floor reflection what to do ?

[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

[SAC]

A quick note regarding the loss of sensitivity in the low end...

A linear sweep will normally result in the observed loss of LF sensitivity. That is why some of the platforms make a 'weighted' sweep an option.

Such weighting allows for sufficient LF information to be applied to retain LF sensitivity while applying less energy to the high end - thus avoiding blowing tweeters with the sweep (as a few have found this to be an attractive option! )

An individual pure sound (sine wave) is not helpful for recording a broadband impulsive response or frequency response. This leaves us with three primary options which are available with three frequency weightings: White, Pink, and Weighted.

White frequency weighting is linear frequency weighting. The energy density over the entire spectrum is constant and the level is the same for all frequencies. This weighting is a bit unfavorable if the stimulating signal is to be reproduced by a multi-way loudspeaker. If one preferred to make the best of the load capacity of the woofer, the tweeter would be overloaded. If, on the other hand, the load capacity of the tweeter were considered to be most important, an unnecessarily poor signal to noise ratio of the low-frequency would result.

A pink frequency weighting distributes the output energy equally in all fractional-octave frequency bands. The energy density of the spectrum decays at a rate of 1/f, or 3 dB/octave.

A 'weighted' frequency weighting option distributes the output energy in a predefined manner. A very useful form of this exhibits an energy density over the lower portion of the frequency spectrum that is constant as is the upper portion. Between the two frequency ranges there is a ‘crossover’ area about 500 Hz where the magnitude is decreased by approximately 20 dB in order to reduce the energy output of the high frequencies, and thus protecting the output devices. Thus the tweeter is stressed less so that an overall higher signal level may be used without loss of response sensitivities in the LF.








…And to the point of “'Only' a frequency response graph?”

First of all, one should understand that the time and frequency domains provide different perspectives of the SAME event or Analytic.

Time domain measurements in acoustics provide many advantages over frequency domain measurements. As we are attempting to evaluate time variant behavior composed of many real and virtual sources and arrival times, the time domain provides a way to atomistically identify and evaluate the behavioral characteristics of the various individual energy sources. And thus having the means by which to identify and determine the various temporal, spatial and energy content attributes and relationships of the component signals, this view then allows us immediate information regarding how their character is modified by various environmental boundary surfaces and ultimately of their character and relationships at the point of reception. We can use this information to effect changes while receiving immediate feedback regarding changes made to any combination of said variables.

And by understanding the principle of superposition (signal combination), we also know that by remediating issues with signal alignment in the time domain, we can effectively address the fundamental contributing causes of frequency domain anomalies. And unless the component signals are ‘minimum phase’, such issues that manifest themselves as frequency anomalies cannot be resolved in the frequency domain (EQ). Conversely, the primary spatial and temporal psycho-acoustic issues are able to be addressed from the time domain view.

Whereas, in comparison to the time domain, the frequency domain simply shows us the sum total of all of the various signals without providing any detail or information regarding the component signals that combine to form the total magnitude of the energy.

In other words, while the frequency response shows us how much stew we may have, it tells us nothing about the various components that go into its making. It is the time domain that does that.

So while both perspectives are ultimately useful, the time domain provides a much more ‘precise’ way of viewing acoustic behavior in such a way as the information that is provided is directly useful in adjusting and tailoring the results we specifically wish to achieve - including that of the frequency response! Thus, while the frequency response shows you what you have when all is finished, the time domain perspective provides the tools to get you there.

In fact, the advent of tools allowing us to examine acoustics from the time perspective has been responsible for the 'recent' radical advancements in the science.

And this is why the primary means of describing the various acoustical room models are expressed specifically in terms of the Envelope Time Curve( ETC) response.

Edit:

For those still confused as to what the time domain perspective offers that the frequency domain does not, take a look at this:

http://www.geom.uiuc.edu/~banchoff/Flatland/

[PaulP]

Originally Posted by SAC

Thus, while the frequency response shows you what you have when all is finished, the time domain perspective provides the tools to get you there.

