floor reflection what to do ?

[DanDan]

Welcome back Marc. It would be good to find out exactly what is causing your dip. Lupo is clearly skilled with reflection hunting, so I suggest that you send him your measurement file if possible. In post 17 I suggested a result orientated approach. That approach would help you reach the best compromise of speaker and listener positions. I am not sure if you want to leave fibreglass on your floor! However if you could find out the cause or causes of the dip, you might take other actions as well. e.g. If you have side wall bounce, you might consider using much thicker traps there, ditto the ceiling, thicker cloud, and so on.
Worth checking the modes issue again.
This sounds very simple in the context of all this measurement and calculation. However, simple can be very powerful. Generate a sine wave and sweep it in the dip region to see if you find a resonance. There may be more than one. When you settle on one, move about and locate the hot spots, typically at a boundary or corner, including the ceiling and ceiling corners.


DD

[DanDan]

Dange

What does ETC stand for?

Etcetera

Lupo, thanks for the pictures and the effort. Unfortunately I am not seeing a better picture in the ETC.
In fact for the first two, the IR looks clearer to me. ARTA certainly seems to be delivering for you, but it is PC only unfortunately. I understand that the ETC is a more complete and accurate picture. Certainly a dB scale is more useful than the strange ones seen in the IR graphs. I will certainly experiment with ETC in terms of reflection hunting in the future. To be frank though, I will do this based on recommendation, your enthusiasm and efforts. I still have not seen one single result from reflection hunting, that would not have gotten by eye, ear, mirror, brain, handclaps.
If I ever do, you will be the first to know!
DD

[DanDan]

Hi Marc, I finally got SAC to show us his solution for the floor bounce.
Here it is.
He reckons two 'broadband' panels as in the diagram should do the trick. Apart for the very obvious problems with these things in the way, I claim that even the two in tandem will have no significant effect on the dip you are experiencing. I have offered test this here, but it would make more sense if you did it. After all it is you that has the dip and this is SAC's suggested solution.
Let's prove it or rubbish it in situ, in the real world.

DD

[SAC]

Amazing! He is still lost!

The two orientations are to show them in comparison! Unlike yourself, we NEVER suggested a horizontally placed absorber on the floor to be reasonable! And the vertically oriented absorber was mentioned to function in a similar manner to how some of use place monitors on mechanically isolated pedestals back several feet behind a mixing board such that the downward dispersion is blocked by the board thus preventing reflections off the board itself!

I am really sorry that the concept of using absorption to address specular reflections or spatial dispersion/coverage is SUCH an obviously complex task for you!

And that the underlying concept of superposition so utterly confounds you!

But thanks for the laughs, even though there is a (very slight) guilty feeling in laughing so hard at someone who is obviously so very very challenged.

You had best simply stick with simply trying to figure out what you are doing, and quit worrying about others, as you very obviously have more than enough of a challenge on your hands!





Oh, and let's not confuse poor dd further. As the OP assumed that the null was due to superposition. But just about all, including myself have suggested that the null could be caused by either a modal null or superposition, or a combination of both. But we now see that dd has no means by which to even construct a test (as it would necessitate creating exactly such a situation controlled superposition by introducing exactly the reflection of the broadband signal from the speaker that he says cannot be mitigated by absorption- as was earlier mentioned! - and he must now rely on the OP to do this!)

And let's not tell him that the entire reflection need not be eliminated, and that it need only be reduced in gain. but we will let him ferret out by how much! Besides, he is still looking for a "~100 Hz reflection".

...And just when you think it can't become any funnier!

[DanDan]

SAC, try to get at least the topic right.
The OP asked a question regarding his 110Hz, 135 Hz dip.
Absorption on the floor was first mentioned by the OP.
SAC suggested that a 'broadband panel' would solve this problem.
Any sane reading of this thread will find this to be true.
Now that a test/proof is imminent we see the pathetic squirming, the shifting of goalposts, the revisionism. Refections off a mixing board seem to be replacing the floor bounce issue. We also see the return of irrational bouts of text laughter. Standard SAC /the banned foxfyr nonsense.
Once again, I maintain that a normal 'broadband' panel, such as a MiniTrap will have negligible diminishing effect on the path of the alleged reflection in the 135Hz frequency region.
It was and is nonsense to suggest it.

DD

[SAC]

Are you on drugs? Or do you need prescription medication? Seriously!

Shifting nothing! Superposition is superposition!

And the CONCEPT of using absorption or a reflective surface to block or redirect downward oriented signal propagation is the same whether one uses absorption or any other reflective/blocking surface to reduce energy gain in a particular area.

But such concepts and analogies confuse dd. As he fails to understand the basis for, and the practical interpretation and use of the ETC, the fundamental Analytic upon which ALL measured responses are simply perspectives, and now: superposition.

Folks, what is sad here is that he continues to chase this very simple concept around as he holds everyone else accountable for that which he clearly has little or no understanding.

Ironically, what has not been ascertained with certainty is the actual cause of the null. Once the actual cause is identified, be it modal or superposition, the solution options are known. One simple way to do this is to obtain a precision ETC response. The ETC will clearly show a reflection corresponding to the time offset/distance necessary to result in the initial null frequency and any harmonic intervals. Likewise, the simple introduction of a piece of absorptive or reflective material in the reflection path during the measurement will result in the ETC response corresponding to that reflection disappearing, or at least being reduced significantly in gain.

The OP had a pretty good concept of this from inception, as did the other contributors who simply posited the potential for there being two potential causes that bore investigation. And we (just about!!!) ALL know how to mitigate reflections.

