[Rclark]

I've even tried all four walls. I can tell though that there is more to unlock, almost at that stage.

[Davey]

Josh,

Is that ray trace on the diagram at high frequency or low frequency? 
Actually, if you look at the MMG woofer panel horizontal polar response you'll see it becomes fairly directional above 1000Hz.  (A deep null at about 2000Hz at 45 degrees.)  The tweeter is much "wider" of course because it's much narrower.

So, I think it illustrates the polar response (when crossover is included) is pretty much cylindrical for most of the frequency range.  (Two cylinders....one on the front and one on the back...out of phase.)  Thus, the single ray doesn't properly characterize the larger situation.

I think the whole concept outlined here is overly simplistic to what's actually/really a very complicated acoustic environment.  A solution in search of a problem......IMHO. 

Cheers,

Dave.

woofer

tweeter

[josh358]

You might be surprised! To be sure, if you crack an acoustics text, you'll find more sophisticated analytical tools. Some of these find use in practical room and loudspeaker design. But for the most part, when wavelength is small compared to room features, the optical ray tracing model, along with some simple statistical tools like reverberation time and coefficient of absorption, work very well for geometric room design.

This is why the typical studio control room, for example, has faceted walls in front -- they do what this barrier does, reflect the early reflections away from the primary listening area, the idea being that the first room reflections don't reach the ear for 10 ms. Here's a diagram of a typical RFZ control room; you can see that it's based on simple ray tracing:



And a room that I worked on that illustrates those principles in action (I didn't do the acoustics or room design, just the signal paths):





At lower frequencies, where wavelength becomes large compared to room features, the timing of reflections can no longer be controlled precisely and the main goals become maintenance of the right coefficient of absorption and, below the Schroeder frequency, suppression of standing waves, which become audible as frequency response anomalies. Many of the tools that work at higher frequencies, such as practical diffusers, refuse to work here.

Fortunately, we seem to be able to get away with a lot of slop at those frequencies. Higher up, we don't need a precise analysis, there are analytical tools that are founded in statistics and psychoacoustics that allow for practical simplification. So there are various useful quantities, such as the ITD (initial time delay) gap, Rt, and interaural correlation, that can be measured and manipulated without solving the wave equation. Similarly, the effects of the loudspeaker beaming to which you refer can now yield quantitative orders of merit that don't require a precise acoustical analysis. Beaming affects the perception of sound quality more than spatial representation, although it's important that polar response be consistent if the best spatial representation is desired. Harman has an algorithm for determining the effect of polar response on subjective rankings that they claim is something like 95% predictive.

That isn't to say that the polar response won't have an effect in a setup like this one, in this case it's been used (apparently) to suppress the sidewall first reflection (that ITD gap again) but also affects the amplitude of the front wall first reflection that's partially blocked by the barrier (feature size and diffraction will also play a role in this barrier, as lower frequencies will tend to diffract around the hard edge). So things do get complex, and in practice, impractical to analyze; the analytical tools exists but we audiophiles don't usually know enough about the materials to make use of them. Even professional acousticians do fine tuning after the fact (and sometimes screw up in a way that they can't fix, as they did in the room in the picture). Fortunately, in practice, it will work pretty well without that degree of analytical precision.

[MGbert]

The diagram shows just a single ray emanating from the rear of the speaker (at a fairly extreme angle.)  What about the infinite number of other sound rays at all other angles?  How are you blocking first-reflection on those?

Cheers,

Dave.

Davey:

Please!  do you want me to draw an infinite number of rays on my next diagram?  I'll NEVER be able to post it!   

Seriously, as Josh posted, the higher frequencies are directional, since otherwise there could be no nulls at the edges of the panels; they would be omnidirectional, which they are not.  Coincidently, those higher frequencies are the ones we depend on for location cues.  They also sound pretty bad when comb filtered, and especially so in a small room like mine.  So this approach only targets part of the first reflection problem, true, but there are already two elegant solutions to controlling the first reflections at all frequencies.  One is called "headphones", and I think the lack of reflections helps create the "in your head" feeling that most headphones deliver.  The striking effect of the FRTs is to create a headphone-like wide soundstage in your room out of your head, even though it only controls a portion of the frequency domain.