This is probably the most important sentence in this entire thread.

Paul P

[Dange]

+1 to Paul P

Originally Posted by 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.

This has been answered by Lupo and SAC.

Lupo's ETC illustrates the effect of the reflection at around 4.40ms

The sound reflected from the floor takes longer to get there than the direct path.

[DanDan]

First of all, one should understand that the time and frequency domains provide different perspectives of the SAME event or Analytic.

Absolutely.
The IR can be manipulated and then displayed in all sorts of ways.
I really don't see how or why ETC devotees repeatedly put forward the notion that it is a superior or the only proper way to view the information. Both types of graph illustrated the floor bounce.
IMHO the frequency response graph showing the comb filtering is the most visually alarming.
However, no matter which view is chosen, it seems to me vital that perspective is applied.
e.g. Vicious looking comb dips are audibly insignificant if they are narrow enough.
When view controls are applied to lean toward the audibly significant, the 'problem'
appears relatively small. Certainly compared to modal issues.


It is just another tool in our kit. In this case it forensically identified the source of the dip. Let's remember that mixary identified this earlier with a tape measure

We have seen in other threads ETC being used to identify a triple bounce reflection. At that time I regarded that bounce as low level and insignificant.
Ditto here.

I will however stick with the carpet :-)


DD

[DanDan]

Lupo,
I tested traps behind my speakers to see if they would alleviate SBIR.
No visible result. However, the test is worth repeating, in the light of these newly developed view manipulating skills!

I work around it. I test and move each speaker in all axes, to get the most even bass. I double check the combined response by RMS averaging and testing again with both driven. Basically I am trying, on a result basis, to use some of the problems to alleviate others.
Bass null in my new treated room, help! w/pics
Quite often I end up using a LF boost on the speaker, or moving the speakers closer to the front wall, occasionally almost touching it. I have seen it mentioned elsewhere (by SAC I think) that SBIR is amenable to correction by Eq. An interesting and largely overlooked point I reckon.


A mirror, measuring tape, and calculator, are of course useful, but Thomas Barefoot has done us a great favour by writing a 2D Wall Bounce calculator. Good fun. John Sayers' Recording Studio Design Forum • View topic - Wall Bounce Calculator 2D

Clarification-

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.

I was referring to my original test. The one using a third octave band of pink noise at 125Hz. This yielded a different result. I don't think the lack of difference can be attributed to 'full bandwidth' measurement. The 85dB SPL was way ahead of background and was limited to one third of an octave.

I fully accept that comb filtering is a full range issue. The OP however asked about a singular one. Quite valid and natural IMHO. I reckon the LF dip is the most potentially significant because music has a lot more energy at LF.
Also the bandwidths of the higher frequency spectacular dips are very small.
I wonder are they that audible? You need to try that flanger test blindfolded!

DD

[Nordenstam]

Originally Posted by DanDan

I really don't see how or why ETC devotees repeatedly put forward the notion that it is a superior or the only proper way to view the information. Both types of graph illustrated the floor bounce.
IMHO the frequency response graph showing the comb filtering is the most visually alarming.

Imagine that you're in front of a 100 channel mixing board. Each channel on the board is fed by a single sound sent through a short delay line. The original sound starts at the very left end of the mixer(or right if you're arab, jap etc ). The next channel have a 2ms delay simulating a work surface reflection. Next channel have a 4ms delay simulating side wall reflection. Next channel at 6ms delay simulating the front wall. Then there's 7 ms simulating ceiling, the side wall on the other side at 8ms, rear wall at 20ms, some twofold and threefold reflection in between there. And so on til the hundred channel mixer is filled up with tightly spaced delay lines. Add cross-feeding to make it all more interesting and like it is in real life. Room acoustics is all about changing the channel level fader for each of those reflections. Any and all such virtual rooms can be "treated" by setting the channel faders appropriately.