Unfortunately for dd, he simply fails to understand how the result of the superposition of similar non-aligned signals in the time domain manifests itself in the frequency domain in the form of comb filtering in the frequency domain which is a very limited perspective upon the larger spatial anomaly regarding the spatial polar lobing that varies with frequency based upon the superposed signals' separation in TIME.

And what the frequency response comb filtering actually 'reflects' (sorry, as the use of this word here will probably send dd into another tizzy) is spatial polar lobing. And the frequency response null simply means that you are spatially located in a polar lobing null! That frequency still exists!!!! But its spatial dispersion has been modified by virtue of the superposition of the direct and reflected signals into spatially dispersed lobes and nulls!

And this problem can be 'cured' if one were able to simply align the two (or more) signals in the time domain - as this problem is commonly done (at least in one plane or possibly at a 'point') through the use of precision microsecond signal delay in arrays and distributed sources.

But in the case where you cannot effectively delay one signal relative to its reflection, the problem is remediated by reducing the reflected energy by at least 10 dB.

Unfortunately, the problem here is not the actual acoustical problem, the ways to identify the acoustical problem, nor the means to remediate it. All are routinely employed and well understood.

The problem is one individual who simply fails to understand basic concepts, and instead follows another around in a dysfunctional manner from post to post as he decries the tools and repeatedly demonstrates a lack of understanding of the basic concepts at hand.

So dd, keep working on this. But start taking ownership of your own ignore-ance of the basic concepts and of the modern tools and measurements and underlying concepts to identify the problem, as well as the BASIC MATH useful in understanding such physical behavior. You see, it has become very tiresome to keep watching you project your ignore-ance of such basic concepts onto others. And if you can't fathom the utility of the ETC measurement, write to the manufacturers of all of the modern measurement platforms and complain to them and blame them for your inability to comprehend the usefulness of the perspective. As ALL of this stems from dd's attempt to discredit the ETC and the concept and tool that Richard Heyser introduced in an attempt to redeem himself for his erroneous earlier pronouncements.

[DanDan]

Drugs not at the moment, you got some? I am however, pagan, black, lesbian, and left handed......
Pretty much an antidote to SACtarianism
Some perspective, SAC/foxfyr has not been with us very long. He caused sufficient trouble to get banned when using his first moniker foxfyr.
A search of his posts will reveal an unmitigated series of rows with many of us regulars. One will not find the words 'thank you' at the end of any of his threads. He comes here to fight.

A MiniTrap, standing vertically as illustrated will not diminish the 'path' of the 135Hz wavefront to any significant extent. Certainly not the '10dB' required. Look at the OC absorption coefficients. This LF wavefront will pass right through, slightly ruffled. More of it will simply go around this relatively little panel. Thus the additive null caused by the different path lengths will not be diminished. We will see this shortly.
Reality as shown by experiment is imminent. The goalposts are desperately shifting, 'maybe it's modal' maybe it's, oh look a bear.....
Yeah right, remember the OP, the frequency shifts with listener distance. Modal?

DD

[dtf]

Even though I am a bit confused on what this thread is still about, I'd like to chime in to ask something which is probably very basic.

With the speed of sound being around 340 km/h, the soundfield builds up relatively quickly. I fail to see how any later reflections would not be part of a measurement to begin with. I assume that any intelligent person would look at later reflections as well after having dealt with the first refl. points and a problem still remaining.

And, isn't a bass-trap already dealing with what SAC calls "time-domain" anyway, since it absorbs mostly reflected sound?

[SAC]

...see if I can address this in the 3-4 minutes I have before running...

When you ask ' aren't reflections included in measurements' - well, yes, depending upon the measurements to which you refer.

They are 'included' in the frequency domain, as no discrimination is made with regards to time - so you see the summation/total of all of the direct and reflected signal and you; in effect, you see the car wreck that resulted - with NO detail as to the component signals that contributed to the situation. Thus you see a freq. response complete with comb filtering - a frequency perspective correlating to the spatial lobing of the signal (and whether you are in a positive lobe or a null at a particular frequency) at the particular spot where the measurement was taken. Specular reflections are addressed by 'intercepting the path' of the reflection where the preponderance of their energy is located. Bass traps are located in the high modal pressure zones to absorb and dissipate energy. And modal regions do not necesarily concur with the incident reflective points of the focused specular reflections.

On the other hand, our hearing and the behavior of sound occur and have real issues with regard to the arrival times that are properly viewed in the time domain - things happen with respect to time.

One perspective of the total Analytic that provides for a complete accounting for the kinetic and potential energy of the system is the ETC response (Envelope Time Curve). This provides detail regarding the arrival time and energy content of each reflected energy 'packet' - which brings us to the last aspect regarding bass traps.

Sound has size. Wavelengths that are larger than the encountered boundaries are modeled differently than those that are smaller than the encountered boundaries. Low frequencies below the Schroeder critical frequency result in what is refereed to a modal behavior, as they essentially pressurize the entire room and result in distributions of high and low pressure. Sound that exhibits frequencies above the Schroeder critical frequency have wavelengths smaller than the encountered boundaries as are referred to as specular reflections - localized focused reflections defined by their arrival time (delay) relative to the direct signal and their energy content as well as their Q - or degree of dispersion.

The ETC allows (some of) us to identify and locate the various individual reflections that combine via a process called superposition that results in destructive modifications to the original source signal. the only way to correct the resulting anomalies - that become manifest in the form of spatial polar lobing and the appearance as comb filtering in the frequency domain, is to correct the combining signals - either by aligning them in time (delaying one) or by effectively removing or modifying the one with respect to the other. This is typically done with either precision delay or by absorption or diffusion to effectively reduce the energy content (gain) of the 'offending' reflection.

