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How do windows affect standing waves and bass trap behaviour?

13K views 11 replies 4 participants last post by  bjs 
#1 ·
I'm curious how a window behaves acoustically at the lower frequencies (say 200hz and lower)?

I've been "allowed" to convert an upstairs room into a music/ht room. The room is basically a 14.5' x 17.5' foot rectangle box (8 foot ceiling) with the end behind the listening area containing a large double width patio sliding door (ie window) opening to a balconey.

I'm wondering how this large window area on the end wall will affect the room's resonant modes?

I'm also wondering how a base trap in front of the window would behave?

Does anyone know (or can point me to where the effect of windows on lower frequencies and bass traps etc) is explained?

Any input greatly appreciated!
 
#2 ·
Welcome to the forum.

Actually, a window like that actually passes bass pretty well. It's not going to pass it all but it's better than a drywall wall. A bass absorber or 2 in front of it will only help the effect and (in the case of a broadband treatment), help negate the harsh upper mid and high frequency reflections. Not that it's a big deal since you'll have to have a curtain over it for light control anyway.

Bryan
 
#6 ·
Bass trap placement is not critical in terms of general decay time reduction. However, to address specific cancellations and reinforcements, it's very critical.
 
G
#7 ·
While many seem to focus on the windows ability to impede the sound transmission to other spaces, little has been said regarding the window's affect upon standing waves within the room.

A flush mounted window will have VERY little if any affect upon standing waves within the room.
In fact, they would only begin to effect the standing waves if they were recessed deeply and if the surface area of the window is large relative to the LF wavelength.

To the degree that the recess is deep and the surface area large, sufficient to interact with the LF wavelengths, the recess volume will begin to act as a coupled space, slightly modifying the standing waves.

Calculating the effect is prohibitively complex (and please note that the room mode calculators which you may access are at best ONLY valid or an ideal rectangular space.

In order to accurately determine real room mode behavior in a room, a cumulative spectral decay curve would prove helpful.



Regarding the windows impact on bass traps, they have little relationship. The only issue here again is the nature of the room modes - and any degree to which a recessed coupled space would affect the total room mode response. Bass traps would ideally be tuned exhibiting a high Q to address the actual resonant room modes frequencies while not not absorbing mid and high frequency energy which should be analyzed via such measurements as the Envelope Time Curve (ETC).

The remaining mid and high frequency acoustical energy should be analyzed in the time domain with the ETC and where the surgical application of absorption and the greater use of diffusion will address the arrival time and intensity of the first order reflections sufficient to create an Initial Signal Delay Gap and the later arriving energy in the form of focused specular reflections is then addressed for excess intensity via diffusion (and only in anomalous situations via surgical absorption).

The use of diffusion will then mitigate the intensity by creating a more diffuse scattering of the specular reflection and simultaneously creating a well-behaved semi-reverberant sound field that decays in intensity via greater diffusion instead of via discrete focused specular reflections.The use of diffusion, rather than absorption, of the remaining focused specular reflections will use the finite acoustic energy to create a well behaved semi-reverberant sound field that imparts a pleasing sense of space - the sense that the room is larger than it actually is.

In a small acoustic space where acoustic energy is finite and no statistically diffuse reverberant sound field exists, there is no need to worry about decay times - except to increase the presence of an already too rare well behaved semi-reverberant sound field via diffusion. And this is accomplished by diffusing the focused specular reflections.
 
#8 ·
...A flush mounted window will have VERY little if any affect upon standing waves within the room...
Are you saying the room will have the same standing waves with a wall there instead of the flush mounted window? If the window is transparent at the bass frequencies then how does the standing wave (in that axial direction) set up? It's a big window...as much window as wall in fact.
 
#11 ·
Agreed. The large windows are difficult to predict. They will not so much shift WHERE the modal issues are as impact the intensity of the modal problems. However, you still have to consider the impact of the space outside the room and it's modes and what will be transmitted back through the glass into this space and how it will interact with the predicted modes inside the space.

In short, measure it.

Bryan
 
#12 ·
Thanks guys, interesting discussion. I've just finished the room conversion this weekend...so time to measure...analyze...learn...then treat.

By the way, there is only empty space on the other side of the big window since it opens to the outside (on the second floor).

I'm interested in the diffusion approach to handling some of the focused specular reflections. In particular while this room is a rectangular shoe box, it does have (in the rear right hand side) a doorway leading to a walk-in closet and ensuite bathroom...the total volume is about 500 cubic feet. So this is a sizeable area (compared to the 2000 cubic feet of the main room) which potentially can be used to augment the reverbant soundfield with some form of phase grating.
 
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