Now imagine that someone removes the input level displays(PFL) from the board, while also replacing the channel faders with endless rotaries that does not have any markings. There would be no way to get any idea of how the room sounds by looking at the levels of the channel faders. It would also be a problem using the channel level controls to achieve anything useful, depending on what you send into the board and what's being strapped on the master bus. Hooking a frequency response measurement tool across the master output of the mixing board isn't that useful. It doesn't give us information about each channel on the board and neither does it give direct feedback on what happens when one is changing the level on a particular track/room reflection. Observing the master VU meters isn't going to do much good either. Even rather large changes in the room will produce overall level changes so small that it'll hard to measure any change in the total level of all reflections. The only measurement tool that gives back what we had before loosing the channel faders and input level metering, is the time vs level view (ETC). Such a tool gives us an idea of the reflection pattern as is and it lets us observe the actual level change as seen when treating any particular reflection.

Have been playing with impulse responses in the 'puter lately. Although I do not have high thoughts for the frequency response graphs as such, it's interesting to try to see the correlation between individual reflections and the resulting overall frequency response changes. Virtual room treatment is easy to do by editing the level of particular reflections in a sound editor and then re-importing the impulse response into the acoustic measurement software. It's very handy to be able to test treatment changes without doing actual changes to the room. It's of course way simplified, but much better than not having the option! More on this is another thread.

Originally Posted by DanDan

However, no matter which view is chosen, it seems to me vital that perspective is applied.
e.g. Vicious looking comb dips are audibly insignificant if they are narrow enough.
When view controls are applied to lean toward the audibly significant, the 'problem'
appears relatively small. Certainly compared to modal issues.

The 1/3'rd octave corresponds to the ear/brains 24 critical bands. It's about masking for simultaneous sounds, like noise. It does not imply that the ear will smooth a slowly sweeping sinewave to the response seen in 1/3 octave graphs! Most people, the average listeners of psychoacustics, can discriminate a 5 cent note frequency difference. 5 cent is only a few Hertz at high frequencies and way less than that a low frequencies.


Looking at the level of each reflection makes a lot more sense if one wants to compare a particular room to what one may experience as a human listener. Back to this picture:


and the variable threshold argument in this old post.

Mapping the virtual mixer describe above, or an ETC, against the picture above is easy. There is NO such figure of merit for frequency response graphs. They do not describe the room in a manner that can be used to predict or build specific room responses. Neither do they show what happens when treating a particular reflection. The only way to have a totally flat frequency response is to be in an anechoic chamber.


It's easy to get a general idea for how this works for the ear. Load up some piece of music and put an echo FX in line. Adjust the delay time between 2 and 50 milliseconds while trying various dB levels at various delay times. Headphones may make this easier to hear. The echo should be very audible at levels above -10dB. Somewhere between -10 to -20 dB may be good enough. The amplitude change from a single reflection at -20dB is about 2dB RMS. Should still be within audible range. With some sound sources, even 20-30dB down may not be good enough! A reflection at -30dB still gives 0.5dB amplitude changes between dip and peak in the comb filter response. It all depends on the excitation sound used in the particular test. Short transient sounds are the most revealing. Generally speaking, below 15-20dB within the first 20ms or so should be rather transparent in most circumstances.

Often times subtle problems does not reveal themselves until some special track comes along. Then it's like WTF is THAT?? It's why I put the floor absorbers back after living without for a while. One day, one track, there was this obvious flanger when I moved my head back and forth. IIRC, there was some sort of synthetic marracas like sound that made it obvious. Putting the absorber at the floor suppresses the flanger. Measuring confirmed the before/after change in reflection level. That it happens to be at the lower edge of the general curve for perception is not totally surprising. We're supposed to be better than the average listeners!

Yet, it's not a big problem. By all means! One can live fine with the -20dB reflection. The point is that it's an example of measurement methodology that holds true for any and all situations.

Originally Posted by DanDan

Lupo,
I tested traps behind my speakers to see if they would alleviate SBIR.
No visible result. However, the test is worth repeating, in the light of these newly developed view manipulating skills!

