floor reflection what to do ?

[Mixary]

hey,

i built a new studio and i am very happy with it. the studio has the 80is "Live End - Dead End" design and my work-place is sideways to the speakers to prevent table reflections. there is nothing between the speakers and the listener. it is an unbelieveable environment to listen to.
no early reflections and no standing waves, i love it

the only problem is the first floor reflecton, that dips the frequency-response round about 15 db at 110 hz. the dip is very narrow.

i put some acustic stone wool on the floor but that didn`t work.

do you have any idea or design suggestions to prevent floor reflections ?

thank you.

Marc

[gehauser]

Originally Posted by Mixary

hey,

i built a new studio and i am very happy with it. the studio has the 80is "Live End - Dead End" design and my work-place is sideways to the speakers to prevent table reflections. there is nothing between the speakers and the listener. it is an unbelieveable environment to listen to.
no early reflections and no standing waves, i love it

the only problem is the first floor reflecton, that dips the frequency-response round about 15 db at 110 hz. the dip is very narrow.

i put some acustic stone wool on the floor but that didn`t work.

do you have any idea or design suggestions to prevent floor reflections ?

thank you.

Marc

Probably a room mode. I had a similar situation - I use a small laptop stand on the side of the room, and I have nothing between the monitors and the listening position.

I had a 15db dip at 65 hz I thought I would never get rid of. I finally resolved it by moving my subwoofer toward the listening position by 1 ft, leaving my speakers where they were. It worked perfectly, once discovered.

At that frequency (110hz), my inclination would be to move the listening position or your speaker position to get rid of the low frequency dip.

[DanDan]

Are you sure that dip is caused by the floor? If so, changes of position, including height may help. FuzzMeasure or REW are brilliant at teasing out these problems and finding a solution best spot. Don't be afraid to place your speakers very close to the wall behind them if necessary. Here's something you may find very useful. I can't double post so try a search for this
Wall_Bounce_Calculator_2D.xls

DD

[Mixary]

thank you for your replies !

i ordered i subwoofer for my o300 but the crossover is at 80 hz. i will see what will happen.

the first measurements indicates, that is the first floor reflections.
when i vary the space beween the micro and monitor the dip will vary in the frequency. the dip frequency is the difference from direct wave to reflected wave multiply 2 ( 180 phase shift).

the to change the height is a problem and will only change the frequency of the dip. i will check the calculator. thank you

what do you think about a wideband absorber on the floor ?

have you other options ?
thumbsup

Cheers

Marc

[DanDan]

How did you measure this dip? I don't see how the frequency of the dip could change with microphone position. An absorber would need to be quite big to have any decent effect at 110Hz. Also most of us are working just fine without floor absorbers.

DD

[jhbrandt]

True. It seems like a modal or speaker placement issue to me.

-John

[Weasel9992]

Originally Posted by Mixary

the first measurements indicates, that is the first floor reflections.

Not at 100Hz it's not...

John and Dan are right.

Frank

[Bubbagump]

Originally Posted by Mixary

the only problem is the first floor reflecton, that dips the frequency-response round about 15 db at 110 hz. the dip is very narrow.

I am confused as to your measurement methods too. Are you measuring at mix position? Or are you moving around the room or? What are you using for your measurements?

[Mixary]

thank you for your replies !

we measured it with professioal measure mic.

here is a mathematical description for a 110 Hz Floor Reflection.


what do you think ?


cheers .... Marc

[jhbrandt]

Reality can be quite different. Yes, that calculation works for a ray model. 110Hz is in the wave region and though you will get a bounce at that distance... have you calculated the comb filtering effect at that distance?

It is a modal null. (standing wave problem)

Are you using a sub? If so, move it to the center of the wall on the floor in front of the mix position. If that changes things, move it away from the wall until you get the best response.

If you are not using a sub, move your speakers away from the front wall by a few inches.

What are your CR dimensions? including the height? As accurate as possible. Thanks.