So while frequency response and waterfall or cumulative spectral decay are useful in identifying the larger than boundary size wavelengths of the LF in order to identify areas of high and low pressure, we need more refined tools able to view the specular reflections more focused nature in the time domain. So the two perspectives of the behavior of sound allow us a more complete view allowing us to better and more precisely address the full range of issues.

[DanDan]

Graphs display measurements in different ways, suited for purpose.
SAC very eloquently illustrates comb filtering with hypothetical software in post 30 here treating live room ceiling
Ironically I note he choses to use a Frequency Response graph to convey the full extent of the damage.

But back to the topic, the original poster's question.
Mixary asked for help with a dip in his frequency response.
It is severe, around 15dB around 135Hz.
He notes the frequency of the deepest dip shifts with listener distance.
With this shift in mind, he uses calculation to show how a floor reflection could be causing the null. IMHO plausible, but there are very likely other mechanisms contributing also. Interesting though. We all have floors.

I posted my methodology of dealing with it. i.e. Using measurements , carefully optimise the speaker positions in all dimensions to minimise the dip.
This approach will also alleviate the problem if it has other sources. e.g. Wall bounce, SBIR, etc.
I try to be helpful.

SAC claims that a broadband absorptive panel, if placed in the path of the damaging reflection, will diminish the level of that reflection. Since the reflection is diminished, the nulling it causes should be consequently alleviated.

I believe the following-
SAC is and has not been helpful. Such a location would be impractical. I have never seen such an installation, ever.
Such a panel will not stop or significantly diminish the level.
The OC absorption coefficients show that.
135 Hz will pass right through and around the panel.

No solution, not helpful, nonsense, and as usual causing confusion.

Happy St. Patrick's Day

DD

[DanDan]

I thought it a good day for this....

Here's the scenario. The little mirror was used to establish the exact bounce location. Both LF drivers included.


125Hz third octave filtered pink noise, for the SoundCheck CD.
B&K 2250 measured 87dBC (Slow)

MiniTrap inserted, front facing speaker for maximum effect.

Exactly 87dBC was again measured.

DD

[dtf]

Originally Posted by Victor_Stoian

actually the speed of sound is 343 m/s and not 340km/h.
Using other units it would be 34cm/ms - meaning the sound "moves" 34cm every milisecond.

Oops! Funny nobody noticed it before...! Sorry about that.

[Weasel9992]

Originally Posted by dtf

Oops! Funny nobody noticed it before...! Sorry about that.

I think in AS, not metric...

Frank

[jhbrandt]

Originally Posted by DanDan

I thought it a good day for this....

Here's the scenario. The little mirror was used to establish the exact bounce location. Both LF drivers included.
Attachment 163371

125Hz third octave filtered pink noise, for the SoundCheck CD.
B&K 2250 measured 87dBC (Slow)

MiniTrap inserted, front facing speaker for maximum effect.
Attachment 163372
Exactly 87dBC was again measured.

DD

Very good! As we knew it would be.

Marc, get a copy of my http://jhbrandt.net/FirstReflectionCalculator.xls

I still think you have modal issues. Can you give us a drawing of your room with dimensions? 3D?

- John

[Nordenstam]

Originally Posted by DanDan

I thought it a good day for this....

Here's the scenario. The little mirror was used to establish the exact bounce location. Both LF drivers included.
Attachment 163371

125Hz third octave filtered pink noise, for the SoundCheck CD.
B&K 2250 measured 87dBC (Slow)

MiniTrap inserted, front facing speaker for maximum effect.
Attachment 163372
Exactly 87dBC was again measured.

DD

Thanks for the effort, but... Are you sure you're measuring relevant data?

There's no doubt that a huge wavefront will find its way around an obstacle. There's also no doubt that a piece of absorption will absorb sound energy even if the wave is significantly larger than the absorber. Otherwise, there would be no effect of bass traps below their typical 2 feet width which equals about 500Hz!

The conclusions seemed so wrong to me that I had to do a quick test on the subject.

Speaker<>mic distance was set to 120cm's. Speaker acoustical center is at 122cm and mic height was set to 118cm above the floor. A bit too low. Didn't think about the mic height before I had done the measurements and I can't be bothered to do it again. The four different measurements are:

1. Nothing on floor
2. Foam on floor. 40x40cm, four layers of pyramid shaped foam for a total of 16cm depth. (about 1.33 feet square half foot thick)
3. Rockwool on floor. A spare piece I had lying about, 55x60cm, 10cm thick, 60kg/m^3. (about 2 feet square 4" thick 4lbs/feet^3)
4. Same piece of rockwool, standing up.

The best position of the absorbers was found to be a little bit further out from the speaker than the mirror on the floor indicated. This was evaluated using realtime ETC measurement.


Here are the frequency response graphs (500ms window):


Nothing on floor


Foam on floor


Rockwool on floor


Rockwool standing up


The corresponding energy time curves:


Nothing on floor


Foam on floor


Rockwool on floor


Rockwool standing up


10dB difference in the reflection! Not bad for a 2x2 feet panel.


Given these results, I'd say that a even a relatively small piece of absorption would be a nice solution to the problem of the original poster.

I don't know about you guys, but I don't have any reason to walk around in the zone between sweet spot and speakers. Having a piece of absorption there would do no harm to my practical work flow.


Edit: solution is not the right word. The small absorber is better than nothing. Throwing a bit more absorption at the problem shouldn't take much effort.

