Gap or No Gap for Soffit Bass Traps?

[mrfye17]

Hi People,

Besides 6" Panels for side walls, back wall and ceiling, I'm also building Soffit Bass Traps (Roxul Safe'n'Sound) for the 4 Corners (Floor to Ceiling) as well as Front/Rear and Side Wall/Ceiling Corners for my Home Studio.

The 4 Main Corners are good (17" x 17" x 48")...2 in each corner 1 stacked on top of the other

My challenges are with the Side Wall Ceiling Corners.

On 1 side I have a window and the other side an in-built closet, Both with a frame approx. 1/2" thick and 13 1/2" from the Ceiling.

My questions are:

1) Do I make 17" x 17" Soffits and leave a gap to clear the frames or do I make
them 13 1/2" x 17" and have them against the wall?

2) In general, what are the Pros/Cons for leaving small gaps behind any of these
Bass Traps (ie; Absorption effectiveness as well as Air circulation and any
other possible concerns)?

P.s. Traps are not been built as permanent into walls. Will probably use Angle Brackets to Install.


Thank you to all in advance for your advice and Input,

mrfye17

[Soundman2020]

Hi. Please read the forum rules for posting (click here). You seem to be missing a couple of things!

Gap or No Gap for Soffit Bass Traps? NEW REPLY UNSUBSCRIBE

I'm not quite sure what you mean here: do you mean bass traps, or do you mean soffits for your speakers, or do you mean bass traps that are built into speaker soffits, such as we commonly do in RFZ style rooms?

Also, what do you mean by "NEW REPLY UNSUBSCRIBE"? Are you saying that you don't want people to subscribe to your thread, or that you don't want new replies?

The 4 Main Corners are good (17" x 17" x 48")

Your ceiling is only 48" high?

1) Do I make 17" x 17" Soffits and leave a gap to clear the frames or do I make
them 13 1/2" x 17" and have them against the wall?

Without a diagram showing the tow options, and some photos of the room, it's hard to say. I'm not sure I even understand the question!

2) In general, what are the Pros/Cons for leaving small gaps behind any of these
Bass Traps (ie; Absorption effectiveness as well as Air circulation and any
other possible concerns)?

If you have a mold problem in your room, where mold grows behind furniture that is against the wall, then you should get that fixed first. While it is possible to leave air gaps to reduce the possibility of mold growth, it's MUCH better to simply get rid of the problem in advance.

The pros from an acoustic point of view is that leaving an air gap is beneficial, as it extends the low frequency performance of the device beyond what it would be of you used the same device with no air gap. So if you had, for example, 4" of OC-703 over a 4" air gap, that gives you similar performance to having 8" of OC-703 against the wall, but much less than if you had 4" of OC-703 against the wall with no air gap.

P.s. Traps are not been built as permanent into walls. Will probably use Angle Brackets to Install.

What is the purpose of these traps? What type of room is this? (Control room, live room, vocal booth, instrument iso booth, ...) What issues do you want them to deal with? Is this for modal issues, SBIR issues, bass tightness, flutter echo, ... ?

There's just not enough info in your thread to be able to give you a better answer.

- Stuart -

[mrfye17]

Hi Soundman 2020.

Sorry for the confusion.

1) The 4 Main Corner Traps are (17" x 17" x 48" high - Roxul Safe'n'Sound)...2 in each corner 1 stacked on top of the other.
Ceiling is 8ft high.

2) By the frames, I meant the window frame and similar frame around the in-built closet.

3) There is no Mold problem...that I know of lol!
It's just that these frames are 13.5" from the ceilng so if I build these Rectangular boxes (Bass Traps) 17" x 17" then they would have to be installed 1/2" or so away from the wall, therefore leaving an air space. But if I made them 13.5" x 17" then they can sit on top of the frames and touching the wall.

4) This Room will mainly be used for Mixing, Songwriting and Recording of Vocals and Guitars.

Hopefully this clears things up a bit Stuart....

