Wood acoustic diffusers have become a decorative item - loved the idea!

[sarumbear]

Lightweight skyline diffusers made of expanded polystyrene than can be suspended using hook and loop (Velcro)....

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They created the diffuser type Skyline which is now a standard term. They have been around since the turn of the century. (Notice the old style website?) They offer a vast range of acoustic treatment products.

 

[kemmler3D]

Well, OK then, based on what the physics profs have taught me about wave propagation, the angle of reflection is equal to the angle of incidence,

I wanted to go back and comment on this because this is what we all learn in school. It holds true for elastic collisions / particles and when the wavelength is very small compared to the object it's colliding with. However, with sound, the wavelengths are usually large compared to the objects we're interested in, sometimes larger THAN. Even in optics you don't get nice clean reflections when the size of objects is similar to the wavelength of light.

This is more like a wave in the ocean hitting a series of docks and piers than a billiard ball hitting a wall.

 

[MAB]

physics profs have taught me about wave propagation, the angle of reflection is equal to the angle of incidence

This is correct and is true when the wavelength is much smaller than the characteristic size of the surface of reflection. But the elements are small (1.7cm cross-section as one example, the same size as 20kHz wavelength in air.) I imagine this is why this board size was chosen, to minimize direct reflection in the audible range from the front size of of the facets. By the time you get to midrange frequencies, the shape of the ends of these individual facets will have no net impact on the reflected sound. The pseudo-randomized surface will present pseudo-randomized reflections with no perceptible change due to the shape of the faces. Following that thought, bats would care about the shapes of ends of the facets!

 

[kach22i]

I like the design Joe Pittman came up with, he posted a PDF in the second post of this thread:

The Search For A Simple Solution With Paper Thin Walls - Audionirvana.org

The Search For A Simple Solution With Paper Thin Walls Having lived in a plaster and wood lath walled house for the past 27 years that was built in the early 1900's I forgot how terrible typical stud walls with drywall are - acoustically speaking that is. Our new house (built in 1947) had the...

www.audionirvana.org

Simple and clean, and if you need more you can still address it as he did at the first reflection points.

 

[kemmler3D]

I like the design Joe Pittman came up with, he posted a PDF in the second post of this thread:

The Search For A Simple Solution With Paper Thin Walls - Audionirvana.org

The Search For A Simple Solution With Paper Thin Walls Having lived in a plaster and wood lath walled house for the past 27 years that was built in the early 1900's I forgot how terrible typical stud walls with drywall are - acoustically speaking that is. Our new house (built in 1947) had the...

www.audionirvana.org

Simple and clean, and if you need more you can still address it as he did at the first reflection points.

It's not clear from this PDF what this does other than decouple the room from the rest of the house... maybe there's some absorption due to damping?

Anyway, the diffuser looks pretty casual in there. Probably works but only for HF, which I guess is what he wanted.

 

[posvibes]

Aside from hi-fi purposes are acoustic diffuser panels useful or advantageous for any other domestic room purposes?

 

[kemmler3D]

Aside from hi-fi purposes are acoustic diffuser panels useful or advantageous for any other domestic room purposes?

If you're doing anything involving music (recording, playing) they could help. Also, if for whatever reason you have a lot of people in one room talking, it might help with speech intelligibility, but that's a stretch.

Basically, they might help any time you might consider adding other acoustic treatments to a space. That's usually only in a hifi or recording/playing scenario IME.

 

[Doodski]

If you're doing anything involving music (recording, playing) they could help. Also, if for whatever reason you have a lot of people in one room talking, it might help with speech intelligibility, but that's a stretch.

In areas like a indoor pool the noise levels can be pretty intense. I've seen stuff used in those environments.

 

[MAB]

Well, OK then, based on what the physics profs have taught me about wave propagation, the angle of reflection is equal to the angle of incidence, wrt a normal line perpendicular to the reflective surface. If the object is to break up the wavefront into smaller bits and send them off in different directions in order to reduce their perceivability (did I just make up a word?) at the listener’s ear, then perhaps what I wrote previously would hold. Or perhaps not. I hear you saying not necessarily,, read the papers, so I suppose the answer is in there.

