cyclecamper

Now I am starting to understand better. The absorbers I am using are velocity-based, so at the walls are not the optimum locations. Sometimes Wikipedia is just not thorough enough for full understanding LOL.

re-reading, it sounds like perhaps Bdiament is talking about where on a wall to place absorption, whereas I was talking about where in the space of the room's volume to place absorption. But I still can't follow what he said. Now I'm home and I can try to follow his links...maybe that will clear things up.

Getting down, maybe this will help: Standing waves acoustic resonance and vibrations on ideal strings - Standing waves are stationary waves room modes sound pressure level between hard parallel walls node antinode stationary room acoustic frequency - sengpielaudio Sengpiel Berlin Bdiament said: "To address the first harmonic of any dimension's resonance, treatment must be applied at the 1/2 point along this dimension. (Again, corner treatment will be "invisible" to first harmonics and addresses only the fundamentals.) Similarly, in order to address second harmonics of any dimension's resonance, treatment must be placed at the 1/4 points along this dimension." I cannot understand the harmonic series you are describing either...at the 1/2 then 1/4.... The first harmonic is the fundamental overtone with one node at the 1/2 point. The second harmonic is the first overtone with two nodes at the 1/4 and 3/4 points. The third harmonic is the 2nd overtone with three nodes at the 1/6, 1/2, and 5/6 points.

ISVR - Institute of Sound and Vibration Research

Reference: https://en.wikipedia.org/wiki/Room_modes and notice the graphic. Bdiament ... Too bad I can't reach your links, but protected mode at work won't let me see it during my break...I will have to check them out when I get home later. But...from what I've been reading, mostly about how room standing waves differ from a string, what you are saying appears to be fundamentally incorrect, or perhaps confusing pressure with velocity? Either you are completely wrong or everything I am reading in my studies is bull or I am misunderstanding something here... The relationship between pressure and velocity complicates understanding... There is a pressure antinode for the fundamental at the wall. There is also a pressure antinode for the first harmonic at the wall. There is also a pressure antinode for the second harmonic at the wall. There is a pressure antinode for the third harmonic at the wall. Etc. etc. etc. Like Wikipedia says: "At the walls there must always be a sound pressure antinode." This is in direct conflict with what you are saying. Remember that for sound in a space the wall is not a terminating constraint like for a string between anchors. Of course to fully absorb the fundamental on first reflection the ideal complete absorber needs to approach the size of half the room on each side, filling the room. For the harmonics the absorber is more effective on the first bounce, but of course there are other antinodes within the space where no absorption takes place. The result is that absorption at the wall absorbs some of the fundamental and some of all harmonics of the standing wave modes for that dimension, but is more effective at the higher frequencies for the harmonics where the absorber depth approaches or exceeds the size of a quarter-wavelength (at the speed thru the absorber, not free air). Again, this is in direct conflict with what you are saying. This is easily evidenced by the effectiveness of the thin foam absorbers on only the highest harmonics.This appears to be fundamentally wrong: "To address the first harmonic of any dimension's resonance, treatment must be applied at the 1/2 point along this dimension. (Again, corner treatment will be "invisible" to first harmonics and addresses only the fundamentals.) Similarly, in order to address second harmonics of any dimension's resonance, treatment must be placed at the 1/4 points along this dimension."

If I had it to do again and still intended to install a solid Masonite or pegboard back, I would probably pre-cut the back panels and put them on with glue and staples from an air stapler (I found that the best electric T50 stapler I could find would not penetrate the Masonite because the limitation was the T50 staples). That way the back panel would also align the frame exactly square. The way I did it, first I attached the supplied triangular corners then cut the backs, and cut the corners off the backs, and stapled them on without glue.

