Diffusing 1st reflections of speakers that measure great on and off-axis - instead of absorbing

[Duke]

Open Baffle speakers attempt to reflect, with a minimum 6 ms delay, a full spectral replica of the primary wave...as per Linkwitz, this delay isn’t perceived as distortion only if it is at least 6 ms and full spectrum.

I totally agree with Linkwitz. Well perhaps not "totally"... Geddes suggests 10 milliseconds, and imo there is worthwhile improvement to be had in going from 6 milliseconds to 10 milliseconds. More is arguably even better, but becomes progressively harder to do in our limited-size home listening rooms.

 

[Plcamp]

Well I guess it is a bit more than 6, because I find it best at 1.25m min.

I intend to remake my OB’s with magnet mounted drivers and an independently mounted baffle that has no flat surface on it. It will also be progressively absorptive.

I guess I am a disciple of stop the early reflections any way you can. It’s as important as both driver timing and dispersion control.

And say all this not having yet achieved it.

 

[Duke]

I guess I am a disciple of stop the early reflections any way you can. It’s as important as both driver timing and dispersion control.

Banish the early, but cherish the late! Which implies, not absorbing the early reflections (as you said, "timing and dispersion control"), because we want the energy which would have been in the early reflections to survive long enough to show up as late reflections.

 

[j_j]

If I understand correctly, Toole finds that the indirect timbre should have less top-end energy than the direct, (conveniently) by a margin which corresponds with typical radiation pattern narrowing due to driver beaming. My own amateurish investigations have led me to a different conclusion: That the timbral discrepancy between the two should be minimal (even less discrepancy than Toole's findings indicate), and that BOTH should be gently downward-sloping. You are obviously vastly more experienced in real-world acoustics than I am, so I'd welcome your comments and corrections.

Nature usually has a rolling-off at high frequencies from a real source, because HF beams, of course, some things like strings don't actually have this characteristic, so there's always a question of "what's natural for the source". That's part of the issue.

Then the speaker, if you're used to an acoustic, you've learned to compensate for it, but for the boxes to have too much direct information with no indirect will suck you right "splat" into the loudspeakers.

May I ask you this: When you speak of comparing the direct vs indirect timbre, HOW exactly do you do so?

If you're listening, that's a natural part of learning to listen, between the hearing compression, precedence effect, auditory history, and the like. There are a lot of issues to consider.

If you mean analytically, you can measure the short-term spectrum of a room impulse response. It's more difficult to relate that to perception that one would like.

And, in fact, the diffuse vs. direct vs. "reverberant" is where a great deal of argument comes about, because the ear does not work like a meter, and there can be diffuse sounds that are not linearly independent as reverberant is supposed to be. (More on that in a discussion of analysis windows, etc.)

Could you go into some specifics about what intereferes with hearing "through" the listening room? THAT's the experience I'm aiming for, and any insights you can share would be greatly appreciated.

Biggest thing is to have the very first reflection from a room at least a few milliseconds later than the direct, and not even remotely specular. I've avoided that discussion because of the chaos upstream, but no, I am not a fan of specular reflections in a listening room, at all.

And I'm not a supporter of LEDE. That's both a question of physics and perception. That's also where an absolutist separation of "small" vs. "large" falls flat on its face.

 

[Plcamp]

Banish the early, but cherish the late! Which implies, not absorbing the early reflections (as you said, "timing and dispersion control"), because we want the energy which would have been in the early reflections to survive long enough to show up as late reflections.

I am referring to reflections at the speaker itself. Unrefined baffle edges, in OB case the frame elements behind reflect. I got good improvement just by covering a baffle/ frame members with two layers felt (reduced diffraction above 2 kHz from edges.) I am now awaiting a waveguide for a new tweeter, want that to further reduce potential for sub 1 ms reflections

I think it implies curved baffles with absorptive treatment in and around driver mounts, plus the separate mount of drivers from baffles, get rid of frame concept...that sort of thing.

ultimately I want a full spectrum rear wave that’s bounced off a fractal dispersion panel located 4’ behind a speaker that doesn’t itself produce early reflection. With minimized early reflections, full spectrum delayed reflections and the zero loading of the room 90 degrees off axis it should be quite good when I am completed.

or so I susoect

 

[Bjorn]

And I'm not a supporter of LEDE. That's both a question of physics and perception. That's also where an absolutist separation of "small" vs. "large" falls flat on its face.