[DanDan]

Lupo, thank you for the tests. They do indeed seem to show a significant change.
Do you have a theory as to what mechanism is causing that? The superposition one would seem to require a reflection with a similar frequency response to the original surely?
My overall point is that absorption will obviously diminish the reflection in a frequency selective manner due to the frequency variant absorption coefficients of the material used. My test clearly shows no reduction of the 125Hz third octave band at the reflection point. Perhaps I should have tested using a sine wave?
I will try this again using FM, with the absorption a bit further out.
Similar tests should show similar results.

DD

[Brainchild]

My overall point is that absorption will obviously diminish the reflection in a frequency selective manner due to the frequency variant absorption coefficients of the material used. My test clearly shows no reduction of the 125Hz third octave band at the reflection point. Perhaps I should have tested using a sine wave?

Isn't the point of this method the elimination of a specific frequency cancellation? It seems an untreated test to determine the cancellation caused by floor bounce using a swept sine, then testing the same swept sine with treatment for the effect on that cancellation.

[SAC]

The resulting tests are based upon an incomplete analysis of the real problem, as well as a misunderstanding of the nature of what the flawed 'experiment' was assumed to prove.

After all, quality results are dependent upon a properly framed question, and the the assumption made, that was then assumed to be proven or dis-proven is flawed. Unfortunately some simply rush ahead with their incomplete premise and then think they are making pronouncements to some singular principle.

First we have the issue of the assumed source of the problem. Despite the fact (that one would have us ignore!) that just about ALL of us suggested that more measurements needed to be taken to determine the degree to which modal issues were at hand, and if the superposition of specular reflections were involved. As if they were, the intensity/gain of the reflected signal would be a significant indication as to the degree to which it contributed to a 15 db null. Likewise, the Q of the null becomes important as well in evaluating the nature of the null.

Unfortunately, instead of doing this, we were treated to the assumption that:

Originally Posted by DanDan

Marc, nice diagram. I see how the dip frequency would change, my apologies for not thinking it through earlier. You could certainly take a look at either your ETC or Impulse response and you should be able to easily see the floor reflection. However, since you already see it with your eyes, and have measured it physically, there is hardly much point. Unless you can't believe your eyes and need further graphical confirmation. ;-)...

Thus, no additional testing to actually evaluate the given circumstances was apparently performed, as we witness a failure of a proper diagnostic process.

And instead, as there was a null at 130Hz or 110 Hz or whatever frequency, that we were looking for a rogue 130 Hz or 110 Hz reflection!!!!

Thus mixing time and frequency perspectives into a hodgepodge that corresponded to neither model nor about which one knows if there is any correspondence to the actual cause of the problem. But we DID have a "nice diagram"! And what more could there be to acoustics than pictures?

So, without knowing the dimensions of the space, one might reasonable suspect that at 110 Hz that we are dealing with a mixed mode’ area where we have both modal and, possibly, specular behavior, depending upon each the boundary dimensions.

As was posted, IF the behavior was entirely specular, the time and corresponding distance corresponding to the difference between direct and delayed signal can be easily calculated for both the initial frequency null, and also the null frequency interval.

But since some reflection point can be located with a mirror, and if we draw a picture, there apparently is no need to verify the various energy paths and to verify if any even correspond, or if there are multiple paths that contribute to the same arrival time. And are there modal issues at play as almost everyone else, including myself, suggested investigating?

But hey, we had a picture!

And then one went hunting for an isolated “110 Hz reflection” in order to disprove superposition and his assumption that the cause of the null was superposition and not a modal null or a combination of modal behavior or specular reflections – assuming that the room dimensions supported specular reflections at that frequency!!!!

One will note that the concept of the Schroeder critical frequency was never addressed at which the behavior at a particular frequency transitions from modal to specular based upon the boundary dimensions and the propagated signals wavelength!

And as this continues, one person continues to mix models and behaviors. If the null is modal, judging the response of a specular test a bit absurd.

Can a broad band specular reflection near the intensity of the original signal create a low localized frequency null – absolutely – as is demonstrated in the posted ETC and frequency response of a direct and delayed identical broadband pink noise signal. Can such a nulling effect be corrected by broadband absorption or diffusion? Absolutely. Assuming such behavior is actually the cause.

But will addressing specular issues correct for modal nulls? And will addressing modal issues correct specular issues? No.
The principles of each are actually pretty simple and straightforward.

But mixing models in an attempt to prove one invalid simply demonstrates a lack of understanding of at least one, if not both concepts.
All that has thus far been demonstrated is that the pursuit of a solution, without understanding the real cause of a problem, is a recipe for frustration and error. But once the real cause, or perhaps better stated as the ranked primary cause of a problem is identified, the correct option for remediation is more easily determined.

And pictures, however fancy they may be, that ignore additional environmental factors are not sufficient to diagnose a problem. And to proceed to run full speed ahead in an emotional attempt to disprove a concept based upon an incomplete understanding of both the principle as well as the real situation only proves that one fails to have adequately evaluated and understood both the nature of the original situation as well as the nature of the principles called into question.

But then, why should anyone actually use the measurement and analysis tools at our disposal to validate actual behavior when we can simply posit a supposed cause and draw a diagram? I think we have an excellent example as to why.

Addendum:
What would be refreshing would be that the various models and behaviors as well as the tools available to accurately evaluate them in detail, ALL be used appropriately to complement each other! And that some cease to pursue an emotional agenda in an attempt to discredit an established and proven method and model of which they, for whatever reason, have chosen to deny - regardless of whether they choose to remain willfully ignore-ant of the additional tools at our disposal....Well, one can hope....

[Nordenstam]

Originally Posted by DanDan

They do indeed seem to show a significant change. Do you have a theory as to what mechanism is causing that? The superposition one would seem to require a reflection with a similar frequency response to the original surely? .. My test clearly shows no reduction of the 125Hz third octave band at the reflection point. Perhaps I should have tested using a sine wave?

