Bass Management vs Full Range Speakers - Page 2 - AVS Forum
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Old 11-21-2012, 01:44 PM - Thread Starter
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Thanks kiwi2,

 

While I think a small piece of it may be stereo or localization issues, I'm coming at it from thinking it is more of a bass summing issue.  I get the flattest response from my fronts using a 100hz crossover.  And I can set the sub level to get things flat with the mains and music sounds awesome.  Great dynamics and bass.  But throw in a movie or music blu-ray and add in the center channel and LFE and now the bass becomes too much again.  I can drop the sub level a couple more db and it sounds good again.  

 

Using lower crossover overs for the surrounds also seems to give much better pans.  But again, I don't think it is as much a localization issue as it is the bass getting summed is too high and drowns out some of the other sounds.  

 

So I'm trying to understand more details of bass management and it sounds like it is just a straight sum.  And this sum turns out to me more than what the mixer heard (again, my guesses).  Sure I can turn down my sub level, but this can never be correct because it always depends on how many channels are playing the same sound at the same time.  

 

And thanks for the tip about trying to plug the ports on my mains, not sure why I never thought of that.  Something to investigate.  Because otherwise reducing the crossover really messes with the integration on my fronts.  I can tweak the distance but it needs to be by quite a bit since at the port tune things are completely out of phase.  And by adjusting the distance that much I feel I've thrown the LFE and other routed bass all out of wack.

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Old 11-21-2012, 01:48 PM - Thread Starter
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Originally Posted by arnyk View Post

The other view is that trying to guess the acoustic response of summed arbitrary speakers in arbitrary locations in an arbitrary room is pretty much like rolling dice.

 

This is exactly the counterpoint to what I'm trying to figure out.....what you mentioned could be better or worse than my "too much summed bass" issue, who knows.

 

I read a couple papers making the point that the only real way to solve this, or at least get better at it, is for mix stages to use bass management and route all bass to the sub.  Until that happens (and even then) there is always guess work and uncertainty.

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Old 11-21-2012, 02:22 PM - Thread Starter
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One particular scene I like to use when I make changes in my system is from TRON when he enters the grid.  Loud explosions everywhere.  With 80hz crossovers all around the bass is loud and just seems muddied up and brings focus to the front of the room.  Switch my speakers to large fronts and 60hz else where and now the bass seems to becoming from everywhere, I'm in the middle of it, I hear things that may have been masked before, yet I still get great impact from the LFE.

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Old 11-21-2012, 03:55 PM
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Bass management electrical summation would approximate what distance between subwoofers used in an acoustic summation?

Electrical summation is well-approximated by two perfect subwoofers that themselves have no distortion, no frequency response variations, no room acoustics, and no time delays due to sound traveling through the air.

Since most modern bass management systems in AVRs include adjustable time delays, that is the approximation of distance.



What does time delay in an AVR have to do with the issue being discussed? The time delay that an AVR uses is the time delay between the subwoofers and the main speakers

We are talking about the difference between mutual coupling on an acoustic basis of bass from the main speakers (AKA distance between main speakers) as compared with electronic addition of bass being sent to one subwoofer at a single location.

There are two views of that. One is that time delay whether acoustic or electrical is time delay.

The other view is that trying to guess the acoustic response of summed arbitrary speakers in arbitrary locations in an arbitrary room is pretty much like rolling dice.


I am talking about mutual speaker coupling as described in the link below. An example would be the comparison of two subwoofers stacked at one location as compared with two subwoofers separated by say 12 feet. They do not couple the same way (AKA different SPL gain).

http://www.rane.com/par-c.html


Coupling or mutual coupling Loudspeakers.

General term describing the combining behavior of two or more drivers reproducing the same frequency. If two or more identical loudspeakers are mounted such that their acoustic centers are close together (i.e., some fraction of a wavelength), their acoustic outputs over some frequency range will combine (couple) and propagate forward as one waveform, thus two smaller drivers behave as one big driver. [This is the simple vague answer, a detailed specific answer requires a great deal more.]