Can you describe the LEDE design, what it actually consist of, and why you don't like it?

 

[localhost128]

Banish the early, but cherish the late! Which implies, not absorbing the early reflections (as you said, "timing and dispersion control"), because we want the energy which would have been in the early reflections to survive long enough to show up as late reflections.

splayed walls (or large (wrt wavelength) planar reflector "treatments" at the sidewalls angled appropriately) under D'Antonio's RFZ (LEDE) design work well for these requirements as it attenuates the high-gain early reflections that are destructive to localization, imaging, and intelligibility while maintaining that specular energy within the room (that would otherwise be absorbed). the redirected specular reflections take a longer flight path thus longer delay in time and help to "redrive" the rear-wall broadband reflection phase grating diffusers to terminate the ISD-gap with a dense, reflection-rich, laterally-arriving exponentially decaying diffuse sound-field (emulating that of large room concert halls) for passive envelopment, & spaciousness.

 

[Duke]

Thank you very much j_j for writing back.

Biggest thing is to have the very first reflection from a room at least a few milliseconds later than the direct, and not even remotely specular. I've avoided that discussion because of the chaos upstream, but no, I am not a fan of specular reflections in a listening room, at all.

And I'm not a supporter of LEDE.

So we want a few milliseconds delay before the first reflections come in, with none of the reflection being remotely specular, but we don't want to resort to something like Live-end Dead-end. This all makes sense to me.

Since you are "not a fan of specular reflections in a listening room, at all", I presume you use a lot of diffusion, whether it "looks like" a room treatment product or not... ??

 

[localhost128]

the challenging aspect of utilizing diffusion at first reflection points is the sizing and spacing requirements to meet broadband criteria such that the reflection is not "colored" or "low pass filtered" as akin to the erroneous and commonly-seen use of thin porous absorbers (that color/eq/spectrally-alter the reflection). the treatment (be it absorption, diffusion, redirection) needs to be broadband in nature over the entire specular reflection's bandwidth.

for reflection phase grating type diffusers (Schroeder QRD/PRD), adhering to minimum seating distance limitations of 3x design frequency wavelength (such that one is in the "far-field" of the diffused returns/generated diffraction lobes - vs the chaotic and distorted near-field + hearing resonances from individual wells), this would require ~12ft distance for a 300hz design frequency (with scattering down to 150hz). ~7ft at 500hz design freq. this can be extremely challenging in home residential-size reproduction rooms.

binary amplitude diffusers alleviate this somewhat (relaxed minimum seating distance requirements) by absorbing LF and spatially diffusing mid-HF content, but temporal dispersion is not provided and that is really key to achieving proper "diffusion" (and thus emulating the physical sound-field that develops naturally in Large Acoustical Spaces).

 

[Bjorn]

Yup. While a binary product like RPGs BAD Arc diffuses primarily above 800 Hz and thus doesn't require a distance of more than 1.3 m (and works well with less too) and remains more energy compared to an absorber, it doesn't diffuse like pure diffuser. In reality it's closer to an absorber than to a diffuser with hard surface.

 

[Bjorn]

Oh, as a sidenote. The old Skyline diffuser that we see in Harman's room and was mentioned in an article someone posted here earlier, doesn't diffuse very much. The design causes a lot of diffraction and hence absorption. It's really an outdated product as a diffuser.