The second, and arguably BEST strategy for controlling first reflections at all frequencies is to put Maggies in a very large room, positioned so all reflections are delayed at least 10 msec compared to the direct signal.  That appears to be a mathematical impossibility in an 11 foot by 11 foot 4 inch room, hence the FRTs.  Stereophony has always been implementation of compromises; although not "perfect", the FRTs seem to improve one aspect of sound reproduction, for very little effort or cost. 

[MGbert]

Josh,

Is that ray trace on the diagram at high frequency or low frequency? 
Actually, if you look at the MMG woofer panel horizontal polar response you'll see it becomes fairly directional above 1000Hz.  (A deep null at about 2000Hz at 45 degrees.)  The tweeter is much "wider" of course because it's much narrower.

So, I think it illustrates the polar response (when crossover is included) is pretty much cylindrical for most of the frequency range.  (Two cylinders....one on the front and one on the back...out of phase.)  Thus, the single ray doesn't properly characterize the larger situation.

I think the whole concept outlined here is overly simplistic to what's actually/really a very complicated acoustic environment.  A solution in search of a problem......IMHO. 

Cheers,

Dave.

Wow, Dave, I can't quite let that one go.  First of all, the bass from a panel may have two half-cylindrical wave fronts (the rear being out of phase) but the highs definitely taper off as the angle off axis approaches 90 degrees - all you have to do is move from side to side of a panel while it's playing music or pink noise.

And as for "I think the whole concept outlined here is overly simplistic to what's actually/really a very complicated acoustic environment.  A solution in search of a problem......IMHO."  Well, I at least have the problem, and found empirically that this helps alleviate it.  So much so, I excitedly wanted to share this with other afficionados here.  When I get the chance, hopefully this weekend, I'll sketch it out why I think the ray approach works, AT LEAST AS A WORKING SIMPLIFICATION, in more detail.  My day job is being a structural engineer.  Many real-world structures are quite complex and as such defy exact analysis (much like acoustics in a room), yet there is frequently a way of visualizing a load path in said structure which in turn leads to a simplified model of that structure which breaks down the problem into a manageable model for analysis purposes.  Does that mean the simplified model replaces the reality?  Heck no, but it does frequently allow one to draw some useful correct conclusions.

But really - am I (and the rest of us) not even supposed to try things that will not perfectly overcome problems?  The saying that comes to mind is "Perfection is the enemy of good enough".  And please accept my apologies if I misinterpreted your intent.  Now if you tried to place FRT in your room, and it didn't perform, then that would be really useful info which would help improve the ray model.  Till then, it seems to describe what I hear in my listening space.

[Davey]

C'mon.  Obviously you don't need to draw an infinite number of rays on your diagram.    But I think you should draw one at about every 15 degrees (front and back.)  I don't think that would be too busy looking.  It looks to me like your FRT panels are fairly optimally placed to try and "block" first reflection points in your room and achieve your objective. 

However, my point (poorly made I guess) is that you've got the wrong speakers if this is a primary objective for you.  Magnepans are just the wrong speakers for small rooms IMHO.  Okay, now I'll get beat up and laughed at by someone who says his 20.1's just kill in a 100 square foot room. 

Anyways, some sort of very controlled directivity speakers....much like the Geddes offerings would seem to be the optimal playback system for your environment.  Had you considered something like that?

Cheers,

Dave.

[Davey]

Josh,

Good info.

I'm not sure our objective with a domestic listening room is the same as for a recording studio.  Neither are we trying to create Boston Symphony Hall.

To me, it's amazing the soundstage or depth or whatever you want to call it, that can be created by just two well-designed speakers optimally placed in a room.  Part of the key to that success is to embrace/use reflections and not minimize them.  I suppose I'm in the minority, but I think a listening room should be fairly "live."  You won't see any "room treatments" in my listening room. 

Cheers,

Dave.

[medium jim]

Josh,

Good info.

I'm not sure our objective with a domestic listening room is the same as for a recording studio.  Neither are we trying to create Boston Symphony Hall.