The difference in level by moving a single panel in a room is very small measured in overall dB's across a large stretch of time. Sound is practically omni at such low frequencies. The speaker shoots sounds everywhere, the SPL meter measures reflections from everywhere, across a relatively large stretch of time. It's likely to have included three digit numbers of reflections from the room in each displayed value. The 2.7 second long impulse response wave files of the no treatment and standing absorber recordings posted above, reads -47.247dBFS RMS and -47.498dBFS RMS respectively. 1/4 dB change on full bandwidth sweep.

And in case this isn't obvious for the casual reader: A bandwidth limited noise would have adressed a specific problem frequency to some precision. The thing is, there is no specific problem frequency band. The frequency response seen is a consequence, not the cause. The real problem is that a time delayed copy is superimposed on the signal. The low end hole is an obvious problem to adress, by all means, but it doesn't mean that the problem itself is isolated to that single frequency band. The time delay is the cause of both the dominant frequency dip and a bunch of other dips across the spectra. I've only shown the most problematic area of the FR, but there are dips all over the place.

Originally Posted by DanDan

My overall point is that absorption will obviously diminish the reflection in a frequency selective manner due to the frequency variant absorption coefficients of the material used.

The low end performance roll off in the typical absorbers is of course not a positive thing. That's how it is with such devices. It doesn't make them any less useful in this application than any other application. The added distance from the standing absorber to the surface gives higher velocity and better low end effect. Using a tuned low end device to adress a broad band problem is not an option. Though it may be a compliment to low end absorption behind normal broadband absorbers, it would hardly be practical in this particular case.

[DanDan]

Many of us have expressed the opinion that the floor bounce could not be the only mechanism at work here. However, let us not forget that the OP stated clearly that the frequency changes with listener distance.
On the basis of the frequency change, we went with his idea that the floor bounce was the most significant cause.
Then some of us suggested remedies.
Mine was a catch-all, make the best of it, which would address the dip irrespective of it's component causes.
SAC proposed that a broadband absorber in the reflection path would diminish the reflection enough to alleviate the dip somewhat.
I simply cannot see how this would happen. I do not think a MiniTrap or other panel in the path of a 135Hz wavefront, would drop the level by the required 10dB or so. My test appears to confirm this view.
I would have thought that third octave pink and an SPL meter were quite valid, in terms of what the ear would actually hear. i.e. no change in the frequency response, no alleviation of the dip we are interested in.
I would welcome any views on that one way or the other.
A fascinating issue has arisen now. Lupo's test shows an alleviation of the dip, in the case of the fibre, but strangely not the foam.
If this test is valid it is surely good news. Could we extrapolate that a mere 10cm of Rockwool, no gap, is enough to diminish SBIR dips?
Again, I find it very difficult to believe that it would.
I will try to duplicate Lupo's test.
Lupo, nothing was changed when generating those graphs right? e.g. You have previously shown how manipulation of the window can result in different FR graphs.
DD

[Nordenstam]

Am not saying the floor HAVE to be the source of the particular problem here. What we do know is that there is a floor there, there will be a reflection from it and with the distances involved and the levels I had in that setup, it'll look like the graphs I posted. He may have other levels of the reflections.

Originally Posted by DanDan

SAC proposed that a broadband absorber in the reflection path would diminish the reflection enough to alleviate the dip somewhat.
I simply cannot see how this would happen. I do not think a MiniTrap or other panel in the path of a 135Hz wavefront, would drop the level by the required 10dB or so.

Well, it does! As shown in my test. That particular reflection (not the total sound field) drops 10dB in level with the measly 2x2' panel. What's being measured is a broadband reflection, not a 135Hz wavefront. If you move the mic a bit the comb filter dip frequency will change, yet the signal is still the exact same old signal. It doesn't have a single specific frequency.

Here are two excerpts from Bob Golds absorption coefficients tables:

Product	thickness	mounting	density	125hz	250hz	500hz	1000hz	2000hz	4000hz	NRC
703, plain	4" (102mm)	on wall	3.0 pcf (48 kg/m3)	0.84	1.24	1.24	1.08	1.00	0.97	1.15

Product	thickness	density	125hz	250hz	500hz	1000hz	2000hz	4000hz	NRC
RHT 40	4" (100mm)	3.5 pcf (56 kg/m3)	1.07	1.01	1.07	1.06	1.07	1.16	1.05

The rockwool I use should be similar to the latter. The most interesting thing is that it worked so well with the wee size!

Originally Posted by DanDan

My test appears to confirm this view. I would welcome any views on that one way or the other.

Such a test needs a dead steady excitation signal (hard to get even with a sine wave) and a SPL measurement device with at least one digit significant precision. As good as your SPL meter is, it's hardly made for measuring zero dot something differences with any accuracy. You'll be hard pressed to have such a steady excitation signal to begin with in a real room with a human in it. The RMS level differences as cited above was calculated from normalized files.

Originally Posted by DanDan

A fascinating issue has arisen now. Lupo's test shows an alleviation of the dip, in the case of the fibre, but strangely not the foam.

The foam does something, but not that much. Waste of money..

Originally Posted by DanDan

Could we extrapolate that a mere 10cm of Rockwool, no gap, is enough to diminish SBIR dips?

This test was for a reflection, not SBIR, although I think the results translate. Problem isn't exactly gone, but it's sure much better than it was!

Originally Posted by DanDan

I will try to duplicate Lupo's test.

Great!

I do each measurement twice to check for correlation (avoiding random stuff to make much influence) and usually do every measurement series twice to confirm the results. Sometimes pays off!

