[Aether Audio]

Noah & Friends,

A picture is worth a thousand words, they say.  Well, I guess we’ll see how many these will generate.  I believe my original postings on the subject of dispersion, its importance and the unnecessary compromise involved in achieving true Constant Directivity performance at narrower angles may not have been understood as clearly as I would have hoped.  The following should clear things up - even if the title has you a bit puzzled.

In this first graphic we have a pair of speakers in a 15’ W X 23’ D room.  Each speaker’s center is 3 feet from the nearest sidewall.  The distance from the wall behind them is not important in this discussion, so I haven’t exactly specified.  That distance would certainly make a difference in the bass performance, but it has no bearing on the output of the waveguide – which is the subject of concern here.

To help you interpret the following graphics, just remember that if a reflection “ray” crosses in front of the listening position – that’s bad.  If it crosses behind – that’s good.  The rest should be self-explanatory.

This first graphic shows us the advantage of true CD performance combined with a 90 degree waveguide.  When the speakers are not toed-in at all, the CD waveguide of 90 degrees gives us the desired “Reflection Free Zone.” Of the two waveguides demonstrated here, it offers a widest possible listening area for a given placement distance from the sidewalls.  Many may say, “Well, that settles it.  That’s what I want.”  To be honest, we would too if the tradeoff wasn’t the excessive time domain errors that result from having to use such a deep waveguide to obtain these results.  I guess if your only concern is theater or sound reinforcement performance, then this is the way to go.  If you have any high-resolution audio requirements though, you may want to think twice...maybe three times even.



The second graphic shows what happens when we move our listening area forward just a little bit.  In this case, we’re looking at a normal Equilateral seating position as is commonly used in 2 channel stereo applications.  We can see that the 120-degree CD waveguide is getting closer to achieving the reflection free zone that we desire.  Still, further improvements could be had and are needed for optimal results.



The third graphic is now showing us what happens when we use a relatively moderate 9 degrees of toe-in.  The Equilateral seating position is necessary to achieve the Reflection Free Zone that we desire.  As we can see, the 120-degree waveguide is able to provide the necessary conditions, but seating is quite restrictive.  This may be acceptable for many that prefer an individualized sweet spot, but an even wider listening area is possible, as we shall see.



The fourth and final graphic shows us what happens when we increase the speaker toe-in to 13.5 degrees.  Up to 15 degrees is actually the preferable amount for most speakers, as virtually all traditional dome tweeters begin to narrow their radiation pattern to an approximately +/- 15 degree pattern above 10 kHz.



As we can see from this last graphic, the Reflection free Zone is quite wide and the listening distance is a bit further back besides.  One can deduce from this that using the Equilateral seating position will widen the listening area even further.  W

We would add to this the fact that even further improvements can be had by moving the speakers further from the sidewalls.  As you may be able to tell, with a slight amount of toe-in, an additionally small amount of speaker inset from the sidewalls can have a considerable effect in widening the Reflection Free Zone.  it will probably make the bass better too.  In the end, everyone's room is somewhat different so your milage may vary.  Nevertheless, it becomes easy to see that small adjustments in placement and toe-in can have fairly profound effects.

Closing on our original point, in all of the above we can clearly see that in most practical applications, the shallower, 120-degree waveguide can easily provide the desired Reflection Free Zone.  It may take a few minor adjustments in positioning, but these adjustments will most likely be made for other reasons anyway.  In so doing, it also permits the level of acoustic center time alignment with the woofer that we all prefer for true high fidelity reproduction.  While the 120-degree waveguide is a bit of a compromise from a dispersion perspective, we at SP Technology believe it offers the best balance of performance features and compromise.  Remember:  Good engineering always dictates some level of compromise.

Oh...and before somebody jumps on the bandwagon and says, "But I thought your speakers sound better with very little toe-in," I have this to say:  While that may be your preference, I actually doubt if that's optimal.  Decreasing toe-in will have the effect of generating a bit more reflection from the sidewalls.  This will be especially true near the lower end of the waveguide's operating range.  Reflections from the sidewalls in the 900 to 5kHz band can have the effect of widening the soundstage and producing more of a 3-D effect.  

It is for this reason many prefer the sound of dipoles or other designs that intentionally bounce sound off the rear and/or side walls.  The image of a pair of  Bose (yuk) 901's comes to mind.  Isn't that what they were known for?  Many folks like this effect but to be honest - it's artificial.  In so doing, one tends to be creating their own form of "holographic synthesis."  In the end there is no "right" or "wrong."  Whatever suits your tastes is fine.  Just be aware of the fact that this effect may not actually be in the recording.  So toe those darn speakers in!

Hope this helped.  It better... 'cause I'm pooped!