I was just looing at the calculator that @sarumbear provided, and realized the diffraction effects I mentioned earlier are built into the calculator. I put in 20kHz as the upper frequency for the panel and the calculator returned 0.86cm square elements (exactly half the wavelength of 20kHz sound in air at Standard Temperature and Pressure!!!)

So above 20kHz this panel will start beaming. Faceting would help, or just making the columns smaller cross section to get to successively higher frequencies. (Or just having my hearing!) Also, the lower frequency determine the height of the column elements, 2kHz give 8.6cm long columns. Very nice! Hope this helps.

 

[kach22i]

It's not clear from this PDF what this does other than decouple the room from the rest of the house... maybe there's some absorption due to damping?

Anyway, the diffuser looks pretty casual in there. Probably works but only for HF, which I guess is what he wanted.

Imagine you have a concrete room, no bass can escape, it just bounces around.

Then imagine you have no walls, sound all escapes, no reinforcement at all.

Then imagine you have something in the middle as most of us do, those walls be they drywall or what ever move and flex, absorbing some of the sound energy and converting it to mechanical energy ie heat.

Now what if those walls flexed so much they did more than bounce and or muddy the sound? What if your walls could take deep bass out like a tuned membrane absorber does? That's more or less as I understand what the thin plywood supported only at the top and bottom does, flexes more than a normal wall and dampens the energy mechanically. In short the walls become a giant membrane absorber, just not a curved one nor one with a very deep space behind it.

Related? I recall being in Europe 30 years ago and some of the glazing systems on the buildings of busier streets had an extra pane of glass offset from the actual window via mounting posts near the four corners of the opening, with an air gap all the way around. I surmised that the outermost plane of glass might reflect higher frequency sounds (car horns) as well as attenuate or absorb some sound by vibrating at a lower frequency like a membrane does. More than one way to skin a cat.

 

[Chrispy]

TL/DR but what made you stop loving them?

 

[AudioJester]

Unfoftunately I have glass windows behind my speakers in one room.
I am waiting for decorative diffuser curtains!

 

[anotherhobby]

Aside from hi-fi purposes are acoustic diffuser panels useful or advantageous for any other domestic room purposes?

If you work from home and do zoom meetings, having an acoustically treated office is amazing. My office is small at only 10'x10', but it's my favorite place to listen to music now that it's treated. After my wife took a week of zoom meetings in my office while I was out of town, she immediately requested I do the same to her office, and she doesn't have a stereo.

I started with only absorption and made panels using 2" OC703, FR701 fabric, and 3/4" MDF for the frames. I put some behind my desk and some behind myself and immediately discovered that I really did not like an absorption behind me. I still had a nasty reflection off the back wall so I ordered a box of 6 total GIK Acoustics Diffusor/Absorbers in 23"x23" size ($490 shipped). Once I got them I played around with placement some more and ended up putting 4 of them on my back wall.

The GIK panels completely changed the room. These are the first diffusors I've had and I'm incredibly impressed by how dramatic the effect is. I hear people frequently say the most dramatic improvements you can make are speakers and DSP. I'd love to see everybody include room treatments in that elite list of things that really make a very noticeable real world improvement in audio quality. I get that not all rooms can be treated like my office is below due to practicality and aesthetics, but most rooms can have some treatments done. You'll get more tangible benefit from room treatments than from an amp or DAC upgrade.

 

[kemmler3D]

Imagine you have a concrete room, no bass can escape, it just bounces around.

Then imagine you have no walls, sound all escapes, no reinforcement at all.

Then imagine you have something in the middle as most of us do, those walls be they drywall or what ever move and flex, absorbing some of the sound energy and converting it to mechanical energy ie heat.