Good luck! Have seen some improvement by treating the offending positions with rockwool. Not as much as I'd like in an ideal world, but it's been better than nothing. Though I haven't tested enough to say much about this.

Originally Posted by DanDan

I work around it. I test and move each speaker in all axes, to get the most even bass. I double check the combined response by RMS averaging and testing again with both driven. Basically I am trying, on a result basis, to use some of the problems to alleviate others.
Bass null in my new treated room, help! w/pics
Quite often I end up using a LF boost on the speaker, or moving the speakers closer to the front wall, occasionally almost touching it.

That's nearly as much art as it's science. Takes much hard work to get it right. Cudos to you for digging the trenches!

Have fallen in love with RPG's room optimizer software for this purpose. The starting points provided corresponds surprisingly well to the rectangular rooms I've tried it in, even if not all walls are perfectly reflecting objects. It's awesome to watch the program go through five digit numbers of virtual moving/re-measuring changes that would take weeks to do by hand. Especially when there's more than two speakers involved.

Originally Posted by DanDan

I have seen it mentioned elsewhere (by SAC I think) that SBIR is amenable to correction by Eq. An interesting and largely overlooked point I reckon.

Now that's interesting! Got more info?

SAC..?

Originally Posted by DanDan

I was referring to my original test. The one using a third octave band of pink noise at 125Hz. This yielded a different result. I don't think the lack of difference can be attributed to 'full bandwidth' measurement. The 85dB SPL was way ahead of background and was limited to one third of an octave.

Got a folder on this machine named "test and beep tones". There are now more than 1500 files in the various subfolders, all of them created to test various ideas. Many of which comes around during internet debates. A good percentage of those files are in a folder named "DanDan". I keep checking up on things, mostly to have a quality control and check that my ideas aren't totally off. Comes in handy when doing forum posts. Recommended!

I've already written why it's impossible to make the narrow band noise change large enough that it'll show up in the SPL meter. But I'll repeat your test and show you what sort of numbers it gives in total energy response. You've mentioned 1/3 octave noise, so the 1/3 octave (four notes) between B(123.48Hz) and D(146.84Hz) should be close enough. Synthesized such a noise (with slow roll off bandpass filtering) and ran the noise through the impulse responses from my measurements, giving the response with and without the rockwool on the floor. The RMS level difference in these no treatment vs rockwool measurements are 0.060dB in the left channel and 0.050dB in the right channel. Also made an extremly sharp filtered noise with hardly any energy outside the band, using FFT filters. Much sharper filtering than just about anyone is practically using. This measured 0.269dB and 0.225dB difference L/R respectively. Still outside the resolving abilities of most SPL meters and real world testing scenarios.

Using a sine wave should give the biggest change of them all. The exact frequency of the prominent dip is a bit hard to find, so I tried with various sine waves. With 137Hz sine wave, the before/after difference was 1.311dB and 1.201dB RMS L/R channel respectively. 138 Hz; 1.170 and 1.296 dB. At only half a Hertz away, 138.5Hz, change rolls sharply off to 0.613 and 0.715 dB. At 139 Hz, 0.055 and 0.019 dB. The 1.3dB change seen at the exact right frequency (plus minus 1 Hertz) is a change that may possibly be resolvable by the nice SPL meter you're using. So if you want to use the SPL meter as a figure of merit, make sure to use a sine wave centered on the exact frequency of the offending dip.

If we assume that we can hear a difference of 5 cents of tuning, the corresponding frequency change is to go from 137 Hz (C#2 minus 20 cents) to 137.4Hz (C#2 minus 15 cents). This is well within the actual width of the dip as measured using the sine wave test.

My room isn't your room, but the overall trends seen in this test should be about the same for your room. The reason why there's so little overall change is because it's only changing one of the 100 faders on the acoustics mixing board!


If you still think this is odd, I urge you to redo the tests you've done with the panel on the floor, except that you use the closest sidewall reflection as the figure of merit. You'll find the same lack of discriminating ability using the SPL meter, the same lack of change in the 1/3 octave frequency response graphs. The objections put forth against those measurement views holds true for any and all reflection measurements.

