QRDude design change disccussion

[collo]

This thread is to allow discussion of a proposed change in the way inverse panels are calculated in the QRDude calculator.

The hoped-for contributors would be those who have already helped with the development so far, namely TerryJ, Lupo and Xenon. All others are welcome, of course.

...........................

An inverse QRD panel is one where the well depths are the opposite to the normal panel.

For a 1D N7 normal. panel, the depths range from 0 to 6 depth units.
For a 1D N7 inverse panel, the depths range from 7 to 1 depth units.

The amount of phase shift introduced by a given well is equal to 360 degrees minus that introduced by the same well position in the normal panel.

When QRDude was first being developed, it was proposed that the deepest well in the inverse panel, which produces 360 degrees phase shift, could be replaced by a zero depth well. At the time there was not enough known by our little group to say with certainty that it was allowable. The calculator was thus produced without this being included.

Since then, 2D panels have been added to the mix, and in researching these, an example of a this substitution has come to light. By none other than the most reputable producer of panels.

...........................

Here is a screenshot showing grab from the QRD overview notes for the calculator along with the product in question - a 2D inverse N7 panel...

Here is the same panel as produced by the current version of QRDude. The difference can be seen at the bottom left corner...

I think it is well worth looking a this again. The benefits in allowing the substitution are a shallower inverse panel and simpified treatment of 2D inverse panels. (Fairly large changes to the code needed, but in the end, a better product)


As a means of investigating this a little more, here is a drawing that will eventually end up in the guide.

The standard equation for determining the well depths of a QRD panel is:
Well depth = (well position squared) mod N .... where N is the number of wells.

The green area shows the depths for a normal N7 panel if the mod part of the equation was not present. The dark horizontal lines show depths which are multiples of one wavelengths worth of shift.
The blue vertical arrows show how the mod operator removes these multiples to give the familiar N7 pattern (shown in grey)

This same approach when applied to an inverse panel would produce the transform we see in the commercial 2D inverse panel shown earlier.

I would like to canvas opinions on this before taking it any further...

[collo]

Had a close look at the huygens diffusion and QRDude scatter drawings for 1D normal / inverse pairings and realised that the substitution is not on for those combinations.

For the 2D panels, which are not generally modulated with such pairings, but use rotations of a single panel geometry, the substitution is still up for grabs...

Here is how a modified QRDude would generate the 2D inverse panel from the previous post... note it's now the same as the commercial panel:

[terry j]

I am happy to do my duty yet again collo.

Ask dumb and stupid questions!

You just posted the second post...so let me get this straight. Orig you thought the inverse could be made shallower, but not anymore (1d qrd).

So it is now solely in the realm of 2d changes??

Right now I'm in a six of one half a dozen of the other frame of mind about it...

So what is to be gained for effectiveness, ease of build, depth etc.

Haha, just pretending to not follow you (where's the wink smiley?)

the second build you show looks like six units (is it), the first is also six units (?) so I am currently confused what any advantage is????

[collo]

Hey, thanks for chiming in!

When using an inverse panel to modulate a sequence of normal panels, the diffusion angles need to cancel out.

Here is a drawing with a 1D normal panel shown in green with an inverse panel super-imposed over it in brown. The scatter drawing shows that the diffusion angles are indeed opposite to each other as required.

Now the situation using our proposed inverse panel, shown in purple.
The diffusion angles are no longer all opposite, meaning that the modulation will be unable to produce the even lobes we are looking for.

An analysis using huygens drawings brings out the same result.

From this, I would say that for panels used in normal / inverse pairings, the substitution should not be allowed. Where an inverse panel is used in a stand-alone situation, substitution is still a possibility.

For 1D panels, an inverse is generally only introduced as part of a modulation scheme, so I would not look at introducing a change there.

When it comes to 2D panels, the situation seems to be the opposite. I've not seen any examples of pairings. In fact the only advice comes from RPG, who advise using a random orientation for their 2D panels. In this situation I'm thinking of treating all 2D inverses as stand-alone panels, and am considering using the substitution.


As for the depth savings, you need to look at the drawings carefully. The QRDude screenshot in the first post shows a panel that has no zero-depth wells and the total height is 7 depth units - including the fins, which sit up 1 depth unit above the tallest block.

The second posting shows the panel if substitution is used. There is now a block with zero depth but no longer any with full depth. The entire bottom layer is disgarded, and the total height is now 6 depth units.

