Should I proceed? Math versus intuition....

laptoppop · Post by **laptoppop** » Sat Aug 02, 2003 7:02 am

Help!

I *really* respect barefoot -- but his analysis of slot absorbers is scaring me!

I've got a tiny room (10x16.5 feet). I've completed the roof work - hanging 19 bass hangers above a 4" thick cloud of fabric wrapped 703 drop ceiling. The ceiling is sloping, so the hangers go from 4"x8' to 48"x8', with the tall part at the rear of the room. The plans are in the studio under construction area - I won't take bandwidth to repost them here.

I'm just about to start building the wall treatments - 8 slot absorbers, 2' wide by 8' tall, and 4 corner slot absorbers, 8 feet tall, and front and back 4" thick 703 absorbers.

I'm trying to have nice broad lows and mids absorbtion. I hope I am misunderstanding Barefoot's analysis - because as I read it, my slot absorbers would actually end up working mostly at the average frequency, and I want more broad absorbtion.

I'm planning on using a variant of John's wall unit design - the depth of the box will go from 2" to 8" giving me a nice angled front. The slats will be 1"x2", 1"x3", and 1"x4". My variation would be to angle every other slat (always a 1"x3") so that all the slots will vary from zero to 1/2", kind of like the beautiful absorber wall at Left Bank.

My concern is that I'll end up with 8 boxes that absorb at the frequency equal to 5" deep, 3" slat width, and 1/4" slot width. I don't want a sharp peak of absorbtion at one frequency, I want more wideband absorbtion. Yes, the cloth behind the slats is supposed to flatten it out - but by how much?

My GUESS is that this is fine. I GUESS that the parts of the box will tend to resonate at a variety of frequencies and will be damped out nicely. I HOPE that the changing slot depth and the triangle shaped slots will create a very smooth frequency response. My FEAR is that my room will be junk, with all the absorbtion at one frequency.

I need the angles on the slats to avoid standing waves, so I'm going to use those regardless. Should I proceed as planned? Should I change my plans -- make each pair of the 8 boxes tuned to different average frequencies? Help! I really really want this room to sound good.

Thanks,
-lee-

knightfly · Post by **knightfly** » Sat Aug 02, 2003 8:52 am

Lee, first of all I would't recommend ignoring Barefoot's comments - I've tried, and he just sooner or later ends up proving his point -

Seriously though, in this case (as in most) I think he has a valid point. How's this for an idea - if you don't HAVE to build the slots immediately, try listening to the room without them - maybe lean some panels up at the sawtooth angle shown, to get rid of fluttter, but nothing more - then, if you still think the room needs more, go from there.

With your plan of varying both the air depth AND the slot width, it would be impractical to try putting "septums" between, making individual "mini-slots" -

Depending on what music software and mics you have, you could concievably record "impulse responses" over the course of construction, and compare the differences - Samplituide, for example, lets you plug a waveform into their FFT algorithm and see a waterfall plot - I've not tried it in this manner, but I think you could hook up a good mic, clap your hands, hit a kick drum, record them, name them something intelligent, then compare them in a waterfall plot to see what's going on in the room. You'd only need about a 100 millisecond or so sample to see what changes in absorption you'd caused.

Just a thought, but remember - ignore the barefoot, prepare to get "stomped"...

Steve

Post by **John Sayers** » Sat Aug 02, 2003 9:21 am

but I think you could hook up a good mic, clap your hands, hit a kick drum, record them, name them something intelligent, then compare them in a waterfall plot to see what's going on in the room.

if you've got a hand gun use it with blanks

:)

cheers
john

giles117 · Post by **giles117** » Sat Aug 02, 2003 5:16 pm

My slots and angled wall work.

I hear the diff in the room, not sure if it is just the angle or not, but my mids are 100 times better than they were before.

Bryan

I Love theory, but Jeez it makes my head hurt sometimes, I prefer practicality.

knightfly · Post by **knightfly** » Sun Aug 03, 2003 12:24 am

"if you've got a hand gun use it with blanks " -

Blanks??!? Blanks???!? Man, no WONDER my wife gets nervous when I'm testing acoustics...

laptoppop · Post by **laptoppop** » Sun Aug 03, 2003 3:31 pm

Right now the sound in the room sucks. The ceiling treatments helped a lot, but there's still a TON of ringing because of rigid parallel walls.

I'm on a tight schedule -- I have to be DONE with ALL construction this month -- and I only have a couple of nights a week and 1 day per weekend to work on it. I've got a helper that I'm paying for some help, but hes out the first 1/3 of this month. I should be able to get 20 hours or so a week for the last 3 weeks from him. Bottom line, I'm hurting for time. The end of the month is a hard deadline - we're living in a hotel while we do some remodeling -- and the studio must NOT be the last thing finished in the remodeling!

I must admit -- I'm still very puzzled. Assuming Barefoot's analysis is correct (and I'm the first to say I admire him which is why I'm even concerned about this) -- it seems like virtually ALL panel resonators would be absorbing at one primary frequency with a little widening due to the air constriction by the cloth. Am I misunderstanding it? Wouldn't that make them tools with very limited usefullness?

John, how does this square with your experience? Seems like you've used slot absorbers successfully in a number of installations and the rooms don't sound like they have a hole in one frequency, but more of a broad band absorbtion. It seems like lots of folks have successfully used slot absorbers for more than just selective absorbtion at a particular frequency.

1) How wide would the cloth make the absorbtion? In terms of a digital parametric filter -- would it be a "Q" of .1 (real broad) or a "Q" of 1 or 10?

