# How do windows affect standing waves and bass trap behaviour?



## bjs

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!


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## bpape

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


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## bjs

Thanks. Yes, I'll have a heavy curtain for light control (as well as blackout cloth blinds behind the curtain) and expect all this will do double duty for mid/high frequency control. That left me wondering if adding a bass trap would add anything worthwhile in the low frequencies but from your answer it appears not (or not much anyway).

I'm expecting trouble from the boxey room shape...that's actually how I got to this site...searching for suitable measurement programs, found REW and now I'm committed it seems...! :yikes:

Hopefully the benefits from bass trapping are real rather than imaginary...all new to me...time will tell I guess...:dunno:


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## OvalNut

> Hopefully the benefits from bass trapping are real rather than imaginary


Oh, they're very real alright.:clap:


Tim
:drive:


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## Mirek

As far as I know, bass trap placement is not critical so that by the window should not make any harm.

Regards
Mirek


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## bpape

Bass trap placement is not critical in terms of general decay time reduction. However, to address specific cancellations and reinforcements, it's very critical.


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## Guest

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.


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## bjs

mas said:


> ...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.


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## bpape

The window will pass more bass than a structural drywall wall. However, it does in fact still present a boundary for modal issues.

Bryan


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## Guest

bjs said:


> 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.


Essentially, yes. But let me qualify this!:bigsmile: As is often the case, several factors determine the specific final answer, and my final answer will assert that while _small _flush mounted windows will have a small effect, large windows such as you are suggesting _will_ have an effect.

Standing waves/room modes are a function of wavelengths that are large relative to the dimensions of the room. 

While higher frequencies where the wavelengths are smaller relative to the structures encountered and manifest themselves as specular reflections are effected to a greater degree. Without going to much into the physics of the wave mechanics, they in effect exhibit differing characteristics. Room modes function as a pressure wave - with areas of low and high pressure determined by the dimensions of the room. (And as so many point folks to room mode calculators, ALL of them necessarily assume a perfect rectangular space with NO alcoves, recesses, or other variations (in dimensions or surface impedance) - as these quickly affect the behavior in an increasingly complex non-linear manner - and you quickly find yourself dealing with what are referred to as 'coupled spaces' - and very complex mathematics!) Each change in a variable can have an effect, with the degree of the total effect depending on the degree of variable for each variable. (pretty simple there)

The main significance of varying room surfaces, with varying transmission and reflective characteristics is to create a variance in the amount of acoustic energy, which will have the affect of creating a variance from the ideal room model assumed in the room mode calculators.

So, generally speaking, small flush windows will have a minimal effect, while larger windows such as you imply, can have a much more significant effect primarily due to changes in transmission losses. And curtains, etc., as so often recommended will have little to no significant effects on the low frequencies at issue - so they are NOT a solution.

I would suggest not using the room mode calculators except as a beginning thumb nail sketch to get a very approximate idea of what you are dealing with. The models assumed are VERY simplistic compared the the average basic room. Thus the prudent path is to measure. This measurement does not care about all of the variances in an assumed ideal model and instead presents a picture of the complex reality of the room with ALL of its variances in dimension and component surface characteristics (their impedance) as well as room contents. In other words, you get an accurate picture of reality - what is - instead of what might be. 

The measurement that you would want to use would be the cumulative spectral decay - commonly referred to as a 'waterfall' plot. But beware! Many measurement rigs do not have sufficiently large windowing capabilities (and noise immunity) to accurately present this measurement. 

I believe ETC/R+DPlus is capable of this. (But I will admit to having little hands on experience with the smaller tools as I am most familiar with TEF and EASERA and their implementations fo TDS and MLSSA.)

But to reduce this a bit- Room modes are primarily determined by the dimensions of the room. But the real answer to your question is that windows will cause a small variation from what an ideal room model calculator program will return. At this point, the only way to determine the real affects is to measure the room. 

And before you dismiss measurements (as they represent 'what is', the reality, as opposed to a guesstimate returned by an overly simplistic calculator), truly effective bass traps are high Q (narrow band) in nature (be they resonant panel, tuned cavity, etc. variations in Helmholtz resonators). And you want to trap the offending peaks _without_ simply absorbing the finite mid and higher frequency acoustical energy as well.(And yes, this does suggest that the overuse of traditional broadband absorptive bass traps _can_ be detrimental to a listening space. A fact that has been _confirmed over and over_ since 1977 and the first implementation of the the LEDE concept by Chips Davis.)

By performing, or having the measurement performed, you are able to essentially ignore the why, and focus on the 'what is' - the reality of the environment. And the 'why' can be left for a lively discussion over your favorite beverage after the room is effectively treated.

{Moving beyond the issue of room modes, the mid and high frequency acoustical energy should only be surgically trapped in order to establish an initial signal delay gap for the early arriving signals and then to use diffusion to mitigate the intensity and to 'spread out' the arrival times of the later arriving focused specular reflections that dominate a small acoustical space. (The easiest way to understand this is to take a look at an ETC measurement.)}

I apologize if I have covered the topic in a simplistic explanation where a more comprehensive explanation might help! If you find this confusing, PM me and I would be glad to give you a call and address the areas that are confusing. And I can shoot you some graphics and example of measurements that will quickly result in this making allot more sense! Its really not too difficult to understand.:bigsmile:


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## bpape

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


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## bjs

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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