Originally Posted by DanDan

Lupo, nothing was changed when generating those graphs right? e.g. You have previously shown how manipulation of the window can result in different FR graphs.

Right. Playing around with the windowing can make the rockwool look worse than the bare floor in the FR graphs. :D

[DanDan]

Lupo, I have very quickly tried to duplicate your test, fearing I might be wrong :-O

I believe what we are seeing here is an illustration of how different measurement methods yield different results.

Right. Playing around with the windowing can make the rockwool look worse than the bare floor in the FR graphs. :D

I have been at my windows and smoothing also, like you I can easily show a much worse response without the panel.

I note that with third octave smoothing there is a 1dB difference, while with no smoothing I can achieve something like your 10dB.

Third octaves are no accident. Historically they were chosen for testing as they give results which are closely related to aurally detectable anomalies.
That is why I chose to use the third octave test in the first instance. (Plus, a bit of laziness) Note also the OP's 'broad dip centred on...'

My second test used swept sine/FuzzMeasure methodology. The result viewed in third octave smoothing is within 1dB of the first test.

IMHO this illustration of the dangers in measurement and interpretation thereof has been fascinating.
Lately I have found discussion with Lupo tends to be so. I greatly appreciate his honesty and demeanor, and to be frank, his greater technical chops.

However in this case, I believe the third octave viewpoint is closer to audibility.
In which case I will not be placing panels on the floor, or in front of my speakers. Neither will I live in vain hope of them combating SBIR.

Best, DD

[Dange]

Lupo, where on your ETC are we looking for the 10dB drop? Around 4.40ms?

Just doing a quick calculation I have the reflection, in theory, arriving at 7.8ms (343m/s over a distance of 2.68m) (2.68m from 2 *sqrt(60^2+120^2) )


EDIT: I'm with you now (i think) the ETC time is relative to the direct path to the receiver? That would be around 4.40ms in that case

[Brainchild]

Isn't this just disregarding the evidence in favor of one's preconceptions?

[DanDan]

Which preconceptions? As we said, tweaking the 'windowing' can give a reverse result. That plus the fact that nobody seems to put panels in these positions seems to me to depict a reality.
DD

[Dange]

Originally Posted by DanDan

Which preconceptions? As we said, tweaking the 'windowing' can give a reverse result. That plus the fact that nobody seems to put panels in these positions seems to me to depict a reality.
DD

You can't just play about with windowing and say it effects things adversely. Windowing has a use. In sound level terms it's the amount of time the level is calculated over.
Say you were measuring the level of a noise of motorway, you'd use a long window as the sound doesn't change much. But for sound that varies a lot you'd use a short window.

In measuring frequency response via FFT, windowing is used in a slightly different way. Basically to improve accuracy of the resulting frequency response..... I can go into the maths, but again the correct window needs to be selected for the type of signal.

People do put panels on walls in these positions to combat 'SBIR'. Which is what Lupo's test demostrates in my opinon. Loudspeaker interfacing with the boundary of the floor. Read the second to last paragraph on here Learn what is SBIR (Speaker Boundary Interface Response).

Is SBIR a term invented by GIK Acoustics? Just never heard it before, but well know the effect it describes. All my Googling points to GIK that's all

[DanDan]

Dange, I don't understand what you are saying.
Both of us certainly can play around with 'windowing' to achieve results such as the following.



However by using the controls to see an audibly significant result we get this.


Looks like up to a 2dB improvement in a small frequency area. IMHO not useful enough to put up with a 2x4 panel standing where my keyboard and mouse normally are.

Lupo's earlier broadband figures showed exactly the same as my first test.

-47.247dBFS RMS and -47.498dBFS RMS respectively. 1/4 dB change on full bandwidth sweep.

Here it is pretty easy to see how one might say 10dB of energy has been removed by the panel.


Overall I am going with the 2dB third octave view. Not enough to put up with a very inconvenient panel, in my room not even possible.

I have tested panels behind speakers with a view to diminishing SBIR. I got no measureable improvement. The popularity of the practice, particularly at GIK has prompted me to do these tests again sometime but for now my test stands.

DD

[Dange]

Ok, I think I see what you've done.

Although, what have you done in the first plot? How come this is different to the third?

In the second plot, you've applied 1/3 octave filtering. Around 100Hz the bands are 80, 100, 125. The software looks in these band and gives an average of their content. You are there essentially decreasing the resolution of the plot. I don't think this is the right thing to do. It does not model how this will be perceived by the ear. All you are doing is making it less accurate.

To me it looks like you have proved that the panel does work in the 3rd plot

[Nordenstam]

Originally Posted by DanDan

I believe what we are seeing here is an illustration of how different measurement methods yield different results.

Almost. We used the same measurement method! This goes to show that the same measurements are up to different interpretations depending on the selected observation tool.

Originally Posted by DanDan

I note that with third octave smoothing there is a 1dB difference, while with no smoothing I can achieve something like your 10dB.

Excellent! You've repeated the tests with the same results. Thanks for confirming it.

Originally Posted by DanDan

Third octaves are no accident. Historically they were chosen for testing as they give results which are closely related to aurally detectable anomalies.

Add 0-20ms windowing to simulate the ear/bran integration time and it's getting closer to what the average Joe hear most of the time.

Yet, the sharp null is still there. The ear/brain hears about 24 frequency bands (and this corresponds somewhat with the 30 bands in 1/3 octave graphs across 10 octaves). What this means is that a noise or several overlapping sounds within a particular frequency band will sound more like the 1/3 octave indicates, due to the lack of discrimination within each of the 24 critical bands of the hearing system. The smoothing corresponds to that lack of discrimination of closely spaced simultaneous sounds. It doesn't mean that we only hear 24 different frequencies. The actual dips in the room are more like a 1/24 octave graph shows it to be. A slowly sweeping sine or low distortion instrument playing through single notes will clearly reveal that the dips are really sharp Q's by nature.

