Above the Schroeder Frequency. Diffraction.

[DanDan]

Is it possible then, to combine the information given by an ETC reading, and transpose that time based information with frequency waterfall plot and determine from the given reflection points that the spectrum arriving at the listening position is bad (absorption) or needs augmenting (diffusion)?

In short, no.

The procedure for evaluating the ETC has been presented multiple times.

Really, just once would just fine, where is it? Without the tape measure and calculator please.

I really don't know why you are banging on about Impulse Responses and ETC. Any one of us can copy and paste definitions from anywhere. As you well know.

to be honest, i'm not confused by SAC posts and for sure it is not nonsens!
But that put aside, simply thinking different than "the four" and yourself is reason enough to get banned? Is the inquisition back to town? Are we here to learn or do we have to "follow" to still remain part of the GS community?

No Victor, it takes quite a lot to get banned. Insults, Bickering, Taunting, all that good stuff will do it. :-)

DD

[SAC]

Originally Posted by DanDan

Really, just once would just fine, where is it? Without the tape measure and calculator please.

LOL! Do I have to post pictures of it so you make sure you are not looking at the impulse response as you did last time? (fact!)

And YOUR tool auto-converts the time offset to distance?

Originally Posted by DanDan

I really don't know why you are banging on about Impulse Responses and ETC. Any one of us can copy and paste definitions from anywhere.

Let's see, you demonstrated that you could not even identify the ETC response, let alone evaluate it (fact!); and yet you need yet another way (the 4th) in addition to 3 ways already described to evaluate a simple gain/time graph.

And yet you are running around bothering other people as you try to identify reflective incident positions with mirrors that ONLY provides possible reflection paths with no necessary correlation to the real paths of reflections which are identified as anomalous.

Do you have ANYTHING to add aside from ridiculous complaints that fail to address real issues?

Go troll another thread and explain to them yet again how to clap your hands.

Originally Posted by DanDan

Insults, Bickering, Taunting, all that good stuff

Pretty much sums up your contribution to this thread.

[DanDan]

The procedure for evaluating the ETC has been presented multiple times.

Really, where?
Once again you resort to insults to cover your lack of anything substantive. You are one nasty piece of work. My real contribution here is to illustrate just that, in order to hasten your banning, which I guarantee you will be quicker than your last life as foxfyr. By the way, why the aliases, why no real name or location? Are you in fact a virus?
DD

[DanDan]

So do we use the frequency transition regions of our room to determine at what frequency we should build/buy or diffusion? And is there a formula to determine an approximate amount of diffusion coverage to obtain a semi-diffuse sound-field based on volume.

Again no I'm afraid.
Little of what has been stated in this thread applies to the typical small rooms we work in these days. Few of us can afford the luxury of beneficial later and diffused reflections. BBC research from way back concluded that a diffuse sound field was achievable by using distributed absorption. We have become a bit bored with just absorption, many don't like a close to anechoic listening space. For those reasons and of course simply curiosity/research diffusion has been a topic of interest of late. Ethan Winer has done some interesting tests which show great benefits from diffusion in a room smaller than one would expect. It has been the practice to keep diffusion some distance away. Ethan's experiments at the very least challenge that. Space Couplers are another interesting device. Little has been published about them, but from what I have seen they can indeed impart a little liveness, some diffusion, while enhancing the performance of absorption by up to 45%, in a small room. A holy grail I would suggest. Take a look at the link below. Ignore the coupling two spaces bit, in deference to our small single rooms, focus on the other applications. Perhaps the most amazing is when a Coupler grid is attached to a wall it has an absorbing effect, plus diffusion. Apparently these are amazing under clouds.
http://www.resolutionmag.com/pdfs/SW...dsmallroom.pdf

DD

[Seamus TM]

So, I have an obvious question, that's probably impossible to definitively answer.
What's a small room?

I know Everest says something like anything 1,500-2,000 cubic feet and under (I think?).

Is that what we are going on?

[Nordenstam]

Originally Posted by DanDan

Really, just once would just fine, where is it? Without the tape measure and calculator please.

Post #21 in this thread: http://www.gearslutz.com/board/5045546-post21.html

[DanDan]

Seamus, this will likely answer all your questions. http://www.ebu.ch/en/technical/trev/trev_274-hoeg.pdf
The BBC R & D archives are brilliant also.
There can be no definitive answer. Acoustically small might for instance mean incapable of supporting a diffuse field down to a specific frequency. The Everest and oher guidelines are from experience. The EBU reckons no less than 30 square metres floor area for a decent control room, 40 for a critical listening room. As you can see that's most EU rooms gone right away. Take a look at the EBU requirement to eliminate very early reflections. You will quickly see why us small room users end up almost anechoic.

Lupo, when I read that thread I find string instead of a mirror. I find an absorptive panel being inserted block a path to identify it's source. I also see distance measurements and calculations. It's all in reverse. ETC is used at the end of all this to view what we already know is there.

Is it possible then, to combine the information given by an ETC reading, and transpose that time based information with frequency waterfall plot and determine from the given reflection points that the spectrum arriving at the listening position is bad (absorption) or needs augmenting (diffusion)?

That haircut is absolutely no help in answering this question, as evidenced by the silence. There is no post anywhere illustrating how an ETC can be of primary use in this very common endeavour. It can only confirm that one has succeeded.

