dangers of parallel walls

[colterphoto1]

Ugh, never again will I build a building with parallel walls.
I"m talking about my workshop building 32x48x10.5, detailed in the General Forum.

Although I left the insulation exposed (with plastic vapor barrier) on
all walls, this huge expanse of concrete v drywall on the ceiling is
too much parallel surface. I got the feeling midway through the project
that I should have put in at least a slightly vaulted ceiling and
should have followed that feeling. In all truth, there is a 1" drainage slope in the floor so the surfaces aren't exactly parallel.

Which begs the
question- how much do you have to go 'off-parallel' to avoid the pingy
reverb that comes from having exactly parallel surfaces? I don't hear
it becaus I'm rarely in the area, but my building is also parallel to
the metal pole barn next door and that's another echo chamber. If I were to back track and position my building slightly different with respect to the neighboring building some 30 feet away, or to put in scissor trusses, how much angle would be necessary to reduce what I'm going to call 'flutter echo'?

arrrrgggghhhhhh

Perhaps if I hung a bunch of chicken wire and fibreglass 'clouds', angled panels or draped materials, might that help?

[DrWho]

1" per foot for each wall, so about 2" per foot if you can only modify one surface. Keep in mind that acoustic treatments behave differently at different frequencies - so whatever you do, make sure it fixes all of the problem.

[Groomlakearea51]

Given shelves, objects arranged on the shelves, and other necessary "clutter" on the walls, it may not be as bad as you would expect. The ceiling in the man cave is sloped, so I avoided that problem and the "field expedient" bass traps in the upper far corners took care of that problem. The bookshelves on either side wall are an array of "cubicles", but the number, depth and position of the books, etc. is "assymetrical" thus diffuses or absorbs the reflections well enough to pretty much kill off any "hand clap" problem. The ceiling expanse is likely going to be the battle ground in the workshop. What about hanging/ suspending some storage slats from the ceiling for plywood, veneer, etc.? Maybe that would break it up enough.

[colterphoto1]

Thanks for the suggestions guys. It's worse in some places depending on where you stand. I noticed this especially before insulation, when the exterior walls were studs with OSB and hardiplank(concrete board) on the exterior side. Standing in the direct middle of the room would give equal reflections from equidistant walls, therefore less bothersome. Walking around to areas where I was some multiple of distances from walls would be horrible (10/20 feet from long walls and 16/32 feet from end walls. In between would be, well, in between.

Marshall, I agree with the wall treatments, and my myriad of shelving, bumpout for the office space, vehicles will help in addition to leaving all walls with exposed insulation. The ceiling is indeed the problem. I thought I'd put the veneer in a verticle space in the office, which will be more temp/humidity controlled. My plan is also to build a 'warehouse' of shelving with 8' high loft overhead but this will be in the far corner away from the work area. (at the nose of the truck/boat garage area)

There will be 'Colter's Wall of Sound' (see Grateful Dead Wall of Sound) at midway point in the room, dividing the garage from workshop space. I may put a rolling track for heavy canvas material behind this. This should help 'shorten' the room from a mid/hi frequency standpoint and give some absorbtive surface.

I think I need something for directly overhead on the shop side. Overhead storage will look cluttered, something I'm trying desperately to avoid.

Mike, how about some plywood panels hung at obtuse angles? What percentage of the ceiling area might I have to diffuse in order to gain some relief? Perhaps it would be enough to get my stuff in there, hang lighting, then experiment with panels directly overhead work areas where I'll be spending larger amounts of time. Also dead center between the stacks at 'listening position' would be a good candidate.

Another candidate would be to build some 'mega' skyline diffusers from builders foam or styro. Or could make them of wood scrap cutoffs, kind of like what Professor Thump has in his basement.

[mark1101]

Mike, You might try Owens Corning 705 (and 705-FRK sheets). This is what I use (plain 705, 2" thick). They come in various thicknesses and I have 2" thick 2'x4' sheets. These are pressed insulation boards. A few of these around the room and they are like sound vacuum cleaners. You could suspend them from the ceiling or fix them to the ceiling in a loose coverage pattern. There is a lot of info on them on the net for use as acoustical treatments and what frequencies they are good for. Work well for me. They are not pretty though. There is also 701 and 703 for high frequency absorption. The 705 is for lower frequencies.

[jacksonbart]

I would level it and start again.

[Bill H.]

Michael,

Found these 705 (package of 6)...................

http://www.readyacoustics.com/index.php?go=products.proddetails∏=OC705UF

[colterphoto1]

Good idea on the Roxul stuff. Let's do some math. The main part of the workshop area will be 20 x 24, thats 480 ft2. One pack of that stuff is 6 sheets at 24 x 48 or 48 ft2.

So hanging one pack or 6 panels will give coverage of 10 % of the ceiling surface. I wonder if this would be enough if care was given to their placement?

I would guess that suspended some distance from the ceiling at some prescribed angle would be best usage? Doc- any help or experience in what you may have seen?

[Bill H.]

Six 2" thick boards of Owens Corning 705 brand 6 lbs. per cubic foot
density semi-rigid fiberglass board – use this material to make 2”
panels

If more are needed, no Sweat................