Cheers,
John

[Mixary]

okay

the distance to my speaker is about 140 cm and the real ear level is 114 cm.
in this position the dip is at 135 hz. the dip is very close but very deep ( from 120 to 150 HZ and 15 db max at 135 hz.
When i increase the distance to the speaker, the dip frequency will also increase.( 200 cm distance = 165 hz dip).

the only modal is at 35 hz (32-38 Hz +- 3 db) and this is the length from the roof.

there are no paralel surfaces except the window surfaces

the reflections are at 0,1 sec linear ( hole frequency range )

i will upload the response soon

cheers

Marc

[Mixary]

Originally Posted by jhbrandt

Reality can be quite different. Yes, that calculation works for a ray model. 110Hz is in the wave region and though you will get a bounce at that distance... have you calculated the comb filtering effect at that distance?

It is a modal null. (standing wave problem)

Are you using a sub? If so, move it to the center of the wall on the floor in front of the mix position. If that changes things, move it away from the wall until you get the best response.

If you are not using a sub, move your speakers away from the front wall by a few inches.

What are your CR dimensions? including the height? As accurate as possible. Thanks.

Cheers,
John

thank you for your fast replay,

the studio is under the roof. the length gable to gable is 10,50 meter and to the acustic insulation about 9,50 meter. the width at the sill is 6,50 meter and behind sill the is on the left side 70 cm the acustic insulation an on the right side 2,50 meter insulation. the roof level is 3,20 cm and 60 cm acustic insulation.

the speaker is 3 Meter to the front gable.

i will post some photos

i will not eleminate any possibility of modals, but our measurments doesn`t indicate it. the studio has no parallel surfaces and a dip from a modal doesn`t
wandering in the frequency by changing the mic position exept there are more modals.

what do you mean " the comb filtering effect at that distance" ?


next week i will get my new subwoofer K+H o810 and we will measure more impulse responses.


thank you


Marc

[jhbrandt]

Originally Posted by Mixary

the studio is under the roof. the length gable to gable is 10,50 meter and to the acustic insulation about 9,50 meter. the width at the sill is 6,50 meter and behind sill the is on the left side 70 cm the acustic insulation an on the right side 2,50 meter insulation. the roof level is 3,20 cm and 60 cm acustic insulation.

Marc,
I'm sorry, but I need accurate measurements to the hard surface walls and ceilings. The insulation does nothing for the base modal calculations.

Originally Posted by Mixary

i will not eleminate any possibility of modals, but our measurments doesn`t indicate it. the studio has no parallel surfaces and a dip from a modal doesn`t wandering in the frequency by changing the mic position exept there are more modals.

No parallel surfaces? all the walls are splayed?? The hard shell of the CR, Right? Ok, Well that's your problem most likely. When you move the mic - the dip changes because the splayed walls are giving a different null at that position.

I keep saying this over and over. Splaying walls to eliminate modal issues only make them more unpredictable. A slight cant will help with flutter echo, but I would never recommend it for LF. It just doesn't work that way. Only sometimes do people get 'lucky'.

I could be wrong, as I have not seen or measured your room myself. This is my best guess based on the available information.

You are going to need a LOT more bass trapping.

Originally Posted by Mixary

what do you mean " the comb filtering effect at that distance" ?

This:

Surface Reflections
center of isosolese fm operator 93.33333333 cm
Operator Distance to surface 114 cm
Speaker Distance to surface 114 cm
Operator Distance to speaker 140 cm
travel distance of reflection 147.3333333 cm
Time Delay = 0.008570516 seconds
primary frequency = 116.679095 Hz
level -0.889443998 db
1ST NULL 58.33954751 Hz
2D 175.0186425 Hz
3RD 291.6977376 Hz
4TH 408.3768326 Hz
5TH 525.0559276 Hz
7TH 641.7350226 Hz


I use these kind of calculations to determine comb filtering... We are assuming that the center of the speakers are also at 114cm. ~shrug~

Please post a drawing of your CR shell with dimensions.. We'll do our best to help you out.

Cheers,
John

[Johnny the Fox]

Well, in this forum section the final answer is always a lot more bass trapping .