Now what if those walls flexed so much they did more than bounce and or muddy the sound? What if your walls could take deep bass out like a tuned membrane absorber does? That's more or less as I understand what the thin plywood supported only at the top and bottom does, flexes more than a normal wall and dampens the energy mechanically. In short the walls become a giant membrane absorber, just not a curved one nor one with a very deep space behind it.

Related? I recall being in Europe 30 years ago and some of the glazing systems on the buildings of busier streets had an extra pane of glass offset from the actual window via mounting posts near the four corners of the opening, with an air gap all the way around. I surmised that the outermost plane of glass might reflect higher frequency sounds (car horns) as well as attenuate or absorb some sound by vibrating at a lower frequency like a membrane does. More than one way to skin a cat.

Got it, a membrane-style absorber. I didn't catch the thin plywood bit. Thanks!

 

[sam_adams]

As @sarumbear mentioned, 2D quadratic diffusers (skyline diffusers) can be made from any material as long as it is rigid. Most are made of wood like pine (heavy) but could be made from balsa wood (lighter) or from polystyrene foam (light). For the DIY crowd, the problem is time plus the cost of materials and having the necessary tools and skills to construct one.

For those who want a quick overview on how QDs work, some theory of operation is covered here. A calculator is also available to experiment with.

The often referenced authority on diffusors of all types is "Acoustic Absorbers and Diffusers - Theory, Design, and Application" by Trevor J. Cox—although it is nearly twenty years old. Chapter 11 of the Cox text covers Hybrid Diffusers. These combine a binary amplitude grating based on a Maximum Length Sequence and an absorbing medium. They have many advantages over quadratic diffusers and can be constructed from lightweight materials. The only DIY issue is drilling hundreds of holes in the grating medium which would take a lot of time.

Another easy to construct MLS-based diffuser-absorbers (diffsorber) uses vertical slats/slots of absorber and reflector:

The operation here is loosely based on the Huygens-Fresnel principle. The image above is an 'extreme' example of a floor-to-ceiling implementation. Panels could be constructed of any size and practically any aesthetic as long as the slats/slots are vertically oriented.

 

[anotherhobby]

Another DIY option for a diffusor is a BAD panel (Binary Amplitude Diffsor), but you need to really love drilling holes or have a CNC handy. I happen to have 2 BAD templates on hand, so I attached them below. Both are 2'x2' and are just inverse of each other. I never worked up the ambition required to make them.

 

[AlfaNovember]

@sarumbear, I wonder if there is any guidance for the placement & coverage of diffusion elements within a room? The various textbooks I've skimmed seemed to focus on auditoriums and studios, rather than domestic residential spaces.

Intuitively, the wall behind the listener would seem to be the first location to address; Toole says "Add absorption or diffusion to center portion of rear wall" (p. 120) but... how much? Do I add diffusion behind me, and experiment until it sounds "right", in the manner that @anotherhobby has demonstrated?

 

[tuga]

I couldn't resist to post this example.

View attachment 241984

Unfortunately it looks like a hi-fi store demo room, not like a sitting room for normal people.

 

[abdo123]

All the diffusion I have seen so far are abysmally thin. little disappointing.

I'm thinking of building one out of Lego in our living room, my partner is a Lego geek and the 1 cm wide blocks allow for diffusion up to 20KHz, I hear up to 16KHz and he up to 14KHz so I might just save myself the time and do 2 cm wide blocks (up to 10KHz Diffusion).

 

[abdo123]

I was just looing at the calculator that @sarumbear provided, and realized the diffraction effects I mentioned earlier are built into the calculator. I put in 20kHz as the upper frequency for the panel and the calculator returned 0.86cm square elements (exactly half the wavelength of 20kHz sound in air at Standard Temperature and Pressure!!!)
View attachment 242082
So above 20kHz this panel will start beaming. Faceting would help, or just making the columns smaller cross section to get to successively higher frequencies. (Or just having my hearing!) Also, the lower frequency determine the height of the column elements, 2kHz give 8.6cm long columns. Very nice! Hope this helps.

that's exactly the width of a 1x1 Lego brick.