John Sayers' Recording Studio Design Forum Archive

Hi,
this is my first post in this forum. After an embarrassingly unsuccessful attempt to build a membrane bass trap, I detected this great, very helpful forum and have been reading a lot for the last weeks, but I could need some more advice...

Task: I want to balance the bass response in my little bedroom studio (=control room), and first of all get rid of an annoying peak at 123Hz(b1)...131Hz(c1). This peak seems to correspond with the height of the room (2.62m=9').
To check the room acoustics, I played a sine wave with a softsynth and raised the tone slowly from 30Hz to 250Hz. The valleys(d#2, 156Hz) and peaks were quite obvious, so I didn't bother to make any measurements yet.

Question 1: Is it effectual to build one or more (fibre glass) corner traps, or do I have to cover the ceiling with fibre glass or maybe drywall?
Question 2: Where should I place a corner trap? I'd prefer the vertical corner between the front wall and the left wall (see drawing), but maybe the horizontal corner between front wall and ceiling would be better?

I forgot to draw the subwoofer: it is placed on the floor under the desk, 1m (3.3') from the front wall and 1.1m (3.6') from the left wall.
I use a Tannoy sub 10, the speakers on the desk are Tannoy Reveal Actives.

Peter,

> Is it effectual to build one or more (fibre glass) corner traps, or do I have to cover the ceiling with fibre glass or maybe drywall? <

You need as many bass traps as you can fit. Basically, the smaller the room, the more bass trapping you need. The best place for rigid fiberglass traps is straddling the room corners, including the wall-ceiling corners. You can put rigid fiberglass on the ceiling, but it won't absorb the lowest frequencies unless it's very thick and/or spaced away from the surface at least several inches.

> Where should I place a corner trap? <

Er, um, well...

Have a look at the Acoustics FAQ, second in the list on my Articles page:

www.ethanwiner.com/articles.html

It's a long read, but it explains a lot about this stuff. Then feel free to ask any followup questions.

--Ethan

Thank you Ethan for your reply, but I'm afraid I'll have to state my questions more precicely (sorry if my English is not too good):

1. If the main problem of the room really is standing waves between floor and ceiling, then do corner traps make sense?
I'm not sure if a 125Hz wave is omnidirectional, or if the direction of wave propagation is important anyway.
I've read that drywall absorbs good at 125 Hz. would it be recommendable to put a drywall (with some air gap) under the ceiling?

2. Supposed I want to build only 1 corner trap: which corner would be the best place for a corner trap?
At the rear wall, there is little space for corner traps, but maybe the rear wall is better?

Peter,

> If the main problem of the room really is standing waves between floor and ceiling, then do corner traps make sense? <

Yes, especially since there are corners at the tops of those walls and you can treat those too. I'll also mention that peak and dip frequencies are often related more to where you are in the room than to the room dimensions. The only way to tell which is which is to measure at a variety of locations. I just added a new article to my company's web site that explains this. Follow the link under my name below, and find "Do room modes even matter?" on the Articles page. I'll give away the ending first: Yes, room modes matter, but not as much as many people think.

> I've read that drywall absorbs good at 125 Hz <

A real bass trap will absorb far more than drywall.

> Supposed I want to build only 1 corner trap: which corner would be the best place for a corner trap? <

One corner trap won't help much of at all.

--Ethan

Ethan Winer wrote:One corner trap won't help much of at all.

--Ethan

sorry for the late response, Ethan. and thank you very much for your help !
Currently I have build two corner traps, and apparently, as you stated, I will have to invest more work and build more traps. Perhaps the glass fibre sheets I use are too thin (50mm=2"), density is about 80kg/cubic metre. Bass response hasn't changed yet.
I will have to buy some more glass fibre with higher density (very pricy here), or maybe rock wool?

Peter,

> will have to invest more work and build more traps. <

Yes.

> Perhaps the glass fibre sheets I use are too thin (50mm=2") <

Yes, that too. Three inches thick is a lot better than two, and four is better still.

> or maybe rock wool? <

Sure, same thing basically. As long as it's rigid and has a similar density.

--Ethan

...>> Perhaps the glass fibre sheets I use are too thin (50mm=2") <

>Yes, that too. Three inches thick is a lot better than two, and four is >better still.

ok, then I will simply use two sheets of glass fibre instead of one. The sheets I use are a bit pricy (10€/m2), but eventually it serves a good purpose - BASS!
Peter

Peter Buhr wrote:...Question 1: Is it effectual to build one or more (fibre glass) corner traps, or do I have to cover the ceiling with fibre glass or maybe drywall?
Question 2: Where should I place a corner trap? I'd prefer the vertical corner between the front wall and the left wall (see drawing), but maybe the horizontal corner between front wall and ceiling would be better?

First, I would rotate your whole listening position and face the left corner of the room. That corner would become the base trap. And would also give you a splayed wall design.

That base trap would be a simple broad band design with two air gaps and two layers of 2" fiberglass (about 6" air gap). Bass traps are more efficient when a density change is introduced.

Standard 2" fiberglass panels would also be applied to the side wall and ceiling up to the location of the near-field monitors so that no sound from the front can be reflected back to the listening position.

If there is room, a rear diffusion GoBo could be added behind the listening position w/ absorption on the opposite side.

Placement of the sub should be experimented with...I wouldn't want it to couple with the corner/floor/desk...need a clean and tight sounding bottom end.

Jim B wrote: First, I would rotate your whole listening position and face the left corner of the room. That corner would become the base trap. And would also give you a splayed wall design.

Brilliant idea ! I have seen so many typical "symmetrical room: face the short wall" assemblies that I just didn't thought of an alternative.