Thank you,

Chris

[DanDan]

Hi Chris, Happy New Year, and Forum
A picture is worth a thousand words.
If I understand you correctly, I would prefer the larger amount of fibre plus the air gap.
Acoustic advice is often given with great feeling, in an attempt to persuade people to do it.
Anyone who does, hears and lives with the result, but many do not get over that initial hump, or don't go at it hard enough.
So particular aspects can be 'over' emphasised IMO.
Modes are very strong and significant in a Concrete Bunker, not so much with a light structure.
The Back Wall is very often seemingly ignored, with only minimal treatment.
IMO it can be THE most important surface to treat. http://forums.musicplayer.com/ubbthread ... ven_matter
The Ceiling is the other biggest area of the room and we often sit at half height.
IMO Clouds matter a lot more than 2- 4 100mm hanging panels suggest.

Words don't convey space, geometry, stuff, to my mind, I'm not visual one might say. Sketchup or even a pencil sketch or a photo would probably put us all 'in the picture' here.
DD

[mrfye17]

Hi and Thank you DD,

I've attached 2 pics... 1 shows the frame around closet and the other shows thickness of frame (approx. 1/2 inch)

I also have a Window on the other side with same type of frame and thickness.

Both frames are 13.5 inches from the ceiling.

These are both on the side walls.
If I make the Soffits 13.5" (height) x 17" (depth) then they can rest on top of the frame, but, if 17" x 17" with a gap is better then I'll do that.

Would you still suggest the 17" x 17" with a gap?

Chris

[DanDan]

Yes. But I remain, same as Stuart I reckon, a tad concerned about the lack of any context here.
e.g. Have you acoustically measured the room? Are there significant modes up there?
I often use the Signal Generator in REW to light up suspect modes to see how loud they are at particular locations.
DD

[mrfye17]

ok....

I came up with a quick design of the room (pics attached)

Size of room is 11.5' x 18.5' x 8' (high).
Will mainly be used for Mixing/Songwriting and Recording Vocals and A/c Guitar.

Panels for side walls, rear wall and ceiling have been made.
Bass Trap Soffits are in progress for the Front/Rear corners and Front/Rear Wall Ceiling corners. (17" x 17" x 48" each)

As shown in pic 2 (Front of Room), there are 2 framed windows and a framed Clothes Closet on the Back Right side (not shown).

All these frames are 13.5" from the ceiling and 1/2" thick (protruding from the wall - as shown in the previous post)

I wanted to build Bass Trap Soffits for the top of these areas as General Bass Trap Absorption but because of these frames I have 2 choices:

1) make them 13.5" x 17" x 48" so they can rest on top of the frames and be flush against the wall, or,
2) make them 17" x 17" x 48" but they would hang in front of the frames, 1/2" gap from the side wall.........

....and don't know which would be better, worse etc.


I hope this clarifies things somewhat.

- Chris

[Gregwor]

Hi Chris,

To simply answer your question, make your bass traps bigger. And yes, having the extra 1/2" gap is a good thing and will improve the bass traps performance.

What the other fellas have brought up are very good points ---

4) This Room will mainly be used for Mixing, Songwriting and Recording of Vocals and Guitars.

Realize that a mixing room and a recording room should have two completely different acoustic characteristics. A mixing room should be transparent not adding anything to the sound reproduced by your speakers. A recording room should add character and vibe acoustically speaking. You can get away with tracking vocals in a mixing room, but acoustic may be lifeless unless you add some variable acoustic devices that you can "open up" to add life into the room during tracking.

The Back Wall is very often seemingly ignored, with only minimal treatment.

Yep. What you should be doing is adding at least 6" of insulation across your entire rear wall (the one at the head of your bed). I mean, the FULL wall. This will probably do more good than anything else.

Have you acoustically measured the room?

This should be step number 1. Do this using the REW instructions on the forum and post your results here. From there, you add one section of acoustic treatment at a time and take a new measurement at each step and post it here. We can all take a look at your file and see if you're making moves in the right direction and suggest the next step. This will ensure a great sounding room all while saving you time and money!

Also, other things I notice in your picture are:
- your clouds are not hard backed
- your speakers are on your desk
- your mixing end of your room is not symmetrical due to racks or whatever
- most people build angled bass traps. Yours are square which eats up a lot of room visually and I don't think them being square is really going to improve their performance as much as wider traps would.
- do you actually have a hard wood floor?