Every acoustician who has done lab tests in free field lab chambers over a reflecting plane knows these floor reflections too well. For instance if you measure tonal noise sources on a table according to iso 11201. Although you may have read something else, my experience is, that the basic rules for interference work amazingly well down to the 100 Hz frequency range.

If you manage to build a real dead end without a big console and small distance to the speaker you basically have an anechoic test chamber with a reflective plane. If you only know rooms full of modes than this effect maybe covered to a point that makes it unimportant or hard to detect.

Once you are aware of this effect chances are you will find it in your graphs more often. I for one can find it in the graphs for my small home studio room with very close monitoring which still has a few modes left to work on.

But as always in the field of acoustics: a lot of effects interact and the short and easy answer may be wrong in your specific case.

John, I may be wrong, but I think your comb filter results look a bit strange, are you sure about the calculation?

Best regards

[SAC]

I find this discussion quite interesting, especially as so many are speculating and debating whether the anomaly is due to a modal null or the superposition of a reflection!

And while one person has suggested using REW of FM, it is interesting to note their lack of suggesting the program be used to actually identify the REAL reflections and their arrival times which would allow one to identify the precise path of the reflection - if indeed it is due to one - and thus treat it and determine is the problem is resolved, or if further investigation of real events (such as modal nulls) is warranted!

(Its rather interesting that this person did not suggest the usual hand clapping as a friend moves a mirror about as if they were a blind pig rooting for acorns! And once finding the reflection in the mirror, still having no idea if that path had any relation to the real causal events of interest!)

Just use the programs to measure the impulse response and derive the ETC response! You can review the REAL (as opposed to the phantom supposed) early reflections, determine their path and treat them as appropriate!

Likewise, if they are absent and not the cause of the particular problem, you will quickly know that too and you can then pursue alternative courses of action as appropriate! And in this case, the frequency response afforded by the same measurement may very well indicate a modal issue if the problem is not due to the superposition of a specular reflection(s)!

In either case, it is much easier to address real acoustical issues than to chase suppositions and feelings. And that is the real advantage afforded by measurements in general and the ETC in particular as opposed to simply chasing reflections in a mirror!

As without such confirmation of real behavior, we are all limited to 'empty' speculation based on the 'possibles'.

And after so much debate over the usefulness of measurements such as an ETC, this case illustrates precisely why it is so much easier to identify and chase what is real (or possibly eliminate an option that is not real), rather than to simply speculate and root about blindly in the hopes of serendipitously and accidentally stumbling upon the source of the problem!

After all, the real purpose of an analytical tool is precisely to help you ascertain the real issues and their characteristics as well as to eliminate various unsupported suppositions, thus allowing you to focus more precisely on real issues. It is not merely to give you a screwdriver and set you about wasting time and energy serendipitously turning every screw in the building!


-----------------------------
Oh, and in a more general vein (not necessarily related specifically to this thread), it has become very frustrating to keep reading of folks thinking that the LEDE concept is simply one end of a room being reflective and the other end being anechoic! That IS NOT an accurate summation of the LEDE concept at all!!!!, but rather one of the many all-too-common presumed misconceptions. Of course, other folks in other threads here have already determined that the LEDE concept is dead and that it died many years ago - also an absolutely incorrect conclusion! And then again, we have many here who think the RFZ acoustic model concept is a virtual anechoic chamber devoid of all reflections, when that is also absolutely incorrect!

The LEDE/RFZ concept is very much alive and one of the, if not THE, predominate acoustical room models employed in many rooms by professional designers! And the coupling of the LEDE concept with the RFZ concept will, I'm sure confuse many who think that the full implementation of each (as opposed to focusing on merely a small portion of each model!) of the concepts are somehow in conflict, or that the later model assumes no later arriving reflections! Fundamental to BOTH (as if they were different!) is the integration of a well behaved semi-diffuse soundfield following an essentially anechoic defined Initial Signal Delay /RFZ gap!

The LEDE/RFZ model is indirect contrast to the 'all reflections are evil; long live the acoustically dead anechoic chamber; you can't have too much absorption" approach championed by many.

