[noah katz]

Hi Bob,

First, I don't mean to be a thorn in your side (I asked some difficult questions awhile back at HTGuide forum), just want to get things straight on what your speakers do.

And no matter what, I'm considering  three AV-2's for my HT L/C/R's based on what others have said about their sound.

Back to the topic, I just looked at the off-axis response plots of the Millenium here

http://www.4sptech.com/version2/revelation-mr1_specifications.html

I'm puzzled by the fact that the 45 deg response doesn't hit -6 dB until 7 kHz.

For that size waveguide I'd expect to see -6 dB at about 2 kHz.

Am I missing something? For comparison, Fig. 3 here

http://www.gedlee.com/downloads/Summa.pdf

shows the response of Geddes' Summa (15" woofer and 15" waveguide) from 0 to 90 deg off axis aty 7.5 deg intervals.

The response is -6 dB at about 900 Hz, and varies little from that level up to 10 kHz.

Thanks

[Aether Audio]

noah,

Thanks for writing.  I'll do the best I can to answer your questions.  Regarding the Summa, I'm not sure how Geddes took those measurements.  It looks to me as though he just rotated the enclosure on a turntable without keeping the mic to source distance constant.  The proper way to do the turntable thing is to make sure the front to back acoustic center of the enclosure is vertically aligned with the turntable's center of rotation.  If you don't, as you rotate the table/enclosure by larger degrees of angle away from the main axis, the "source" will actually be moving further away from the microphone if it's acoustic center is behind the turntable's center of rotation.  Then, when you do curve overlays as in the Summa graphs, the output appears to be decreasing.  That's because it is - the source is actually moving further away with increasing measurement angles.

In the Summa's case though, the HF driver is physically positioned considerably further back in the cabinet with respect to the woofer due to the extreme depth of the waveguide.  That places it's acoustic center a considerable distance further back than the acoustic center of the woofer as well.  Using Geddes" approach, one may achieve a flat frequency response on-axis and the drivers may be in phase at the crossover frequency, even though the acoustic centers are significantly offset from front to rear.  Nevertheless, the "in-phase" condition is a function of the inherent delay of the crossover network working to generate sufficient delay in the high-pass section to bring the H.F. driver in alignment/phase-sync with the woofer at the X-O frequency.

You can see the whole effect a lot easier if you look at the group-delay vs. frequency plot of the system.  I imagine that in the Summa you would see a finite amount of low frequency delay up to the crossover frequency, and then see a slide to even more delay once the frequency increases beyond the X-O point.  Actually, it may not be all that bad as the inherent delay of the filter network may be sufficient to bring everything from the X-O point on down into better time alignment with the H.F driver's output above the X-O point.  Still, I doubt the L.F. delay is sufficient to completely offset the physical setback of the H.F. driver.

Regardless of total phase rotation issues, the fact is that the physical source of radiation is different between the two drivers in the system.  That means there really is no true vertical axis alignment point from a signal level (SPL) standpoint.  Even if Geddes were to try and set the enclosure's median front/back acoustic center directly over the turntable center of rotation, there would still be a larger SPL drop-off from the H.F. driver than there would be from that of the woofer.  That's what one appears to see in his published graphs as well.  Between 200 & 300 Hz, the level doesn't drop off near as much as it does from 300Hz on up.  Because of this effect, I for one, can't really tell what the actual beam width of the system is.  

I guess that the main thing is that the curves are all pretty smooth and that's the most important thing.  Regardless of any delay issues, etc, the Summa looks to be a pretty darn good performer.  My only point in any of the above was to make clear that I can't really draw any comparisons to our stuff from that of Geddes... looking at his graphs.  I have the utmost respect for Earl and would probably never put down anything he designed, and especially so in the case of the Summa.

So in light of all the above, I will say that I made some effort in maintaining a constant distance from the source as I rotated the mic around the speaker.  The best way to do this is perform an impulse type test and adjust the mic distance such that the impulse indicates the same distance from the source with every change in angle.  When you do that you won't see near as much vertical magnitude offset as you do in the Summa's graphs.  In fact, there's so little change that it makes it hard to tell one graph from another when you do a set of curve overlays.  That's why the Revelation's graphs are all separate on our website.

Finally, we don't show the off-axis response past 45 degrees so you don't see much decrease in H.F. SPL magnitude - as it should be if the system provides constant directivity out to that angle.  As you noted, the response is down -6dB at 7kHz and that really doesn't meet our published spec.  It should be -6dB at 10kHz.  The reason it's not is that those graphs were from an earlier prototype wherein the throat of the waveguide hadn't been optimized yet.  I didn't have time to do new ones before T.H.E. Show, which we attended last January.  I wanted something up on our website before we went so I just used those.  Sorry for the confusion, I'll try to get new ones posted as soon as I get a chance.