Mark Gander, VP of pro marketing for JBL, puts it this way: "The correct maximum distance that mutual coupling occurs depends on what you want to define as the limit of coupling. For example, is it the maximum approaching +3 dB, or when it reverts to unity gain? It's a gradual transition, so either 1/4 wavelength or 1/2 wavelength separation distance is just a rule of thumb." [Gander sites Lee Henney's Radio Engineering Handbook, 5th ed., Ch. 11 'Loudspeakers and Room Acoustics,' as a useful source that shows the response from groups of pistons at various distances, following the work of Klapman (Klapman, S. J., "Interaction Impedance of a System of Circular Pistons," J. Acoust. Soc. Am., vol. 11, p. 289, 1940.)"

John Murray, Digital Audio Lab Manager, Columbia College Chicago, explains: "Mutual coupling is when multiple drivers produce relatively more output at the low end of their response curve than a single driver. This occurs when the drivers are close enough together to have less than 90 degrees of path length difference for the wavelengths of interest at a given listening position. This phenomena is position-dependant, and is, in fact, what causes the high level portion of lobing. At shorter wavelengths (higher frequencies) and listening positions farther off-axis of the drivers, the phase difference becomes destructive which results in the nulls of lobing. Even with drivers that are touching, mutual coupling over a wide listening area are virtually always below 300-500 Hz.

Chuck McGregor, Technical Services Manager for EAW, says, "It depends." And goes on to explain: "There is no such thing as a correct, maximum source spacing distance for mutual coupling. First one has to define what they consider as mutual coupling (i.e., at what level off-axis does one stop considering the sources as no longer being mutually coupled) and then what is the criteria is for a particular situation, meaning the frequency to which one requires the coupling and the overall angle over which it is needed. Based on this stuff you can figure out the maximum, acceptable driver spacing.

Harry Olson analyzes this (although he does not call it mutual coupling) as a double or doublet source in his book, Acoustical Engineering, section 2.3.

McGregor's personal view: "First, mutual coupling will occur on-axis even if the center-to-center distance of two sources is 100 light years. At 1/2 wavelength separation they cancel completely at 90 degrees off-axis. At 45 degrees off-axis the signal is roughly 6 dB down, i.e. the equivalent of one source. Does this mean their mutual coupling beamwidth is limited to 45 degrees or is it limited at the 6 dB down point? Maybe it is 3 dB down over a lesser angle?"

McGregor offers this as a detailed definition: "Mutual coupling is when the outputs of two or more acoustical sources producing the same signal combine (couple) and propagate forward as one waveform. In this way, two smaller drivers can behave as one larger driver. While any number of sources can mutually couple, for clarity this discussion will focus on two sources.

The amount of coupling directly on axis between two sources producing the same signal will result in a 6 dB increase in level. On-axis, the spacing of these sources has absolutely no effect on this result. However, because the two sources must be physically separated, the coupling decreases off axis as the path lengths from each source to the listener increasingly differ. This is because the two waveforms become increasingly out of phase. For a given source spacing, the higher the frequency is, the more quickly off-axis this occurs. Likewise for a given wavelength (frequency) the wider the source spacing, the more quickly off-axis this occurs. Thus, the amount of mutual coupling at any point off-axis depends on both the source spacing and the wavelength (frequency) of the sound being produced.

What is considered mutual coupling? The broadest definition is that any multiple sources producing the same signal whose outputs acoustically combine to produce an increase in level over that of one of the sources means mutual coupling is taking place.

For audio purposes, changes of 3 dB and 6 dB are often used as typical criteria for acceptable increases or reductions in level. For two sources, a 3 dB decrease from the on-axis coupling of 6 dB occurs when the path length difference from the sources to the listener is 1/4 wavelength. This equals a 90 degree phase shift between the two waveforms. Likewise for two sources, a 6 dB decrease from the on-axis coupling of 6 dB occurs when the path length difference from the sources to the listener is 1/3 of a wavelength. This equals a 120 degree phase shift between the two waveforms. It also results in a level that equals the output of one source, meaning the amount of mutual coupling is effectively equal to zero.

Thus, the best definition for mutual coupling is defined by what is acceptable for any given situation. This can vary from the full, on-axis mutual coupling to the effective absence of mutual coupling, which is where the level decreases to be equal to or less than of one of the sources. Contrary to popular notions, there is no particular driver spacing that results in mutual coupling. Mutual coupling is defined by the level below which mutual coupling is not longer considered mutual coupling. This is an arbitrary level but, in any case, it cannot be below the level of one of the sources. The source spacing and the wavelength (frequency) of the signal then determines the angle over which the combined wavefront falls within the chosen definition of being mutually coupled."
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Old 11-21-2012, 04:37 PM - Thread Starter
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Thanks for that JPC, that's the kind of information I was looking for to help me understand things.