This depth saving is the major advantage to using the substitution. A smaller advantage is that it becomes easy to see the orientation of the panel when multiples are used in an array. A final advantage, and one that is not much use to us DIY'ers, is that the single zero-depth well is the perfect spot to place your manufacturer's logo..

[PaulP]

Originally Posted by collo

When it comes to 2D panels, the situation seems to be the opposite. I've not seen any examples of pairings. In fact the only advice comes from RPG, who advise using a random orientation for their 2D panels. In this situation I'm thinking of treating all 2D inverses as stand-alone panels, and am considering using the substitution.

In this picture from RPG the orientation is far from random :



Most pictures I've seen have the panels all with the same orientation.
A couple more :

This depth saving is the major advantage to using the substitution. A smaller advantage is that it becomes easy to see the orientation of the panel when multiples are used in an array. A final advantage, and one that is not much use to us DIY'ers, is that the single zero-depth well is the perfect spot to place your manufacturer's logo..

I find that the zero depth well looks kind of odd. It sticks out too much.
But it does make it easy to see the orientation of the panel.

Paul P

[collo]

Nice find Paul,

all the same panels and no modulation apart from some spacing between groups of panels.

Seems like the issue of periodicity for their 2D diffuser is not a large enough problem for them to follow their own advice (see part C - also includes their description of the qrd sequence as "optimized", presumably referring to the substitution)

[Schaap]

The hemiffusor is a Omniffusor without a top row and last column RPG Diffusor Systems.
Why did they do that? To avoid too much symmetry?

[collo]

Turn it round the other way...

The hemifusor is the correct 2D QRD panel.

The omnifuser has an added row and column - presumably to give a symmetrical appearance.

[Schaap]

Originally Posted by collo

Turn it round the other way...

The hemifusor is the correct 2D QRD panel.

The omnifuser has an added row and column - presumably to give a symmetrical appearance.

Aha, touché! Yeah, a lot of people dislikes asymmetric looks so it's more a marketing thing I guess.

[Lupo]

Hi Bill and all!

Never got around to replying to this. Don't know what to add!


Question: what is the response for other effective frequencies, beside the base design frequency? What happens at the integer multiplies? No difference? Increasing difference as it gets further away from base frequency?

And.. Is it just one corner piece that's diferent? This is perhaps herecy, but guess it doesn't matter much with one single piece out of the 49.


Paul: impressive pics! Where's that live room?

[PaulP]

They were stolen from this site Akustar. Looks to be a French acoustic
treatment supplier. This site still has large holes in it.

Is it just me or are Europeans more interested in diffusion than elsewhere ?

Paul P

[collo]

Thanks for dropping by, Lupo.

We're basically applying the mod N arithmetic to the deepest well. This converts it from one that produces 360 degrees shift at the design frequency to one that produces zero degrees of shift. The situation is the same for multiples of the design frequency.

It is only one cell affected in the N7 and higher, although the rarely used N5 2D inverse has 9 cells changed.

Using the substitution brings the results into line with the products that are commercially available - although really, there are only variations on the N7 out there.

The resulting inverse panels are the same depth as the non-inverse panels, meaning that for any order, the choice between the two comes down to how they look.



There doesn't seem to be any barker style modulation used with 2D panels, so any QRDude support for non-optimised 2D inverses would be purely academic.

In the interest of keeping the calculator as simple as possible, I'm going to make a judgement call:

For 1D inverses, the subsitution will not be used. (There is plenty of choices for normal panels using the phase wheel. Inverses would normally be used for modulation, so should have the correct maths)

For 2D inverses, the substitution will allways be applied. (No barker style modulation possible - panels only to be used in arrays of the one design)

I was thinking of using a disabled menu option as a reminder:

Paul, that French site is a good find. They also have some nice lighweight plastic N7's...

[Lupo]

Looks like a good upgrade! :)

This too...: Akustar - diffuseur de Schroeder Skyfusor - (babelfish english) Awesome!

Also recently found a local company that produces transparent absorbers to be used on windows etc. It's 2010! :D

[Lonely Raven]

Originally Posted by PaulP

Is it just me or are Europeans more interested in diffusion than elsewhere ?

Paul P

I agree with this. I have friends from Europe and parts North that are heavy into diffusion. And IMHO, one of the best listening rooms I've *ever* been in, was 90% diffusion and 10% questionable absorption. It sounded awesome in that the reverb used in the recordings, seemed to surround you as the listener.

