# Bass traps for asymmetrical set-up in L-shaped apartment?



## streetcore

Hi,

This is my first post here and I'm hoping to get some advice regarding bass traps or acoustic treatments for my apartment. It's a rented space, and it's not very big, so I want them to be as unobtrusive as possible. My setup is 2-channel only and I've already purchased Jim Smith's book "Get Better Sound" and have achieved some benefits from his setup tips. I've also purchased a Behringer ECM8000 mic and have tested the room with REW. Based on the results I was able to further optimize my speaker and listening positions. I've attached a pic of the best graph I was able to achieve by moving the speakers and mic position, and I'm wondering if I can improve on this with some DIY bass traps.

I've been doing some thinking about it, and cutting out cardboard templates, and I think I could make some 24" tri-corner traps that would be fairly easy to install in the positions shown in the Sketchup drawing below. I was also thinking I might be able to put a soffit style panel across the upper rear wall over the desk, and perhaps install some downlights to dress it up a bit. I may also try building some bass-trap planter boxes similar to the ones offered by RealTraps. 

Any thoughts on these ideas? Anything else I should consider since the room is L-shaped and my set up is asymmetrical? Right now I have blinds and thin curtains over the large windows, and I was also wondering if I would be better off starting with some heavy curtains or blinds instead of bass traps.

Thanks very much and any advice would be much appreciated.


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## bpape

While those locations will help address excess bass ringing in the room, in my opinion, the front right corner is a prime location for bass absorption. The right speaker is much more boundary and somewhat corner loaded than the left which is going to cause it to sound much richer and fuller than the other one. You'll also want to address the early reflections off that right wall which are going to be more intense and arrive earlier in time than the ones off the opposite wall.

Bryan


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## streetcore

bpape said:


> While those locations will help address excess bass ringing in the room, in my opinion, the front right corner is a prime location for bass absorption. The right speaker is much more boundary and somewhat corner loaded than the left which is going to cause it to sound much richer and fuller than the other one. You'll also want to address the early reflections off that right wall which are going to be more intense and arrive earlier in time than the ones off the opposite wall.
> 
> Bryan


Thanks for your response. Actually, my first thought was to fill that right corner with a SuperChunk or Studiotips Corner Absorber trap, but I thought it might be better to distribute the absorption around the room a bit. It sounds like it might be better to completely fill that right corner first.


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## bpape

You will get some benefit in decay time by spreading things out. That said, that corner and the close right wall still need to be done. Then you can maybe do the upper rear corners. 

Bryan


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## streetcore

Thanks again for your advice. I had a mirror out last night and with my current setup the first reflection point on the right wall is in the seam between the door and the wall where the door is hinged. So treating that point is going be difficult. I may be able to move the speakers forward and my seat back to put the reflection point on the back of the door, but I don't think I have the room and I'm not sure I want to hang a panel on the door. Maybe I'll start with the right corner for now and see what happens. 

Cheers,

Andrew


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## bpape

Even without getting at the reflection area, having a panel directly beside the speaker will help minimize the uneven boundary loading.

Bryan


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## streetcore

bpape said:


> Even without getting at the reflection area, having a panel directly beside the speaker will help minimize the uneven boundary loading.
> 
> Bryan


That would be much easier, and not too obtrusive. I really don't want to fill the corner with a superchunk, but would be willing to put a tri-corner in the upper right, and a 2'x4' panel on the right wall beside the speaker. Would you recommend 4" thick with FRK facing out for both of them?


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## SAC

Brian is trying to help you, but I fear your concern for aesthetics has already significantly limited your options.

Let me summarize:

First, your choice of speaker placement lacks Left-Right symmetry.

The significance of this is that the imaging, localization is pretty much hosed unless you can effectively establish that. 

Brian has politely suggested a partition and bass traps.

But realize that this reflective partition will necessarily need to be large. It must be large enough to be larger than the wavelengths down to about 400 Hz (at least 3 ft by 3 ft per each pathway) and large enough to cover the multiple reflective pathways affecting the seating area.

This is further complicated by the fact that you will have to make this boundary perform similarly to the other side boundary – while also surgically controlling the high gain, early arriving reflections. Thus you will be selectively applying absorption to the reflective boundary. A mirror is not sufficient to determine this complex amalgam of behaviors. You NEED to use the ETC response to identify specific reflections and to adjust gain to both effectively control the various reflections as well as to create acoustical symmetry. And don’t’ forget the ceiling!

As to a comment regarding your questions: A 2'x4' panel would not be large enough to control 'well controlled' distribution down to 400 Hz where the wavelength is ~2'8". And not having a reflective surface there is already equivalent to having an absorber there! You just have no idea regarding what the energy is doing and how it correlates to the other boundary surface! You need an ETC response for each speaker to discover what is actually happening! that will determine the exact nature of the boundary, be it reflective, absorptive or a combination of the two.

Much of this could be alleviated if the room were rearranged for optimal sound quality by relocating the speakers to the end of one of the alcoves, providing a degree of symmetry… And then using the ETC to surgically treat the side walls and ceiling as necessary.



The other major fly in to ointment is room modes. You have what is known as a coupled space. You will effectively have modes that are conditioned by 3 regions: Each of the side alcoves, and also by the entire space. Compounding this is the similarity in the dimensions of each space! They will sum. Prediction is beyond the already severally restricted generally available tools.

