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Old 17th June 2013, 09:15 PM   #141
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As you know I've been involved in the directivity issue since the 4430 days and my opinions on the importance of constant directivity have vacilated over time.

Its good to point out that we are really talkng about smooth directivity or directivity that is constant over a partial range. Conventional speakers do not have uniform directivty over 10 Octaves.

We also would be better off talking about polar uniformity. There are a number of units out there with constant directivity and very nonuniform polars. If they beam in one plane they broaden in the other. I think "constant directivity" was a catch phrase for marketing and it generally emphasizes the wrong aspect. Good drivers can have polar uniformity or at least slowly changing polars, and that is a more rigorous requirment than the 1 dimensional directivity curve.

When we talk about directivity, more than anything, we are just setting the relationship between the speaker's power response and its axial response. If you know any 2 curves you also know the third: axial response curve - directivity index curve = power response curve. This gets you directly to the fundamental question of loudspeaker design: what response do we optimize? Anechoic response? Power response? Room response?

Opinions alone are not the answer here, we need to rely on listening tests, many of which have been done. I like to look at the early tests of Toole where he rank ordered several dozen speakers and then compared all the measurable parameters of them. My conclusion is that there is a strong correlation between axial response and listener preference and a very poor correlation with power response.

This alone makes you wonder if directivity matters at all?

Lipshitz and Vanderkooy did an interesting study where they independantly adjusted axial response and power response. The could make power response flat, axial response flat, power response roll down hill with flat axial. Axial rise with flat power. The only natural combination was with flat axial response and a falling power response (rising directivity). Flat axial with flat power response was too bright. Flat power response achieved by rising axial response was way too bright.

They even found that holes in power response were fairly innocuous.

If you look at the Soren Bech studies then he goes into the audibility of reflections, primarily based on their direction of arrival. He finds most reflections are fairly "invisible" at least at the levels you might expect to naturally occur in a home listening room. One exception was the floor bounce that could give problems up into the midrange and the rear wall bounce. The reduced side wall energy that a "more directivity" speaker would have didn't seem to buy you much. (If the reflections are inaudible, then lowering them isn't beneficial.)

When you talk to audiophiles you run into the attitude that "I can hear everything" and "everything is important". Based, I think, on this there is a growing consensous that directivity is important and maybe part of the magic between the great and the merely good.

I'm not so sure.

I do like having smooth polars and a response that doesn't vary with listener position. The ability to have several listeners on a couch and the same initial arrival response for all. I also know that people will look at room curves and want to equalize them. This is a big can of worms that gets you somewhere between axial response and power response. Having a smooth directivity function may help in that regards: it won't drag response away from the axial or anechoic response to a totally different power response curve.

Smooth polar curves are also generally an indicator of low reflections. Thats a good thing too. But is there a requirement of constant directivity for a speaker to sound good? Can anybody give evidence?

David S.
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Old 17th June 2013, 09:40 PM   #142
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Originally Posted by speaker dave View Post
Lipshitz and Vanderkooy did an interesting study where they independantly adjusted axial response and power response. The could make power response flat, axial response flat, power response roll down hill with flat axial. Axial rise with flat power. The only natural combination was with flat axial response and a falling power response (rising directivity). Flat axial with flat power response was too bright. Flat power response achieved by rising axial response was way too bright.
Did they also test flat axial and flat power response equalized to different house curves? Now this would have been interesting.
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Old 17th June 2013, 11:33 PM   #143
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Originally Posted by speaker dave View Post
If you look at the Soren Bech studies then he goes into the audibility of reflections, primarily based on their direction of arrival. He finds most reflections are fairly "invisible" at least at the levels you might expect to naturally occur in a home listening room. One exception was the floor bounce that could give problems up into the midrange and the rear wall bounce. The reduced side wall energy that a "more directivity" speaker would have didn't seem to buy you much. (If the reflections are inaudible, then lowering them isn't beneficial.)
Reflections clearly aren't "invisible" at any reasonable level. I suppose he must be talking about some particular effect of reflections? It's awfully easy to disprove the notion if "hearing" a reflection can involve any difference it causes. A room with one glass side wall and all the other walls covered in 10cm of insulation, for example, or a mirror placed next to one speaker.
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Old 18th June 2013, 01:42 AM   #144
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Did they also test flat axial and flat power response equalized to different house curves? Now this would have been interesting.
"Equalized to a house curve" of course wouldn't be flat. They did't try to control the room curve but to turn the speaker into a system with independently variable power response and axial response to see if any particular combination was better or worse than others.
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Old 18th June 2013, 02:02 AM   #145
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Originally Posted by dumptruck View Post
Reflections clearly aren't "invisible" at any reasonable level. I suppose he must be talking about some particular effect of reflections? It's awfully easy to disprove the notion if "hearing" a reflection can involve any difference it causes. A room with one glass side wall and all the other walls covered in 10cm of insulation, for example, or a mirror placed next to one speaker.
The experiment was to recreate a typical living room system within an anechoic chamber. A large number of speakers surounding the test subjects were set to mimic the arrival direction, strength and response of all the initial room reflections. Based on that setup they could vary the strength of any particular reflection. At some level every reflection would be audible as either a directional shift, a response change or an echo. These levels of audibility were compared to the real levels found in a typical room.