[Mixary]

john,

i am very buisy and i will post plots and measurements next week.


thank you very much


Marc

[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. ;-)
So, what will you do about it? Who does any of us about it? We all have floors.
An absorber would need to be quite deep to have any significant effect at 110Hz.
Here's how I would deal with it.
Search for Wall_Bounce_Calculator_2D.xls
This may help you find a complementary boost at 110Hz by changes of speaker position. Remember height is just as important as the other dimension. This is a 2D device, but you can run the numbers a second time using height as one of the two D's.
There is an even more direct, more certain route.
Measure the frequency response from one speaker at a time. Vary the speakers position, find the least damaging spot. Don't forget height. Measure using the other speaker, tweak for optimum, average the results. Check using some averages with some of the earlier different positions to confirm that you have the best combined spot. Double check by driving both speakers, and try a few combined moves, e.g. both closer to the front wall.
This is all done very easily and quickly. It provides a certain path to the best compromise. Unless you are prepared to work on a giant sponge!

Best Regards, DD

[SAC]

Originally Posted by DanDan

An absorber would need to be quite deep to have any significant effect at 110Hz.
…..
This may help you find a complementary boost at 110Hz by changes of speaker position.

An absorber would need to be quite deep to have any significant effect at 110Hz…and a complimentary boost and measure tweak and average????

I dare say this snipe hunt is out of control! ;-)

If said issue is a result of superposition, which it might be, for an initial frequency null of 110 Hz, the frequency of the reflection is NOT 110 Hz requiring an absorber effective at 110 Hz!

This is a function of time!

The result of the direct/reflected signal time delay offset manifests itself in frequency, not because the reflection is ‘at’ a particular frequency! And the propagated energy at that frequency is most likely below the Schroeder critical frequency and will most likely manifest itself as modal behavior, unlike the energy above that frequency which will be specular. For specular energy, the math describing the behavior in terms of the initial null frequency and null frequency interval is pretty simple.


The delay can be expressed as a time in seconds or a distance in feet or meters.
The delay predicts the “initial null frequency” (INF) and the null frequency interval (NFI) (e.g.: the distance in Hz between nulls).

INF = 0.5 (c/D)
D = 0.5 (c/INF)
C = {D(INF)} / 0.5
NFI = c / D
D = c / NFI
T = D / c
D = Tc

Where:
C is the velocity of sound on feet or meters
D is the distance in feet or meters of the delay (beyond the direct signal)
T is the time in seconds


Thus, for an initial null frequency of 110 Hz, and a velocity of sound of 1130 feet/second;
110 Hz = .5 (1130 / D)
D. the distance of the signal delay = 1130 / (110/ .5) = 1130 / 220 = 5.13636 ft = 5.1 feet

The delay in time is = D / c = 5.13636 ft / 1130 ft/sec = .004545 sec = .0045 = 4.5 msec


And the null frequency interval would be:
NFI = c / D
= 1130 / 5.13636 ft
= 220 Hz

Assuming the null is caused by a reflection, you simply need a broadband absorber capable of mitigating a simple reflection delayed by 4.5 msec that travels an additional 5.1 feet further than the direct signal.

An initial ETC would have verified the existence of such a reflection, as well as a simple and quick verification of the precise path (for which a vertically mounted absorber would work just as well as a horizontal absorber laid precariously on the floor!). And it would have provided ALL of this directly without any “high end maths”(sic) and the path quickly confirmed as well without knowing ANY of the additional information gathered in a very roundabout manner.

If indeed the cause is a reflection ! Which the ETC would have also confirmed or denied! And if not, pursuit of a modal solution would most definitely be in order.

Witness the perils of living solely in the frequency domain!

[DanDan]

Why not try a 'broadband absorber' on the floor? Shall we say something reasonable, a 2 -4 inch panel in the specular path to the mic?
Lets see the ETC or just the Impulse Response, before and after. I reckon the reflection spike will be severely diminished, but the frequency response dip will remain absolutely unchanged. If you can filter the ETC is some way, it should show considerable absorption at HF, none at LF. This pointless experiment should emphatically prove that you need thick absorbent panels to affect Bass frequencies. Kinda basic.
Also where is all this going? You are unlikely to be able to work with large areas of your floor covered with fibre.
No-one knows, and many very much dislike the fact that SAC, aka the banned foxfyr, repeatedly makes these completely unhelpful and mysteriously insulting contributions. It is unfortunate.