I've never had that experience in heavily damped rooms, only heavily diffused.

I think that's why I'm so gung-ho for diffusers in the back of my home theater though many tell me not to. In fact, I've seen photos of some high end cinema in Europe, and often I'll see diffusers around the surround speakers!

[Lonely Raven]

Originally Posted by Lupo

Looks like a good upgrade! :)

This too...: Akustar - diffuseur de Schroeder Skyfusor - (babelfish english) Awesome!

Also recently found a local company that produces transparent absorbers to be used on windows etc. It's 2010! :D

I just found these earlier today in fact.

Diffusors - RPG Diffusor Systems - The Clearsorber Deamp

[Lupo]

Originally Posted by PaulP

Is it just me or are Europeans more interested in diffusion than elsewhere ?

Don't forget the wizards from Oz! :)

Originally Posted by Lonely Raven

I just found these earlier today in fact.

Diffusors - RPG Diffusor Systems - The Clearsorber Deamp

Looks really good. Am contemplating some for the window in front. Originaly intended to have drapes in front of the window when working, but got addicted to the daylight so the drapes are stuck at the sides of the window most of the time. Transparent absorbers and/or the diffusers would be a very high tech and neato addition to the treatment!

[PaulP]

Originally Posted by Lupo

Quote:
Originally Posted by PaulP
Is it just me or are Europeans more interested in diffusion than elsewhere ?

Don't forget the wizards from Oz! :)

I hadn't forgotten terry j .

I was referring more to rooms that I've seen on the net. Quite a few have
wall to wall diffusors. I haven't seen many like that from North America or
Asia.

I think there must be a cultural component. I have a French book on
speaker design and one of the models has always attracted me. It's got
the tweeter firing straight up from the top of the box and there's a
parabolic mirror on a stand above it that disperses the high frequencies
all over the place.

My mother-in-law has a set of speakers, which I bought for her late husband,
Magnasphere's from Germany :

The upper midrange and tweeters are back-to-back domes, no cones.
The things radiate everywhere and sound good when not too far off a
bare wall. No treatment in her room. They sound great. While I was going
around to all the stores listening to some CDs I brought along these had
by far the best presence.

So why do Americans shun diffusion/dispersion ?

Paul P (sorry to have gone so far off topic)

[collo]

Done:

thanks for the feedback ...

[PaulP]

Originally Posted by collo

Done:

Here's the location of QRDude 2.08 for those who don't already know where
to find it : QRDude.

Thanks again for all the work you've put into this collo

Paul P

Edit : I just realized that 'Done' was a link. Subtle !

[SAC]

Originally Posted by PaulP

...
Quite a few have wall to wall diffusors. I haven't seen many like that from North America or Asia.

... I have a French book on speaker design and one of the models has always attracted me. It's got the tweeter firing straight up from the top of the box and there's a parabolic mirror on a stand above it that disperses the high frequencies all over the place.
...

So why do Americans shun diffusion/dispersion ?

Adopting a very 'smart aleck' tone (in order to provide a bit of info in a very wry humor) - because controlled dispersion and Q are consistent with what we now know about intelligibility.

Uncontrolled Q (dispersion) in a source is a PROBLEM, not a positive attribute. And the use of diffusion and absorption as a sink are in direct response to the need to control such uncontrolled 'dispersion'.

Sometimes there is a benefit to paying attention to physics and not simply to what looks cool aesthetically!

If you want 'omnidirectional' sound, I have a completely abysmal sounding dodec you can borrow. Its great for driving a room for measurement, yet horrendous to listen to. I guess I could use a Bose 901 or a dipole - if they were easier to transport...

Listening to line arrays in the near field and dipoles in small acoustic spaces are far too popular now too! Just look at the list of 'hi fi' systems costing in excess of $25K! They dominate! All at the expense of physics! And then we have the 'oh so popular' and fundamentally flawed Bose 901 direct-reflecting abomination that flies in the face of what we know now about intelligibility. And ALL are repudiated by the physics of intelligibility in a small acoustical space. Yet we continue to hear audiophools touting their amazing(sic) attributes.

I might suggest that it makes far more sense to avoid creating problems just so we can employ more tools like absorption and diffusion to remediate the problems that uncontrolled dispersion and resulting superposed reflections create.

