my new diffusor, thanks to gs

[jrp]

Hi everyone,
My fist diffusor is finished (almost). It is a 125cm*80cm*20cm qrd, sequence is 23. Well width is 29mm
I like to thank all the people on this forum for their help on understanding and designing this diffusor!
I went through a lot of work cutting the old wooden roof from my parents house into 29mm stripes for this one and two more to follow. One day on the table saw.
The build uses no solid backplate, well bottoms are sitting on 4 wooden blocks each.
Another question, how should i fill this? I am thinking of stuffing it with mineral wool. Should i seal the little gaps between the wells first? Someone told me it should be airtight. Somehow i can´t belive that this would make any difference so i would be interested if someone could clear this up!

[localhost127]

wonderful work! might i inquire as to the type of wood used - especially for the well dividers (looks like 1/8"?). also, what is the ~ weight? thank you!

[Jens Eklund]

Looks great!

Seal the gaps or you’ll end up with a very nice looking absorber. Fill the cavities with something that prevents the well lids to resonate.

[Glenn Kuras]

Originally Posted by Jens Eklund

Looks great!

Seal the gaps or you’ll end up with a very nice looking absorber. Fill the cavities with something that prevents the well lids to resonate.

+1

JRP if you think about what happens when the sound goes into the well you can then see why having gaps is not a good thing.

[jrp]

Thanks!
The Fins are 3mm poplar (or cottonwood?) plywood. Well bottoms is 12mm Hemlock.
Those are quite different materials, but with the dark mahonia scumble it turned out quite even. Should be around 25-30kg.
not sure if i do see though... not sure if i can imagine what happens when sound travels into the wells.
If it has mineral wool in the back then i could imagine that some sound will travel through the gaps and be absorbed. And yes, it would be a little less stable maybe.
So sealing the little gaps is not about making it air tight like when you build a resonator (which would make no sense to me right now since its more of a reflector) but about dampening the whole thing to reduce resonance.
Is spraying a little bit of expanding foam into the backs of the wells an option? That would be a lot easier than using silicone.

[boggy]

Beautiful work.
I think you don't need to seal gaps because air circulating, for a small room it's can't be too much absorption in your situation, from my experience, but if your new diffuser vibrate and make noise because this, then seal the gaps. Anyway, put heavy wideband absorber behind your new difusor and that will work.

[Jens Eklund]

Originally Posted by boggy

Beautiful work.
I think you don't need to seal gaps because air circulating, for a small room it's can't be too much absorption in your situation, from my experience, but if your new diffuser vibrate and make noise because this, then seal the gaps. Anyway, put heavy wideband absorber behind your new difusor and that will work.

Why take the risk? The absorption range is likely to be in the midrange and not the lows where it might be useful. I would definitely seal the gaps! The effect of gaps as a result of poor construction was the reason high absorption coefficients was discovered in the early days of the diffuser. It should be avoided unless you want to build a very complicated and expensive absorber.

[jrp]

It doesn´t feel like anything is resonating, at least not that i could hear. When i installed the diffusor i did some measurements and there was no big reaction in frequency responce. The reflections look a lot nicer now.
But sure, why take the risk.
If it wasn´t so beautiful as it is i would be tempted to seal it and start drilling holes to make it into a diffusor/helmholz resonator. Seems like an ideal combination to me. I am not going to try that, would be very sad if i somehow ruined it.
So how should i seal it? Once i start using construction foam there will be no going back... Just imagine it to be hard to get silicone all the way down those wells...

[Jens Eklund]

You seal the gaps to prevent Helmholtz resonance. You fill the wells to prevent membrane resonance. I have personally never built a QRD in wood (way to much work I say ) so I cannot give any specific advice.

If you need low frequency absorption, you might be able to use the entire diffuser as a part on a membrane absorber:
building membrane absobers

[boggy]

Originally Posted by Jens Eklund

Why take the risk?