DD

[SAC]

Originally Posted by DanDan

Why not try a 'broadband absorber' on the floor? Shall we say something reasonable, a 2 -4 inch panel in the specular path to the mic.
Lets see the ETC before and after. I reckon the reflection spikes will be severely diminished, but the frequency response dip will remain absolutely unchanged. If you can filter the ETC is some way, it should show considerable reflection control at HF, none at LF. This pointless experiment should emphatically prove that you need thick absorbent panels to affect Bass frequencies. Kinda basic.
DD

More inconsequential gibberish by one whose every post is a personal attack as he has run out of anything of actual substance to post.

And our very learned friend who is afraid of such "high level maths" as presented above fails to understand that an absorptive panel, placed close to a source to deal with specular reflections acts equivalent to a panel with larger surface area placed further away.

And all you have to do is to reduce the total gain of the specular reflection sufficiently (~10dB), and the issue is effectively resolved. But I guess they didn't teach that in frequency response school.

Such is the plight of those stuck solely in the frequency domain, where the frequency response simply tells us that 'something' is happening, without the ability to show us what; along with a refusal to understand what the various additional domain measurement tools afford to us. Such are the 'benefits' of being a Flatlander!

And for those a bit confused by just what a comb filter 'means' - as the frequency response tells us little other than that 'a frequency' is missing - it it corresponds to a spatial polar lobing, which can be either horizontal or vertical in orientation (or even both) caused by the superposition of two or more signals. And where there is a frequency response comb filter, it indicates that you are in a spatial polar lobing null for that frequency. Other frequency dependent lobes exist, but at the present location you are in areas coinciding with their lobes and not nulls.

Keep clapping.

[DanDan]

I hope the test gets done, the results will speak for themselves.
Just in case, though, since there is a considerable depth of unreality here. How about we fully prove the point that depth is necessary for LF absorption, let's try a thin layer of absorption also. Shall we say a rug?
By the way, I recommend a search of posts by SAC, foxfyr. The results of this test will also speak loudly as to the consistent irrelevance and nasty demeanour of this poster.
DD

[SAC]

Again, gibberish, reflecting no understanding of energy content versus frequency content.

So now ONLY bass traps are sufficient to control specular reflections that cause comb filtering.

For as you will now note, it is proposed that one must employ bass traps in order to tame specular reflections in order to mitigate comb filtering - as the initial null occurs at a frequency not determined by the frequency content of the reflection , but by the time offset of the reflected energy itself. Apparently it requires "high level maths" to appreciate the problem with this approach. Time, frequency, frequency, time...its all so confusing.

What still really confuses DD is that the very seminars and workshops using the very tools that ole DD derides as meaningless (not to mention the content of the workshops extolling the utility and usefulness of the ETC in particular and of a well controlled semi-diffuse soundfield) were personally lead by 'questionable'(sic!!!) folks such as Russ Berger and Peter D'Antonio and involved many others not limited to Don Davis, Don Keele, Alton Everest, Dr. Eugene Patronis and MANY others, and B&K was one of the very first to jump on the measurement tools to license and manufacture them.

Not to mention Richard Heyser, the obviously MOST uninformed and confused individual who introduced the ETC measurement. One has to seriously suspect such a consortium of obviously uniformed and confused folks who should read more of ole DD's treatise on the universally adequate applicability of the frequency response.

But what do they know?

Obviously more than the ole clapper himself.

And in another exciting, yet terrifying, bit of news... I have heard an unsubstantiated rumor that says the AES is about to withdraw the collection of Richard Heyser's papers and replace them with DD's treatise on the universal applicability of the frequency response to solve whatever ails you. Apparently it is reputed to work on all ailments except for ignore-ance.

And all of this from someone who takes great pride in parading his repeatedly mentioned self-admitted inability to understand how a SIMPLE gain versus arrival time plot can be productively used to identify, isolate, and treat specular reflections and the issues they create.