Cool. But of course though it can correct timing deficiencies in the speakers, and can achieve some room EQ at one listening location or an averaged response, no electronic correction can remove the room's extra reflections and their timing problems, nor can it remove the standing waves that cause frequency response to vary wildly from one location to another only centimeters away. Only altering the room's characteristics can, which is why that must be the first step.

http://www.roomeqwizard.com/REWhelp.pdf Sounds like I can use my Radio Shack SPL meter, with or without my Behringer measurement mic / preamp combination, and just use my laptop's built-in sound card.

Jud, I was hoping not to buy a USB mic, but use my existing Behringer mic and my existing mic preamp, and purchase some kind of A/D that might be repurposed later for recording perhaps? The miniDSP UMIK-1 might be cheaper, but would it be very useful repurposed for anything else after I'm done?

Thanks Jud I'll check it out. Ralf11, I'm not looking for room EQ measurements, or I would use my RTA. Room EQ cannot correct early reflections or standing waves. The RTA or the equivalent via a soundcard gives some useful data for room treatment, but I'm hoping for a lot more, maybe ideally some impulse time/frequency waterfall plots like MLSSA produces...I might be expecting too much, I've never used REW yet myself!

A teensie bit off-subject...I'll comment on making the absorption panels. I need to start by stating that it's great that GIK supports the DIY community, and I don't want my comments misconstrued as derogatory, just review. I am fully supportive of their DIY product. The GIK frames were convenient, but for making 30 panels perhaps I should have just made a panel-cutter slide for my table saw and saved some money making my own. Making jigs and clamps, buying tools, etc. pays off as you scale up. The GIK frames have openings in the sides, which increase the potential surface area assuming there is space between adjacent panels. It also makes the finished panels lighter. And a bit weaker; one end piece arrived broken and they replaced it immediately, as soon as I sent a picture of the damaged one. There are cheaper ways to make larger frames for continuous coverage of a large area. But the GIK frames are a convenient size to handle and hang, and for one or two people to work on. The fact the frames screw together with pre-drilled holes is convenient, and holds together well with sufficient strength. I used glue, and with the corner braces I could have just used my air nailers into the frames quicker than an electric screw gun for the supplied screws. But their screws are much stronger than a headless wire brad shot from a finish nailer, and any larger nail would split the MDF. Even a small wire finish brad from a finishing air nailer could split the MDF when shot into the edge of an MDF board, and long wire brads tend to bend and wander as the shoot into MDF, so they can end up coming out the sides. So the GIK screws are a good way to go. I should have set up a square-edged jig on a piece of plywood to assure everything came out square, but instead I used 4 corner clamps which aligned the corners at 90 degrees while I screwed them together, but I had to continuously clean up the glue that squeezed out of the joints. Some got into the clamp screws, and caused problems with the leadscrews. The supplied screws into the pre-drilled holes were less than perfect. The clearance holes were nice; though counterbored a bit deeper than I would have preferred, they never broke out because they made a wise choice of screw head and counterbore shape and size. However, MDF has no strength in tension and splits easily when anything is driven into the cut edge, and screwing the screws into the small pilot hole into the "end grain" (if MDF has a "grain" LOL) which has no strength, about 1/4 of the time the MDF split in its core from the screw to the edge of the board (not thru the smooth surface of the MDF, but thru to the other "end grain" surface at 90 degrees to the penetrated edge). The boards did not crack from over-tightening, they cracked before seating just from the screw driving into the pilot hole. I tried different screwdriver speeds, even driving in & out repeatedly, with or without glue. Nothing made any difference. The holes should be placed 1/16" inch farther from the edges of the boards so the board doesn't split to the edge, and screws one size smaller in diameter utilized, but the size of the heads was pretty good, so I guess they chose a workable compromise. There are special screws which are designed for MDF, but screwing anything this large into the edge of MDF can split it, and often does. But with glue on the screws and in the joint it doesn't really matter. The supplied triangular corners help strengthen the frame a lot, and the thin Masonite used is sufficient. I hand-nailed them on, but if I had it to do again I'd use an air stapler with a narrow staple (not shooting a headless wire finishing brad either). The Masonite they chose is smooth on one side, but rough on the other, and the rough side adheres to glue very well. I used Titebond III. Attaching the corner braces early means you can't easily staple the fabric on and then stuff the frame from the back. The big problem with the frames is their finished interior size. The Roxul to stuff them with is not available in the size of the inside of the finished frame! Cutting the Roxul with an electric carving knife took more time than the pre-cut frame pieces saved me, and caused a lot of waste, mess, and waste material cost. Perhaps the frame sizes were dictated by how many pieces they could cut from a full sheet of MDF or trying to target a common finished size of 2' X 4'. But if it was about an inch and a half larger in each dimension the standard Roxul batts would drop right in! I put standard pegboard on the backs of most of the panels. Thinner solid Masonite would cost only half as much. When I added another stuffed box behind the pegboard, it creates an absorbent Helmholtz, but the holes in the MDF are too plentiful, too large, and the chamber behind not deep enough to absorb really low frequencies. I didn't do the math. It also acts a little like a damped mass-loaded diaphragm. Can't really tell what the heck I created until I try to measure what it does to a room I guess... I used cheap muslin fabric intended for wedding tablecloths. It was too visually transparent and showed the imperfect surface of the Roxul. I will probably attach a second layer. Now I know why people use burlap. I did not want to spend the money for special-purpose cloth that's acoustically absorbent, the room will probably absorb too much high-frequency sound anyway. I also tried some nicer fabric which stretched in all dimensions, but then I should have sanded the frames and used better fiberglass instead of the cheaper Roxul.