There are also a lot of other things happening at other frequencies, besides the big low end dips. The sharp low end dips are small parts of the total problem the reflection is causing. Also did the same tests using 200Hz to 20kHz sweeps. Thought it may come in handy to illustrate that it's not a sub 200Hz problem per se. It's not even worth posting the graphs. The only difference to be seen in the full bandwidth and limited bandwidth measurements is that the information below 200Hz is missing in the latter graphs. The rest looks the same in the FR graphs and the difference is also very very small in the ETC's. The sweep/impulse is linear, having equal amounts of energy across the spectrum. The sub 200Hz info only contains 1/120 linear part of the full 24000Hz spectrum! The RMS level difference in the after-treatment-files, including and excluding sub 200Hz info, is 0.007dB. Comb filtering is a broadband problem.

Originally Posted by DanDan

That is why I chose to use the third octave test in the first instance. (Plus, a bit of laziness) Note also the OP's 'broad dip centred on...'

I bet a cute cat picture there wasn't 1/3 octave smoothing on his measurement.

Originally Posted by DanDan

IMHO this illustration of the dangers in measurement and interpretation thereof has been fascinating. ..

However in this case, I believe the third octave viewpoint is closer to audibility.
In which case I will not be placing panels on the floor, or in front of my speakers.

Treating the floor may seem odd. That's a valid objection! We're so used to hearing a reflection from the ground. It's probably somewhat easier to ignore than other reflections. Speaker polar dispersion patterns also comes into play. It is however an early reflection like any other early reflections, with the same cause and consequences. Some solve the floor reflection issue by appropriate placement of a console, setting it to block the floor reflection.

My console is made as small and open as possible so as to not interfere with the sound field. Have a small mono speaker in the middle and the console is enough to block the floor reflection from that one. But it doesn't do anything to the sound from the mains so I've been using floor treatment. Have gone back and forth between having absorbers on the floor or not and ended up using it. It's not occupying any space I use anyway. Am actually employing those crappy pieces of foam for this purpose. Ghasp! Spread out a bit to cover more than the 40x40cm's used in this test, but still, it obviously doesn't do much to the low end. The reason for using foam is because of the stepping-on-it issue. Thanks for giving the incentive to do something about it! Will look into replacing it with some rockwool pieces. That said, the 6" foam does work well enough to alleviate the otherwise obvious flanger effect from the floor. It's easy to hear it when playing pink noise on a single speaker. Just did that right now, again. A small sanity check for my own sake! Moving the foam back and forth from the correct spot makes it very obvious that the flanger comes and goes. It can be more gone with a better absorber, but the foam is just good enough to take it from annoying to not particularly distractive.


The important thing to keep in mind here is that all of the different measurement viewpoints describe the same physical phenomena. All the views have a direct cause, effect and consequential change in the displayed measurement value. The measurements seen in this thread and the peculiarities of the different frequency response graph options holds true for measuring any reflections. Rotating the room 90 degrees sideways, making the floor reflection a sidewall reflection, or 180' to make it a ceiling reflection, would give the same aural sensation of a flanger or not, the same measurement results and the same viewpoint dilemmas when observing the change. This exercise exemplifies measurements for any early reflection point treatment.

Each measurement view comes with its own set of particularities and caveats. Let me once again expound on this topic. It's essential to understanding measurements! What looks like a gross anomaly on a 1/24 octave graph doesn't look half bad in 1/3 octave. The change is much less again when viewing the total deciBel level in the (full range, >10 octave) SPL meter or RMS measurements. All of those views are indirect responses to the underlying cause. The cause of the actual change that is really happening, physically in the room, is that the level of a particular (relatively loud) time delayed copy of a sound is changed(plus some incidental reflections being trapped as well). That's the only thing going on in real life. All the different measurement methods will do is to show this more or less directly. Some measurements are vague and indirect, some goes straight to the core of the actual physical change. The FR graphs are vague in the sense that they are looking at the comb filter pattern resulting from superpositioning of a vast number of reflections. With the typical default views spanning 500ms, hundreds of reflections are typically included in the measurement. Makes it rather iffy to see the direct cause->consequence of the energy contribution from a single reflection amongst the hundreds of them.

When I measure something, I always try to find the most appropriate measurement tool. Preferably one that shows the value I'm trying to measure directly. There are always many ways to go about viewing the same practical events and they all have their pros and cons. Ie, an oscilloscope shows the frequency of repeating waveforms by having a calibrated time grid on the screen, thereby making it possible to see how much time each cycle take and calculate the frequency using 1/cycletime. It is, however, much quicker to see the cycles per seconds displayed directly in a frequency counter. That oscilloscope example is 100 times easier than trying to reverse-engineer the comb filtering pattern resulting from the superpositioning of hundreds of reflections. That's of course why one typically concentrate on a single dip in the resulting frequency response and try to see the difference there. It can show that things are moving in the right direction, but it doesn't show how much the total energy level of the reflection is diminished. Concentrating on a specific part of the comb filter response also creates a danger of overlooking the change that happens in the rest of the spectra, as seen in this thread.


Here are some typical ways to measure the before/after reflection point treatment we've been doing:


Frequency response graph, no smoothing, 500ms window: shows about 15dB difference in the first comb filter dip, an impressivly sharp dip at 4500Hz approaching 30dB difference, plus a lot of other differences all over the frequency spectra. Definitely a broadband issue!