[DanDan]

Lupo, thanks for doing more tests. I was about to do the Sine. All the test are showing the same thing, DOH! Probably a nonsense statement technically but If we could radically filter the ETC reflection it would show a similarly tiny difference at any of the comb dips.
It is well understood that third octaves are an approximation. They are however the result of much research and work very well in many many situations where we need to bring science closer to human experience. In my first test I was trying to simulate the OP scenario. I still think it is a viable analogy. I got the zero difference result. The FM test with similar smoothing 2dB at worst. Now your Sine test shows no appreciable difference, plus illustrates the extremely narrow band of the anomaly.
Surely one must accept that this is not a big issue, and to even find it requires very creative use of measurement techniques. The 10dB of energy loss shown in the ETC reflection is spread throughout the spectrum. Any individual anomaly, as in comb dip, will be a tiny fraction of that. This we have proven. Furthermore individual dips are so narrow in bandwidth that they are most likely totally inaudible. To cap all this if we were to apply the A curve to the dip spectrum, we would arrive at a much lower number than 10dB.
Having said that, I appreciate that you hear flangeing, and intuitively I just cannot ignore a hard shiny floor under a speaker.
I have carpet, and I will be recommending it for listening rooms. That at least is a clear and useful result from our discussions here.
DD

[Dange]

Originally Posted by DanDan

I have seen it mentioned elsewhere (by SAC I think) that SBIR is amenable to correction by Eq. An interesting and largely overlooked point I reckon.

The way it effects phase?

[johndykstra]

from what I understand, (I think it was actually Rod, but SAC certainly may have said it as well), because speakers are stationery, reflections from the front wall come back and combine with the direct signal... this relationship never changes. Regardless of where you are in the room, this relationship is constant. any other type of room mode is location specific, thus eq is not an option.

[DanDan]

Correction. On reflection :-) SBIR cannot be amenable to correction by Eq.
I must have picked that up wrongly, apologies to whoever I am misquoting.
If by SBIR we mean the dip(s) caused by the destructive reflection from the front wall, then Eq cannot fix it. An increase in level at that frequency (Eq) will also increase the power of the cancelling reflection.
However, as we know soffit mounting creates a +6dB LF reinforcement.
I believe moving speakers close to the front wall, tends toward the soffit condition. The resulting LF increase can often be very welcome, e.g. with smaller speakers. With larger active speakers, one can often simply turn down the LF. The shift of SBIR frequency upwards, plus an overall LF enhancement touched by a little corrective Eq, can sometimes lead to an optimum speaker position, pretty much touching, or very close to the front wall.
DD

[SAC]

thumbsup

EQ cannot correct non-minimum phase anomalies caused by the superposition of the reflected signals with the direct signals.

But it Can (potentially) adjust the source's direct signal level emanating from the speaker itself such that it has the potential to adjust for the effective increase in the LFs due to SBIR and the reduction of the effective volume into which the energy is directed by the addition of the boundary.

And proper in-wall/soffit mounting can indeed help with this (as well as the additional benefits of assisting the potential for reducing some of the early reflections by a more highly controlled Q and dispersion patten of the direct signal - assuming you are also aware of the addition issues that must be addressed as noted above.

[Nordenstam]

Hi!

Came to think about this.

Originally Posted by DanDan

If by SBIR we mean the dip(s) caused by the destructive reflection from the front wall, then Eq cannot fix it. An increase in level at that frequency (Eq) will also increase the power of the cancelling reflection.

Comb filtering results in equal amounts of added pressure and removed pressure. If there's a sharp dip, there's also a sharp peak somewhere. The peak can be EQ'ed. Since SBIR doesn't move with listening position, this seems like a good place to use an EQ if that's so desired. Perhaps that's what SAC wrote above and I'm too dense to realize it. :D


Been thinking about this room EQ thing. What about the impulse response? Surely adding an EQ to correct non-minimum phase behaviour will make the impulse response worse..? Is this a problem in real life?