Well, as usual my response is rather lengthy.  That's why I don't get around to responding as often as I should.  I always know that's what it will turn into before I'm through.  Hope this helps.

Take care,
-Bob

[noah katz]

Hi Bob,

Thanks for your informative response.

But leaving speculation about Earl's measurement methods aside, I'm still puzzled why your speakers don't show more directivity at much lower freq.

Thanks

[Aether Audio]

noah,

I guess the confusion is due to a couple of issues.  First, it's most likely due to our definition of controlled directivity and beam width.  I guess I've sort of mixed definitions in the past and haven't been clear.  I should know better, I harp on this stuff all the time - let alone come up with my own definitions.  So if I've confused or mislead you or anyone else, I apologize.  It's totally my fault.

For what it's worth, my thoughts have been that most folks aren't interested in the "standard" definition of beam width.  Who cares what angle the speaker exhibits a -6dB reduction in amplitude across the board?  Nobody typically listens at such an angle anyway.  My definition has been to describe at what frequency and angle the system exhibits a -6dB roll off of the highest frequencies.  At greater angles yet the waveguide does provide a level of true constant directivity, but again - who cares?

Although our waveguides do exhibit what would be termed a classical "constant directivity" at angles beyond +/- 45 degrees, it is over a considerably narrower range than the span of frequencies outlined above (i.e., - the -6dB @ 10kHz spec).  Actually, I need to go back and change the spec because the CD performance only goes out to about 5kHz and the angle is more like +/-60 degrees.  

Thanks for calling me on it, I've never concerned myself too much with the whole issue as most folks don't have a very deep understanding of all the technical parameters - let alone seem to care.  You are the first person to ever seem to be interested in such details and are the first to catch me not having my "ducks in a row" in a technical spec.  It's good that someone comes along once in a while and challenges you - keeps you honest.  Although, I will say we certainly weren't intentionally trying to deceive anyone.

In defense, I will say that we've claimed "controlled and constant dispersion" - not actual constant directivity.  (I know, sounds like a play on words.)  You can see from the graphs that this is pretty much true at angles up to 45 degrees and out to (what should show) 10kHz.  It may not be true "constant directivity" but compare those graphs to any traditional 2-way speaker and you'll understand my thinking.  

Falling short of true constant directivity is mainly because of two things.  First, as you can see from our graphs the highs have a tendency to continue rolling off at lower frequencies as the angle increases.  That's because of the throat depth in the immediate vicinity of the tweeter.  We had to make the taper of it such that we get the loading we needed for L.F. extension to 600Hz.  That, by default, limits H.F. extension at increasing angles off-axis.

The other is because of the shallowness of the waveguide.  It simply is not deep enough to narrow directivity at smaller angles off-axis.  Beyond +/- 45 degrees you start to see the effect of constant directivity much more dramatically, but the last couple of octaves are pretty attenuated.

The fact is that our waveguide is a bit of a "mixed" bag of performance specifications.  It has CD properties, but it is not a true constant directivity waveguide - and as I said,  I've never claimed that it was - at least not in the classic definition of the word.  To get that you need a monster of a horn like Geddes is using.  

Although true CD performance certainly has its advantages in many applications, it also has some pretty serious drawbacks as well.  Reference my comments in the previous post regarding time delay issues, for one.  With such a deep horn (OK, "waveguide") you're never going to get anything resembling time alignment.  More than anything else, transient response ( I prefer "envelope" response) suffers greatly from failure to achieve reasonable time alignment and is far more important than dispersion - at least for high resolution audio anyway.  If you're designing a PA system, that may be another story.

The other main issue is as stated above as well - SIZE!  Look at the thing Gedddes has built.  It's a monster!  How many folks have room in their home for something that big?  And as a near field monitor? - ain't gonna happen.

So...we don't really offer true constant directivity, or at least not over a very wide band - and to be honest, I don't think anybody really cares.  It's simply not as important in a smaller room setting such as is found in most homes and studios, for which our stuff was designed.  Our waveguide does improve and control dispersion to a great degree though, and it offers a host of other advantages besides - and it won't take up half the room either!  I think the positives more than justify it's use and will be perceived by most to offer tremendous value, irrespective of the whole CD issue.

I hope I cleared things up this time.  I also hope you're happy now that you've managed to thoroughly humiliate me before my audience. That's the worst part about vanity - it never lasts.

-Bob

[noah katz]

Bob,

"First, it's most likely due to our definition of controlled directivity and beam width."

What is your definition? From the reading I've done on waveguides, it's the angle at a particular freq at which the level is -6 dB from on-axis.

