Low-end buildup in rehearsal room.

[danex]

Hi all,

I'm looking for some help with my rehearsal space.

The dimensions are 10' 3" x 11' 4" x 10' ceiling. There's shelves on 2 opposite walls at 7'. See below:

3J TOP 2.jpg

I've treated a lot of the walls with acoustic foam, which has helped a lot with the reflections from the walls, but at this point I'm having a problems with low-end buildup. Whenever the bass player hits a G, the note rings exceptionally loud where I sit at the drums. See below (Bass amp left, Drums right):

3J Corners E.jpg

I'm wondering what corners I should tackle first... should I treat the underside of my overhead shelves?

3J Corners A.jpg

3J Corners B.jpg

3J Corners 3.jpg

Any help is greatly appreciated it.
Thanks,
-Danex

[Soundman2020]

Hi there, Danex, and Welcome!

I've treated a lot of the walls with acoustic foam,

As you already discovered, "acoustic foam" isn't much use for low frequencies! In fact, it isn't a whole heap of use in general, since it only treats the high end and some of the mid range.... which is why acousticians and studio designers don't use it much. Despite what the manufacturers try to tell you, foam isn't a good treatment, and isn't widely used in professional rooms.

I'm having a problems with low-end buildup. Whenever the bass player hits a G, the note rings exceptionally loud

Right! For two reasons. One of them is this: "The dimensions are 10' 3" x 11' 4" x 10' ceiling" That's almost square in section, and not far off from being a cube. Your problem is something called "room modes": All rooms have a series of frequencies where the entire room resonates, because at those specific frequencies, the wave-length fits in exactly between two walls, or between the floor and ceiling. When that happens, the wave bounces back and forth between the walls exactly in phase with itself, and the energy just builds and builds and builds. Then when the note that caused the mode suddenly stops, the mode does NOT stop... it carries on "ringing" for a while, because it has stored so much energy from bouncing around. That ringing can last for a second or more, after the original note stopped.

So you have room modal problems. Room modes are associated with the distance between walls: that sets the frequency where the modes can occur. In your case, the three lowest modes are at 50 Hz, 55 Hz, and 56 Hz. Guess what? Those are all very close to a G. Between G and A, actually, but close enough that either will trigger them.

The next problem is that all room modes start and end in the corners of the room, and your drum throne is right there, in the corner! So not only is the room roaring at around 53 Hz, but you have your head in the exact spot where it is roaring loudest!

I'm not surprised you are unhappy.

Alright, so that's the technical explanation. Now for the "what can you do about it". The answer is "bass trapping". You need massive bass trapping in the corners of that room. Bass trapping is a general term for all types of acoustic treatment that are good at sucking up bass frequencies. It comes in various forms, such as porous absorption, membrane traps, panel traps, perforated panels, slot resonators, and others. It does NOT come in the form of acoustic foam! Foam is no use at all here, because it is way, way too thin to do anything to such low frequencies. You can use porous absorption, but it has to be VERY thick to get down to 50 Hz. The wavelength of a 50 Hz tone is about 22 feet, and there's a basic rule-of-thumb that porous absorption needs to be at least 1/16th of the wavelength in order to be slightly effective, and 1/8th wave is better. At 22 feet, 1/16th is roughly 17 inches, and 1/8 is roughly 34 inches. Thus, if you go with porous absorption, it would need to be about 3 feet thick. That's not as bad as it sounds, because the best place to put the absorption is in the corners (duh! Because that's where the problem is loudest!), and there is a type of bass trap called a "superchunk" that is triangular, 36" deep, and fits in a room corner very nicely, without taking up too much space. This is what it looks like:

Superchunk-8_737.jpg

It's the yellow thing in the corner.

Here it is being built:

Superchunk-3_707.jpg

In your case, you also have a problem in the vertical direction (floor-ceiling), so you will need bass trapping in those corners too, like this:

superchunks-08-vertical-and-horizontal.jpg

They way you make them is very, very simple: Go to your local Home Depot or other favorite hardware store, buy a pack of Owens Corning OC-703 insulation panels, and cut them up into triangles where the short sides measure 36", then stack up those triangles in the room corners, floor to ceiling, as you see in the photo. That's it. You can also make a light wood frame for the front, and put some attractive fabric on it if you want, because superchunks are not exactly pretty!

I would suggest putting one in the corner right behind your head, another one in at least one other corner of the room, and two more in the wall/ceiling corners along the top of the walls on either side of the drum kit. That should go a long way to controlling the modal issues in the room. It won't kill the 50 Hz issue completely, but it should calm it down quite a lot. It's a small room, it's almost a cube, and that makes it tough to kill the modal problems completely, but this should get you a very noticeable improvement.

There are other devices you could build to get it completely under control, but they are harder to design and build, because they have to be tuned specifically to your modal frequencies, and that's not easy to do.

If you can't find OC-703 at your local hardware store, let me know what they do have, in their stock of house insulation: note down brand name and product number for all of their mineral wool and fiberglass insulation. Let me know what the do have, and I'll tell you which one is best for your case.


- Stuart -

[danex]

Thank you so much Stuart!

That all makes sense to me, I really appreciate the in depth explanation.

I do have a couple of questions:

• For the Wall-Wall corners, do I need to treat the entire length of the corner (floor to ceiling), or can I treat above my head and still notice a difference?

• I have some acoustic panels (2' x 6' x 2" thick), will it be useful to place them diagonally, in the wall-wall corners? I was thinking of leaving an air gap behind the panel. Bad idea? Does the space behind need to be filled up in order to trap?

• Any reason for me to treat the corners where the walls meet the bottom of the shelves? Or should I just focus on the real corners where the walls meet the ceiling of the room, instead of the shelves?

Thanks again for your help!
-Danex

[Soundman2020]

• For the Wall-Wall corners, do I need to treat the entire length of the corner (floor to ceiling), or can I treat above my head and still notice a difference?

The most important parts of that corner are up near the ceiling and down near the floor. So you could leave out a part on the middle without too much detriment, if you have to. If you do that, I'd suggest that you do the corner opposite that one, on the other side of the room, from floor to ceiling.

• I have some acoustic panels (2' x 6' x 2" thick), will it be useful to place them diagonally, in the wall-wall corners? I was thinking of leaving an air gap behind the panel. Bad idea? Does the space behind need to be filled up in order to trap?

2" thick isn't very useful for low frequencies. Yes, placing it at an angle with an air gap behind it is valid, and can help to extend the low frequency response, but even so 2" isn't a lot. If those panels were 4" or 6", then they would be more useful. I'd suggest using one or maybe two of those panels on the ceiling, directly above the drum kit, to reduce the reflections from up there. But don't over-do it! Leave most of the ceiling free. Bass trapping and broadband absorption also has the side effect of "sucking out the highs", which can make the room sound too dull and lifeless. There are ways of dealing with that too: so if it turns out that you think the room is too "dead" after you put up the superchunks and ceiling treatment, then drop back in here again, and we'll help you recover some of the life without damaging the bass control. It might not be necessary, though.

• Any reason for me to treat the corners where the walls meet the bottom of the shelves? Or should I just focus on the real corners where the walls meet the ceiling of the room, instead of the shelves?

The corners under the shelves won't be a problem for modes. Modes only form between the actual boundaries of the room: walls, floor and ceiling. In fact, the shelves are probably already helping a bit to "blur" the modes in the vertical direction, which is a good thing! But they are not causing any new modes. Maybe some reflections, but not modes. If you are planning to record your drums with mics on them, then a couple of inches of 703 spray-glued under the shelf would be useful, but it's not necessary for modal treatment.

- Stuart -