[DanDan]

Lupo, indeed, second thoughts are worth while here. Ignoring SBIR is a bit like ignoring the floor reflection.....LOL
The first dip from SBIR can be up to 20dB according to Genelec.
I am now fusing all these input streams in my mind. Another test is forthcoming. This time, a Bass Trap immediately behind the speaker, a couple of feet from the wall. Close to speaker, high energy. Far from wall, high particle velocity, large gap.
I think we would be best served to use the terms SBIR for the reflection, and call the Boundary Bass Reinforcement Effect, something else.
As you correctly note, reinforcements can be subtracted from with simple level. Cancellations however can not be added or subtracted to or from.
DD

[Nordenstam]

We're certainly getting into the finer peculiarities of treatment now!


Was thinking about the floor today as I compared the measurements of my own room with another masterers room. We both have the exact same dip at ~70 Hz. And it turns out we both have the exact same distance from woofer to floor.... Fits exactly with the dip in the graph.

Going to try showing some absorption in and right next to the speaker stand and see what happens!


As for wall treatment - I have been able to cut these reflections to -20dB by ever so carefully moving things around 'til it worked. Have only used the 10cm slabs for this. Adding more absorption did not help! What did help was to have the absorption in the right place, which have been a hit and miss for me - when it measures right, it's right.

[DanDan]

As for wall treatment - I have been able to cut these reflections to -20dB by ever so carefully moving things around 'til it worked. Have only used the 10cm slabs for this.

Finer indeed. Which wall treatment are you referring to here? Side wall reflection killer I presume?

DD

[Nordenstam]

Originally Posted by DanDan

Finer indeed. Which wall treatment are you referring to here? Side wall reflection killer I presume?

Was refering to both floor(first) and sidewall(second) SBIR. You've mentioned that you've seen zero effect from sidewall SBIR treatment and I've mentioned how I've treated those.

The first topic was the reflection down to the floor and straight back up to the speaker. Not the most typical one to treat. The corresponding ~70Hz cancellation in obvious while measuring and I figured it was time to do something about that. Problem is to find somewhere practical to put the absorber. Not easy given the fact that the speaker stands are where the absorbers should go.

[Mulmany]

If I follow... Can you make a stand that is a trap? Wouldn't this solve the issue of needing to treat were the stand is?

[DanDan]

Getting a bit confused. I would like to restrict the use of the word SBIR to the front wall. The term has too many meanings already! The side wall reflection effect will change with frequency like the floor one?. When testing front wall SBIR, I placed traps behind the speakers leaning against the front wall. No improvement in LF eveness. Not the greatest test in the world, will try again. My new thought is to have these traps right behind the speaker, well out from the wall. I suppose I have wandered off topic really, the only connection being the destructive reflection aspect.
DD

[Dange]

Originally Posted by DanDan

Getting a bit confused. I would like to restrict the use of the word SBIR to the front wall.

'SBIR' can occur with any surface close to a speaker if you think about it. Though it's a term I've never come across before to describe the effect. I'd argue that it's not an academic term because it is SBIR and not LBIR (Loudspeaker Boundary Interface Response). The word 'speaker' is never used to describe a loudspeaker by researchers and academics formally.

Personally I'd put the term SBIR to one side and start to think about sound in terms of waves, their phase and what happens when they meet.

If you think about where you place a bass trap, you don't put it in the middle of a room. You put it against a wall as this is where sound pressure levels are greatest so the absorber will be most effective.

If you can get your head around phase and deconstructive and constructive interference of sound waves then it all becomes clear.

[Dange]

Originally Posted by Lupo

Hi!

Came to think about this.



Comb filtering results in equal amounts of added pressure and removed pressure. If there's a sharp dip, there's also a sharp peak somewhere. The peak can be EQ'ed. Since SBIR doesn't move with listening position, this seems like a good place to use an EQ if that's so desired. Perhaps that's what SAC wrote above and I'm too dense to realize it. :D


Been thinking about this room EQ thing. What about the impulse response? Surely adding an EQ to correct non-minimum phase behaviour will make the impulse response worse..? Is this a problem in real life?