So please dan, we all await your next incredibly insightful utterance regarding that which you take great pride in not understanding and misrepresenting. After all, you did write a paper 'on it' once. right? I guess it wasn't in either the AES or ASA journals, as that is where all of the myriad papers extolling the use of such time based measurements as the ETC, as well as the collected works of Richard Heyser himself, are published... You see how lucky we are folks? Where else can one go to learn that such new-fangled ideas are not only unfounded, but of little or no use!?

[DanDan]

Sound Absorption Coefficients, ASTM C 423
Mounting: Type A – Material placed against a solid backing.
Product Thickness, Octave Band Center Frequencies, Hz
Type in.(mm) 125 250 500 1000 2000 4000 NRC
701, plain 1 (25) .17 .33 .64 .83 .90 .92 .70
2 (51) .22 .67 .98 1.02 .98 1.00 .90
703, plain 1 (25) .11 .28 .68 .90 .93 .96 .70
2 (51) .17 .86 1.14 1.07 1.02 .98 1.00
705, plain 1 (25) .02 .27 .63 .85 .93 .95 .65
2 (51) .16 .71 1.02 1.01 .99 .99 .95

I think it safe to say the 110Hz will pass right through and be reflected 80-90 percent of full level, while higher frequencies will be seriously attenuated. The earth it does appear is round.

For those so far and luckily unaware, SAC used to be foxfyr before he was banned from GS. I have been told he was once called MAC on HTS, also banned.

Here is a typical result from stream of bilious posts.

11th July 2009, 07:03 PM foxfyr
This message has been deleted by jayfrigo. Reason: repeated insults, bickering, and unnecessary escalation.

A search of posts will deliver consistent evidence of this behaviour.
One can also note a complete absence of any positive contribution to any thread, or any semblance of helpfulness. Not one.
Seeming SAC has learned from his bannings. He has learned how to work just within the rules of fora. Unfortunately we now have to learn to work around this litter.

May I express my regret to readers of this thread for having to see this unpleasantness. You will note it started with SAC post 15, a stream of insults directed towards me.


DD

[SAC]

Geesh, and he still thinks that what is causing the comb filtering is simply frequency!

I would ask if you have any understanding whatsoever of event behavior in the time domain, but the answers speak for themselves.

It must confuse you to all ends that the same signal, differing ONLY in its arrival time relative to the direct signal, will cause differencing initial null frequencies and differing null frequency intervals in comb filtering!

Just one more thing to be confused about, isn't it!?

Yup Dan, its ALL frequency. We have NO use for any perspective other than a frequency response plot.

And apparently, referring to the oft made suggestion to simply root about serendipitously with a mirror in the attempt to locate first order reflection points, which, while the process can be quite amusing to watch, does not necessarily result in the location of the actual reflective paths that are responsible for the anomalous behavior, and unlike the ETC which provides a snapshot of ALL of the real specular reflections incident at a given point, thus quickly and accurately providing much insight into the actual nature of the problem - is considered insulting. It is interesting that one might find such a comparison insulting, but yet the facticity and practical limitations of the more general process are insufficient to cause one to consider more reasoned alternatives. Oh well. And who says that acoustics has to be dull???


Apparently ALL of the significant measurement platforms that have been developed since TEF that include time domain measurements are all simply superfluous eye candy for the fools who happen to appreciate the lessons taught by superfluous and ignorant folks such as Richard Heyser, as well as the myriad folks who have mistakenly learned from his errors and routinely utilize the methods which he introduced, such as Russ Berger, Peter D'Antonio, Don Davis, Don Keele, Dr. Ahnert, and a list of acoustics luminaries, (or should that be 'slackers'), far too long to list here.

Dan, thanks to you we now know they are ALL wrong, and that any of the major measurement platforms, including the only one you seem to be somewhat familiar, Dirac,(at least in name as you also keep listing Odeon as a measurement platform as well - when it is a modeling environment!), by B&K, also includes the ETC as a fundamental measurement and most surprisingly were among the first to license TDS (complete with the ETC) when it became available. But hey, apparently they were wrong too.

