Bass traps for asymmetrical set-up in L-shaped apartment? - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #11 of 48 Old 08-23-11, 06:26 PM Thread Starter
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

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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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post #12 of 48 Old 08-23-11, 06:41 PM
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

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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post #13 of 48 Old 08-23-11, 07:08 PM Thread Starter
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

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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post #14 of 48 Old 08-23-11, 08:40 PM Thread Starter
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

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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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post #15 of 48 Old 08-24-11, 08:51 AM
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

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.

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post #16 of 48 Old 08-24-11, 09:14 AM Thread Starter
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

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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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post #17 of 48 Old 08-24-11, 11:20 AM
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

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.

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post #18 of 48 Old 08-25-11, 08:54 PM Thread Starter
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

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


Last edited by streetcore; 08-25-11 at 09:21 PM.
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post #19 of 48 Old 08-26-11, 02:31 AM
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

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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post #20 of 48 Old 08-26-11, 04:17 AM
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

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.

Last edited by SAC; 08-26-11 at 04:50 AM.
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