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Old 11-21-2012, 05:02 PM
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My WAG would be that room effects far outstrip any acoustic coupling effects, and that what is being heard are different frequency responses caused by the different locations of the source of the bass. Using the quarter wavelength concept for coupling at 100 Hz speakers need to be less than three feet apart to achieve acoustic coupling. At fifty Hz, a little over five feet apart and coupling goes away.
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Old 11-21-2012, 05:27 PM - Thread Starter
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My WAG would be that room effects far outstrip any acoustic coupling effects, and that what is being heard are different frequency responses caused by the different locations of the source of the bass. Using the quarter wavelength concept for coupling at 100 Hz speakers need to be less than three feet apart to achieve acoustic coupling. At fifty Hz, a little over five feet apart and coupling goes away.

Right but think of it this way. My mains are 10ft apart so little if any acoustic coupling. But turn on bass management and now the bass from those speakers is perfectly coupled since it is summed to the subwoofer.


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Old 11-21-2012, 09:03 PM - Thread Starter
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See post 14 on this link, pretty much what I'm experiencing.

http://duc.avid.com/showthread.php?s=82838d6d98a366c63509eb46efac9be5&t=271980&page=2

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Old 11-22-2012, 07:15 AM
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Right but think of it this way. My mains are 10ft apart so little if any acoustic coupling. But turn on bass management and now the bass from those speakers is perfectly coupled since it is summed to the subwoofer.
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No. Acoustic coupling happens because the drivers are within a quarter wavelength of each other. Period. If they are not, whatever happens is by definition not acoustic coupling So a single sub outputting well below the crossover point has no acoustic coupling, because there is no other driver for it to couple with.

I'll say it again. IMO, the differences relate to room effects, which measurably audibly and without serious question anywhere in the civilized world differ when you move the source of the low frequency sound. The room really dominates low frequency reproduction in rooms that are in homes. unless you take everything outside, getting it at least 60 feet away from any building or reflective surface (other than the ground) and run your test out there, there is just no reason to think that bass management is, in your case, trumping the known effects of changing the location of bass output inside a room. IMO

Although the vagaries of differences between bass managed and multisource bass are interesting to contemplate, they're just not likely to be the most relevant issue unless you're in a theater sized room with dimensions well over the wavelength of the bass you are reproducting.
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Old 11-22-2012, 07:27 AM - Thread Starter
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Right, but without bass management I have no acoustic coupling of my mains... But if I engage bass management then the bass from the front channels is perfectly electrically summed (similar effect to acoustical coupling).

I agree there are also room affects, but from what I can tell there is no way around the acoustic vs difference in level, which when summing 5 or more channels can be significant. To me it seems I can shoot for flatter response or proper level but not both.
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Old 11-22-2012, 07:36 AM
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My WAG would be that room effects far outstrip any acoustic coupling effects, and that what is being heard are different frequency responses caused by the different locations of the source of the bass. Using the quarter wavelength concept for coupling at 100 Hz speakers need to be less than three feet apart to achieve acoustic coupling. At fifty Hz, a little over five feet apart and coupling goes away.

Right but think of it this way. My mains are 10 ft apart so little if any acoustic coupling. But turn on bass management and now the bass from those speakers is perfectly coupled since it is summed to the subwoofer.

True, given that your mains are set as "small" and the crossover point is set high.

Since the main artifact I see is around 80 Hz. Since that is likely to be at or above your crossover frequency, the question of bass summing is somewhat blurred.

Fact is, room acoustics below 150 Hz are often as much the consequence of the room and speaker placement as anything else.

If you want really good control over bass below 100 Hz, you might consider adding a second identical sub per the Harman papers.
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Old 11-22-2012, 07:38 AM
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Right, but without bass management I have no acoustic coupling of my mains... But if I engage bass management then the bass from the front channels is perfectly electrically summed (similar effect to acoustical coupling).

Electrical summing is easy to predict if the sources are in phase or the relative phases are known. Simple electrical engineering 201.