Frequency response graph, 1/3 octave smoothing, 500ms window: about 1dB difference in the largest response dips and some smaller differences in higher frequencies.

Same graph with 1/2 octave smoothing gives 0.5dB difference in the largest dip change, 0.4dB with 1/1 octave smoothing. I'd say the non-smoothed response is easier when it comes to determine if things are moving in the right direction. Anyhow, the FR graphs gives an indirect view of what's going on. What's really happening is still the same old change in level of a particular early reflection(plus some later stray reflections). The early reflection will cause comb filtering that will affect the whole frequency spectra. To evaluate the total effect, all the frequency response changes have to be taken into account. It's very hard, if not impossible, to guesstimate the total change in all of the peaks and dips that changes across the full spectra. So we may as well stick to full bandwidth measurements to see what the total change is during some time interval. The interesting thing is what time interval we chose to look at.

Have mentioned a 1/4 dB RMS difference in the before/after impulses, but that was wrong. It was measured on normalized files. Bad idea! It skews the relative levels. Without normalization, the before/after difference across the full spectra is 0.116dB. Even less than the 1/1 octave smoothing indicates, because this measurement is "smoothed to ~10/1 octaves"! About the same result would have been seen with an insanely sensitive SPL meter excited by perfect white noise. Those RMS levels was measured across the full 2.7 seconds long impulse response files. It includes all reflections in the room and a lot of noise floor. The level difference observed will grow as the observation window is targeted in on the time point when the actual physical difference occurs. By decreasing the RMS window to the first 50 milliseconds, difference increases from 0.116dB to 0.530dB RMS. Increasing to 0.835dB difference when measuring the first 10ms, 3.659dB with a window from 3ms to 6ms and a 5.662dB RMS difference when looking at the single millisecond from 4 to 5 ms. Recall that the actual reflection is around 4.4ms. Zooming in at the very peak of the highest reflection around 4.4ms, measuring the sample point peak level, shows a difference at that sample point of 10.4dB. The difference at the reflection maxima increases to 10.9dB when viewing the intersample peak level information, which is using an even finer time resolution than the sample point peak level meters. It's not possible to find an even smaller time slice that'll display a larger difference that that. It's the limit as the measurement time span approaches zero. These last measurement, covering a small slice of time around the particular reflection in question, is the ones that provides the most direct information of what's been achieved by treating that particular point in time. I wouldn't use peak level or intersample peak level information to evaluate acoustics though. It's too discriminating for the purpose! The reflection treatment affects a bigger area of time than the nanosecond information provided by digital peak level measurements. There's also the usual caveat that these impulse response measurements doesn't take both kinetic and potential energy into account, like the ETC's does. Which is part of the reason to use ETC's.

Here's the same before/after treatment as in the FR graphs above, seen in the ETC's:


Long time span view, the particular difference in question still obvious.


Short time span, difference very obvious!

The ETC view shows the difference before/after on the particular reflection in question. It's obvious what and how much is happening. It's WYSIWYG! No weird changes due to window and smoothing choices, just a level change displayed in dB. What less can one ask for?

I'll let the readers decide what they deem most easy and appropriate for measuring the effect of treating a particular reflection.


There's also of course the implication that the measurements can be used to find the offending reflections in the first place. I'm sorry if this comes across as another religious post. Think it's important enough to spend time on this. Am also still having geeky excitement about the subject! The excitement is not only because it puts a decibel level meter on each particular reflection point in the room, so to speak. It's first and foremost a door that opens up for the extremely interesting territory of pushing the room response towards some predetermined goal. It's very hard to get anywhere without knowing where one is going!

Found a very nice post in a Norwegian hifi-forum some days ago. A user (Tolsen ) had used the information provided here lately to write a small and easily understood post, using far less words than I'm able to. It described the why's and how's of ETC's in a manner that took a couple of minutes to read. Seems no one in there is having any problem understanding it. It really is both simple to learn and effective to use.

Originally Posted by DanDan

Neither will I live in vain hope of them combating SBIR.

Interesting. What do you do with the SBIR?

Originally Posted by Dange

Just doing a quick calculation I have the reflection, in theory, arriving at 7.8ms (343m/s over a distance of 2.68m) (2.68m from 2 *sqrt(60^2+120^2) )

Minus direct path 120cm.

Originally Posted by Dange

EDIT: I'm with you now (i think) the ETC time is relative to the direct path to the receiver? That would be around 4.40ms in that case

thumbsup

The impulses have been normalized both in level and time (peak at 0dB and 0ms).


...
10.30 pm saturday night.. Have sown this years kitchen garden, repotted some plants and mastered some nice electronic music inbetween writing this. But still, posting here, 10.30pm saturday night.. Hereby officially giving up on the "life" thing.


Have a great weekend everyone!


Best regards,

Andreas Nordenstam

Edit: now with pictures. Think I spent too much time from uploading the attachments to making the original post..

[DanDan]

Dange, again I don't really understand your point. 'Only' a frequency response graph? Let's remember the original question was quite specific, a 15dB dip around 135Hz, that seems like a frequency response question to me. The third octave test showed no change in the area of interest. Windowing of the FM measurements and Lupo's can show almost anything one wants to see, include a graph which indicates that the dip is worse with the panel.
Regarding the overall story of floors. I have seen it written elsewhere that we are very used to floor reflections so we can put up with them easier than a low ceiling. I wonder.
An important overlooked point of interest. I have carpet with underlay!
Seems like I am lucky, this should alleviate some of the higher frequency combing.
Final detail, being very fair and honest I placed a glass picture on the floor to get a clear reflection for my test.
DD