 

[localhost128]

Yup. While a binary product like RPGs BAD Arc diffuses primarily above 800 Hz and thus doesn't require a distance of more than 1.3 m (and works well with less too) and remains more energy compared to an absorber, it doesn't diffuse like pure diffuser. In reality it's closer to an absorber than to a diffuser with hard surface.

concur. with BADs, you're just getting a single "splash" of spatially dispersed reflections (albeit low enough in gain as to not be triggered by the ear-brain for localization and imaging) since it is a flat/planar reflective panel - but RPGs provide spaced/delayed reflections in time and thus are simply a better mechanism for emulating the sound-field physical characteristics that develop naturally in Large Acoustical Spaces. ie, "better diffusion"; more dense/reflection-rich sound-field, and "random".

Oh, as a sidenote. The old Skyline diffuser that we see in Harman's room and was mentioned in an article someone posted here earlier, doesn't diffuse very much. The design causes a lot of diffraction and hence absorption. It's really an outdated product as a diffuser.

i have seen photos of the Harman test rooms with 2d skyline-type diffusers deployed about, but i have never seen an explanation detailing why those specific devices were chosen, their placement, and measurements (time-domain analysis) providing objective measurements and analysis of the test room. surely the real-world behavior of the acoustical devices deployed into the room is relevant when understanding the context of the different loudspeaker preference tests?

it's akin to how these discussions get muddied as many assume "diffusion is diffusion" and use the term globally. ie, "i tried diffusion at my sidewalls and did not prefer it" ... well there are a myriad of different types of diffusers, their bandwidth/beheaviors, vastly differ - etc - so it is critical to understand the context of the acoustical device, its performance, and taking that into consideration when one states they "liked" or "did not like" diffusion.

 

[Duke]

Localhost128 and Bjorn, thank you for going into detail about diffusion types. I'm guilty of using the term generically, so I have some homework to do!!

 

[j_j]

Thank you very much j_j for writing back.



So we want a few milliseconds delay before the first reflections come in, with none of the reflection being remotely specular, but we don't want to resort to something like Live-end Dead-end. This all makes sense to me.

Since you are "not a fan of specular reflections in a listening room, at all", I presume you use a lot of diffusion, whether it "looks like" a room treatment product or not... ??

Dead is my preference.

 

[youngho]

concur. with BADs, you're just getting a single "splash" of spatially dispersed reflections (albeit low enough in gain as to not be triggered by the ear-brain for localization and imaging) since it is a flat/planar reflective panel - but RPGs provide spaced/delayed reflections in time and thus are simply a better mechanism for emulating the sound-field physical characteristics that develop naturally in Large Acoustical Spaces. ie, "better diffusion"; more dense/reflection-rich sound-field, and "random".

I think this is why RPG seemed to prefer the term "diffusor" to indicate products that diffuse in both space and phase, as well as why they have often included information on scattering versus diffusion (as well as absorption) for their products.

i have seen photos of the Harman test rooms with 2d skyline-type diffusers deployed about, but i have never seen an explanation detailing why those specific devices were chosen, their placement, and measurements (time-domain analysis) providing objective measurements and analysis of the test room. surely the real-world behavior of the acoustical devices deployed into the room is relevant when understanding the context of the different loudspeaker preference tests?

The placement of some of the products shown here (https://docs.google.com/presentatio...tsaowbpiFvjs_epoBvaTv28o7w/edit#slide=id.i123) are in line with recommendations in Toole's book, though I have a feeling that most were chosen for their adjustability. Have you looked here? http://www.aes.org/e-lib/browse.cfm?elib=14873

it's akin to how these discussions get muddied as many assume "diffusion is diffusion" and use the term globally. ie, "i tried diffusion at my sidewalls and did not prefer it" ... well there are a myriad of different types of diffusers, their bandwidth/beheaviors, vastly differ - etc - so it is critical to understand the context of the acoustical device, its performance, and taking that into consideration when one states they "liked" or "did not like" diffusion.

Or like "wider dispersion is more pleasing" or "down-sloping off-axis is preferred"? Similarly, I think it's critical to understand the context of the individual listener's preferences, their musical context, and taking that into consideration when one states they "like" or "did not like" specific attributes like clarity, timbre, or reverbation/broadness/loudness.