New/old house; I'm starting with room treatment, and setting up my family room (half-vaulted ceiling) as a multi-purpose audio room mostly for listening, practice (bass guitar mostly) and eventually perhaps a little bit of recording. It's going to be less than ideal because there's a kitchen at one end and things will rattle no doubt. But it has some cubic volume space to it, and I live alone so I can do whatever I please. My first steps were based on what's expedient and affordable. I've started by making 30 2'X4' absorption panels based on the GIK Acoustics DIY frames, 4" thick Roxul, with perforated pegboard on the back. A few have an additional staggered-wall extension frame sistered on, and another 6" of Roxul behind the pegboard and then a closed Masonite back. A few have a thick stack of acoustic ceiling tiles in that back chamber instead of more Roxul. I've got a little used Auralex I bought for cheap, but it isn't very effective except at high frequencies. Admittedly, I didn't do the math, I'm hoping for relatively broadband absorption and can rearrange their placements many ways if I need to adjust what bands are absorbed. If it gets too dead there are connecting hallways and rooms that can use absorptive treatment and some panels might end up in other rooms. I'm probably also making some very large convex curved diffusers, and some fractal diffusers. I don't know whether I'll get around to any RPGs or skyline diffusers. First I'm doing the broadband absorption every LEDE room needs. Absorption to address the reflections from the speakers to the walls and the early reflection points to the listening area and near all corners. Some diffusion to the sides and behind. Some ceiling treatment and some overhead clouds since the room has the height to accommodate them. But then, halfway thru hanging absorption and diffusion (if not sooner), I need to take baseline measurements and determine where I am and what I still need to deal with. If additional bass traps or diffusion are required, they will need to be tuned properly. I'm a programmer by profession, but new to actually doing any room measurement myself. I imagine I'm going to want to know, at the least, room frequency response and what the reverberation times are in various frequency bands. So, what hardware / software do I need to start gathering in order to use REW for room measurements? I have: - A Behringer RTA / EQ. - Inexpensive Behringer measurement mic, and many other mics. - Various inexpensive mic preamps with phantom power. - Cables etc. - Alienware laptop. - I think I'm getting an iFi micro DSD??? What I don't have: - any way to connect the mic pre to the laptop besides the built-in sound card. So do I need some kind of A/D to take the output of the mic preamp into the laptop's USB? Is it practical to attempt using the laptop's built-in sound card for measurement purposes? What additional hardware should I accumulate in order to use REW?

I'll take it! Message me!