ideally want response to be flat all the way to the top octave. Mass-rolloff is allowed, of course, but ripple in excess of 3dB is not.

What other waveguides are available that meet this criteria? There must be several examples of waveguides with exceptionally smooth response.
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Old 16th June 2013, 05:25 PM   #139
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Speakers with uniform directivity sound more natural because the reverberent field has spectral balance.
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If one could make a source that confined ALL the energy into the desired angular coverage, the reverberant and direct spectrum would be identical, this is the “grail” for dealing with room acoustics
Doesn't the reflection frequency response from room boundaries and furnishings also have a great deal to do with reverberant spectrum? Except for treatment of early reflections, are room treatment guidelines the basically the same for CD, DI, and speakers without directivity controls?

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Old 17th June 2013, 06:42 PM   #140
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Absolutely, I agree, 100%. But I think that if the speaker's directivity is peaky, then there's little you can do with the room to correct it. Except of course make the room completely absorbent, like outdoors. Then the directivity wouldn't really matter, at least in terms of spectral balance, since the direct sound would be all you heard. Directivity would still matter in terms of coverage though, of course.

The thing is though, rooms are never anechoic. I'm not sure that's even a desirable goal, except for making acoustic measurements. But I do agree that confining all energy into a desired angular coverage is the "grail" for dealing with room acoustics. That's why I like constant directivity cornerhorns - they do the magic trick of confining all energy into a desired angular coverage, bounded by the adjacent walls. They also prevent self-interference from reflections off the nearest walls, because the sound source is flush with them.

I also think that the DI-matched two-way is a useful compromise, which is probably why it has remained so popular for so long. Altec and JBL had been loosely using that approach for many years before JBL optimized the paradigm in the 4430 model in the early 1980s. That's when I learned about it, and began to adopt it for some of my loudspeakers. It isn't constant directivity, but it is smooth, something I would call uniform directivity. I think it gives a natural sounding pattern that's useful for most rooms.
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