To me, it's amazing the soundstage or depth or whatever you want to call it, that can be created by just two well-designed speakers optimally placed in a room.  Part of the key to that success is to embrace/use reflections and not minimize them.  I suppose I'm in the minority, but I think a listening room should be fairly "live."  You won't see any "room treatments" in my listening room. 

Cheers,

Dave.

+1

Jim

[josh358]

Josh,

Good info.

I'm not sure our objective with a domestic listening room is the same as for a recording studio.  Neither are we trying to create Boston Symphony Hall.

To me, it's amazing the soundstage or depth or whatever you want to call it, that can be created by just two well-designed speakers optimally placed in a room.  Part of the key to that success is to embrace/use reflections and not minimize them.  I suppose I'm in the minority, but I think a listening room should be fairly "live."  You won't see any "room treatments" in my listening room. 

Cheers,

Dave.

Control rooms are generally less reverberant than home listening rooms, the objective being to allow the mixers to hear what's "on the tape," e.g., a minor dropout or bad splice. Olive and Toole have studied this and found that mixers preferred a more reverberant acoustic at home. So I think there's a lot of truth in your surmise.

However, the issue is more complex than how reflective the surfaces they are. The brain determines the space of a venue by calculating the delay between the direct sound and the reflections. (There's some pretty amazing processing power in that!) So -- if the rear wall of a concert hall shell is 15' behind the players, the initial reflections from the wall will be delayed by about 30 ms. But if the wall behind a pair of dipoles is only 5' behind the speakers, the first reflections will be delayed by only 10 ms and the brain will be presented with conflicted cues, the room reflection and the recorded reflection. Worse, the room reflection will tend to predominate, in part because the brain tends to reject ambiance coming from the same direction as a sound source, assuming instead that it's part of the original sound. In practice, what I've found happens -- with me, anyway -- is that my brain makes a compromise, and I hear the sound somewhere between the wall of the listening room and the virtual wall in the recording.

Here's a diagram that illustrates some of the psychoacoustic effects of delayed reflections at various levels:



In practice, dipoles seem to gain depth as they're moved further out into the room up to about 15' or even more.

The objective of delaying the first reflections by at least 10 ms is to get them out of the image shift region (in which the apparent source appears to move away from the loudspeaker in the direction of the reflection) and into the spatial impression region, so that's one criterion that applies both to studios and home listening. I think it's also the objective of using diffusion in front -- the diffusers reduce the amplitude of the early reflection (though not, from the figures I've seen, all the way down to the desired >-20 dB threshold), without absorbing the reverberant energy and making the room unnaturally dead. As such, diffusion allows a small room to behave acoustically more like a large one. It also reduces interaural correlation, which is negatively correlated with the subjective quality of reverberation.

One problem with all of this is that we're using the mechanical equivalent of artificial reverb, and a one-size-fits-all solution doesn't really work, since recording are made in venues of various sizes from small studios and clubs to cathedrals. I've known people to absorb the rear wave because they listened mostly to chamber music and didn't want the effect of a large hall.

[medium jim]

No Jim I.messed with them a lot. You should see all the spots taped out. I had just never tried this specific orientation, with something blocking.

Remember I've only had these seven months!

So you really have no idea how stock Magnepan's sound after they have had a chance to break-in and to have found their sweet spots?  That said, you have no way of knowing if the PG mods made them better or just different.

Jim

[Rclark]

Calm down Jim. I'm talking about a small improvement in soundstage, nothing more.

[medium jim]

Calm down Jim. I'm talking about a small improvement in soundstage, nothing more.

Oh I'm very calm!  The way you wrote it, one would think you had an epiphany of the grandest order.