The only way to determine the distribution of these modes is to measure and map the space.
I appreciate your reluctance to employ obtrusive bass trapping. But as you are opting to go the porous route, you don’t have much choice. Tri- corner traps are very cool looking, but they will not be extremely effective, especially anticipating the degree of trapping you will require. Also, while a straddled panel may be more convenient to make, they are not as effective as SuperChunk style, where you benefit from more porous material. And you are going to need more coverage area to be effective. 

One additional ‘trick’ you can use is to use adjoining spaces as bass ‘sinks’ and to, where possible, leave the doors open and allow the energy to be distributed into a larger volume. Just realize that in doing so this will modify the modal distribution as well, so you will need to explore these options and measure and map the space in order to determine the best options – hoping that such manipulation will not place your listening position in a null.

And again with regards to your question: Right now you have lots of uncontrolled reflections about which you have no idea what is happening. And considering the limited volume of bass traps, I would not worry about FRK facing at this point - especially as it is hard to predict just where and how such reflection would be directed and at what gain. Again, ETC measurements made with simple panel template mock-ups would provide much needed information that would quickly and accurately determine the effect of the use of a reflective FRK surface...

Again, I would suggest that you use the waterfall response and determine the precise behavior involved. And then use this as guide for determining the optimal treatment, rather than simply estimating you needs based upon aesthetic concerns without any idea of what are the real problems.

Dependent upon the results, you may want to seriously consider some of the tuned Modex modules (RPG), placed in high pressure regions determined by modal mapping of the space. 

But, as always, the choices are yours. Simple measurements will reduce the range of reasonable options from 'unlimited', to a more manageable range conditioned by the actual behavior. If you need any assistance, please don’t hesitate to holler. (But also realize that positing ‘what ifs’ without actual measurements of actual behavior will necessarily receive an indeterminate answer!onder: Good luck!


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## streetcore

SAC said:


> Brian is trying to help you, but I fear your concern for aesthetics has already significantly limited your options.


I am very appreciative of Brian's assistance, and I certainly hope I didn't say anything that would suggest otherwise. Unfortunately, this is not a dedicated listening room or home studio, so I do have to take the aesthetics of any acoustic treatments into consideration. Due to the placement of doors and windows, and the lack of electrical outlets on some walls, I also have very few options for equipment placement. So it's certainly a less than ideal situation, but it's one I have to live with for now.



SAC said:


> Brian has politely suggested a partition and bass traps
> 
> But realize that this reflective partition will necessarily need to be large. It must be large enough to be larger than the wavelengths down to about 400 Hz (at least 3 ft by 3 ft per each pathway) and large enough to cover the multiple reflective pathways affecting the seating area... A 2'x4' panel would not be large enough to control 'well controlled' distribution down to 400 Hz where the wavelength is ~2'8"


The wall beside the right speaker is 4' wide from the rear corner to the door. I understand it's almost impossible to give definitive answers, but should I consider covering most of that wall, if not the whole thing, with 4" thick panels?

I'm generally pretty happy with the sound I'm getting and the imaging, but sometimes I find the bass pretty boomy, which is why I started looking into bass traps. Adjusting my listening position and the speakers has made a difference, but I thought some room treatments would also be worth considering, as long as they didn't take over the apartment and make it look like a recording studio. 

I definitely appreciate the advice both of you have provided and it's given me lots to think about. I was all set to rush out and buy some insulation a few days ago, but I think I'm going to slow down and spend some more time learning about the various options.

Thanks again,

Andrew


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## SAC

Please take the time to make a few measurements (waterfalls and ETCs).
This step can significantly bring what may seem to be a pretty overwhelming issue down to a manageable task.

These will tell you exactly what is going on and will provide an inventory of behaviors you would want to focus on treating.
Armed with such information, not only will the many possibilities be reduced to a few realities, the options for effectively treating them can be better assessed. Many of the most egregious issues can often be treated rather easily and surgically -_ without _the need to rebuild the entire house!:laugh:

Then you can explore the best options for each, maximizing their effectiveness while complimenting your home to the greatest extent possible.

And the effects of incremental changes can be quickly and objectively assessed. 

I know many are scared by the thought of measurements, but they are actually quite easily done and WILL make your options more defined.


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## streetcore

SAC said:


> Please take the time to make a few measurements (waterfalls and ETCs).
> This step can significantly bring what may seem to be a pretty overwhelming issue down to a manageable task.


Thanks again for all the info. I had to do some searching to find out what an ETC was, but figured it out and understand now that it's part of the Impulse Response. So what you're saying is I should measure the Impulse Response for each speaker separately? Sorry if this sounds like a stupid question, but do I just do this with the balance control on my amp? Should I do the same for the waterfall measurement, or should that be done with both speakers on?


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## SAC

You do not adjust anything unusual with your amp.

You measure the test signal as generated by a program such as REW generated through each speaker separately.

Thus you have one sweep generated with the left speaker, and without moving the mic, another generated through the right speaker.

The goal is to obtain a response of the signal from one loudspeaker and to then 'observe' the energy propagation from that one loudspeaker within the room, and of its various pathways and incidence with object boundaries within the space, until it reaches the measurement mic.

