Can a Room 'Leak' Bass?

[Rutgar]

Can a room 'leak' bass? I understand how a fairly solid room with minimal openings can 'hold' bass in, thus causing peaks and dips. But can the opposite be true? Can a room due to openings in the corners, windows, flimsy walls etc., let so much bass out that even with powered woofers, the bass performance is underwhelming? And in such a case, would Bass Traps offer little to no benefit?

 

[DonH50]

There are at least two issues... Walls flex and openings/windows etc. can let sound out thereby "leaking" energy that would otherwise be constrained in the room. Technically that means less energy in the room for you to hear, but in practice you'd never know it.

In-room modes could vibrate walls and lead to coupling that reduces the bass at certain frequencies, but by and large I think room modes are orders of magnitude more of a concern that what the walls themselves are doing. In fact, the walls probably contribute little to nulls, and by vibrating absorb energy and might reduce the peaks a hair.

One last factor is that window wells and other openings can act like resonators at certain frequencies, thereby creating peaks and valleys in the response.

 

[cjfrbw]

I think has to do with the stiffness or springiness of the air in the room. Open a window, softer spring and you change the resonance modes, as well as using some of the bass energy to vibrate the outside air instead of your boody. The room is like sitting in a speaker cabinet. Infinite baffle or vented, things will change.

 

[DonH50]

Hmmm... Not sure I follow the air springiness bit, but it did make me realize I assumed everything was closed. Openings, like an open door or window, change the effective volume and can create a LF resonator structure (like a Helmholtz resonator), either of which could "kill" the bass.

 

[Rutgar]

Sorry, I don't get the 'air springiness' either. As far as windows go, I was referring to them 'leaking' through the glass. Opening a window to the outside seems completely undesirable to me.

Open doors to other rooms, on the other hand, I can see having an effect. And closing them would make sense to me (if the leaky bass thing really applies).

 

[DonH50]

In that case, I stand by my original post. More concisely, I find it hard to believe walls and windows will flex enough to significantly change the frequency reponse in a room. Change, yes, but I really doubt you could hear it.

 

[GaryProtein]

I agree with Don. Unless the walls are clearly flexible, the dimensions are much more important than the construction materials.

Having said that, I don't think you can, as they say, "pressurize" a room with bass. A room with four walls will have a stronger bass than a room with three or only two walls because the listener will not hear as much reflected bass, but the room isn't pressurized in the usual sense of the word.

 

[Mark Seaton]

Can a room 'leak' bass? I understand how a fairly solid room with minimal openings can 'hold' bass in, thus causing peaks and dips. But can the opposite be true? Can a room due to openings in the corners, windows, flimsy walls etc., let so much bass out that even with powered woofers, the bass performance is underwhelming? And in such a case, would Bass Traps offer little to no benefit?

Hi Bill,

At the lowest frequencies, very solid construction can result in significant gain which is often 6-12dB/octave below a frequency which correlates to the longest dimensions of the room. The differences in the amount of gain would be exemplified by a basement with all walls formed by the foundation vs. a room with multiple windows on every wall. Above this low frequency (12-25Hz in most rooms), room modes and related acoustic interactions dominate. "Leaky" walls will tend to reduce the max and minimum of the modal peaks and valleys related to the simple dimensions of the room. This is generally a good thing for sound quality.

Unless your point of reference was a room with huge peaks and over emphasis of ranges, I would not expect what you describe from just a leaky room. Are you able to measure the bass response in your room? If not, an investment in something like an OmniMic, XTZ Room Analyzer or similar would be well worth it. Most likely the problem is weak translation of your speaker's bass to your listening position over important frequency ranges.

This can happen in a room with rigid or flimsy walls/surfaces. Where I have seen plenty of head-scratching situations are cases where opposing walls differ greatly in construction or rigidity. A flimsy wall opposite a brick wall is a great example where you can get some big holes in the response, especially as you pull away from the solid wall toward the flimsy one.

 

[Ethan Winer]

Can a room due to openings in the corners, windows, flimsy walls etc., let so much bass out that even with powered woofers, the bass performance is underwhelming? And in such a case, would Bass Traps offer little to no benefit?

In fact the opposite is true. The ideal room would "leak" all the bass out through flimsy walls, and then the bass response inside the room would be perfectly flat. You wouldn't have any response peaks or "room gain," but you wouldn't have any nulls either. So it would be flat, just like outdoors. I often joke that the ideal room would have walls made of cardboard. Note the key is nulls, which like peaks are caused by reflections off the walls, floor, and ceiling. When a room lacks bass, the cause is nulls. Look at this response, which is typical:

The biggest problem is clearly the nulls, where much of the energy is reduced by a staggering 20 dB. This is the cause of weak bass.