-Bob

[Bill Baker]

Okay, let's experiment. I was going to post this in my original post of "SP Continuums with EXtreme Hurricanes" but this works nicely. Perfect timing Bob.

 Before leaving last night, I decided to try a different set of amplifiers with my continuums. The ASL Monsoon 100 watt mono blocs. I have the speakers set up about 8 feet apart which leaves about 4.5ft from each side wall and they are 7 feet from the wall behind them.

 I started with alistening position of 10 feet from the speakers with no toe-in. The presentation was very good with good (not great) center imaging and a decent soundstage outside and behind the speakers.

 Skipping all the other setups I tried, let's move on to what worked out the best in my room. The speakers are still 8 feet apart but I now have the listening position also 8 feet back. I adjusted toe-in a few degrees at a time and yes, ended up with 10-15 degrees that sounded the best.

 I want to try a wider arragement going with 10 feet apart which will leave me with about 3.5 feet from each sidewall. My room is 30+ feet deep so I can still make this happen but the Continuums are so damn heavy, I have to wait till I get another cup of coffee in me. Maybe tomorrow.

 Till then, I will experiment with the Bella speakers in a completely different part of the room.

 By the way, the Monsoons, which are completely stock at the moment and are a few grand cheaper than the Hurricanes, do a great job with the Continuums. So good in fact, that while I was telling a good customer about them on the phone yesterday........... anyway, he bought a pair of Monsoons.

 Bob, thanks for taking the time to post these thoughts along with pictures to show everyone exactly what your words mean. Most people need a "brain cam" to understand what the hell you are talking about. Pictures are the next best thing. I am lucky in that I have spoken to you so much that I can visualize what you are speaking of (most of the time). If not, I do get off the phone with more new information that I have to soak in for a while.

[skrivis]

Noah & Friends,

A picture is worth a thousand words, they say.  

Excellent info. I wish more manufacturers would give this kind of info.

[skrivis]

I was just poking around on the web site and didn't see any graphs of the polar response. How sharply does the response drop beyond 120 degrees?

Have you also looked at reflections from the ceiling (and floor in some rooms) as additional data points for producing reflections? It seems that toeing the speakers in will also increase the reflections from floor and ceiling at the listening position, won't it?

The cross reflections are made worse by toeing in too. In a small (width) room, this could become a factor.

[Aether Audio]

skrivis,

I was just poking around on the web site and didn't see any graphs of the polar response. How sharply does the response drop beyond 120 degrees?

I don't have any recorded data on that angle for our waveguides since we changed the tweeter/throat assembly.  I did a quick look at it months ago but I don't remeber much in the way of details.  I should have saved the plot, but I didn't 'cause I hadn't gone to any lengths to do the setup for that test the way I should have.  It was sort of a "OK, it looks pretty good - on to other stuff." kind of thing.  I need to be three people to get all the darn documentation done around here.  My main concern was the -6dB angle, which is 10 kHz.  Even the plot we have of that was from the first prototype and not as good as it is now.  Any electro-acoutic technicians out there that want a job and are willing to work for the "joy of the experience"?

Have you also looked at reflections from the ceiling (and floor in some rooms) as additional data points for producing reflections?

No...and it's not really necessary.  At the shorter wavelengths governed by the waveguide, simple "ray-tracing" (like the graphics above) will tell you where the reflections will be.

It seems that toeing the speakers in will also increase the reflections from floor and ceiling at the listening position, won't it?

Nope.  If your listening position is fixed, the ceiling/floor reflections are governed by the distance between that postion and the speakers.  Again, the greater that distance, the more reflections you'll get.  Well, then there's the actual height of the speaker involved too.  Wherever the tweeter/waveguide is in the vertical dimension, will also determine the reflection angle(s) and distances to a large degree.  I guess there's one more issue.  The tilt forward or back of the speaker can significantly effect the ceiling/floor reflections too.

The cross reflections are made worse by toeing in too. In a small (width) room, this could become a factor.

Well, if you're in a really narrow room and/or are seated a considerable distance from the speaker, that could be somewhat true.  But then again, those reflections will be later arrival reflections than those from the nearby sidewall.  The earlier the first reflections, the worse they are for altering the timbre of a source.  The shorter the path length, the higher the frequency effected by the resulting "comb filtering" effect.  Don't get me wrong though.  Longer delay reflections can be a real pain too - but they exhibit their effects at lower frequencies.  The human ear is more sensitive to changes at high frequencies than anywhere else in the spectrum.  That's why everybody argues over which preamp or CDP is the best.  It's those subtle high frequencies we just have to get right.  

Adding to that, those longer path-length reflections will be broader-band in nature... and hence less annoying.  We're talking flatter power response reflections here.  To a large degree, we want those coming back at our listening position (from behind us of course) - nice and diffuse though.  Don't think so?  Just try covering all the walls in your room with as much acoustic deadening material as you can shove in there.  You'll find out real quick what I mean.