Jim

[MGbert]

As promised, here is the latest iteration of my First Reflection Trap (FRT) layout, designed to eliminate as many early reflections from the front, sides, AND back of the room by trapping (delaying)  them so they become later reflections.  I replaced the 48” high by 31” long FRTs in front of the room with new FRTs made of 6 foot high by 4 foot long luann panels, and added 2 other 6 X 4 FRTs to the outer sides of the speakers.  Here is the revised floor plan:




You will note that, per request, I showed a few of the sound trajectories on this plan.  I think it makes the point pretty clearly that the 2 FRTs on each side together do a remarkable job of trapping the first reflections from the front, and except for the relatively low acoustic energy of reflections which make it over the 6 foot tall barriers, each reflection needs to take a long path in order to finally escape towards the listening position.  The sketch was getting pretty crowded, so I’ll leave it as an exercise for the reader to trace sound trajectories which, if not trapped, would have been early reflections from the rear and side walls.  Short answer – almost no early reflections (< 10 msec) from the rear!  As you can see by the pictures below taken at the listening seat, you see the side of the panel facing away from the speaker.  This is very important: the FRT must be angled so you’re looking at this side, without the actual sound producing panel (ie, the black grill cloth portion of the Gunned MMG shown) being blocked from view; otherwise, all the myriad sound trajectories bouncing off that inner side will be directly audible, and the result is a sonic mess thanks to the comb filtering.  Although it is instructive to lean forward and hear what it does to music – it makes you wonder how you ever tolerated comb filtering before.











If you have larger Maggies, then simply substitute as high a panel that won’t damage your ceiling when you move it.  Luann panels are typically sold in 8 foot by 4 foot widths for less than $10USD apiece, so the cost of trying this is relatively small.   And if you don’t need to cut them, and decide it’s not for you, then you can return them.  But if, like me, you are forced to use a dedicated smallish room for listening, it’s hard to imagine someone not appreciating the way this mod cleans up the sonics.

The net result of this is an almost solid “phantom center” and exceptional clarity to each instrument, as well as a sense that the room is larger due to the delayed first reflections, not to mention slightly enhanced bass energy since you’ve basically created a transmission line in your room.  It looks like that, too – I feel like I’m a bug who found my way inside a Bose radio.  Doing a treatment like this has zero WAF, unfortunately, unless perhaps you use clear plexiglass panels for the FRTs outside the speakers, and cleverly make the FRTs in front of the room into decorative oriental “room dividers”, so long as the back remains acoustically reflective.  Never has a mod encouraged listening with eyes closed or lights out!

And yes, I need to brace the panels so they’re not flopping over.  Basic laziness; they sound really good as is!

[MGbert]

Josh,

Good info.

I'm not sure our objective with a domestic listening room is the same as for a recording studio.  Neither are we trying to create Boston Symphony Hall.

To me, it's amazing the soundstage or depth or whatever you want to call it, that can be created by just two well-designed speakers optimally placed in a room.  Part of the key to that success is to embrace/use reflections and not minimize them.  I suppose I'm in the minority, but I think a listening room should be fairly "live."  You won't see any "room treatments" in my listening room. 

Cheers,

Dave.

Dave,

We almost see eye to eye, then.  My approach uses the room reflections, only it manages them by making it harder for the pesky early ones to ruin things.  The FRTs do NOT absorb, they simply reflect/deflect.

Having said that, consider that the recording studio is the place where the record producer decided that the recording sounds "good".  So maybe having a listening environment not too dissimilar to a studio may not be a bad thing when listening to these recordings.

MGbert

[MGbert]

Here's a diagram that illustrates some of the psychoacoustic effects of delayed reflections at various levels:



In practice, dipoles seem to gain depth as they're moved further out into the room up to about 15' or even more.

The objective of delaying the first reflections by at least 10 ms is to get them out of the image shift region (in which the apparent source appears to move away from the loudspeaker in the direction of the reflection) and into the spatial impression region, so that's one criterion that applies both to studios and home listening. I think it's also the objective of using diffusion in front -- the diffusers reduce the amplitude of the early reflection (though not, from the figures I've seen, all the way down to the desired >-20 dB threshold), without absorbing the reverberant energy and making the room unnaturally dead. As such, diffusion allows a small room to behave acoustically more like a large one. It also reduces interaural correlation, which is negatively correlated with the subjective quality of reverberation.

One problem with all of this is that we're using the mechanical equivalent of artificial reverb, and a one-size-fits-all solution doesn't really work, since recording are made in venues of various sizes from small studios and clubs to cathedrals. I've known people to absorb the rear wave because they listened mostly to chamber music and didn't want the effect of a large hall.

Josh:

Where did you find that graph?  There is an awesome amount of info represented by it.  I'd like to learn more.