I would suggest before doing either measurement that you establish and mark the precise position of the measurement mic capsule placed at the spot occupied by your head at the listening position, by establishing a secure plumb bob from the ceiling (you can pull it aside when not dong measurements). By having this reference point available, you will be able to make subsequent measurements at the exact same point allowing comparison of the various generations of measurements. (Also, you will not move the speakers)

(Don't worry about the exact position of your ears! This is an issue referred to as inter-aural cross correlation - IACC- and it is well understood and we need not worry about that here.)


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## streetcore

Thanks, I'll give it a try tonight. I have done some measurements with REW and still have tape all over my floor marking the various measuring positions. Instead of hanging a plumb bob from the ceiling, I attached a string and small weight to the mic so I could position it over the tape on the floor and get repeatable results.


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## streetcore

SAC said:


> You do not adjust anything unusual with your amp.
> 
> You measure the test signal as generated by a program such as REW generated through each speaker separately.
> 
> Thus you have one sweep generated with the left speaker, and without moving the mic, another generated through the right speaker.


Okay, I've been trying to figure out how to send the test signal from REW to each speaker separately and I'm stumped. I've been reading the help files and searching the forums, and can't find any settings in REW that will do this. I did find a post on avsforum that said you should measure one speaker at a time by disconnecting all but one speaker. Is that the best way to do this, or am I missing something in REW?


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## bpape

You're not missing anything. Just disconnect one speaker. 

The results will give you insight into what's happening. That said, given the limitations, it's likely to be an exercise in frustration. Knowing what is going on and where it certainly a good thing but if you can't deal with it, then it's not really much use. It will simply prove to you what needs to be done.

The front corner and right wall WILL need to be addressed. The symmetry is what it is and this is the best way to address it. Given the limitations on available space, I would agree that covering more of that wall would be desirable.

These are common problems in real world, non-dedicated spaces and we just try to address them the best we can given the limitations.

Bryan


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## streetcore

bpape said:


> You're not missing anything. Just disconnect one speaker.


Thanks for the clarification. You're probably right about it being an exercise in frustration, but I have the gear to do the testing and I am keen to understand what the sound "looks" like in my room.


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## bpape

Don't get me wrong. Measuring the room is certainly worth doing. Even if you can't treat everything, it can show you how potentially small changes in speaker and seating position can improve frequency response.

Bryan


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## streetcore

I ran some more tests tonight and here are some waterfalls and ETCs for each speaker. I'm trying to figure out how to interpret the results, and I'm not sure if I did them correctly, so right now they don't mean much to me. Any feedback would be much appreciated and I will keep reading and trying to learn more.

Thanks.

SPL Right Channel
 

SPL Left Channel
 

Waterfall Right Channel
 

Waterfall Left Channel
 

Impulse Right Channel
 
 

Impulse Left Channel


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## Ile

Page 16 design would help to 60Hz peak and maybe little to 30Hz dip.
http://downloads.bbc.co.uk/rd/pubs/reports/1992-10.pdf

Thinner 6" version with metal front at page 4. (tuned to 80Hz)
http://downloads.bbc.co.uk/rd/pubs/reports/1995-07.pdf

As you see from other graphs, same amount of bare wool wont be as effective that low freq.


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## SAC

Can you post the raw response (wav or the native REW) files so that we can open it and adjust the windowing?

Just a bit of insight, Sound travels approx. 1.13ft/ms, thus displaying an ETC window to 1sec captures the direct and indirect signal travel up to 113 feet. My guess is that there is no appreciable specular reflections after about maybe ~50ms which would encompass energy traveling ~56.5 feet (more than twice the length of the room...). Thus beginning with a horizontal/X-axis time window of ~50 ms should be adequate to get a good overview - and one can zoom in from there as necessary for more detail.

Also, I would estimate that one would not need more than a window of about 40 dBfs on the vertical gain scale will be needed displaying from a little above 0dbfs to ~-40 dBfs.

Are you using loopback device correction to address hardware propagation time delay? (While necessary, it may be a moot issue as configured as I am not sure if we are able to see the total time of flight from speaker source to measuring mic.)*

Also, I fear a critical bit of data is not displayed. 

While it may seem advantageous to translate the time axis and to assign t=0 to (generally) correlate to the arrival of the direct signal, we do not have the total time of flight from the source speaker to the measurement mic in order to determine the base time, and hence distance the sound traveled before being received at the mic. Thus, all we can do with what is displayed is to determine the difference in time and distance of each indirect signal relative to the direct signal.

While translation of the origin on the time axis is very useful for adjusting delay lines in multiple unit arrays and distributed systems, it is *not *useful for determining the distance and direction of the specular energy.

Thus, it would help if you would carefully and _*as precisely as you can*_, measure the distance from the center of the speaker baffle (assuming that is somewhat close to the acoustic center of the unit) to the mic capsule. We will need to adjust the distance calculations obtained from the ETC plot by that distance. The reason for this is that the signal does not travel from the point starting at the arrival of the direct signal to its arrival time (yeah, I know that sounds a bit absurd!), but instead the signal for all direct and indirect signals travels from the source speaker to the measurement mic, and thus in order to demonstrate how one can determine the precise actual distance of travel and the location of incident boundaries, we will need to know the complete information describing the total time/distance of flight.

After one understands this concept, it is not too difficult to explain a few other techniques to determine the precise path of travel.

(And if there is a way to overcome this in REW, I would_* greatly*_ appreciate knowing it!)

With the raw data that we can massage, we can illustrate, explain, and suggest a few ways to make very practical use of the ETC response.