Well, there's some more food for thought.  Did you ever guess it would be this complex gentlemen?

-Bob

[BobRex]

One of the values of these graphics is in identifying the "true" reflection points of a room.  Most audiophiles would just blindly assume that the first reflection point needed to be treated.  Now, since many people are Mathlexic and afraid to do the calcs, they'd just walk around with a mirror until the points were located.  While this is a valid methodology, it makes an assumption of 180degree dispersion.  That is rarely the case, as these drawings show.  

So it looks to me that toeing in the speakers (at lease SPs) can eliminate the need to treat the first reflection point.  Am I thinking correctly?

[skrivis]

The dimensions of floor and ceiling reflections are similar to side-wall reflections. In fact, assuming an 8' ceiling and 4' speaker height, the reflection from the upper and lower surfaces will be in front of the listening position, no matter how you toe-in the speakers.

As for how toeing-in the speakers effect this: With the speaker's axes perpendicular to the plane of the listening position, the floor and ceiling on-axis reflections will be directed to the sides of the listening position. As you toe-in the speakers, you will move those reflections _toward_ the listening position.

I was visualizing this, but I also did some scribbling on paper. I don't see where I'm making a mistake.

[skrivis]

skrivis,

Well, if you're in a really narrow room and/or are seated a considerable distance from the speaker, that could be somewhat true.  But then again, those reflections will be later arrival reflections than those from the nearby sidewall.  The earlier the first reflections, the worse they are for altering the timbre of a source.  The shorter the path length, the higher the frequency effected by the resulting "comb filtering" effect.  Don't get me wrong though.  Longer delay reflections can be ...

At some point, the Haas effect will separate the reflections from the initial event. We then begin to perceive an "echo" instead of just a smearing of the initial event. (Echo isn't always the right term. From flanging, reverb, chorus, echo.... it's a whole spectrum, but I'd say that from reverb on it's an addition to the original event.)

[BobRex]

As for how toeing-in the speakers effect this: With the speaker's axes perpendicular to the plane of the listening position, the floor and ceiling on-axis reflections will be directed to the sides of the listening position. As you toe-i ...

And as the last drawing indicates, it is possible to move the reflections beyond the listening area.  To me, this is the best scenario. With bothe the 90 and 120 waveguide, the reflection area is potentially out of harm's way and shouldn't interfere with listening.  Additionally, moving the reflections beyond the listening position adds to the reflection time, possibly taking it beyond the point where it interferes with the primary wave and into a timing area where the brain can better decode this signal without mixing with the primary.

I won't debate the floor/ceiling reflections, my feeling is that carpeting will limit the floor effect which just leaves the ceiling as an area possibly needing attention.  That seems to be the last area to ever be treated.  Me, I'm lucky enough to have a cathedral ceiling with the peak running along the listening axis.  Asymmetrical (vis-a-vis the ceiling peak) placement guarantees that I've lessened the impact of ceiling reflections.

[skrivis]

And as the last drawing indicates, it is possible to move the reflections beyond the listening area.  To me, this is the best scenario. With bothe the 90 and 120 waveguide, the reflection area is potentially out of harm's way and shouldn't interfere with listening.  Additionally, moving the reflections beyond the listening position adds to the reflection time, possibly taking it beyond the point where it interferes with the primary wave and into a timing area where the brain can better decode this signal wi ...

But using the proper toe-in seems to me to fire the ceiling reflection right forward of the listening position, using my figures of 8' ceiling and 4' high speaker.

You may be ok with your cathedral ceiling, but what about people with a regular flat ceiling?

My listening room has my right speakers about 6' away from the wall, left speakers about 10' away from the left wall, sloped ceiling (although not enough to be cathedral or valuted or anything) and back wall about 15' behind the listening position, although the back of the room is very irregular in shape and also has various openings and even an open stairwell to the basement.

I guess I needn't worry, but what about the poor schnook with the small NYC apartment?

[ooheadsoo]

With the speaker's axes perpendicular to the plane of the listening position, the floor and ceiling on-axis reflections will be directed to the sides of the listening position. As you toe-i ...

The listening spot will have reflections from the ceiling regardless of whether the speakers are toed in or not, which is what I think Bob is getting at.  I think you're focusing on just the axis and forgetting the rest of it.

[skrivis]

With the speaker's axes perpendicular to the plane of the listening position, the floor and ceiling on-axis reflections will be directed to the sides of the listening position. As you toe-i ...

The listening spot will have reflections from the ceiling regardless of whether the speakers are toed in or not, which is what I think Bob is getting at.  I think you're focusing on just the axis and forgetting the rest of it.