For most of the reflections the real life levels were found to be lower than the level of detection. In other words you could turn off a number of simulated reflections and not hear a difference. The exceptions were the floor bounce in front of the speaker and the rear wall behind.

How would higher directivity help us? If we found that early side wall bounces were typically audible then a more directional speaker might just get those bounces to fall below the threshold of detection. This was not found to be the case.

If higher directivity isn't helpful with regard to particular reflections, and late power response spectrum shape is a low level effect (relative to the early sound) then how important is speaker directivity?

David S.
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Old 18th June 2013, 02:26 AM   #146
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Thanks for the clarification. Seems to conflict with my experience... more than a little bit. How large of a room were they emulating? Did they try different spectral content for the reflections for determining the level of detection? Should I just go look this up somewhere?
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Old 18th June 2013, 03:05 AM   #147
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When considering the original question posed by Wayne, I think it only natural to consider the room and application BEFORE generalizing what performance parameters to prioritize. CD may not take top priority in a large space or well damped/treated....or in the nearfield. I know my desktop mains have a significant discontinuity here at and below 2khz but I never miss it at such close proximity.
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Old 18th June 2013, 04:34 AM   #148
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Originally Posted by speaker dave View Post
"Equalized to a house curve" of course wouldn't be flat.
Would be to your ears.

Quote:
They did't try to control the room curve but to turn the speaker into a system with independently variable power response and axial response to see if any particular combination was better or worse than others.


In my mind if we leave out the physical impact the room imposes on our hearing we are only half way there. Not to say there is anything wrong with the body of research. At first glace would appear to agree with my experience. Would also appear to be a good jumping off point to further refine our theory(s).

Should we start a checklist of sorts? A "Start here, follow these rules and it'll be magical, lists. LOL

1. Proper wavefront generation
2. Proper wavefront propagation
3. linear phase crossover (active or passive) +/-1 or more octaves
4. smooth natural on and off axis response spectraly balanced
etc.

not necessarily in that order or inclusive of those mentioned.
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Old 18th June 2013, 05:10 AM   #149
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Thanks for chiming in, Dave. I see you as one of the grandfathers of the matched-directivity two-way design approach, so your input here means a lot.

Mayhem, I generally agree with you about needing to define the room before talking about what kinds of directivity matter. I am most interested in small rooms, those typically found in homes. Large rooms are easy to make sound good, just like outdoors. But indoors, we have all these tricky boundary reflections to deal with.

Quote:
Originally Posted by speaker dave View Post
As you know I've been involved in the directivity issue since the 4430 days and my opinions on the importance of constant directivity have vacillated over time.

Its good to point out that we are really talking about smooth directivity or directivity that is constant over a partial range. Conventional speakers do not have uniform directivity over 10 Octaves.
...
Lipshitz and Vanderkooy did an interesting study where they independently adjusted axial response and power response. They could make power response flat, axial response flat, power response roll down hill with flat axial. Axial rise with flat power. The only natural combination was with flat axial response and a falling power response (rising directivity). Flat axial with flat power response was too bright. Flat power response achieved by rising axial response was way too bright.
...
If you look at the Soren Bech studies then he goes into the audibility of reflections, primarily based on their direction of arrival. He finds most reflections are fairly "invisible" at least at the levels you might expect to naturally occur in a home listening room. One exception was the floor bounce that could give problems up into the midrange and the rear wall bounce. The reduced side wall energy that a "more directivity" speaker would have didn't seem to buy you much. (If the reflections are inaudible, then lowering them isn't beneficial.)
...
I do like having smooth polars and a response that doesn't vary with listener position. The ability to have several listeners on a couch and the same initial arrival response for all. I also know that people will look at room curves and want to equalize them. This is a big can of worms that gets you somewhere between axial response and power response. Having a smooth directivity function may help in that regards: it won't drag response away from the axial or anechoic response to a totally different power response curve.
I wouldn't say my opinions on the importance of constant directivity have changed, but I would say that my position compared to other audiophiles has changed. Like I said in the first post in this thread, not so long ago, directivity wasn't even on the list of concerns for any audiophile I knew of. In that environment, I probably came across as being obsessed with directivity.