------------------------------

An aside:

*My request for a new feature would be to make the total time of flight the default time view (assuming the required loopback compensation is done, unlike gain and freq calibration which are optional, time calibration IS necessary!!!) Then one can then translate the T=0 time value origin for the limited cases where such translation is of practical use. If this issue were addressed (or at least made the default view if it is possible), then just about all of my other concerns would simply be issues regarding bells and whistles.

Also, a selectable dB SPL scale would be of MUCH more use than a dBfs scale which, while very useful for setting digital signal line levels, is not of use for sound pressure level measurements. And, again, as we are concerned with relative differences in gain and not absolute gain, gain calibration is not necessary.

For this curious about gain calibration (and the notion that many seem to feel it necessary), gain calibration is only necessary if one is conducting environment certified noise level analysis (NLA) measurements. And if that is the case, the entire rig will need to be certified and calibrated -something that is far outside the scope of what is possible here. The other use would be if more than one response made in different locations/environments were being compared there was required an objective reference baseline for comparison, but one would be hard pressed to imagine such a practical scenario where such would be the case.

--------------------------------------------

As far as the modes are concerned, I have quite a few questions and there is likely more work to be done.
I assume the waterfalls were taken with the mic in the listening position. 

The problem with this is that you are most likely in the nearfield of he speakers and you will be dealing not only with a measurement of the modal support of the room, but of a high level direct signal of the speakers.

In order to determine the lowest supported mode of the room, the easiest way to do this is to place a speaker in the front floor corner of the room, and then to place the mic on a tall stand facing the rear diagonally opposite ceiling corner 

And you may also be interested in the cancellation of signal due to superposition (folks need to learn this term!) that refers to how waves sum when multiple sources are used, and also when virtual sources in the form of reflections are manifest, creating spatial polar lobing. This polar lobing consisting or regions of reinforced response alternating with regions of phase cancellation, will appear to vary with frequency depending upon where you are located in the field. It is this polar lobing that is responsible for the pattern seen in a frequency response called comb filtering. It is important to understand that there is no such 'thing' as comb filtering (it is just a pattern in a display!). The real 'thing' that actually exists is the behavior known as spatially distributed polar lobing caused by the combination of two spaced real or virtual sources. And this will contribute to the modal measurements made at the listening position when more than one LF source is driven, as well as by possible contributions by virtual sources (boundary reflections) called SBIR.

Thus a few related variables will need to be systematically researched and identified and/or eliminated as contributory variables. 

Its not hard, but a few well thought out steps may be necessary to isolate the specific causes contributing to the total bass response. Thus, measurements will tell you what is happening, but you will need to think a bit in order to apply an understanding of the various acoustical physics concepts that can be at play depending upon the setup. The reason is that if the behavior that you are viewing is a result of multiple causes, you need to identify the particular cause that effects each 'part', and correct that. One solution will nor magically correct issues caused by multiple causes. Thus, this is not a limitation of the measurements! It simply requires that you be at least as smart as your tools that can expose such behavior. And depending on the circumstances, you may need to conduct a few small experiments to isolate each contributory cause so that they can be corrected.


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## streetcore

Here's the mdat file from last night. I hope that helps, and I can also try to alter the graphs tonight when I'm home from work. The distance from the tweeters to the mic is 8' 8" as measured with the Bosch laser measuring device, so it's pretty accurate.

Thanks,

Andrew


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## SAC

Below is the ETC windowed for normal viewing.



Something is not right - in addition to the pre-causal behavior (energy arriving prior to the direct energy!!!). And I do not think the pre-causal energy is due to mechanically coupled and transmitted energy. The floors are not 'sprung' wood are they? But more than that, I have reason to doubt ~7 direct and indirect sources within ~-5 dBfs spread over ~38ms.

Also, for our purposes, can you measure from the center of the speaker baffle (not the tweeter) to the mic capsule to the proper precision. (The precision of the measurement is determined by the measuring device. The precision is to the nearest tenth of the smallest unit division...meaning, for instance, if the marking of a measuring device is marked to tenths of a unit, then we can estimate to the nearest tenth of that scale - meaning we are precise to the nearest 100th. Or if the device is marked only to integer units, then we can estimate precisely to the nearest tenth of the unit. Stating the measurement to less or greater precision than that determined by the unit sale is a mistake. This is important as the total calculation determined by the use of he measurement is determined by the least precise measured unit. So we will benefit from the most 'correctly' precise we can be... )

I know many are of the opinion that the tweeter possesses some magic quality, but to clarify this issue - the tweeter is ofter a concern ONLY in terms that its dispersion is normally the most limited (narrow). Thus in order to be 'on axis', as the tweeter tends to beam, we think interns of the tweeter being the limiting agent.

But here that is not our concern. We are concerned with the_ energy_ that is emitted by the loudspeaker. And in this frame of reference, the tweeter emits much less energy than the other drivers. Here we are concerned with what is often referred to as the acoustic center of the unit - similar to the center of mass or a unit's acoustic 'moment'. In other words, the 'point' from which all of the energy appears to radiate from the entire loudspeaker. And as we have not measured the actual acoustic center, we can use the geometric center of the loudspeaker's front baffle as the reference point.

Thus, the more precise we can be with that measurement, the better off we will be using that measurement summed with the data from the ETC, where the limiting precision will be that of this distance measurement from speaker baffle to mic.