And check out my latest version of FRTs.  The second set might make even chamber music afficionados happy, since the rear reflections are well controlled - at least to my ears.   

MGbert

[MGbert]

C'mon.  Obviously you don't need to draw an infinite number of rays on your diagram.    But I think you should draw one at about every 15 degrees (front and back.)  I don't think that would be too busy looking.  It looks to me like your FRT panels are fairly optimally placed to try and "block" first reflection points in your room and achieve your objective. 

However, my point (poorly made I guess) is that you've got the wrong speakers if this is a primary objective for you.  Magnepans are just the wrong speakers for small rooms IMHO.  Okay, now I'll get beat up and laughed at by someone who says his 20.1's just kill in a 100 square foot room. 

Anyways, some sort of very controlled directivity speakers....much like the Geddes offerings would seem to be the optimal playback system for your environment.  Had you considered something like that?

Cheers,

Dave.

On the face of it, you're right, Maggies don't play well in small rooms.  Unless the pesky short delay reflections are managed. 

I happen to LOVE the Maggie sound, and I refused to give up on the notion that they could be made to do well in a smaller room, which by the way wasn't that small when I first got my MMGs.  The size shrunk when I realized that symmetry was important, so after moving shelves around my 19 foot length became 11 feet.  I took it as an engineering challenge to make them work in such a small space, since room size is a frequent topic of conversation on Planar forums.  I honestly believe I found a way to make them sing in a small space, and like many engineering solutions it is functionally elegant and aesthetically awful.  If someone else here can take the concept and increase the WAF, I'd be all ears - no pride of authorship here.   

I keep saying small spaces, but I think the FRT principles could benefit some larger rooms as well.  More to come.

MGbert

[MGbert]

(can't edit so have to post anew)

 Getting Magnepans right seems very much like focusing a pair of lenses. It will be fun to one day soon have software that just reads our entire room and everything in it (camera based), and is able to give us the exact location we need to be for perfect imaging. With Magnepans it's a game of inches and sometimes all you want is to just have it right the first time, not a voyage of mistakes.

Focusing lenses is a great analogy.  As far as using a computer program, even optometrists, with all their fancy equipment, have to rely on the final step of a patient putting on the lens.  Sometimes, a further tweak is still necessary.  Besides, I have a natural, professional distrust of answers for complex problems coming from a computer, mostly because some people trust them blindly when they can be perfectly wrong.  GIGO, literally.  So enjoy the journey!

FWIW, here is a step-by-step approach to mapping out your room for possible early reflection problems and possible solutions using the FRT concept.  Please don't take offence at the level of detail!  I've been burned too many times assuming someone would just know the next step...

1) Get some graph paper, and draw a scale floor plan of your room. I even go so far as cutting out "paper dolls" representing the furniture, to scale of course. Easier to move those around on paper than the real thing!

2) Use a mirror to find all the places on your walls that you can see a speaker while sitting at the listening position (LP). You should find 4 at least. Note those points as closely as you can on your floor plan.

3) Draw lines on your floor plan from the speaker to the reflection point, then from the reflection point to your LP. Also draw lines from the speakers directly to the LP.

4) Measure the length in feet from the speaker to the LP. Then measure the shortest path from the speaker to the first reflection point to the LP. Subtract the direct distance from the first reflection distance. Each foot of difference is about one millisecond of reflection delay. Ideally, each difference should be at least 10 milliseconds longer than the time it takes the main signal from the speaker to reach you at the LP.

5) The trick is to make each reflected delay 10 milliseconds long. That is where FRTs come in. They do not have to be paneling; any acoustically reflective surface will work. Get creative with the furniture you already have; one fellow on that Asylum place just moved some homemade abfusors he had tucked in corners. Just remember that if the line you draw hits a surface at say a 30 degree angle, it bounces off at the same angle. So angling the FRT is a key part of ensuring the reflection point is made longer than 10 milliseconds.

That basically is how I came up with the floor plan sketch I posted earlier. No in depth acoustics knowledge; just drawing trajectories on paper! Also remember that there is almost no mid/treble sound coming from the edge of a planar speaker, so "reflections" from the edge don't count.  Angling the speakers at a 45 degree toe-in helps this all a lot, since most of the pesky early reflections from side walls get directed behind the speakers, where it is relatively easy to manage them.