Isn't it amazing that ALL of these folks and companies are so misguided and do not simply claim all of this to be snake oil and simply produce exorbitantly priced frequency response meters along with a mirror for folks who admittedly simply cannot understand the utility and meaning of the additional responses addressing different aspects of the larger acoustical Analytic (as postulated by Heyser after Gabor and Nyquist)? But I guess its simply a waste of time to ask if you are aware of anything to which I reference...

Gee, and according to you even FM &/or REW were 'coerced' into including the basic implementations of the log squared! As apparently such a simple addition of this view renders it appropriate to made available separately as a "Pro" version. Now that is a scary notion!

Here's a suggestion: We know that you cannot fathom a practical use for all of this new-fangled witchcraft (if we want to call 45 year old concepts 'new' - which unfortunately to many, they are!), so when we talk of them, remain silent and enjoy your self-admitted ignorance.

Let us enjoy our flights of fancy using the tools the major development firms are releasing in our/their flights of acoustic fancy as evidenced by the myriad use of such tools in the specs and works of Berger, D'Antonio, and in the processes of analyzing and re-mediating rooms where we do not assume that all must be anechoic and reflection free.

Just be careful, as if you actually do choose to read any of the works of folks such as D'Antonio, Heyser and so many others, you will be attacking them and decrying their methods and reasoning as well. And I guess that we should be all calling RPG and demanding they start releasing spec sheets limited only to frequency response plots for use in treating frequency anomalies along with mirror location specs in order to eliminate all reflections as well, and that they cease to propose acoustic room models stated in ETCs rather than in frequency responses.

It must be nice to subsist in a small world where only the frequency response is allowed as you denounce and attempt to censor any mention of methods other than what you presume to be adequate! But you need to face facts and realize that you are unable to social engineer the world for the rest of us, as we are indeed entitled to propose and use exactly these tools, and there is ample precedence presented by folks with MUCH more credence than you possess to support them both theoretically and practically.

Your local chapter of the Flat Earth Society must have rousing meetings as other disbelieving folks are chided for unwittingly risking getting so close to the edge! As after all, those sailors proved nothing! They just showed that the edges of the flat earth are more widely spaced that we had suspected!

The bottom line? If a comb filtering null is due to a reflection, the ETC can easily confirm or deny this, as well as identify and provide sufficient information to assist in identifying the reflection and its characteristics, as well as the ability to evaluate one's efforts to remediate it. And this all could have all been performed easily and quickly by one person in much less time that it would have taken to read the posts here.

And with the Oscars coming up, I would like to nominate you for best reprisal of the performance as King's minister in the Charles Laughton/Maureen O'Hara version of The Hunchback of Notre Dame, as when the King suggests that scholars have proposed the earth to be round, one erudite protector of all things proper keeps chirping up with his "No its not!" Splendid performance!

[DanDan]

Up to SAC's joining in at post 15 we were have a civilised discussion, teasing out a little issue. His first post contained the usual snide, meaningless without seeing the context, remarks about others. There appeared to be some techical content, but not really. He then came up with this virtual notion of apparently thin floor mounted broadband absorbers.
Well, let's see one.
This insane unreality needed to be backed up by some theory, so he made up one. Let's see,

a thin absorber close to a surface works as well as a thick one and a gap

Wow, so we have all been radically mistaken all along. Thin Carpet on the walls is much easier than big Bass Traps. Now, let's install those mythical floor absorbers. OK, maybe we will have to lose a few pounds to be able to walk on them.

As ever, foxfyr's contribution is irritating, of absolutely no value. He has been of no help to anyone (ever). Furthermore in this case he is technically not just simply wrong but fully unhinged.

DD

[SAC]

dd, you are a hoot, in lieu of a more appropriate term. But your red herring strawmen are as humorous as they are vapid in their misinterpretation of what I said. But hey, what's new?