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## streetcore

Thanks again for all detailed info. A lot of it is going over my head, but some it is starting to sink in. I will take a more accurate measurement when I get home from work. My speakers look like ones below, so should I measure from the mic to the point between the mid and bass drivers?


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## SAC

Yes, that would be a better point from which to measure.

Also, using the picture as a source, note how the cabinet extends a bit in front of the speaker to the left side (viewing from the front).

This small jump discontinuity will function as a source of diffraction and reflection - causing energy to emanate from that cabinet corner as if it is yet another small speaker, further acting to superpose with the direct signal and thus harming intelligibility, imaging and localization.

Assuming one does not have the option to remove the impediment, it would be preferable to position the speaker such that the cabinet face was flush with the speaker baffle front in order to (try t0) act as an extension of the speaker baffle to minimize this effect.

I mention this simply as a point of illustration, as I know the picture may not be of your particular installation.
It seems everything is out to get us, doesn't it?


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## streetcore

SAC said:


> Something is not right - in addition to the pre-causal behavior (energy arriving prior to the direct energy!!!). And I do not think the pre-causal energy is due to mechanically coupled and transmitted energy. The floors are not 'sprung' wood are they?


Sorry, but I just noticed the date on the mdat I posted was Aug 23rd. I meant to upload the file from the 25th, but was rushing before I went to work. I'll redo the ETC's tonight and post the graphs based on your recommendations.


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## streetcore

Here's a couple new graphs with the distance from the mic to speaker baffle marked on them. I've also attached the mdat. I tried to measure as accurately as I could with the Bosch DLR130 laser measuring device, which is accurate to 1/16" or 1.5mm. However, it was hard to line the edge of the device up perfectly with the mic, and I might have moved it slightly when measuring.

I read what you said earlier about the pre-causal behaviour. Are those the peaks on the graph before 0s? My apartment building is made of concrete, with parquet wood floors, and I live on a busy street. So there are often loud traffic noises outside. I thought that might have been the cause for some of the peaks, but I ran the tests a couple times and tried to wait for the traffic to die down, and the graphs look similar. So I don't think that's it.

I also hope I'm doing the measurements correctly. I have the ECM8000 mic pointed straight up and tilted forward about 20 degrees. The top of the mic is set to ear level when I'm in my listening chair, about 36". The measurement settings are "256k log sweep from 20 Hz to 300 Hz at -12.0dB taking 5.9s". I'm using the Tascam US-144mkII as the USB preamp.


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## SAC

Why are you band limiting the sweep from 20 Hz to 300 Hz? Technically, we are not even driving the room with energy that will have substantial specular behavior.

Use full range sweeps! And leave off all smoothing for any responses. And process/convolve only unsmoothed responses.

Do not try to play with the measurement by thinking in the frequency domain.

As far as the graph, there should be more definition to the energy arrivals. And it is highly unlikely that you will have reflections persisting at 25ms (~283 feet of travel including bounded incidence) that are barely 2dBfs down from the direct signal (or ~-6.5 dBfs at ~44ms (after 49.7 feet of travel including bounded incidence!). 

And with a severe lack of L/R symmetry, you should not have such a plethora of high intensity early arriving reflections except off the floor (especially without substantial energy being delivered that behaves specularly at those frequencies)

Plus something is amiss with the pre-causal signal arrival of ~-2dBfs at ~-2ms relative to the direct signal - unless the signal and drivers are THAT much (~2.26 feet) out of alignment....

Try a full range sweep. We'll figure this out!


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## bpape

20-300 is easier to identify (with smoothing off), where any frequency response problems are more specifically rather than a full range sweep where it's a guess at best.

Bryan


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## SAC

If one wants to examine the modal range from 20 Hz to 300 Hz, use the proper stimulus and response.

Truncation of sweeps has unintended consequences, and folks would do well to stop thinking solely in the frequency domain. 

For insight into modal behavior from 0-300 Hz, generate the waterfall response driven with a full range signal and then window the response appropriately.

In order to generate the ETC response, drive the system with a fullrange stimulus.

There is a fundamental conflict in trying to use a gated signal from 20 Hz to 300 Hz to generate an ETC that provides insight in to specular energy behavior while restricting the stimulus energy to frequencies that behave (primarily) modally and expecting to see the full range (where our region of concern is actually from about 300 Hz to 10,000 Hz) specular energy behavior.

Thus, for an examination of the specular energy region, drive the space with energy spanning the full range of interest - at least 20-20,000Hz for instance (0-22Khz is even better). ( As an aside, I would normally also suggest using frequency weighted sweep were this feature available in REW. This makes good use of the pink noise behavior and improves the signal-to-noise ratio for the high frequencies simultaneously with more energy being applied to the low frequencies and less to the high frequencies in order to avoid the potential to blow up tweeters, where the difference between bass and treble in the linear plot amounts to only 18dB between 20Hz and 20kHz, in the octave band plot to only 10dB, and the mostly high-level mid-band range is even lowered a bit (5dB).)

I wouldn't recommend using a Porsche GT3 to tow a 26 foot long boat or house trailer either.


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## bpape

Understood completely. That said, using a tool that's well supported here which does do sweeps is very easy and relatively accurate for assessing the basics of what's happening in the room. Yes - you can certainly run a full range sweep and then look at just the 20-300 area.