One other trick: disconnect one speaker, and play some music in mono through the other speaker with many different bass notes up and down the scale. Move that speaker around until all the bass notes are about the same volume. Then start graphing with the speaker located in that spot!

And now you know as much about speaker placement as I do; probably more, since I claim no special expertise other than playing with placement in my room.

MGbert

[josh358]

Josh:

Where did you find that graph?  There is an awesome amount of info represented by it.  I'd like to learn more.

And check out my latest version of FRTs.  The second set might make even chamber music afficionados happy, since the rear reflections are well controlled - at least to my ears.   

MGbert

The illustration is from Floyd Toole's superb Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms. Highly recommended for anyone who is interested in the present state of our knowledge, and unlike some highly mathematical acoustics texts it's accessible to non-engineers (I know you are). A book that's very practical for DIY acousticians is Everest's Master Handbook of Acoustics. It has practical treatments of absorption coefficients, quadratic residue diffusers, etc. For those interested in understanding acoustics on a more theoretical level, I recommend Beranek's Acoustics. That's at the level of an undergraduate course and is invaluable for understanding e.g. the directional characteristics of drivers.

By the way, just to clear up an apparent misconception, the reflection-free zone concept as applied to studio design doesn't rely on absorption. It works the same way your setup does, with reflective surfaces, but is adapted to soffit-mounted dynamic monitors rather than to dipoles.

Studio acoustics are generally drier than home acoustics. This is done not because it sounds good, but because it lets you hear better what's on the master tape. Natural ambiance is pleasing and necessary to recreate a sense of space, but it can reduce detail and color the sound (see the illustration). When listening to a recording, you have to hear every little glitch. I spent years listening to studio sound and in most regards it never equaled the sound we get at home. (Dynamics were the main exception -- uncompressed masters and monitors that could play at natural levels, as almost all home systems can't.) *But* the goals of early reflection control are the same at home and in the studio. They're a consequence of psychoacoustics. You can get a better understanding of the effects of this stuff by reading the instructions for a digital reverb unit! They tell you what settings to use to imitate a given acoustical space.

[josh358]

I honestly believe I found a way to make them sing in a small space, and like many engineering solutions it is functionally elegant and aesthetically awful.  If someone else here can take the concept and increase the WAF, I'd be all ears - no pride of authorship here.   

I think you've hit the nail on the thumb.

Directional control requires big ugly things because of the wavelengths involved. About 1' at 1000 Hz, 10' at 100 Hz. The waves just diffract around anything small.

One of the few exceptions to this -- dipoles and other phase cancellation designs, e.g., cardioids. But they have a limited effect.

You can make a small sound source directional by using ultrasonics and relying on the nonlinearity of the air to shift the frequencies down, and there are commericial directional speaker systems that work that way, but the SPL's are very high and there are some real questions about the health in long-term listening.

Near field listening can help, if that's OK in your setup. But it isn't a cure all.

This I think is one of the biggest unanswered questions in audio -- how do you take the room out of the picture without making something huge and un wife friendly.

One interesting possibility is to make fresnel deflectors, e.g., triangular wedges with absorption on one side. I believe there's one guy who does this in studios, his technique is closely guarded but I reverse engineered the idea from reading a description of what he'd accomplished.

BTW, speaking of dipoles, rather than adding the second barrier, why not just adjust the toe-in of the speakers so that the sidewall first reflection is at the dipole null? I've experimented with that and it works quite well.

[medium jim]

Josh, excellent posts! However, there is so much variances from recording studio to recording studio.  Ideally a recording studio should be as neutral and as dry as possible.  Then there are the secrets of placing mike's, or even in the case of the old days, moving the musicians to achieve a more balanced recording.

Nevertheless, the science is valid and the first waves are the most important to tame.  I have strategically placed furniture to achieve this end.  Important also is to tame front wall with both absorption behind the panels and diffusion in the center of them.

I have a room more like a studio, this works as I'm running all tube gear that warms it up anyway.

Jim