A thin acoustic absorber huh?
I stated that that in order to mitigate a reflection causing the initial null frequency at 110 Hz, that you did not need to absorb 110 Hz as you mistakenly assumed! As you continue to live exclusively within the frequency perspective with little or no awareness or understanding of the role of time. And you have utterly failed to ascertain the real casual nature of the problem, be it modal or specular! Duh! And the adequacy of a broadband absorber was in direct contrast to your erroneous assertion that you need a bass trap capable of effectively addressing 110 Hz signals! And if the 110 Hz is below the Schroeder critical frequency, what does that tell you? But as usual, you miss the point.

But its interesting that you now feel it necessary to use bass traps to mitigate first order reflections!

My alternative suggestion was that a simple broadband absorber such as is normally used for any first order reflection is suitable. Maybe the friend you holds your mirror can explain that to you...But I doubt you will understand even then!

And on the floor? LOL! I suggested that a more prudent alternative would be to position a vertically mounted panel close to the signal source in order to intercept the source vector, just as is commonly done to mitigate work surface reflections with monitors mounted on pedestals behind the work surface. But as usual, you missed that too.

But keep trying. Your strawmen and red herrings simply continue to demonstrate that you neither understand the nature of comb filtering nor the tools that effectively identify the real reflections involved that allow us greater acuity in determining what is really happening and allowing us to focus on the actual causes, rather than your serendipitous rooting about for acorns, many of which have little if nothing to do with the actual issue at hand.

[DanDan]

So a thin panel in front of the speaker stand, say a MicroTrap will diminish the level of a 110Hz signal? Amazing! I, and seemingly the rest of the world thought it would pass right through, relatively unaffected. The Owens Corning figures show absorption coefficients of 0.1 in that frequency area. So by what mystical mechanism would such a panel reduce the level of that path?
By the way your attempts at insult by references to pigs are empty. Pigs are noble creatures in comparison.

DD

[SAC]

He still utterly misunderstands what was stated as he illustrates his ignorance of the cause of the comb filtering as he simply repeats the same nonsense again and again..

MicroTraps and 110 Hz reflections....

Fascinating.

Folks, beware of such demonstrated ignorance of how sound works in the time domain. Is it any wonder that he fails to understand and repeatedly decries the "high level maths" relationship between arrival offset times and the distance traveled?

[DanDan]

So, was SAC's intrusion into this thread helpful? Did anyone benefit?
Why does he continually do this?
DD

[SAC]

There are two simple possible causes of the problem. Well, OK, three, if we count ole DD's fantasies. But of the substantial issues, one is a modal null, and the other - the one initially supposed - is an initial comb filtering null caused by the superposition of an early arriving reflection which we know would correspond to a signal delayed relative to the direct signal by ~4.5 msec.

This issue of the root cause can be settled by a variety of empirical exercises, or the nature of the contributing causes can be quickly identified by the appropriate measurements - namely an ETC to verify or eliminate the notion of an initial null caused by the superposition between the direct and a high gain reflected signal arriving ~4.5 msec later.

And, if the null is not caused by an early arriving reflection, then addressing a modal issue would be in order.

Pretty simple.

But where it went off the tracks is that one guy here does not understand the ramifications of signal behavior in the time domain nor the significance to the Schroeder critical frequency, and instead persists in living instead in the frequency domain and wants to mis-characterize a time based issue in terms of frequency. As a result he is off chasing a "110 Hz reflection" as he thinks that is the cause of a null at 110 Hz. (Its no wonder he fails to understand the usefulness of the ETC plot that provides a time based view of energy in terms of gain and arrival times! After all, how can it be of any use if there is no indication of frequency? Oh well...)

The practical result of which is that we are now reduced by his illogic to having to use bass traps to control early arriving reflections.

At first it was a bit comical, but now it has progressed to complete absurdity as he still fails to realize what he does not understand. He is still trying to solve a time based issue that manifests itself in the frequency domain as a frequency issue! The real irony is that if his reasoning were correct, we could simply EQ the issue and be done with it. But you can't address non-minimum phase time based issues in the frequency domain, and we aren't chasing a 110 Hz specular anything.

So we'll just humor him and let him persist in his notion that the solution is to address time based issues in the frequency domain as he fancies the need for using bass traps to control early arriving reflections as he still hasn't grasped the significance of a time offset.