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## streetcore

Doing a full range sweep seems to have made a big difference and I think I'm getting a proper looking ETC's now. I think had also read that it was easier to limit the sweeps to 300Hz, so I guess that's why I was doing it. 

Here's some new graphs. Please let me know what you think.

I was also wondering if disconnecting the speakers is the only recommended way to make these measurements. Is there any reason why I can't disconnect the input for right or left, or just use the balance control? Disconnecting the speakers is the least convenient way for me, so I would prefer one of those other methods as I continue my testing.


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## bpape

Using a balance control or disconnecting a channel from the source can certainly also work.

Bryan


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## streetcore

I did some more testing last night, and I'm starting to understand how to use the Impulse measurements to identify the various reflection points. By taking some measurements and experimenting with a piece of 3" upholstery foam and heavy blankets to treat the right speaker reflection points on the floor, back of the door and ceiling. I could easily see how the Impulse response changed when each reflection point was treated.

Below is the right speaker Impulse with no treatments. The three spikes after the initial spike at 0 correspond to the floor, back of door and ceiling. The next picture shows the Impulse with foam on the ceiling and blankets on the back of the door, and you can see a drop in the second and third spike.

However, I'm still not sure where to start with the proper acoustic panels and/or bass traps. If we forget about aesthetic concerns for now, what would you guys suggest? Would a Super Chunk in the right corner and some panels at the reflection points be a good place to start?

Thanks again for your patience and assistance while I work through this.


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## bpape

Yes. That would be a good place to start in addition to right next to the speaker. Impulse response can be very useful in terms of finding specific reflections but will not show you boundary interactions. 

Once you individually measure the left and right channels, you'll see pretty quickly the difference due to the boundary proximity.

Bryan


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## streetcore

What do you think if I treated the corner like the picture below? It's basically a Studiotips Corner Absorber plus a 3" panel with a 1" air gap on the side wall beside the speaker. Rather than wrapping each panel in fabric, I was thinking about a screen that would sit in front of the insulation. It might look neater and require less fabric.


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## bpape

You could certainly do that and it will help a good deal with the difference in boundary gain between the 2 speakers and also likely address at least some of the near wall reflections on that side. Not going to fix all the issues but it's a good starting point.

Bryan


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## SAC

A couple of comments...

As your primary concern is modal behavior, you would do well to address the modes first.

Regarding the proposed traps, you might consider a Superchunk style corner trap using the 'cheap pink fluffy stuff" and utilizing several pieces of plastic orchard bird netting to create several partitions that will act to support the material such that it will not compress as much over time.

Also note that a 3 inch panel mounted adjacent to a wall will not act as a bass trap. in fact, a 3 " panel with a 3" gap, utilizing Rockwool, will only exhibit an ~.3 absorption coefficient at 100 Hz! In fact, a 4 inch panel with a 4" gap is about the 'smallest' you want to use for a broadband (mid and higher) absorptive panel.

To be effective below 100 Hz, you need to go with at least a 6" panel with a 6" gap ([email protected] Hz and [email protected] Hz).

AFMG Reflex simulations curves are available for the above.


As far as the ETC...

I am not sure how much you want to go into this, as it can quickly become a extended discussion of the theoretical and conceptual underpinnings that are quite different than that with which most are familiar. We can certainly do it, but its not worth it if no one is interested and if we just get comments from folks saying that their 'brain hurts'.

Stated quickly, the ETC can indeed identify each reflection, provide information about the nature of the reflected energy, and provide specific detail not only about its precise path, but also identify the precise location of boundary incidence! 

In other words,_ it can tell you precisely where to place treatment material appropriate to your desired response model. And it can verify its effectiveness._

Below is a very quick perusal of one the ETC responses annotated generally.
If you are serious in following up and determining the paths and specific points of boundary incidence for each anomalous reflection, let me know - and post the mdat files.

Also, I will provide a synopsis of the manner in which the reflections can be identified and also resolved into their paths and walked back to their boundary incidence locations. (As soon as I figure out the interface sufficiently to attach a PDF...)


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## SAC

Here is the description of the basic steps to identify, determine path and incident points of an indirect signal, be it a reflection or diffraction.


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## streetcore

SAC said:


> Regarding the proposed traps, you might consider a Superchunk style corner trap using the 'cheap pink fluffy stuff" and utilizing several pieces of plastic orchard bird netting to create several partitions that will act to support the material such that it will not compress as much over time.
> 
> Also note that a 3 inch panel mounted adjacent to a wall will not act as a bass trap. in fact, a 3 " panel with a 3" gap, utilizing Rockwool, will only exhibit an ~.3 absorption coefficient at 100 Hz! In fact, a 4 inch panel with a 4" gap is about the 'smallest' you want to use for a broadband (mid and higher) absorptive panel.
> 
> To be effective below 100 Hz, you need to go with at least a 6" panel with a 6" gap ([email protected] Hz and [email protected] Hz).


So there's seems to be some consensus that a Superchunk in the right corner is a good start, but the effectiveness of any further treatment of the wall beside the right speaker is still questionable. 

If I was going to use a 4 to 6" thick panel of OC703 with a 4 to 6" gap, could I skip the Superchunk and just fill that 4'x8' side wall with the panel from the back wall to the door? Something like the picture below might suit my aesthetic concerns better, and be easier to build, than separate corner and side panels, especially if then need to be so thick.

Thanks again for the incredibly detailed response. I'll take some more time to read and digest the ETC info.