Jim B wrote: That bass trap would be a simple broad band design with two air gaps and two layers of 2" fiberglass (about 6" air gap). Bass traps are more efficient when a density change is introduced.

this answers the question I wanted to ask today

Jim B wrote: Standard 2" fiberglass panels would also be applied to the side wall and ceiling up to the location of the near-field monitors so that no sound from the front can be reflected back to the listening position.

Do you mean these panels should absorb sound that was reflected from the back of the room? Or do you mean they shall absorb low frequencies, that are emitted omnidirectional from the stereo speakers?
(I just tried to tested the direction of emittance of tones: even 2000Hz tones are well audible beside (90°) the speakers, so maybe panels beside the speakers would absorb low and mid frequencies, too.)

Jim B wrote: If there is room, a rear diffusion GoBo could be added behind the listening position w/ absorption on the opposite side.

there is not enough space (the room would look rather uncomfortable with an additional wall in it, I think), maybe the shelf is a good diffusor, too.

Peter wrote: Standard 2" fiberglass panels would also be applied to the side wall and ceiling up to the location of the near-field monitors so that no sound from the front can be reflected back to the listening position.
Do you mean these panels should absorb sound that was reflected from the back of the room? Or do you mean they shall absorb low frequencies, that are emitted omnidirectional from the stereo speakers?
(I just tried to tested the direction of emittance of tones: even 2000Hz tones are well audible beside (90°) the speakers, so maybe panels beside the speakers would absorb low and mid frequencies, too.)

Yes. In a Live end/dead end design...the front 3rd of the room is kept absorptive so that minimal first reflections reach the listening position (center of the room)...no matter where they come from. The rear wall is kept live and diffusors are added to remove slap echo while retaining a healthy reverb time. Sometimes diffusion is required on the side walls (when walls are parallel) and on the ceiling just behind the listening position to help treat room modes.

Of course, any corner is a canditate for a bass trap...but sometimes sub placement is even more critical. I love the Tannoy monitors, but haven't heard the sub. I wonder what the crossover point is...

First, LEDE in its true implementation causes some really dead sounding rooms, that's why it's no longer recommended by most acousticians - second, a rectangular room is NOT a candidate for corner placement of the mix position, because it leaves unbalanced reflection problems, the lower frequency of which can't be readily absorbed without giving up a couple of FEET of floor space - third, by the time you control all the reflections coming back from square corners to the mix position (in addition to ceiling absorption for flutter echo control), you'll likely have no high frequencies left in the room.

I've run a simple ray trace of a more square small room than yours in the hope of doing just what Jim suggested, and the area of absorption required to "fix" all the inherent problems would have made the room nearly anechoic. Not a pleasant room to work in.

Also, that room is too small for diffusion to work well in - probably the simplest approach would be something similar to the Auralex Max-wall approach, where splayed temporary panels are placed either side of the mix station along with a sloped false "ceiling" panel overhead, using an absorbent on the insides. Then, fairly heavy absorption at the rear of the room (with all that glass, you might build a portable trap to place in front of the glass while mixing) and 4" rigid fiberglass in all corners (the one by the door might be tricky) to tame bass, and you'll be as close as you can get in that size room.

If you can't visualize what I'm talking about, I'd be glad to mark up your drawing - let me know, and I'll put it on my list... Steve

knightfly wrote:First, LEDE in its true implementation causes some really dead sounding rooms, that's why it's no longer recommended by most acousticians - second, a rectangular room is NOT a candidate for corner placement of the mix position, because it leaves unbalanced reflection problems, the lower frequency of which can't be readily absorbed without giving up a couple of FEET of floor space - third, by the time you control all the reflections coming back from square corners to the mix position (in addition to ceiling absorption for flutter echo control), you'll likely have no high frequencies left in the room.

I haven't had a chance to read about the evils of the LEDE design...but I still view it as a starting point in studio design. Certainly the concept of removing first reflections by using absorption and retaining reverb time with the use of 3 dimensional diffusion has it's merits. I would not be quick to throw the baby out with the bath water.

I've run a simple ray trace of a more square small room than yours in the hope of doing just what Jim suggested, and the area of absorption required to "fix" all the inherent problems would have made the room nearly anechoic. Not a pleasant room to work in.

I only suggested placing absorption in the first third of the listening area. The splayed wall and rear diffusion would help control the room mode.

Also, that room is too small for diffusion to work well in - probably the simplest approach would be something similar to the Auralex Max-wall approach, where splayed temporary panels are placed either side of the mix station along with a sloped false "ceiling" panel overhead, using an absorbent on the insides. Then, fairly heavy absorption at the rear of the room (with all that glass, you might build a portable trap to place in front of the glass while mixing) and 4" rigid fiberglass in all corners (the one by the door might be tricky) to tame bass, and you'll be as close as you can get in that size room.

I would suggest that diffisusion could be used effectively....yet some of the products available would definitely not work in this space.

A reflector side wall design would also work well in this space. Yet, the shelves and furniture remain a problem of symmetry.

Jim B wrote: I love the Tannoy monitors, but haven't heard the sub. I wonder what the crossover point is...

the crossover is at 80 Hz. The sub10 blends very well with the (active) tannoys, I like it!

Steve,

For me it's a bit difficult to understand why there would be a bigger problem with low frequencies when the mix position faces a corner.
But maybe such complex problems can only be found out with a raytracing program you use.

I assume that there would be problems with unsymmetrical reflections in my room, because it is not square and therefore the reflections for the left and the right speaker would be very different.

Should a sloped false ceiling panel connect with the sidewalls (means "airtight", covering the whole ceiling) or did you mean some kind of hangers? the absorbent should not face the floor, right?

if you can't visualize what I'm talking about, I'd be glad to mark up your drawing

that would be nice!