Acoustical summing is far more difficult to predict in a real world room with real world speakers. Now knowledgeable person would try to predict it, especially when speakers are like 10 feet apart.
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Old 11-22-2012, 08:02 AM
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From an old thread, (2008), but still relevant:

http://www.avsforum.com/t/1068195/dual-subwoofers-of-different-sizes/90#post_15011679

It includes the Rane quote posted by JPC above, but it also has some other viewpoints.

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Old 11-22-2012, 08:37 AM - Thread Starter
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True, given that your mains are set as "small" and the crossover point is set high.

Since the main artifact I see is around 80 Hz. Since that is likely to be at or above your crossover frequency, the question of bass summing is somewhat blurred.

Fact is, room acoustics below 150 Hz are often as much the consequence of the room and speaker placement as anything else.

If you want really good control over bass below 100 Hz, you might consider adding a second identical sub per the Harman papers.

 

Yes, the bump in one speaker around 80hz is a problem for me, but using a 100hz or 120hz crossover I can get rid of it.  I already have two identical subs.  :)

 

while I'm using my room for an example I'm still just talking general terms about this whole summing thing.  

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Old 11-22-2012, 08:40 AM - Thread Starter
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Electrical summing is easy to predict if the sources are in phase or the relative phases are known. Simple electrical engineering 201.

Acoustical summing is far more difficult to predict in a real world room with real world speakers. Now knowledgeable person would try to predict it, especially when speakers are like 10 feet apart.

 

Agree.  And I'm starting to realize there is no "best" way.  Electrical summing to a subwoofer will not replicate the acoustic sum on the mix stage.  But turning off bass management and trying to replicate the acoustic sum of a mix stage or theater is also a crap shoot.  

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Old 11-22-2012, 10:40 AM
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Electrical summing is easy to predict if the sources are in phase or the relative phases are known. Simple electrical engineering 201.


Acoustical summing is far more difficult to predict in a real world room with real world speakers. Now knowledgeable person would try to predict it, especially when speakers are like 10 feet apart.

Agree.  And I'm starting to realize there is no "best" way.  Electrical summing to a subwoofer will not replicate the acoustic sum on the mix stage.  

Neither will acoustical summing.
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But turning off bass management and trying to replicate the acoustic sum of a mix stage or theater is also a crap shoot.  

Trying to duplicate one room and speakers with another room and speakers is pretty much mission impossible.

The one thing you didn't try is adding another stereo crossover and creating new wideband mains by crossing over each subwoofer to a a front speaker. Not recommending, just saying... ;-)
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Old 11-22-2012, 02:36 PM - Thread Starter
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The one thing you didn't try is adding another stereo crossover and creating new wideband mains by crossing over each subwoofer to a a front speaker. Not recommending, just saying... ;-)

 

That's been on my list to try :-)  Problem being I only have Audyssey MultEQ so not much correction in the mains as compared to the sub.  

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Old 11-23-2012, 08:17 AM - Thread Starter
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Spent some time doing more measurements last night.  What I found was that when playing one speaker plus sub and adjusting level to get flattest response, and then running both speakers with subs the bass level was boosted about 2-3db (to clarify it was 2-3db above higher frequencies rather than being flat with them like with a single speaker).  I then worked a bit with speakers position and crossovers and went back to using a higher crossover of 120hz to get the flattest response.  Since I have a sub next to each of my mains his higher crossover doesn't seem to have a negative impact at the moment. So the combination of higher crossover and dropping sub level 2-3db yields the bass response below (1/12 octave smoothed).  

 

 

 

Will give it some more listening today to see what I think.  But initial testing it sounds pretty good.  

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Old 11-23-2012, 08:56 AM
 
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sounds largely different with the gap between 20hz and 30hz full with a line that is the same as the rest, care to comment ?
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Old 11-23-2012, 09:07 AM - Thread Starter
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sounds largely different with the gap between 20hz and 30hz full with a line that is the same as the rest, care to comment ?

 

Was the question directed to me?  Not sure I follow.  Are you saying the 20-30hz range has now been filled in?  Well yes and no.  Of the two charts in my first post, one shows mains full range which roll off below 40hz, the other plot shows using 80hz crossover with subs which adds in the 20-40hz range missing when fronts run full range.

