I found OC 703

ScottMayo · « **Reply #20 on:** 30 May 2005, 04:44 pm »

Quote from: jgubman

Scott,

you wound up making a tube trap out of sonotube? Never thought about that before, how'd it turn out? How many holes did you wind up drilling in it?

Haven't done it, yet. Tomorrow, after the holiday, I plan to start calling around for 24" sonotube. When the plywood bending got difficult I thought of a *lot* of alternatives - sewer plastic pipe, custom plastic extrustions, stove pipe - but sonotube looks to be the right cost, size and general acoustic deadness for the job.

Rives Audio suggests that I want a lot of holes, randomly scattered - approaching 20% drilled-out surface, if the tube will survive that. (It should - unlike using sonotube for subwoofers or concrete, this application won't put the tube under a lot of pressure.) I expect to drill 1/4" holes until I get really, really tired of drilling.

ScottMayo · « **Reply #21 on:** 30 May 2005, 05:26 pm »

Quote from: Kevin P

Wow... if you decide to stop writing software you would make a good writer.

Thanks! It's the story of my life - I'm an ok writer, an ok circuit designer, I can almost compose music, I can vaguely draw. Luckily for me I really can write software, or I'd starve.

Of course, with all the programming jobs going to India, I might starve anyway. But as long as the Defense industry continues to hire good ol' Americans - and there are obvious reasons why they prefer to do that - I should be fine.

However, just to be sure, if anyone wants to audition VMPS gear at my place, and then place an order...

Quote from: Kevin P

Hope it all works out in the end.

Not more than I do, I guarantee it. But I figure the reputations of Brian C, John Castler, Rives Audio, my contractor and my sanity are all on the line here, so how can I fail?

(Don't answer that.)

ScottMayo · « **Reply #22 on:** 31 May 2005, 02:49 pm »

Quote from: ScottMayo

Haven't done it, yet. Tomorrow, after the holiday, I plan to start calling around for 24" sonotube.

Found some. If people want to try this approach, a 12' tube of 24" sonotube runs just under $100 (I called around to get that price - YMMV.) If you've got woodworking skills and gear, this is more expensive than building a quarter tube out of cheap plywood; but less expensive than getting "bendywood" and doing it the "right" way. Most folk can get 2 traps out of a 12' piece - because my ceilings are high and Rives specified floor to ceiling traps, I'm not going to do quite that well.

And, something I mentioned over on the Acoustics Circle - I found (maybe) acoustic cloth for 0.99$/yard, 60+ inches wide. You have to buy a lot (50 yard minimum) and the stuff might be junk - I haven't got samples yet. http://www.magnafabrics.com/category/340.htm If anyone has dealt with them, let me know. If it's not cheesecloth, I expect to wrap my traps and other treatments in this stuff...

ctviggen · « **Reply #23 on:** 31 May 2005, 03:16 pm »

I'm not sure I understand the theory behind the sonotube and drilling holes in it. What are the holes for? Are you filling it with insulation?

ScottMayo · « **Reply #24 on:** 31 May 2005, 03:55 pm »

Quote from: ctviggen

I'm not sure I understand the theory behind the sonotube and drilling holes in it. What are the holes for? Are you filling it with insulation?

Yes to the insulation - 1" thick, lining the inside walls.

The idea is that the sonotube provides a form for the insulation, and the usual resonant cavity to eat low frequencies, but also acts as a diffusor for mid and high frequencies. The holes (as I understand it) allow air pressure (formed by the standing wave) to move into and out of the tube, but due to the insulation lining the inside, the air doesn't move quite freely. In an electrical circuit (which I understand better), this would be a series resistor and a parallel capacitor, and it would smooth out the amplitude, eating some energy in the process. I assume there's a physical analogy to air movement, and by leveling out the pressure via sucking air through the insulation over and over, more energy gets eaten.

It's interesting that Rives specified a random arrangement of holes - apparently this makes the trap more broadband in effect. That part I don't understand. But it makes sense that a trap that also offered diffusion would be part reflective and part transmissive. And, of course, the *really* low frequency stuff (I have a mode at 20Hz) is going to go right through that cardboard, holes or not...

My belief is, in a smaller room, where diffusion is less of a win, a tricorner flat trap makes more sense and is simpler. But since I have the room for a fat, long tube and I need the diffusion on the rear surrounds, the tube makes some sense.

There is also a certain coolness factor in a pair of floor to ceiling columns, wrapped in black cloth.

ScottMayo · « **Reply #25 on:** 5 Jun 2005, 03:57 pm »

http://users.net1plus.com/scottm/room.htm

Updated with the next part of the Saga of the Room from Hell

ctviggen · « **Reply #26 on:** 6 Jun 2005, 11:47 am »

I think the randomness of the holes is probably related to frequency. If you have evenly spaced holes, there's probably a certain frequeny range at which the holes interact with sound. If you vary the distance of the holes, then you increase the range of frequencies (though decrease the effectiveness at any one frequency). It's probably like adjusting the Q of a resonant chamber. Are there tops and bottoms placed on the tubes? Or are they open?

Another arrangement is a tube with a bottom and top. The top tube has a hole that's conincident with a smaller tube. By adjusting the big and small tubes' diameter and length, you can adjust the resonant frequency and the Q (basically the frequency range over which the trap is effective). But this is a low frequency trap only.

zybar · « **Reply #27 on:** 6 Jun 2005, 12:25 pm »

Quote from: ScottMayo

http://users.net1plus.com/scottm/room.htm

Updated with the next part of the Saga of the Room from Hell

Any updated pictures?