As for bass traps, in a room with completely flimsy walls bass traps aren't needed. But by flimsy I mean cardboard, not thin sheet rock or thin plywood.

--Ethan

 

[Rutgar]

Ah, so a room 'leaking' bass is just another one of those crazy things you read about on the Internet. I was reading on some web site the other day that suggested this, which is why I brought it up. Stuff like this makes me think of that commercial where the girl says she is dating a French model she met on the Internet (http://www.youtube.com/watch?v=bufTna0WArc).

So thanks everyone for the replies.

Ethan, I can tell you that the 8 Mondo Traps I bought have helped. One interesting thing I found out, is that by moving the 2 Mondo Traps I initially had below my screen to Corners in the alcoves (with the doors all shut), I actually get better performance. So evidently, not all corners of a room are equal.

 

[Ethan Winer]

Stuff like this makes me think of that commercial where the girl says she is dating a French model she met on the Internet (http://www.youtube.com/watch?v=bufTna0WArc).

LOL, that's great.

So evidently, not all corners of a room are equal.

Yes! Which corners are good for bass trapping depends on a lot of factors that make it difficult to calculate: Where within the room your speakers are placed, where you sit while listening, and which frequencies are being considered. My article Pink noise aids placing bass traps shows a good method for finding the best places for a limited number of traps.

--Ethan

 

[Rutgar]

A question for you Ethan (or anyone else that may know the answer)...

When mounting a Mondotrap flat on the ceiling or wall, what is the desired air gap or spacing from the wall (if any), for maximum performance?

 

[DonH50]

Rule of thumb is to mount off the wall equal to the thickness of the trap, but that's as much asesthetics as anything. Since there is a null at the wall (or close to it) you get more absorption getting them a few inches off where the waves are moving. I think Ethan has plots on his site that show the difference, and I'm pretty sure he mentions how to hang them.

Ethan probably has a better answer, but FWIWFM - Don

 

[Ethan Winer]

I do not have a better answer. My book and Acoustics FAQ both explain why the theoretically ideal gap is the same as the panel thickness. But for thicker panels, say 3-4 inches or thicker, the gap can be larger for even more benefit. You could probably have as much as one foot of air space to extend the absorption even lower in frequency without getting a "hole" at some frequencies.

--Ethan

 

[DonH50]

Geez, misspelled your name, sorry, corrected! My keyboard has been going flaky...

In penance, while I was thinking about it, just ordered your book.

 

[Rutgar]

Thank you both, Don and Ethan. I will mount the panels using the recommended 4" post bases. I just wanted to make sure before I put more holes in the wall.

The ceiling will be a bit more challenging. For one thing, the ceiling slopes a bit on one side where I would want to place a trap.

 

[DonH50]

To hang my absorbers I used hanging plant hooks, the kind made for indoor ceilings (toggle bolts that go in a small hole then expand to hold) and short lengths of chain. I used two hangers per panel, centered along the long axis and a bit (6") back from the edge so the chains are hard to see from below.

 

[Rutgar]

I will keep that idea in mind. I want to put some thought in this. I don't need a Mondo Trap falling on my head (or worse... my speakers!)

 

[flyer]

In fact the opposite is true. The ideal room would "leak" all the bass out through flimsy walls, and then the bass response inside the room would be perfectly flat. You wouldn't have any response peaks or "room gain," but you wouldn't have any nulls either. So it would be flat, just like outdoors. I often joke that the ideal room would have walls made of cardboard. Note the key is nulls, which like peaks are caused by reflections off the walls, floor, and ceiling. When a room lacks bass, the cause is nulls. Look at this response, which is typical:

The biggest problem is clearly the nulls, where much of the energy is reduced by a staggering 20 dB. This is the cause of weak bass.

As for bass traps, in a room with completely flimsy walls bass traps aren't needed. But by flimsy I mean cardboard, not thin sheet rock or thin plywood.

--Ethan

A simple example; why do we have bass when listening in our car? Because the walls of the car are such that they allow a sufficient amount of bass energy to permeate causing few reflections and nulls.

Ethan is very right (and he should

 

[Nightlord]

Well, if you ask my neighbours, my cinema-in-construction was definitely leaking bass enough to give me a call one I had completed the test scenes from "War of the Worlds". Now the word 'leak' vould of course be turned into a definitio debate, but let's not go there.

( Am rebuilding inner walls to improve sound insulation, but am afraid I'll never reach the target I set for the theater, was expecting the light concrete blocks in the walls to bemore efficient than they've proven to be... )