But I wasn't really - I chose not to use any of the horns with sharp edges in them because they sounded spitty to me. Instead, I always used horns that traded a little bit of directivity for smoother response. The radials I chose had pretty constant directivity in the horizontal, but they had collapsing directivity in the vertical. And I still think that's fine.

Now that waveguides have become the rage, this makes me a little bit on the side of saying be careful not to trade everything for polars. You can find a constant directivity horn from the 1970s that has really constant beamwidth, but there are a whole lot of anomalies in the trade. So my position now - while not having changed - still prefers uniform directivity but not at the cost of sound quality in other respects.

As for uniform directivity loudspeakers, my favorite design type has always been constant directivity cornerhorns. But they're somewhat uncommon, really, because most rooms aren't well suited for that kind of setup. So when I saw your 4430 design, I immediately embraced that approach, the matched-directivity two-way design.

In hindsight, I think the reason the constant directivity cornerhorn sounds so good to me is it eliminates any self-interference from reflections off the (front) wall that would normally be behind the speakers. I must admit to thinking it helps that it eliminates self-interference from the ipsilateral side wall too. Maybe not - Maybe it's just the removal of the front wall reflection that's most important. But I would think that if the distance to the ipsilateral wall is in a certain range, it could be as big a problem as the front wall. My guess is if it's really far, it doesn't matter. Or if it's really close (like flush, within 1/4λ), then it also doesn't matter. But in between, I would think the ipsilateral wall could make problems something like the front wall does.
Back to the matched-directivity two-way design approach, I should clarify something, briefly. I've had a bad habit of using the phrases "DI-matched" and "directivity-matched" interchangeably. When I say either one, what I mean is matched-directivity, not matched-index. Specifically, I'm talking about matched in the horizontal. I tend to like an approach of matching the horizontals using the crossover point and the horn's horizontal wall angle and setting the vertical pattern using the crossover to place the vertical null angles just outside the vertical wall angle.
Extra points if the horn's vertical beam is controlled down to the crossover point, but this is rare, in practice. Seems like the verticals are usually about equal to the horizontals at the crossover point - starting to do the pattern flip thing - and the nulls cut into that. Larger horns would push the vertical control point lower in frequency, but would also increase the distance between adjacent drivers. So like all other things, the size becomes a balance of competing priorities.

As you might guess from my infatuation with the constant directivity cornerhorn approach, I definitely agree that the bounce off the wall behind the speakers is the most objectionable, and that floor bounce might also be. This midrange anomaly is fully audible, and room treatments are ineffective. A soffit mounted speaker with baffle flush to the wall prevents it, as does the constant directivity cornerhorn configuration. But short of that, I kind of like using helper woofers or flanking subs to smooth those notches, sort of like a truncated array. Where self-interference from one source creates a deep notch, another source in another position fills it in. Not as good as flush soffit mounting or cornerhorns, but definitely worthwhile for stand-mounted speakers as are commonly employed.
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Old 18th June 2013, 05:28 AM   #150
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I would be shocked if floor bounce is among the most objectionable, frankly. Not saying it isn't, necessarily, just saying I would be a bit shocked. I know in cases where you end up with an especially bad lower midrange / midbass cancellation, you can hear it pretty well and it's not good, but it also seems less bad than it measures, and (at home) it's often in the range where vertical room modes are also getting in on the fun so that makes amateur analysis kinda difficult.

Anyway, my point is our brains grow up hearing everything bouncing off the ground/floor all the time, and we evolved with a ground too (and probably to at least some extent, a floor). It's the most normal reflection. Or, maybe it would be more accurate to say it's the expected (by the brain) reflection?
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