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## SAC

A few suggestions that are not only more effective, but much less expensive.

For the corner bass trap, stop messing with expensive OC703 panels and the additional complexity of complex multi-element face frames and simply make a Superchunk corner trap that straddles the entire space that you are imagining being treated with a corner trap and adjacent panels.





The additional space that is filled by the resultant hypotenuse is negligible in terms of room use, but substantial in terms of bass absorption.

And fill the entire corner region with cheap _fluffed_ 'pink fluffy stuff' available at any home improvement store. Just provide a few horizontal layers of plastic orchard bird netting (cheap at any nursery supply or Wal-Mart in season) dividing the Superchunk into sever 2-3' compartments. You can fill these with fluffed 'pink fluffy stuff' and the netting will minimize the insulation settling and compressing due to gravity as the mass will be less in each compartment and it will not all press on the lower portions.

This will outperform the Superchink made with the more expensive lower performing semi rigid OC703.

{Part of the reason the corner placement will outperform the planar application is that some of the energy reflects ;laterally off the wall surface and is directed 'sideways' through the absorbent material, effectively increasing the reactive area whereby the energy interacts with the absorptive material. In other words, porous traps are velocity based devices. Velocity goes to zero near a boundary, so by moving the absorbent material further out from the wall, you lower the effective frequency that corresponds to the maximum velocity quarter wave spacing, and then you also further increase the amount of interaction by having some if the energy be redirected off the wall as it travels 'sideways' over a longer path through the absorbent material, thus increasing the energy dissipation.}

And its MUCH simpler to construct as well! A case where the easier cheaper alternative outperforms the more expensive and complex variation.


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## streetcore

I just noticed you added a picture to your post. I guess we are on the same page.

I'm up for cheaper and more effective, but I'm not sure I'm following you. Are you suggesting something like this?


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## SAC

Yes and no!

I am NOT proposing that you radically cut the depth almost in half!!! That will have significant detrimental effects on a porous velocity based absorber. 
If you want some 'trim unobtrusive' treatment, you need to explore pressure based tuned resonant traps - the easiest of which to obtain are the RPG Modex_ tuned modules_.

Leave the original wall interface dimensions and simply straddle that region with the face frame and fill.


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## streetcore

I get it now. I was going back to my original thoughts about a Superchunk trap with a 24" face and 16" depth. The other drawing I did was based on a Studiotips Corner Absorber, which has more depth. Thanks as well for the additional info about why the corner placement will outperform the planar application. I was wondering about that. 

I think this final idea might be doable. However, I am a bit concerned about using lose pink fibreglass, and don't want those fibres working their way out into the room and my lungs. Would there be any problem sealing the face of the trap with plastic?

Cheers,

Andrew


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## SAC

A few thoughts. 

First, the fibers are not loose, and I always wonder why folks think these things are 'active' and that the fibers somehow grow feet and migrate.

Also, just to allay additional fears and to put things into perspective, Fiberglass is an irritant ONLY.The pink fluffy stuff has minimal binder as opposed to the semi-rigid material (and also a good reason to explore Knauf Ecose semi-rigid equivalents). Funny, I seldom hear anyone concerned that sawdust is listed as an a carcinogen and that 'natural' fibers such as cotton, wool, hemp, etc are literally MUCH more dangerous than Fiberglass or basaltic fibers that if breathed/ingested, harmlessly dissolve in less than 2 weeks. Compare that with natural fibers that do NOT dissolve and cause such ailments as byssinosis. 

Of course, it follows that best practices and (un)common sense are ALWAYS advised regardless of whatever materials are employed!

All of that said, you enclose the material behind a loose weave cloth boundary such as unbleached muslin reinforced with supportive orchard bird netting. You then build a face frame with a stretched materiel that straddles the actual trap leaving about an inch gap between the outer and inner fabric panels. Thus any casual contact, etc., with the external surface never disturbs the contents. There should be no reason for there to be any disturbance of the material, let alone movement other than settling, at all.

I hope that allays a few fears and makes things a bit simpler (and cheaper!)


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## streetcore

SAC said:


> I hope that allays a few fears and makes things a bit simpler (and cheaper!)


I was at Home Depot last night pricing things out and I don't think it's going to be any cheaper to use pink insulation. In fact I think it might be more expensive and more complicated.

The cheapest pink insulation I could find was Owens Corning R12 3.5"x15"x47", 20pcs per package, for $38.99. The plastic netting was much more than I expected at $25.99 for 36"x25'. Metal net was cheaper at $16.97 for 48"x25', but it would be harder to work with.

If my calculations are correct, I would need at least two packs of insulation, maybe three, plus the netting and wood for the framing. In order to hold the netting in place, I think the frame would also be more complicated and require more wood than a frame built to hold rigid fibre board. In fact, many of the designs I've seen for traps built with OC703 have no frame at all.

So the cost of the corner trap with pink insulation would be at least $100 plus the wood for the framing and the fabric to cover it. 

However, I can also get 4 sheets of 3"x24"x48" unfaced OFI48 here in Toronto (the equivalent of OC703) for about $45. One package is enough to build a Superchunk with a 24" face, and two packs would be enough for an SC with a 36" face. If I built the Studiotips Corner Absorber I would only need six sheets, would only have to make a couple cuts, and might not need any framing at all. 