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Old 11-23-2012, 09:55 AM
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Primetime, Show me the waterfall plot, I'd like to see your decay time.
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Old 11-23-2012, 11:57 AM - Thread Starter
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Primetime, Show me the waterfall plot, I'd like to see your decay time.

I'm no expert in waterfalls or how to do it in OmniMic.  But here is the bass decay plot and curious as to your thoughts and input.

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Old 11-23-2012, 06:04 PM
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Primetime, Show me the waterfall plot, I'd like to see your decay time.
I'm no expert in waterfalls or how to do it in OmniMic.  But here is the bass decay plot and curious as to your thoughts and input.

Can you plot the data as a CSD?
Pick as long time length as you can (says up to 250ms, I'm hoping you can plot longer, say 500ms at least), and just 1/12 octave smoothing, and 10hz thru 200hz for bass freq analysis.
http://www.daytonaudio.com/OmniMicV4/index_OmniMic.html
WFscreen144.png
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Frequency Response: Waterfalls

Waterfall plots are used by driver and loudspeaker designers for driver selection, to identify resonances or reflections, and to view driver and waveguide behavior.

The Waterfall feature becomes available when you click the Waterfall button (above the Frequency Response graph, next to the Smoothing control). Waterfalls are calculated from the impulse response.

What does a Waterfall mean?
A waterfall is an attempt to illustrate on a 3-D graph how the energy decays or is radiated over a range of frequencies. OmniMic includes three different styles of waterfall processes, selectable via the "Waterfall Type" menu.

A "Cumulative Spectral Decay", or "CSD" waterfall shows a series of time slices approximately indicating the contribution to the total response that is made after the time instant shown in the axis going into the screen. When a loudspeaker is driven with an electrical impulse, the pressure it creates should ideally also represent a pressure impulse. But loudspeaker drivers aren't ideal so they also generate resonances -- pressure waves that decay more slowly at various frequencies. The effects of echoes can hide the resonances in a CSD waterfall, but at higher frequencies the echoes can be removed by "Windowing" the calculation to only include the part of the Impulse Response that occurs before the first reflection (from a surface such as a wall or furniture) reaches the OmniMic. Careful choice of positioning within the Impulse Response is critical, because the effects of any reflections included within the selected portion will contaminate all regions of the graph up to that point on the time axis. Below some frequency determined by where the Impulse Response is clicked and how far along on the time (depth) axis a trace exists, meaningful calculation cannot be done. The graph curve is chopped off at those points on the waterfall display.

The CSD waterfall calculation process introduces some spurious side effects, so the graph should be viewed in general terms. Exact values along the curves of waterfalls are not usually reliable, rather, the positions and sizes of decaying forward-approaching ridges on the graph indicate frequency and relative intensities of resonances.

CSD waterfall curves can now be shown with different degrees of smoothing, and can also be use with long time lengths (to 250 milliseconds) for viewing effects of room reflections.

I'm used to REW plots, like this one of mine, where you can show decay time thru much longer say 750ms, and also see the decay plotted reference to 105db to 45 db, but i'll try and work with your software.
I'm trying to see if you have modal ringing issues.....
dec%252030%2520seat%25206%2520low%2520ear%2520h20.jpg

(just side note: you can see my 60hz hum, this is when I have the computer plugged into the same circuit as the wall with the lights on and going thru my AVR - a classic ground loop hum, turned off lights that 60hz hum is gone)
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Old 11-23-2012, 07:51 PM - Thread Starter
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Looks like I need to update to version 4 of Omnimic to get more flexibility on the waterfall settings.  Sent in my request to current link to download.

 

I do have REW and RS SPL meter but haven't used in a while since OmniMic is so much simpler and don't trust my RS meter.

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Old 11-27-2012, 08:29 PM - Thread Starter
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@mtbdudex,

the 

Got the updated version of OmniMic but still doesn't have waterfalls like you would find in REW.  The CSD image you show is what I can get but can't extend the decay time out far enough, the db scale is relative not absolute and it only seems to plot above 200hz.  So the bass decay I posted earlier is the best I'll get from OmniMic.  From that plot it shows my worst point around 80hz requiring about 350ms to drop 20db.  I wish it showed more but I'm too lazy (at least right now) to pull out my usb soundcard and fire up REW.....I'm too addicted to the simplicity of OmniMic.

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