What SAC is saying is that you put a loudspeaker near a wall you get an enhanced bass response due to 'SBIR'. If you adjust the output of the loudspeaker by EQ you can compensate for the enhanced bass response at the source, cutting the bass output of the loudspeaker. I.e. the net result is flat in the frequency response. I think it means that you and your loudspeakers are all in the 'peak' of that room mode, close to the wall(s). All the EQ is doing is reducing the amplitude of the 'peak' by driving it less, you're cutting the frequency that drives that room mode. Take the wall away, or move the loudspeaker away, and you need to take the cut out of the EQ to get a flat response again.


What do you mean by make the impulse response worse? Impulse and frequency response are two ends of the same thing. FFT your impulse response and you get the frequency response.

[DanDan]

If you think about where you place a bass trap, you don't put it in the middle of a room. You put it against a wall as this is where sound pressure levels are greatest so the absorber will be most effective.

Dange, I don't think this is correct. Bass Traps are surely most effective at quarter wavelengths from a boundary, where particle velocity is max.

DD

[Dange]

Originally Posted by DanDan

Dange, I don't think this is correct. Bass Traps are surely most effective at quarter wavelengths from a boundary, where particle velocity is max.

DD

Yes you are correct. In my head I had it right. I was thinking about the bass trap as an entire object. I.e. the absorber and the gap behind it. They go 'against' the wall. I was trying to make the point that you don't put one in the middle of the room

[DanDan]

Dange, maybe not the middle of the room, unless it is doubly functioning as a couch or overhead feature. However, a quarter wavelength from the wall may be quite practical. Also I would say the opposite would be true of membrane and panel absorbers, I presume they work best on the boundary?

I am curious to see if Traps more or less attached to the back of the speakers, 2-3 feet from the wall will diminish SBIR more than at or slightly off the front wall.
I think the front wall reflection, with it's consequent up to 20dB dip deserves particular attention. That dip is often right in the midst of a very important musical zone. It uniquely doesn't change with frequency like the side, floor, and ceiling, reflection dips. It would be good if we could all settle on a term for that one alone, rather than First Comb Filter Dip Caused By Front Wall Reflection. I can't even remember what SBIR stands for by it seems to me that most use the term to describe FCFDCBFWR
DD

[Dange]

Originally Posted by DanDan

I think the front wall reflection, with it's consequent up to 20dB dip deserves particular attention. That dip is often right in the midst of a very important musical zone. It uniquely doesn't change with frequency like the side, floor, and ceiling, reflection dips.

What do you mean by not change with frequency?

[DanDan]

Not an april joke, a mistake. Like yourself it was OK inside in the head, but the fingers typed something else. I meant 'frequency of dip changes with distance' as in the OP here. The front wall reflection seems to be immune to that, thus amenable to subtractive eq at least.
DD

[Nordenstam]

Originally Posted by Mulmany

If I follow... Can you make a stand that is a trap? Wouldn't this solve the issue of needing to treat were the stand is?

Perhaps worth a try?

Been showing some mineral wool between the stand legs and leaning onto to the stand. Zero effect observed. Think it's vital to get the absorber at perpendicular incidence and halfway betwen speaker and floor (1/4 wavelength). Have a hunch the angle of incidence is the most important factor.

Been finetuning some early reflections lately. Particularly annoyed by the closest sidewall reflection. The only way I could get them low enough was to very carefully try various amounts of rockwool at various angles. Too much rockwool made it worse. Too small an angle made it worse. Perpendicular was not the answer either.. A tricky thing to get right! Ended up with two 10cm panels with a 3cm airgap, partial overlap and about 30' angle on the first absorber and 45' on the second. The reflection is still at -19dB and I'd like it to be at -25dB..

Originally Posted by DanDan

I would like to restrict the use of the word SBIR to the front wall. The term has too many meanings already!

