Door as bass trap?

[Alexanderc]

I’ve wondered about this for a long time…

My listening space is a small office c. 11x13 feet. The entrance is directly opposite the right speaker (behind and to the right of the listening position).

At night and if my family is home I keep the door closed. Other times I generally leave it open.

When it’s open, does the open door function like a door-sized bass trap? It’s about 6 feet to the wall opposite the entrance (which also has a door that’s usually open, a bookshelf, some other irregularities...it’s not just a flat wall that reflects straight back). I’ve read things that make me wonder if it’s more complicated than that, but I bet someone hereabouts knows the answer.

Honestly, I don’t feel like I hear any particular difference in the sound open vs. closed, so it’s really just curiosity motivating me to ask.

 

[pozz]

There's a difference between a true acoustic boundary and an apparent one. True boundaries for bass will be brick, concrete. So usually the facade, not the walls of interior spaces. Doors are meant for isolation rather than absorption ("trapping"), where you need definite flexure. And isolation in domestic construction targets midrange noise, speech and so forth.

At bass frenquencies the door will simply be ignored, especially if it's a standard hollow-core and nothing's been done to seal the edges. At most, if open, it will shift the position of certain standing waves.

 

[Alexanderc]

There's a difference between a true acoustic boundary and an apparent one. True boundaries for bass will be brick, concrete. So usually the facade, not the walls of interior spaces. Doors are meant for isolation rather than absorption ("trapping"), where you need definite flexure. And isolation in domestic construction targets midrange noise, speech and so forth.

At bass frenquencies the door will simply be ignored, especially if it's a standard hollow-core and nothing's been done to seal the edges. At most, if open, it will shift the position of certain standing waves.

Thanks. So no bass-specific difference is likely, you’re saying, which makes sense. What changes might it cause? Would the door only be reflective of wavelengths equal or less than it’s width (or something)? Am I overly optimistic that it will be something simple and obvious like that?

 

[pozz]

Thanks. So no bass-specific difference is likely, you’re saying, which makes sense. What changes might it cause? Would the door only be reflective of wavelengths equal or less than it’s width (or something)? Am I overly optimistic that it will be something simple and obvious like that?

It's pretty much that simple. All material will pass, reflect and absorb some sound. The absorptivity (if that's a word) depends on how porous and how much air it allows to pass (based on air velocity, effective at mid and high frequencies), and how flexible and massive it is (based on air pressure movements, effective at lower and bass frequencies). So it will depend on the resonant qualities of the door, but I'd assume it would absorb something in the low mids. From what I know of specilized acoustical construction doors are treated almost strictly as barriers and absorption is not seriously considered. The main associated measure is sound transmission class (STC) to quantify isolation.

There may be some effect on imaging or spaciousness if it's open or closed, but I'd expect it to be minor unless you're in a treated room or the door happens to be close to your listening position.

 

[dasdoing]

it is, BUUUUUT

it depends on the amplitude of the sound that comes back from the outside. if there is a small room behind the door it will actualy amplify the bass.
obviousy any bass that escapes there and doesn't come back is "absorbed".

btw: it doesn't matter if the door is open or not for bass

 

[Alexanderc]

I probably should have said "broadband absorber" rather than bass trap from the beginning, but I am grateful to have the insight of those who know more than I. Thanks! My experience has been that turning Audyssey off and on made a much bigger difference than opening/closing the door, and it's nice to have an explanation for that.