Those not familiar with current acoustical physics are indeed doomed to repeat such problems.

Oh, and the use of the proper analytical tools can easily help one determine the optimal amount and placement of absorption and diffusion, rather than geographic location in the world. I think you will find the designs of those using such tools do not exhibit applications featuring such great differences.
And as far as it being a geographical issue, you might stop to consider the embarrasing fact that Bose, and their flawed 'direct-reflecting' system, is an American company.

And as far as the widespread use of diffusion in small acoustical spaces, I would suggest you take a look at more of the professionally designed studios, not the least of which is represented by Blackbird Studio in Nashville. In fact, I would go so far as to suggest that diffusion is a very widely employed tool in most professionally designed studios - despite many here on this site too often dismissing the usefulness of the technique in the small acoustic space.

How's that for a (good natured and admittedly wry) response?



BTW,back to the subject at hand, may I add that QRDude is a WONDERFUL tool! And unlike a fair amount of jest in the prior posting, this is a most sincere compliment! Kudos!!!

[PaulP]

Originally Posted by SAC

Uncontrolled Q (dispersion) in a source is a PROBLEM, not a positive attribute. And the use of diffusion and absorption as a sink are in direct response to the need to control such uncontrolled 'dispersion'.

I think there's more to it than that. For me there is something magical about
spaciousness that in some ways trumps accuracy. There is an airiness you
get from bouncing high frequencies off a bare wall that you just don't get
from a regular speaker in an typical room, though a room analysis would
probably show awful numbers.

I'm prepared to believe that a normal front-firing speaker in a properly treated
room is capable of spaciousness, I'm betting my upcoming room treatment on
that. But the usual sound I hear in supposedly good listening rooms is much
darker. I once listened to a Mackintosh system with line array speakers in
a probably rather dead showroom. The sound was beautiful, but it wasn't airy or
spacious. It was warm, cozy, but not spacious.

There is some similarity I believe with lifestyle differences between, for
example, Europe and North America. European cars are light and nimble,
American cars are heavy and tough. European speakers are small, American
ones are huge. Even European pop songs are sung two or three tones
above those from North America. Ok, I'm generalizing, but I find it
interesting to think about.

And I'm only talking playback here. I realize that discussions in this forum
are mostly about control room type rooms, and recording studios, for
which accuracy is paramount. I'm hoping to build a room that is both
for recording and listening, which is both accurate and spacious. SAC
gives me hope that this is indeed possible.

Paul P

[SAC]

We should take this topic elsewhere. (Edit...I do not mean the original thread subject! I mean our tangential 'digression'!!;-)

But 'spaciousness' is not created by uncontrolled dispersion! It is created in a small acoustical space (per Schroeder) by the 'creation' of a well behaved semi-reverberant sound field. In bounded space, uncontrolled dispersion is not the same thing as diffusion!

Simply creating a wealth of early and late arriving specular reflections throughout the room resulting in beau coup comb filtering and spatial polar anomalies does not 'spaciousness' make! And it most certainly destroys intelligibility. Otherwise the Bose direct - reflecting model would actually be worthy of respect.

The irony is that this issue is fundamental to understanding the behavior of sound and intelligibility in a small acoustical space. (...in other words, "acoustics". ) And such an approach has been 'eliminated' for over 35 years - ever since we became able to identify exactly what was happening acoustically within a small bounded space. This concept has become fundamental in the study of modern models of acoustics.

Again, start reading in Davis & Patronis Sound System Information, D'Antonio & Cox Acoustic Absorbers and Diffusers. Theory, Design and Application, Philip Newell Recording Studio Design, Everest Master handbook...or any number of current texts regarding the behavior of acoustics in small bounded spaces. At some point we need to acknowledge a baseline of acoustic knowledge and not simply be doomed to reinvent the 'acoustical' wheel - as fun as remaining 'a Flatlander' may be...

As far a geographic distinctions, acoustics does not vary with geography. And it is ironic that much of what I refer has been developed by European researchers as well; including, but not limited to, notable folks such as Peter Mapp, Dr. Manfred Schroeder, Dr. Wolfgang Ahnert, Ivo Mateljan, and many others. America has its own embarrassment in having to live down the 'direct-reflecting' unintellibility technology.

[collo]

Originally Posted by SAC

We should take this topic elsewhere....

The original purpose of the thread has been fullfilled, so feel free to take it in other directions. BTW, thanks for all the nice comments...