George

ctviggen · « **Reply #28 on:** 7 Jun 2005, 11:34 am »

I didn't realize it yesterday, but the Handbook of Acoustics has a section that details frequency response for hole spacings for different types of absorbers.

ScottMayo · « **Reply #29 on:** 7 Jun 2005, 01:24 pm »

Quote from: ctviggen

I didn't realize it yesterday, but the Handbook of Acoustics has a section that details frequency response for hole spacings for different types of absorbers.

I think that book goes on my Must Have list. In the meantime, can you summarise? Or is it the kind of think that doesn't reduce well to a paragraph? I'm at the hole-drilling stage...

ctviggen · « **Reply #30 on:** 7 Jun 2005, 01:49 pm »

I can do this tonight. The book is at home. What the book lists is various thicknesses of material on the outside, then thicknesses of the mineral wool on the inside of the material, and hole spacing, and how all of this is related to the center frequency for the absorber. I believe they also relate different configurations (round, like what you're building, corner, and flat) to frequency response. I'll look again tonight.

ctviggen · « **Reply #31 on:** 8 Jun 2005, 01:05 pm »

This isn't as relevant as I thought. They only give data for a perforated panel absorber. Judging from the the graphs, the fewer holes (in terms of percentage of perforation), the lower the resonant frequency. Percentage of perforation can be calculated using 100 (r/(w+r)), where r looks to be the diameter of the hole and w is the distance between edges of the hole. The book lists other formulas, too. It appears that the larger the hole diameter, the lower the resonant frequency (though this depends on the thickness of the panel material). However, the higher the percentage of perforation (the larger the number of holes), the better the broadband absorption will be. In other words, not only does the resonant frequency change, but the overall absorption increases.

ScottMayo · « **Reply #32 on:** 8 Jun 2005, 02:04 pm »

Quote from: ctviggen

This isn't as relevant as I thought...

Matches more or less what I expected, though. Which book is it? The big Handbook, or the much smaller Master Handbook of Acoustics?

ctviggen · « **Reply #33 on:** 8 Jun 2005, 02:48 pm »

There's a big one? The one I'm using is the Master Handbook of Acoustics:

http://www.amazon.com/exec/obidos/tg/detail/-/0071360972/qid=1118242034/sr=1-1/ref=sr_1_1/103-8572761-0799828?v=glance&s=books

If there's a larger book, let me know the title. While I like the book I have, I don't think it goes into enough detail.

ScottMayo · « **Reply #34 on:** 8 Jun 2005, 05:42 pm »

Quote from: ctviggen

If there's a larger book, let me know the title. While I like the book I have, I don't think it goes into enough detail.

http://www.amazon.com/exec/obidos/tg/detail/-/047125293X/102-5162332-3702534?v=glance

I suspect it will have enough detail. It's also >$200 anywhere I've looked.

ctviggen · « **Reply #35 on:** 8 Jun 2005, 06:05 pm »

That does look like a good book. However, there does not appear to be any chapters on diffusion or absorption of sound, and none that relate to control rooms, mixing studios, and listening rooms. As an engineer, I think it would make a great read. However, I'm perplexed as to when and why one should use diffusion (a topic covered in the Master's HB, but not that well, I don't think), and this book doesn't appear to have any information on this subject. For instance, diffusion apparently helps even out the sound in a room, but is diffusion beneficial for a room that strictly designed for listening? I fear the only way I'm going to find out the answer is to put diffusion into my room and measure the results and/or listen for results.

ScottMayo · « **Reply #36 on:** 8 Jun 2005, 08:26 pm »

Quote from: ctviggen

For instance, diffusion apparently helps even out the sound in a room, but is diffusion beneficial for a room that strictly designed for listening? I fear the only way I'm going to find out the answer is to put diffusion into my room and measure the results and/or listen for results.

This is the sort of stuff that drove me to buy expert help.

As far as I can tell, diffusion is best used directly behind a speaker to break up the direct echo off the rear wall. You don't want that echo, as your ear will use it to compute the room size, which is distracting. But just loading up on absorption in every direction tends to make a room too dead. Diffusion's a nice compromise - the echo radiates off in all directions, so the ear hears it as vague ambiance instead of a distracting fixed echo. Less loss of imaging and less "sense of room", without total deadness.

Or at least that's the theory as I understand it. I hope to know more in a couple weeks, as my diffusors are built but not yet bolted to the wall.

ScottMayo · « **Reply #37 on:** 9 Jun 2005, 03:30 am »

http://users.net1plus.com/scottm/room.htm

Updated with part four of the Saga of the Room from Hell.

warnerwh · « **Reply #38 on:** 9 Jun 2005, 06:39 am »

Hope your room goes more smoothly in the future. Built a custom room myself last summer and I'm no carpenter or electrician or drywaller but it got done and is the best sound I've heard. Only bad thing I did was stick the power screwdriver bit through my thumb as I slipped while installing resilient channel. Wish I knew how much a good room could help before. You'll be impressed when you're done is my point and hopefully forget(yeah right) the ordeal. Great story though!

ScottMayo · « **Reply #39 on:** 12 Jun 2005, 03:30 pm »

Quote from: warnerwh

Hope your room goes more smoothly in the future. Built a custom room myself last summer and I'm no carpenter or electrician or drywaller but it got done and is the best sound I've heard. Only bad thing I did was stick the power screwdriver bit through my thumb as I slipped while installing resilient channel. Wish I knew how much a good room could help before. You'll be impressed when you're done is my point and hopefully forget(yeah right) the ordeal. Great story though!

Thanks!

Future updates announced in "The Acoustics Circle", where this thread really belongs.