[Letitroll98]

After nine months in my new digs I've finally gotten around to frequency sweeps in my spare bedroom audio room.  A 2k dip was handled with repositioning my acoustic panels, but the expected 50Hz room dip was still there and I'd ditched my bass traps years before.  So before building new ones I was thinking, darn, those two single hung windows would make perfect 2'x3' infinite absorbsion bass traps if left them open in these beautiful spring days.  Why didn't it work?  No change whatsoever in the room dip, the very typical sharp 10-12db notch at exactly where it's supposed to be, no mystery there.  As it's about the same size as the old listening room I know I can get around a 50% reduction of the attention by constructing new bass traps.  But why didn't the open windows work?   Is the area of the windows too small?

[JLM]

Yes, an open window makes for the theoretically perfect bass trap.  Except for the issue of sound transmission in and out, neighbors hearing your sound and you hearing theirs.  Plus there's the temperature extremes to consider. 

[youngho]

It would depend very much on the location of the windows relative to the specific mode or boundary causing the interference dip, but keep in mind that the walls around the windows would still support reflections or standing waves, so it could be the area or the location or both.

[Letitroll98]

It would depend very much on the location of the windows relative to the specific mode or boundary causing the interference dip, but keep in mind that the walls around the windows would still support reflections or standing waves, so it could be the area or the location or both.

I also think placement and size were the issue, but I don't have enough experience or data to confirm it.  As with most windows they're close to the center of each wall, a corner room, the opposite of where you'd put base traps.  My old traps were two 4' sections stacked to be floor to ceiling in each front wall corner, 2-3' deep, can't recall exactly.   All that was only 50% effective at smoothing a similar dip.  I thought the infinite absorbsion characteristics of an open space might compensate for the size difference, not so much it seems.

JLM, where I live the temperature never varies more than 2 degrees plus or minus 72° and the neighbors are all deaf.  Well, it is very quiet here with the chickadees, sparrows, junkos, and titmouses making the most noise outside of these windows.  And being mid May in the Laurel Highlands the experiment was reasonable, becoming less so as the year progresses.  Thanks for your responses guys.  Anyone have an idea on the math for this situation, just for fun.

[richidoo]

Open or closed the 50Hz energy will go right through a window because it flexes. It will also go right through flimsy modern new home construction walls, like 2x4 with vinyl siding and long floor and ceiling joist spans.

50Hz is very long wavelength (22 feet) which is why it required such big absorbers to have moderate effect. Any room dimensions smaller than 22' will cause the wavelength to fold and create a mode. Damping with standard open friction absorbers may require very large area and thickness, which will dry out the room too much unless you also use an effective diffusion mask on the front.

Consider the floor or ceiling may be resonating with that low freq. If possible to brace it may be worth a try.

A diaphragmatic membrane absorber tuned for low bass (like Acoustic Fields) would be more effective than bare absorber panels. An active bass trap is the most effective for bass cancellation at any freq.

Also placing the LF absorber close to the energy source you are trying to damp will increase the efficiency dramatically.

Corner preference for bass absorbers is just industry dogma. You have to move treatments around and measure to find the best location. Closer to the source is more valuable general advice than in the corners.

[youngho]

Richidoo has a good point. The single-hung windows likely don't reflect much bass when closed, anyway, hence virtually no difference whether they're open or closed. Also, at 6 square feet, they likely represent <10% of the surface area of the boundary on which they're mounted, anyway, plus the locations as you noted.

[Letitroll98]

Great response guys, just what I was looking for.   Rich, I thought the reason for corner placement of bass traps was because that's where the nodes are most strongly re-enforced.  I will try moving them around as you're not the first I've heard this from, what could it hurt and I might learn something.  So the open windows aren't absorbing anything, no energy is being transformed into heat.  Rather than being infinite absorbers they're zero absorbers, which explains the duplicate measurements.

Yes, I'm aware of the tuned bass traps, I believe there's a diy version out there.  I was playing around with the idea when I built my old ones out of Safe n Sound.  I imagine it won't be hard to find the plans again.