[colterphoto1]

it's a budget thing, running out of money

with different thicknesses affecting different bands of frequencies, I'd want to track down the bandwidth where the offending POOOIIINNNNGGGG occurs. That should be easy with an eq, I'll just play with the bands until I find which one makes the sound go away. one step closer.

[PrestonTom]

The costs always increase on these projects. Someday, I will tell you about my "simple" kitchen renovation....

The good news is that the Roxul is less expensive than the Dow Corning. It is comparable in performance. When comparing, match according to the pcf (pounds per cubic ft) and the thickness. Incidentally, these products may be hard to find. They do not carry them at Home Depot or Lowes. You need to go to a "real" building supply store.

Good Luck,

-Tom

[mikebse2a3]

I doubt splaying the walls or ceiling would be the best answer in your situation.

At this time you just have a large empty space causing lots of bad echo and once you begin to place objects in there it will serve to diffuse the sound and break up some of this. Have you thought about some cheap poly cylinders at some point and the more creative you are at using the space 3 dimensionaly with whatever you plan to use the space for the better things will sound.

Some of the best Klipschorn sound I ever heard was at the dealers when they had a room full of speakers all along the walls. All the different 3 dimensional box sizes served as very good diffusive devices and the various size speaker boxes themselves acted as tuned resonater bass traps.

Not sure I would spend any money until I began to use the space and then once you have a firmer plan on how you would like to utilize the space begin to work on the acoustics.

mike tn

[DrWho]

Is it desireable in your situation to have the ceiling "sealed"? A
diffraction grating to couple your room to the above attic would
probably yield the most benefit, and probably be the cheapest
too....the downside is you'll have to put holes in your brand new
ceiling... []

I might also recommend measuring what's going
on to determine what will be the most direct and cheapest solution. I
suppose you're trying to measure with your ears by playing with EQ and
walking around and extrapolating on known behaviors, which is certainly
a valid approach, but I've found that it's usually enlightening when
I've got hard fast numbers to describe what I'm experiencing...and then
sometimes it's just confusing all get out, but thems opportunities for
learning something new.

Anyways, I bring it up because I'm not
quite convinced that we know for sure what you're experiencing, so I
kinda hesitate suggesting anything because I'm not sure if it's
directly addressing the problem or not. I know what flutter echo sounds
like and I think that's what you're describing, but the source could be
all over the place....and it's likely to change dramatically as you put
stuff into the room.

[Bill H.]

Dr.Who,

Good Point . Question, could the "Flutter" also be something as simple as the room is Too Airtight ? Thus no way for air waves to disapate????????

[colterphoto1]

Thanks Doc. Puncturing the ceiling is not an option. Nice drywall and R40 insulation up there, the thermal properties of the room are more important [] than the acoustic properties.

I looked this up:

Flutter Echo

Short echoes in small reverberative
spaces that produce a clicking, ringing or hissing sound
after the original sound signal has ceased. Flutter
echoes may be present in long narrow spaces with parallel
walls.

I don't know about hissing or clicking, but there is a very fast poiinnngggchchchchchch that dies out quickly, so it's very rapidly spaced individual echoes. I think that is flutter echo.

[colterphoto1]

It's amazing what you can find when you Google something:

ACOUSTIC EDUCATION
Solving flutter echo problems
If two opposing reflective surfaces of a room (parallel wall to parallel wall or floor to
ceiling) there is always a possibility of flutter echoes. Successive, repetitive
reflections, equally spaced in time, can produce a perception of a pitch or timbre
coloration of music and a reduction in the speech intelligibility within the room.
Flutter echo can be reduced in one of two ways, with the use of sound absorption
or sound diffusion.
Flutter echoes can be acoustically treated with careful placement of sound
absorption materials such as foam or wall panels on the walls or ceiling tiles,
baffles or banners in the ceiling. The idea here is to absorb the sound wave at
one or both surfaces and keep that sound wave from reflecting of the surface back
towards the noise source.
Flutter echoes can also be acoustically treated with the use of sound diffusers.
Sound diffusers are multi-faceted, slotted or curved materials that are reflective in
nature and are designed to scatter or redirect sound waves. The sound diffusers
can break up flutter echo within a room by taking the sound waves and sending
them in different directions and eliminating the repetitive reflections caused by
reflective, parallel surfaces.

[colterphoto1]

check this out at All Church Sound!

http://www.allchurchsound.com/ACS/R_D/kitchen/index.html

their solution is halved sonotubes used as diffusion- look familiar? This is nearly exactly what PWK put in his listening room. Looks sharp and does the job for not much $$$.

I wonder if I made them out of bent pegboard and filled them with loose fibreglass if a combination diffusion/absorbtion might be effective? Nearly all of the surfaces in the room are hard (it being a shop), so some absorbtion might be good depending on the offending frequency. Glass stuffed in a half round shape would have a variety of thicknesses, therefore appealing to the 'broad spectrum' absorbtion that so many go on about.

Best to wait until I get the space loaded up, just wanted to have weapons at the ready.

M

[mark1101]

You are probably going to have to measure it out and find what issues you have. I'd take the Doc up and get him over once everything is moved in.

[colterphoto1]

Quite right Mark. Perhaps a project for a Spring Pilgrimage!

[artto]

uhhmmmm.........can you say Masonite? You can bend it, curve it, twist it, hang it, panel it, cut it to size. Cheap.