If we compared the cost to build a large triangle as we described above, I would need 16 - 24"x48" sheets cut into two diagonally. That would be about $180, but I wouldn't require any netting and minimal framing. So I think the costs are actually pretty close.

So I hate to say it, but I'm kind of back to where I started, :dontknow: and I think the cost and convenience of working with the rigid fibre board is leading me back in that direction.


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## SAC

Very ‘interesting’ calculations and comparisons, as the performance of the two variations are not even close to being equivalent. 

At Lowes, one can source Roxul Safe’n’Sound 8-Pack 24"Wx4’L x R-30 Fiberglass Insulation Batts for $47.87, which has the additional benefit of also lacking a formaldehyde based binder.

BTW, you do* not *want insulation with a Kraft/FRK applied layer.

Also, I would consult Bob Gold's database or obtain measured specs from the manufacturer before you assign equivalency in absorptive performance of the semi-rigid material! Neither thickness nor density are valid indicators of equivalence - rather gas flow resistivity is.

Considering that R-30 is rated at a thickness of 8.5 inches and can be fluffed substantially beyond this for our purposes (I would suggest ~10 inches), which is actually useful, the effective depth is actually substantially increased.

Assuming that it is fluffed to 10” deep and placed vertically, you could make a trap that is nominally 30 inches deep (at least on the wider side) and 48” wide! 

It would take at least 22 - 4’ semi rigid panels to achieve slightly less than the same volume of fill provided by ~8 panels of fluffy stuff. And due to the variance in gas flow resistance, the performance would not be equivalent.

A quick search found Dalen 7-Foot by 20-Foot Bird-X Net 3/4-Inch Mesh for $7.27. We can find it for the prices you cite, but just how many traps are you considering making!? That’s enough netting material to put full sized traps in every corner of your house AND the neighbors! And as we previously mentioned, we have routinely sourced this at Wal-Mart for ~$5, and less if you are savvy with the end of the summer clearances in their garden department, eliminating the need to even check with Farmtek. 

And after all is said and done, with the referenced materials, we could literally make several corner traps for the price of a semi-rigid design that each outperform the semi-rigid option, not to mention that a 24” faced 4” semi-rigid panel with a 4” gap is not even in the same ballpark performance-wise, and that is far lower performing than an equivalent Superchunk format constructed of semi-rigid panels that is still outperformed by the suggested 'fluffy stuff' construction with lower gas flow resistance.

What we have proposed is not some abstract concept. It has been done many times in reality, for substantial savings.

So, I’m not trying to give you a hard time, except to say that with just a LITTLE research, you can dramatically cut the costs and increase the performance. We make this as difficult as we like. And in the end, the compromises you make only come back to seriously degrade the comparative performance. But if it were me, I would be interested in obtaining the maximal return with respect to actual acoustical performance with respect to the investment of cost and effort. But of course each is free to do as they choose. My interest is that you don’t make it any more difficult than it is! Good luck!


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## streetcore

I don't mind you giving me a hard time, especially since you're providing so much useful information :T and I obviously don't know what I'm talking about. :dontknow:

I didn't really understand that the thickness of the pink stuff varied so much and that it could be fluffed up to make it thicker. My comparisons were only based on the dimensions of the low priced R12. I actually expected it to settle a bit after installation, and I thought maybe it should be compressed to make it denser, so I was thinking that 3.5" of pink stuff would occupy approximately the same volume as 3" of OC703. 

I now understand that I can get much thicker insulation for a bit more money. The OC R-40 is available at my local HomeDepot for $59.77 with a rated thickness of 11". Maybe I should consider that?



SAC said:


> What we have proposed is not some abstract concept. It has been done many times in reality, for substantial savings.


Can you provide any links to traps that have been built like this? I did a lot of google searching last night trying to find examples of people who had built traps with pink fluffy stuff, but couldn't find any. In fact, most of the results I found where people saying not to use it. Examples include: "The pink stuff is good for in the walls only. Completely useless for bass traps or diffusers", "you need density for bass traps- the pink insulation(probably wall or ceiling insulation) won't do much for you", "The pink stuff just isn't dense enough for bass traps. You want something like Owens and Cornings 703 or 705". 

I assume we can disregard those statements since we're talking about a trap with a thickness of 12" or more, but it would still be nice to see some examples.

As for the netting, I'm sure if I look around a bit I can find a better price, but that was the lowest price, and smallest quantity, I found find at my Home Depot. 

Anyway... thanks again for the additional info. I'm sure I'll figure this out eventually.


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## streetcore

So I got some R40 pink insulation 11"x24"x48" and have been experimenting a bit with it in the right corner. It's going to require some cutting and/or squishing to fit it into my triangle, but I think the layout below will be the easiest. It's two pieces standing vertically in an L-shape, and I'll build a brace with netting to support the top layer for a full 8 feet. Before I start cutting I thought I'd see if you guys had any comments. How critical is it to fill the empty space in the triangle as seen below? Since the spaces left are so small, I can't imagine it would make much difference. 

I've also attached some before and after waterfalls, and it does seem to make a difference. I did some quick listening tests too, and there did seem to be a bit more clarity and less boominess (is that a word?)

Thanks,

Andrew


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## monalissabeverly

Hi! I've been living in the same apartment for a long time and will pay my rent for two more years in advance. But then I got an offer to go to a concert and perform there. It's going to be three to four months. I decided to rent the apartment. But the landlord insisted on a license agreement. What is that?


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