Greg

[DanDan]

Looks like a very normal plan. It should work well.

You might think of adding to the cloud, to omit the gap at the Front Wall. There is a lot of Vertical Modal Action there.
I have always been curious about hard backing clouds. I would love to hear more on that Greg.

I noticed that the speakers are a little distance from the Front Wall. Find the optimum location for them and the corresponding best listener spot by testing. More often than not almost touching the FW is best.

The lack of Reflections and Modes in a well treated mix zone is ideal for vocals. That transparency, lack of room colouration, may also work well for acoustic guitar. Great clarity, but if the guitar doesn't have a really sweet balanced tone within itself, cheap will show itself! Violin, Sax, or particularly Drums will probably sound a bit odd in a deadened room. That said KD Lang's drums sound very nicely odd. I read they were recorded in a closet. I am not sure you have the space for both a dead back wall and some live elements. A full wall treatment covered in a high density of wooden lath would be a good start. Unfortunately your ceiling is low so an absorbent or angled hard cloud over the recording area would be advisable. You could try angled wall elements, reflective, plywood if you encounter flutter echo. It often appears after initial treatment.

Corner, or any LF traps, bigger the better. Soffit Style packs a lot of fibre, but they intrude into the room more than the old wide SSC. I would not recommend small SSCs. Note, Rectangles will work fine too. Also any of these LF traps can have Laths or Pegboard added to reflect HF back into the room.

DD

[Gregwor]

I have always been curious about hard backing clouds. I would love to hear more on that Greg.

With soft clouds, you'd be entirely relying on it to dampen the sound before it hits your ears. Angled hard back clouds will dampen as well as reflect the sound towards the rear wall. That extra distance the sound has to travel will increase your ITDG. More absorption on the rear wall will be yet more fibre for the sound to slow down through and attenuate. Hopefully this gets the first reflections closer to the 20ms and -20dB!

Greg

[Soundman2020]

Angled hard back clouds will dampen as well as reflect the sound towards the rear wall.

Hard-backed clouds can also help to "break up" modes, to a certain extent, if they are large enough and massive enough. Shows up as lower, broader Q for the mode, with steeper decay and less ringing. Among other things...

- Stuart -

[DanDan]

Afaik an angled boundary does not result in significantly weaker modes does it? So I am having difficulty in believing a small area would do so.
I can imagine a full angled boundary sort of encouraging wavefronts in one direction, like an LF horn, but a small area has probably small effect.
A really hard back would prevent the double passage of sound through the fibre. Perhaps causing, allowing, it to go around the obstacle has some benefit?
Is there any test evidence, or more specific theory, to suggest adding a backing to a regular cloud is beneficial?
DD

[Soundman2020]

Afaik an angled boundary does not result in significantly weaker modes does it? So I am having difficulty in believing a small area would do so.

... As I said: "... if they are large enough and massive enough." Take a look at the cloud in the corner control room: it covers about half of the entire ceiling. Take a look at the cloud in Studio Three, or Steve's room: between about a quarter and one third of the total ceiling area. I'd agree that small clouds don't have much effect, but large ones can.

I can imagine a full angled boundary sort of encouraging wavefronts in one direction,

Think of a wave tank (water filled). With a plane wave (or "planar wave" if you prefer), if you put an object in it's path that is small with respect to the wavelength, then it has little to no effect on the passage of the wave. But put in a large one, and you get a decent effect. There's a "shadow" behind the object, and diffraction around the edges, plus other effects. So, for example, with a modal issue at 125 Hz, the wavelength is about 9 feet... so a cloud that is 9 feet wide should have a good effect on that... But even an eight foot or seven foot wide one still would have some effect... (the corner control room cloud is 14 feet wide... Good down to at least 80 Hz...)

A really hard back would prevent the double passage of sound through the fibre.

Nope! Mids and highs would see the hard-back and the insulation on that, not the ceiling. So assuming that you have enough insulation on the cloud, you would still get the same effect as if the waves hit the ceiling with insulation on it. They still get the double-passage through the insulation. No change there. If you have 4" of 703 on the ceiling, or 4" of 703 on the cloud, the effect is the same for mids and highs. Normally-incident waves would still "see" 8", and randomly-incident would see progressively longer distances at higher angles, just like they do for a soft ceiling. And lows see BOTH the cloud AND the ceiling.