I take it that SBIR means the reflections that comes back and disturb the speakers attempt at producing sound. Some of those reflections are destructive, some are constructive. The orientation of the reflecting surface is only a practical concern. Front wall, floor, sidewall, ceiling doesn't matter.

Originally Posted by Dange

What SAC is saying is that you put a loudspeaker near a wall you get an enhanced bass response due to 'SBIR'. If you adjust the output of the loudspeaker by EQ you can compensate for the enhanced bass response at the source, cutting the bass output of the loudspeaker. I.e. the net result is flat in the frequency response. I think it means that you and your loudspeakers are all in the 'peak' of that room mode, close to the wall(s). All the EQ is doing is reducing the amplitude of the 'peak' by driving it less, you're cutting the frequency that drives that room mode. Take the wall away, or move the loudspeaker away, and you need to take the cut out of the EQ to get a flat response again.

Yay!

Originally Posted by Mulmany

What do you mean by make the impulse response worse? Impulse and frequency response are two ends of the same thing. FFT your impulse response and you get the frequency response.

FFT'ing the impulse response only gives the presumed outcome if there is no time delay involved. Rooms are not minimum phase!

Originally Posted by Dange

Yes you are correct. In my head I had it right. I was thinking about the bass trap as an entire object. I.e. the absorber and the gap behind it. They go 'against' the wall. I was trying to make the point that you don't put one in the middle of the room

They do work very well in the middle of the room(or 1/4 wavelength)! For practical reasons, having them closer to boundaries are usually the way to go.

Originally Posted by DanDan

I think the front wall reflection, with it's consequent up to 20dB dip deserves particular attention.

Isn't 20dB a bit much? That's a reflection at nearly exactly the same level as the loudspeaker. Have a hunch the large dip often times seen at such a frequency is the response from several refelections.

Originally Posted by DanDan

That dip is often right in the midst of a very important musical zone. It uniquely doesn't change with frequency like the side, floor, and ceiling, reflection dips.

You do mean that there is the same listener independent effects going on at all boundaries, right? There's nothing special about the front wall. Often times there's less energy radiated in that direction, making it easier to tame than other reflections like the sidewall. This does of course depend a lot on the polar dispersion pattern of the speaker.

[Nordenstam]

PS:

Originally Posted by Dange

What do you mean by make the impulse response worse? Impulse and frequency response are two ends of the same thing. FFT your impulse response and you get the frequency response.

It IS possible to use equalizers to make the impulse response from the speaker better. If there are minimum phase errors in the speaker/source, these can be corrected for the better using EQ.

Using EQ to correct room errors (non mimimum phase) will almost invariably lead to a worse impulse response than it started out with.

[DanDan]

Hi Lupo, glad to see you are still boldly going.....
I am curious as to your side wall reflection problem.
I cover as much of the wall as necessary to eliminate visual bounce throughout the (small) mixing area. Typically it takes two 120x60x75 mm panels spaced from the wall by 75. To be honest I haven't wondered if this is erroneous and tested for actual reflections. I trust the geometry. Also the measured and more importantly the audible benefit is quite clear. An important detail, I leave a layer of membrane on the back of these panels, for LF modal control.

Ignoring the back wall for the moment. The reflection from the front wall is unique amongst the close speaker reflections. Ceiling, Floor, Side Walls, all cause reflections which cause the big dips (and more). THESE DIPS CHANGE FREQUENCY WITH LISTEN DISTANCE.

IMHO this is a very important distinction. Because of it, this Front Wall SBIR Bounce/Dip is amenable to negative eq. The combination of such Eq coupled with the LF reinforcement effect of the front wall gives us unique tools to address this big issue.

The 20dB is from Genelec's website. I am not sure what you are saying Lupo, but I reckon they mean -20dB relative to the direct path from the speaker. Quite plausible I reckon with a concrete wall.

DD

[Dange]

Lupo here's a challenge, can you, or anyone, explain the difference between minimum and non-minimum phase.........without going into s plots and poles and zeros!