"For what it's worth, my thoughts have been that most folks aren't interested in the "standard" definition of beam width. Who cares what angle the speaker exhibits a -6dB reduction in amplitude across the board? Nobody typically listens at such an angle anyway."

I'd think anyone who has read about and understood the benefits of waveguides.

I believe there are three:

1) Improved loading and efficiency of the tweeter, allowing lower XO freq.

2) Improved power and room response at high freq because of the constant directivity w/freq

3) Narrower directivity down to lower freq, minimizing early reflections that impair imaging

The last one in particular is why I'm interested in the beamwidth; my HT setup is highly asymmetrical with the right speaker 3' from a glass wall, and the left speaker 12' from a wall.

The pink noise signature of the LCR's are all very different, which is what I'm trying to address, along with improved dynamics and higher output than what I have now.

Thanks

[Aether Audio]

noah ,

What is your definition?

Well to be succinct, my definition is the frequency at which the SPL is -6dB relative to the nominal level - just as I had originally stated.  Actually, that is the tradional definition so I was essentially correct to begin with.  But I was thinking of "beamwidth" along the lines of Constant Directivity when I made the statement that I've "mixed definitions," and in that case, the definition is somewhat diferent.  I guess where I've really mixed definitions is in my own head.  I've gone back and forth so many times in my own thinking that I've essentially confused myself as much as anyone.  Sorry.

In a classical CD design, there are two -6dB points - sort of.  A CD device will exhibit a -6dB drop in amplitude at some mid-band frequency relative to the lowest frequencies (usually those below the crossover frequency) produced by the system.  In such a case, the frequency response looks "shelved down" by -6dB across a relatively wide range of frequencies compared to those below (approx) the crossover point.

OK, so now we'll call this "shelved down" area of the response governed by the waveguide/horn device our new "nominal" level.  At some high frequency there will be yet another drop off of the response by another -6dB.  Now then, the band of frequencies between these two points is the actual "beamwidth" of the CD device.

You'll notice Geddes' graphs of the Summa cuts off at 10kHz.  That's because his waveguide starts to drop off at that "second" frequency somewhere beyond 10kHz.  Essentially, he's defining the constant directivity "beamwidth" of his waveguide as that between the -6dB angle at the mid-band range and on up to 10kHz.  If he were to extend his measurement to say...20kHz and then use our definition or the "standard" definition of beamwidth as I've outlined above, the actual angle would be much less than the +/- 45 degrees or so that he has claimed as the actual beamwidth.

And that's fine.  No waveguide or horn will exhibit constant directivity out to 20kHz at angles much beyond about +/- 15 degrees.  We're all more concerned with response in the mid-band on up to maybe 10kHz.  It's a given that virtually all transducers will begin to exhibit beaming effects above about 10kHz.  Our main area of interest regarding directivity is the mid-band (and possibly lower) range of frequencies.  That range is the practical, "real world" and useful area of constant directivity performance.

And you are correct about the fact that there are those that do care about such things in detail.  My comment was refereing to the fact that "most" folks don't as a result of the fact that they really don't understand it all.  If you hang around here at AC long enough, you'll see that technical specifications aren't anywhere near the primary focus of most folks.  Good or bad, that's just the facts.

As soon as I get a chance, I'll post new graphs showing the full off-axis response out to +/- 90 degrees.  Then you'll be able to see where the CD performance of our waveguides begins to kick in.  It would be nice to acheive full CD response at +/- 45 degrees, but even though we don't our off-axis response is a lot better (narrower) than what you'd get from a standard baffle-mounted tweeter design.  The trade-off was preserving the time domain integrity of the system and I believe that is a far more important aspect in high resolution audio.  

Even with the CD performance offered by the Summa, in most rooms one has to concern themselves with speaker placement due to room effects at low frequencies.  That being the case, some of the advantages of the Summa or any other true CD design are lost... or minimized anyway.  The mere act of optimizing speaker placement for low frequency performance of the system, will reduce early reflections from the room's side walls at higher frequencies.  Therefore, CD response at higher frequencies won't offer as significant of an advantage as it might in other applications.  

Where true, wide-band CD performance has it's greatest advantage is in sound reinforcement applications.  Typically In a large concert venue, the speaker system is often a large overhead center cluster and is well away from any interfering side walls.  In that case the designer wants to achieve a coverage pattern over a given listening area with a frequency response that is as constant (flat) as possible.  Outside that area of coverage he wants the response to drop off at all frequencies as evenly as possible as well.  That's where the Constant Directivity device has its greatest advantage and is why it was developed in the first place.  

