Constructing John's DIY Wall Units

[oedipus_complex]

Hi

I've been checking out the DIY wall units that are linked on the homepage of this site. They look just like what I need, and I plan on getting started building them. However, while some of you more experienced guys may think these designs are self explanatory and completely adequate, I have no idea where to start. There are so many questions I have, here are some of the top of my head:

What insulation do you use?
What are the dimensions of the insulation?
How is the insulation attached?
How are all the MDF segments joined to each other? Glue/screws?
Do you need to gap fill the MDF at the joins? If so, what with?
Do you put a top/bottom on these absorbers? Is it just more MDF?
How is the cloth attached? What type of cloth do you use?

Also, what on earth do the slats do? The diagrams indicates they are just thin bits of wood attached to the front, seems strange (keep in mind I know very little about acoustics).

If anyone knows links to the answers to the questions I have, or if they feel like answering them here, I would be most gracious

Thanks again
Anthony

[John Sayers]

Anthony - check out this studio at the construction site

http://johnlsayers.com/Studio/Mainpage/MP-laptoppop.htm

and read up on absorbers here

http://www.saecollege.de/reference_material/index.html

cheers
john

[oedipus_complex]

Thanks John, now I understand the slats.

Some questions I still have are:

Do you nail or glue the wood together?
Are the slats on the box made of MDF also?
How much insulation should go behind the slats? As much as you can fit? Do you glue it in, or is it just free standing?
Does it matter if there is a gap in between your resonators and the ceiling? How much is too much?

Thanks allot

[knightfly]

Do you nail or glue the wood together?

Both. The tighter the better, so glue and screw wherever possible.

Are the slats on the box made of MDF also?

Whatever you want/have/can afford, as long as they're not lightweight or too thin - 1/2" would be minimum, thicker is better. This needs to be factored into the calculator (in this section; watch out for others on the net, nearly ALL are WRONG)

How much insulation should go behind the slats? As much as you can fit? Do you glue it in, or is it just free standing?

Typically half the total depth should be insulation, and that should be the REAR half. (EDIT - insulation should go on the FRONT half in a slat resonator, I confused this with PANEL TRAP construction; sorry - Steve)Then cloth over the frame, and slats in front of the cloth.

This part ALSO applies ONLY to panel traps, NOT slats - You can use impaling clips to mount the insulation to the rear of the trap - if the back panel is thick enough and you can't find impaling clips, get a heavy pair of nippers or something similar, drive 16d nails partway into the back panel, clip off the heads, and "impale" the insulation on the nails. If you use nails, put them about every foot in each direction and slant them slightly UP to the FRONT so the insulation will tend to stay to the rear of the box.

For slat absorbers, you can keep the insulation up against the slats/cloth by using heavy twine across the rears of the frame members - roofing tacks with big heads would work, driven into the rears of the studs at 2-foot intervals and winding the twine around the nails before installing the insulation.


Does it matter if there is a gap in between your resonators and the ceiling? How much is too much?

No - slats aren't usually for that low a frequencies so they're more directional, you can leave a few inches with no problem. Some build them half height, then stack them... Steve

[Maxx]

I have also couple questions regarding these resonators since I'm 'bout to build these too.

How do I tune the resonator to right frequensies?
I have the Helmholtz calculator (corrected version) and some room mode calculators.

In Helmholtz calculator i can calculate the frequency by modifying the slot/slat widths etc. but how many slats do i need to adjust to one frequency that it starts to work? 5, 10, 50 100? Or since i'm building 6 wall units and 2 corner units, I make all resonators to work on different frequencies? What if i attach the slats randomly and hope it works as broadband?

In wall-units the slats are angled, how do i measure the "Depth from wall" in Helmholtz calculator since it differs from side to side?

Sorry if these questions are answered elsewhere, I just couldn't find the right thread.

Maxx

[knightfly]

First, please note the EDITS in my earlier answer, this is important -

how many slats do i need to adjust to one frequency that it starts to work?

As a rough guide (this isn't a very EXACT science) a typically designed helmholtz will absorb around 60-75% at resonance, sometimes slightly more - this is partly a function of the percent of depth of the absorbent inside, 50% fill tends to return around 70% (again, ROUGH estimate) - so, if you use 70% then each square foot of absorber face would result in approximately 0.7 Sabins of absorption AT RESONANCE.

There is usually about a 2 octave range, again this is dependent largely on insulation fill. More insulation, wider range of absorption, but LOWER "Q" (less than 70% absorption)

Random slats usually won't widen the band as much as expected, because the wavelength vs. absorber size causes the absorber to "average", more or less. Same with depth from wall; you can get a rough idea by using the AVERAGE depth, so if yours goes from 6" deep to 12" deep, use 9" -

Some texts claim that in order for variable depth to make much difference, you need to build in DIVIDERS, or septums, between sections of the absorber - but part of what makes John's designs work where they're used is their ANGLE also helps kill flutter, re-direct early reflections...

I'm still looking for good, down-to-earth info on Helmholtz stuff, so please understand that this isn't a definitive answer, just my interpretation... Steve