I was thinking of a cone. I was just fiddling a bit more and I think it would actually be a rectangle as it intersects the rectilinear walls of the room. The actual propagation wavefront is closer to spherical, so perhaps you're right.

[ooheadsoo]

Whatever shape you choose, the listening spot is sure to fall within its volume, unless you're not sitting inside the beam - I think.

[skrivis]

Whatever shape you choose, the listening spot is sure to fall within its volume, unless you're not sitting inside the beam - I think.

The rectangle is the intersection of the 120 degree cone of dispersion and the walls, floor, and ceiling of the room. The spherical wave front propagates from the front of the speaker itself. And it's back to the drawing board for me so I can visualize this in more detail. hehe

[ooheadsoo]

I know what you mean, and the listening spot is going to be inside that reflection no matter the toe in, which is what I think Bob is saying.

[Aether Audio]

skrivis & ooheadsoo,

You guys are on the right track with the last couple of posts.  As I was reading the earlier one by skrivis I was thinking, "He's not seeing the expanding wavefront/cone of propagation."  Then he brings it up.

That's just what I was going to point out.  With or without toe-in, the wavefront will have expanded to such a wide area in the horizontal plane that you will still get reflections from the ceiling at the listening position.  You have to picture a cone expanding outward with infinite radii perpendicular to the axis of propagation.  But the time the cone reaches the ceiling there will be rays crossing directy in front of the listening position.  Here again, a 90 degree waveguide would certainly help reduce the effect, but toe-in won't change things much except for maybe the last octave or so.  Technically skrivis was correct to begin with to a certain degree, but I don't think it would make enough difference to outweigh the benefits of toe-in with regards to sidewall reflections.

Personally, I think floor bounce is the bigger issue of the two.  Most tweeters are less than 48 inches from the floor, so the floor will be the first source of spurious reflections.  If the ceiling is higher than the standard 8 foot, this is all the more true.  Of course, if the tweeter is at the midpoint between the floor and ceiling, then both will contribute equally (sans any carpeting, etc.).  Logically it follows if the tweeter is higher than midpoint, then the ceiling will be the major culprit.

In all of this I think it's interesting to note that these facts suggest that tilting a speaker back a few degrees could really help with the floor reflections.  Actually, I've played with putting the Timepieces down low to the floor and then tilting them back by about 30 degrees or so.  Hey, it actually sounded pretty darn good.  Didn't have quite the sense of height, but you could tell there was more of a "listening back into the recoding" effect.  It also seemed to give a longer delay time to the reverberation coming from behind (which makes sense).  I wouldn't necessarily recommend it for everyone, but it was interesting to experiment with.

Anyway, time to try and save my marriage.  I have the most understanding wife a man could ask for, but everyone has their limits.

Nighty-night!

-Bob

[skrivis]

I think that there may well be an underlying fallacy here.

When the sound hits the side walls, it will be re-radiated hemispherically. It doesn't much matter where on the side wall it hits, you're going to get it back at your ear anyway.

We're not talking about beams of light here or anything. You won't see a reflection coming off the wall with a 90 degree included angle and then continuing in a straight line.

It also doesn't really matter whether the wave is reflected from the side wall close to the speakers or closer toward the listener. The pathlength and resulting attenuation is going to be similar. The directivity of the SP Tech 120 degree waveguide may reduce the amplitude of the reflections from the points on the side wall near the speakers, but I don't know that the end result will be any better. It would be interesting to measure the SPL at the listening position and see whether the total reflected power arriving at the ear is less with the waveguide and toed-in speakers than without one or the other.

[Watson]

I think that there may well be an underlying fallacy here.

When the sound hits the side walls, it will be re-radiated hemispherically.

Do you have any data to back this up?  There will obviously be both direct and diffuse components to the reflection, but I would think that the wavelength of sound is so long relative to surface irregularities in the paint on an average wall that the majority of the reflection should be direct reflection (where the angle of incidence is equal to the angle of reflection) not diffuse.

[ooheadsoo]

Geddes calls the region in question here the "geometric" or "ray acoustics" region.  Clearly, as Bob has already mentioned, higher frequencies, above 500hz according to some, begin to act like rays.

[skrivis]

I think that there may well be an underlying fallacy here.

When the sound hits the side walls, it will be re-radiated hemispherically.

Do you have any data to back this up?  There will obviously be both direct and diffuse components to the reflection, but I would think that the wavelength of sound is so long relative to surface irregularities in the paint on an average wall that the majority of the reflection should be direct reflection (where the angle of incidence is equal to the angle of reflection) not diffuse.

No, I was just thinking about the whole thing last night and tried seeing what happened in a wave table (a bowl of water). I didn't see ray-like behavior there, thus my speculation.

If I'm wrong, I'm wrong.