Which risk!?

Originally Posted by Jens Eklund

The absorption range is likely to be in the midrange and not the lows where it might be useful. I would definitely seal the gaps! The effect of gaps as a result of poor construction was the reason high absorption coefficients was discovered in the early days of the diffuser. It should be avoided unless you want to build a very complicated and expensive absorber.

Sorry, some absorption in midrange can't cause a "plane crash" in small room acoustics, at least, this midrange sound wave energy can't be easily diffused (if we really need this) with constructed diffuser, because it's too shallow, then we will have only midrange (plain, ordinary, non diffused) reflections and, why we need it so badly, in the (small) control room?.
This construction will still work well like any other good QRD diffuser for frequencies for which it's designed, in a small room, and it's hardly became only a "very complicated and expensive absorber" like you said.

[Jens Eklund]

Originally Posted by boggy

Which risk!?

Sorry, some absorption in midrange can't cause a "plane crash" in small room acoustics, at least, this midrange sound wave energy can't be easily diffused (if we really need this) with constructed diffuser, because it's too shallow, then we will have only midrange (plain, ordinary, non diffused) reflections and, why we need it so badly, in the (small) control room?.
This construction will still work well like any other good QRD diffuser for frequencies for which it's designed, in a small room, and it's hardly became only a "very complicated and expensive absorber" like you said.

Boggy; the OP has chosen to make a N23 QRD with a well width of 29mm … I say it’s quite safe to assume that he wants diffusion (and not absorption) in the midrange, right?

Also, a fined QRD as deep with such narrow wells would even if sealed and sturdy cause quite a lot of absorption due to diffraction so why add to this if it can be avoided?

[jrp]

Diffusor as part of a plate resonator? you mean the diffusor being the plate?
I was thinking of controlled helmholz resonation. I have a problem with 200 and 300 HZ in my control room i´m setting up right now. Unfortunately at the position the Diffusor is supposed to be (rear wall) there is a preassure minimum for those frequencies.

The diffusor is absorbing midrange? that´s interesting.
The Reason i chose these dimensions was to get as broad band as i can and keep it practical to build. I wasn´t designing to aim at anything specific. It´s intended for a 30sqm live room.
Although there is some strange stuff going on in my room in the 7-10k region. At 7.6k if i turn my head i can make the sound disappear.
QRDude says something about 650-6800 hz for this diffusor if i remember right.
I am building a second one 47 QRD right now for my control room. I´ll be sitting about 4.50Meters away when mixing. Same building blocks as the first one (29mm hemlock) but about 150cm wide and 27cm deep.

[Jens Eklund]

Originally Posted by jrp

Diffusor as part of a plate resonator? you mean the diffusor being the plate?

Yes, if not too heavy. The cavities behind each well is probably to small compared to gap area to be able to produce a resonant frequency (Helmholtz) in the bass region. It usually ends up in the midrange (where you normally don't want a diffuser to absorb but to scatter the energy.

[jrp]

well, with 25-30kg i think it´s a bit to heavy. Nice idea though. I was thinking of using a big picture i got printed on a 1cm board (don´t know what material right now) in a resonator design... Would be awsome. Keep the picture hanging and attack some modes at the same time... maybe someday.

[Chris_Nicks]

wow.. love your n23! im planning on making an n13.
guys, what do you suggest stuffing the diffusers with? styrofoam or rockwool? :D

[Sebg]

not styrofoam.

[Jens Eklund]

Just rediscovered this thread.

Originally Posted by Sebg

not styrofoam.

Why not Styrofoam? If glued to the well lids, it will effectively prevent vibration (since the material is stiff and non resonant) and thus energy losses (absorption) due to the membrane effect. If wool, the opposite (or at least lower Q and perhaps a slightly lower frequency of resonance compared to empty wells) thus absorption is likely to occur, and again; the frequency range of absorption this might cause is way too high to be positive.