2) Is there a chance that while slot absorbers are not AS efficient at secondary frequencies, that they are resonating even with partial waves, and a lot broader than full wave analysis would indicate? (or maybe again, I'm misunderstanding)

3) Has anyone taken slot absorbers into a testing lab? Any results?

I'm really unsure about how I should proceed -- but I need to finish this room. One thing I could do is build 4 pairs, with deliberate variance between them. For example, build 2 using 1x2s, 2 using 1x3s, 2 using 1x4s, and 2 using 1x6s. I'm concerned that the resulting sound would not be as smooth as the mixed version, but the average frequencies would be different.

Help!

-lee-

Ethan Winer · Post by **Ethan Winer** » Mon Aug 04, 2003 1:52 am

Lee,

> there's still a TON of ringing because of rigid parallel walls ... I have to be DONE with ALL construction this month <

I usually try not to turns these forums into a sales venue, but you're struggling with such a simple problem. If you'd just buy a few of my company's MiniTraps you could be done by next week.

--Ethan

Post by **John Sayers** » Mon Aug 04, 2003 10:21 am

Lee- don't make all the slots the same - you seem to be trying to incorporate every possibility into all the slot units. Make each one different aiming for different frequencies, then you will get the broad band you are looking for. Apart from the ceiling yourt room is a square plywood box and you must break up those parallel walls. I suspect your walls are acting like Ethan's panels as they are

cheers
john

laptoppop · Post by **laptoppop** » Wed Aug 06, 2003 1:45 am

Yah, John, I guess I was trying to put everything into one type of box -- and even throw in a couple of new things.

I owe you some pics for an update - the ceiling is pretty much done now, and the walls have the final layer of wallboard over the wood. Of course, if I had just known about glueing the last layer...oh well. Bottom line is that I'm hearing dead silence inside the room now - for my purposes, very reasonable isolation.

I'm building 4 boxes on each side, about 2' by 7.5'. Should I leave them as full boxes, or seperate them into 2 boxes each, with slightly different tuning?

Should I just forget about splaying the slots, or should I splay every other slot and just use different widths of wood to tune the boxes to different average frequencies?

What widths of wood for my little room? My original plan was 1x2, 1x3 and 1x4. Would wider slots be good? I'll have deeper slots in the corners, so it may not matter.

Thanks for all the info!

-lee-

laptoppop · Post by **laptoppop** » Wed Aug 06, 2003 1:53 am

Ethan Winer wrote:Lee,

> there's still a TON of ringing because of rigid parallel walls ... I have to be DONE with ALL construction this month <

I usually try not to turns these forums into a sales venue, but you're struggling with such a simple problem. If you'd just buy a few of my company's MiniTraps you could be done by next week.

--Ethan

Ethan,

I appreciate the way you handle yourself on the various boards - You have done a good job of giving out info without making sales pitches.

I'll be honest - I've got very rigid walls and I just don't believe a few of your traps would break up the echoes enough. I've got a tiny little room, but I'm trying to make it sound exceptional. I believe the pretty standard rule of thumb is to splay the walls by at least 12 degrees. Your traps wouldn't do that. The slot resonators I'm building will give me walls that are effectively splayed without killing my space.

Thanks again for your participation - I think we'll have to disagree on this one, but thanks.

-lee-

Ethan Winer · Post by **Ethan Winer** » Wed Aug 06, 2003 2:52 am

Lee,

> I believe the pretty standard rule of thumb is to splay the walls by at least 12 degrees. Your traps wouldn't do that. <

Actually, I was thinking of MiniTraps, not the wood panel traps with the angled fronts. Whatever. I'm happy to help in all cases.

--Ethan

laptoppop · Post by **laptoppop** » Sat Aug 23, 2003 6:46 am

Please excuse me for reposting these, but I think these questions got buried a couple of posts up:

I'm building 4 boxes on each side, about 2' by 7.5'. Should I leave them as full boxes, or seperate them into 2 boxes each, with slightly different tuning?

Should I just forget about splaying the slots, or should I splay every other slot and just use different widths of wood to tune the boxes to different average frequencies?

What widths of wood for my little room? My original plan was 1x2, 1x3 and 1x4. Would wider slots be good? I'll have deeper slots in the corners, so it may not matter.

Opinions?

Thanks,
-lee-

knightfly · Post by **knightfly** » Sat Aug 23, 2003 8:05 am

Lee, here's a plot of the axial modes of your room - I assumed an average height of 10.3 feet, which is really close to your width - this would be worst case, since your ceiling is sloped from 8 to 13 feet or so.

[/img]

knightfly · Post by **knightfly** » Sat Aug 23, 2003 8:20 am

This is how I'd do slots for your room - but remember, I've only studied about this kind of thing, ask Barefoot or John before committing -

knightfly · Post by **knightfly** » Sat Aug 23, 2003 8:39 am

For the 170 hZ slot, you need to use 1-1/2" thick slats to get low enough without too much depth - if you set inside depth at 5 sloping to 8 inches , the average depth will be 6.5" - used with 2x8's spaced 1/8" apart, this works out to 170 hZ. Even if you don't get as wide a response as once thought, the slope front will treat flutter as well as help RFZ.

For the 220 hZ trap, you could use 3/16" slots with 2x6's, same box size and depth.

For the 300 hZ trap, you could use again the same box, same slope, but use 2x6's with 3/8" slots.

Your ceiling is already sloped, so doing this on the walls should eliminate those as a parallel wall problem, at least around the mix position... Steve