Young-Ho

 

[Duke]

Dead is my preference.

Thank you for clarifying. Must admit dead is not mine.

 

[j_j]

Thank you for clarifying. Must admit dead is not mine.

To clarify, I mean the listening room. The recorded acoustic is something entirely different and should vary (real or simulated).

 

[Duke]

To clarify, I mean the listening room. The recorded acoustic is something entirely different and should vary (real or simulated).

Thanks for clarifying; I should have been more clear too. I figured you meant the playback room.

 

[Bjorn]

concur. with BADs, you're just getting a single "splash" of spatially dispersed reflections (albeit low enough in gain as to not be triggered by the ear-brain for localization and imaging) since it is a flat/planar reflective panel - but RPGs provide spaced/delayed reflections in time and thus are simply a better mechanism for emulating the sound-field physical characteristics that develop naturally in Large Acoustical Spaces. ie, "better diffusion"; more dense/reflection-rich sound-field, and "random".

The curved BAD Arc increases diffusion compared to the flat version because the holes are not in phase with each other. BAD Arc is a much better product compared to a flat BAD. People describe still hearing some specular reflections from flat BADs, but isn't able to do so with the Arc.

BAD Arc is a great product for using on rear wall when you don't have the distance to broadband diffusers, you prefer preserving energy from side walls compared to only absorption, and especially great for home theaters when you already have the cues from the mix and lateral exponential diffuse decay isn't important.

i have seen photos of the Harman test rooms with 2d skyline-type diffusers deployed about, but i have never seen an explanation detailing why those specific devices were chosen, their placement, and measurements (time-domain analysis) providing objective measurements and analysis of the test room. surely the real-world behavior of the acoustical devices deployed into the room is relevant when understanding the context of the different loudspeaker preference tests?

it's akin to how these discussions get muddied as many assume "diffusion is diffusion" and use the term globally. ie, "i tried diffusion at my sidewalls and did not prefer it" ... well there are a myriad of different types of diffusers, their bandwidth/beheaviors, vastly differ - etc - so it is critical to understand the context of the acoustical device, its performance, and taking that into consideration when one states they "liked" or "did not like" diffusion.

Harman uses Skylines several places as you see below.

When testing (blind) speakers they remove the side wall treatment so they experience the full effect of the horizontal off-axis response of the speakers.

Harman's white paper of the room reveals a lack of understanding smalll room acoustics and much is based on RTxx measurements. There are no relevant measurements. And as you see they use polys, which we know don't really diffuse the sound but focuses certain frequencies at certain angles. And the absorptive panels they use are thin (2"/5 cm probably) and offer now broadband treatment. The general treatment is also spread out a lot without any clear goal of treating specular reflections. The rear wall is quite dead and I would say opposite of preferred psychoacoustics when you have this distance.

Generally the room is poorly treated without much understanding of small room psychoacoustics IMO. Unless of course it's been changed in later years, something I'm unaware of.

Has this effected their blind tests of speakers? Hard to say for sure but especially with the spread out of bandlimited treatment, no proper lateral diffusion and fairly dead rear wall I would assume speakers with wide dispersion would be preferably over speakers with lower beam width.

That being said, treating a room for several seats is more challenging as it will make it less optimal for one seat as you end needing to absorb more surfaces and hence make the room more dead.

 

[mitchco]

Having been involved in a couple of LEDE builds with Chip Davis a long time ago, he explained it to me as a tool to be used by recording engineers to hear any comb filtering in the studio before the control room acoustics had its way with the sound. As one of those recording engineers, I can remember it worked very well. The "translation" was amazing, which is what is all about when recording, mixing and mastering on the music production side.

His whitepaper and subsequent certification of LEDE rooms is an interesting read and much more than just room treatments. While not my cup of tea for a regular listening room, (I prefer more live than dead, and not split between front and back), I did appreciate it for the tool it was. Amazing what one could hear from the recording studio that sounded like being right in the same room when listening through the monitors in the LEDE room. It was uncanny.