Is there any test evidence, or more specific theory, to suggest adding a backing to a regular cloud is beneficial?

The corner control room, REW measurements, immediately before we put the cloud in, and immediately after we put it in:
Purple is before, green is after.

FRANK--REW--FR--30--1200--before-after-cloud2.png

Waterfalls:

BEFORE:

FRANK--REW--WF--30--1200--before-cloud2.png

AFTER:

FRANK--REW--WF--30--1200--after-cloud2.png

Note the mode at about 109 Hz. Not a huge difference, but very much worthwhile. Decay time for 100 Hz band went down from 190 ms to 140 ms. Also note the "smearing" of the the mode at 53 Hz. It starts off narrow, high Q, then ends up a bit broader, lower Q (level increase is due to an inadvertent increase in the sub volume, not the cloud!).

It works.


- Stuart -

[DanDan]

Thanks Stuart. Some clarification. A hard back will of course reflect back through the fibre up to a point or frequency. But most soft clouds would have an air gap, so that frequency would typically be at least an octave lower.
Unless I am missing something, your tests show improvements caused by hard back clouds. But I would love to see a test of hard back versus fibre only, both with similar air gaps.
It is unlikely that anyone has such a test so we may never know if hard back has benefits over simple fibre or not. Strikes me as similar to Hangars. We are assured the 'work well'. No doubt, but I wonder if simply filling the same space with fibre might perform just as well or better.
DD

[Soundman2020]

Unless I am missing something, your tests show improvements caused by hard back clouds. But I would love to see a test of hard back versus fibre only, both with similar air gaps.

Fair point, but I can't imagine a typical home-studio builder wanting to do such a test! It's hard enough getting a sufficiently large cloud up there, without then having to take it down and put the back on, only to raise it again....

On the other hand, I'm not aware of any mechanism from a soft cloud that could account for the modal "smearing" seen here, at both 53 Hz and even down at 35 Hz. That can't be explained by porous absorption. Porous absorption will lower the frequency, yes, as we have discussed on other threads, (which didn't happen here, interestingly enough....), and it will reduce the decay time, but it won't "smear" the mode out horizontally (across a broader range), as it did here. There's only about 3" of insulation on the cloud, so it's feasibly to postulate effects down to about 160 Hz from that (using the "3.5% of wavelength" theory), but not at 35 Hz. Not to mention that the 3.5% theory assumes random incident sound, but axial modes, by definition, are NOT randomly incident! They must always be totally normal to the surface (or the would not be axial....), so the 7% rule would apply to modes. And below the lowest mode, you are into pure pressure territory ("pumping"), which is, once again, always normal to the surface, by definition, and once again the 7% rule applies. 7% wavelength at 3" depth is 320 Hz... (which is not even pressure territory either!). So 35 Hz is not a porous absorber effect, no matter which way you cut it. I mean, don't get me wrong! I totally agree with Andre on his 3.5% and 7% theory, since I have sen it work in practice numerous times! Absolutely, it is correct! That's why I use thick porous where I need simple treatment that goes low. But we are talking "way too low and way to thin" here. You'd need something like 28" to get down that low.... which is why I do superchunks at 36", if there's enough space...

OK, so there's also an air gap above the cloud that ranges from about 4" to about 17" (the cloud is angled, of course), but for an air gap we are talking true 1/4 wave effect (so 25%, not 3.5%, since there's no insulation in the gap, and the 3.5% or 7% theory is for insulation, not for air...), so that air gap above the cloud would only explain effects down to 200 Hz. Maybe if you wanted to be really optimistic, you could expect effects an octave lower, but that's still only 100 Hz... 35Hz is nearly three octaves lower.... Even 53 Hz is two octaves lower. No way the gap is doing that.

So I'm confident that it is the hard back itself that is responsible for that very low frequency smoothing. Well, along with a few other little "tricks" that I built into that cloud, and which I'm not prepared to share in public.... Not all of my "secrets" are out there for free!

- Stuart -