The very first application for the CD device was in professional, live-sound settings and it still offers the bulk of its advantages for such use. In smaller room settings such as recording studio and home applications where there are nearby boundaries, the true CD device, while still certainly an advantage to some degree, is seldom taken full advantage of because of the efforts involved in low frequency optimization.  At frequencies below the functional range of the H.F. CD device, it offers no advantage whatsoever and yet those frequencies must still be dealt with if one wishes to truly optimize the system.  In the applications that our products were designed for, much of the efforts involved to acheive broad-band CD performace such as what the Summa offers, would be wasted.  And in so doing, the time domain response would suffer greatly for it.  That would not be a good set of compromises if you ask me.

Hope this clears things up.

-Bob

[noah katz]

Hi Bob,

Thanks for the further explanation.

"As soon as I get a chance, I'll post new graphs showing the full off-axis response out to +/- 90 degrees. "

That would be most interesting; any idea when you'll get to that?

Not sure I understand why CD isn't useful in any room, unless the speakers are placed so far from boundaries and the listening position is so close to them that early reflections aren't an issue.

My LCR's will be on/in wall; I was thinking of builsing the LR baffles angled in say, 30 deg, to minimize the sidewall reflection, and to perhaps allow forgoing the center channel entirely (it's causing me no end of grief in designing my 105" RP screen setup).

I seem to remember someone reccommending no toe-in; why would that be and do you have a general reccommendation, or is it mostly a matter of try it and see?

Thanks

[Bill Baker]

I seem to remember someone reccommending no toe-in; why would that be and do you have a general reccommendation, or is it mostly a matter of try it and see?

 I will leave the technical answers up to Bob but in regard to toe-in on these speakers, I do feel it will be a mater of experimenting. Some key factors will be the distance between the speakers as well as listening position.

 With my Bella II's, I have found that different dgrees of toe-in were required depending on where in my very large room they were located as well as how far apart they are. In most situations, slight toe-in provided the best results but it was different in all applications.

 Do some simple trial and error and see what works best for you and your application. One thing I will say is that the ceneter imaging and soundstageing is every bit as good in the Essence as it is in the MR series. Just presented differently.

 If you do go ahead with teh Essence, feel free to contact me and I will be more than willing to provide you with a few teaks that will make a work of difference.

 Good luck and enjoy!!

[noah katz]

Bill,

"If you do go ahead with teh Essence"

Actually it's the AV-2's I'm considering for HT and AV-1's for audio only.

Thanks

[Bill Baker]

Actually it's the AV-2's I'm considering for HT and AV-1's for audio only.

 Sorry, looking back at your original post I now see this.


 I take it you will have two seperate systems? If so, might a suggest a different arrangement. Going withthe same number of speakers, I would look at using two AV-1's for the mains of the HT setup and one AV-2 for the center. This will allow you to use a pair of AV-2's for you two channel rig. Of course, if you went ahead with purchasing the combination you mentioned, you would be able to experiment with them in both systems.

 I will be setting up a system that will double as a HT and 2-channel setup using AV-2's across the three front channels and a pair of AV-1's in the rear. Being more of a dedicated 2-channel guy, the main concentration of this system will be 2-channel vinyl

You picked a good time to look at the Bella speakers as I am offering 10% off the selling price (not the retail price) through April 15th.

[Bill Baker]

Sorry Noah,
  Looking back, I am not sure my post was correct. Are you looking at the Essence AV-2 or the Bella SP/AV-2?

 Either way, I can help you tweak these to best fit your preferences.

[ooheadsoo]

I've got a pair of Essence AV2 kits coming my way, Bill.  Any tips well appreciated

[Bill Baker]

I've got a pair of Essence AV2 kits coming my way, Bill. Any tips well appreciated

 Sure thing. Can you tell me what you are getting in terms of any additional circuits? Are you getting the crossovers with them or building your own?

 I have been spending the last few weeks tweaking the Bella 2's and the results, based on everyone that has heard them, are phenominal. There are a few things that we will not release at this time but we can get your AV-2's sounding sweet

 If you send me a PM, that would be great.

[noah katz]

Bill,

"Are you looking at the Essence AV-2 or the Bella SP/AV-2?"

The Essence; I would be going the kit route.

Thanks

[skrivis]

Not sure I understand why CD isn't useful in any room, unless the speakers are placed so far from boundaries and the listening position is so close to them that early reflections aren't an issue.

What I got out of what Bob said is that CD is very advantageous when the user does not know how to optimize speaker placement in the listening room. It may be less so in other circumstances.

I would be interested to know what the frequency response of reflections are in typical listening rooms. The flat power response approach seems to assume that all frequencies will be reflected equally.

I can also envision instances where you would not at all want CD of higher frequencies. (You can apply room treatment to the listening room walls that will effectively handle the high frequencies. If you're controlling high frequency reflection to some extent, then you probably do not want CD since it will give you more to try and control over a wider area.)

The question then becomes : Where is CD beneficial, and where is it harmful?