I firmly believe that in a small room:-
Destructive refective surfaces are blatantly obvious to the eye.
A mirror can refine this.
Audibly troublesome reflections can be distinguished from beneficial ones by ear, eye, or even measuring tape!
This is very simple well covered stuff. I suggest a healthy interest would veer towards Ethan, Russ Berger, Jeff Syzmanski, and others, who attempt the seemingly impossible, i.e. make the small room sound big.

DD

[Nordenstam]

Originally Posted by DanDan

Lupo, when I read that thread I find string instead of a mirror. I find an absorptive panel being inserted block a path to identify it's source. I also see distance measurements and calculations. It's all in reverse. ETC is used at the end of all this to view what we already know is there.
That haircut is absolutely no help in identifying the good the bad and the ugly and what to do with them and where. There is no post anywhere illustrating how an ETC can be of use in this very common endeavour.

For first reflection points, mirror works fine. For twofold reflections, more elaborate geometrical constructions can be done in say sketchup. Or one can have two helpers with two mirrors. Then what? I'd say this method is highly useful in a small room, where twofold, tripple, quadruple etc reflections all have potential to create strong interference.

That's where the continous ETC measurements and absorptive panel comes into play. Start out near the speaker and keep moving it away til it hits a surface. It doesn't matter how many points the reflection have been visiting along its path, it'll still work. Try absorber, diffuser, deflecting the sound using splayed panels or polys.. Do what works in the situation at hand.

Don't know about you guys, but I must say that this way of working have helped me a lot!

[johndykstra]

Sorry, I keep seeing people use the term "haircut" explain please?

[johndykstra]

Even with mirrors, doesn't it require knowing something about the dispersion of sound from your speakers? And even if we know the array's behavior on paper, how the transducers outer most edges of radiated sound hit not only the first wall, but then figure the multiples... Granted I realize that we are not aiming to treat a pinpoint reflection so as to have everything sound good with one's head in a vice, locked in a fixed position; but as a mixing bubble, or zone. Still it seems to be a useful tool.

I'm still unsure as to why my first hypothesis is incorrect...
Let's say for the sake of arguement, that room modes have been taken into account. Humor me. If we were to take readings from the ETC and determine a location from which a reflection is coming, couldn't we take a cross section of the waterfall plot at that moment in time and determine what frequencies are contained in that reflection and treat it accordingly? For example, wouldn't it tell us how low a frequency our diffusor should be effective to?

[SAC]

First, we address the energy content in reflections, we are not trying to EQ a reflection! A waterfall is simply a frequency plot. It provides NO information about any component reflection of which a given sound field might be comprised. Precisely why we address such issues in the time domain and NOT the frequency domain.

Andreas, the mirror trick provides basic ray tracing, for which one does not even need a mirror.

And all it does is provide the possible paths a reflection might take.

It provides no necessary connection to the real reflections nor their actual gain, as such paths are influenced by such additional real factors as speaker Q (dispersion), speaker orientation, actual surface material and acoustical impedance, and any number of real world factors that impact actual reflection paths and gain. And while he ETC provides a detailed summary of the sound field, the mirror trick provides no usable data.

So while one method tells you what MIGHT occur, the ETC tells you exactly what IS occurring. And room tuning is about dealing with what is really happening, and not with dealing with might occur if such factors as a different set of speakers were used, or there were different boundary treatments in place.

The ETC will also tell you the density of a sound field, the degree of diffusion, whether a particular reflection will cause tonal or image shifts or whether it is benign an requires no treatment.

The ETC provides a tool to evaluate the real specular events and to know exactly how the reflection is manifest in terms of gain and density. The ETC also makes it easy to determine if a particular arrival time has more than one reflection incident at the same time and to evaluate each path independently as well as their sum.

And each spike corresponds to a unique IDENTIFIABLE refolection of which its individual characteristics are knowable.

Some people prefer to treat imaginary rooms based upon incomplete and imagined speculation.

Others prefer to know what is actually happening, and prefer to make appropriate adjustments based upon the real behavior of the room and equipment therein, and then have the means by which to evaluate the changes made regarding real events to verify the effectiveness as well as any interaction with other factors.

And we haven't even breached the common concept of tailoring a manufactured reflection for purposes of say, triggering the Haas Effect. One wonders how they will do manage both its secondary arrival time and gain with a mirror given the mirror provides NO information about the source signal!!

Check the documents, they are unequivocal regarding the value and importance of the ETC response.

[DanDan]

Lupo, thank you, that is the first piece of text I have seen illustrating an actual method of using the ETC. I will certainly experiment with it.
When you say two fold reflection it sets me wondering. Lets take the simplest possible case, a side reflection path. The tweeter/speaker radiates sound, this bounces off the side wall, classic pool shot mirror type scenario. The next bounce will hit an area behind the sweet spot, the level of course significantly diminished. OK maybe the producer will hear a reflection, but I reckon more than 10dB down and thus insignificant IMHO. Furthermore if we treat the first spot, using the mirror to finely identify how much area we need to cover ( and I think we will all agree that absorption is the treatment of choice) there is no secondary bounce. I am to say the least sceptical, but you say you are finding this useful so I would warmly welcome further on this from you.

The treatment of side reflection points and ceiling cloud is a pretty established practice.
I think the assumption that one is actually controlling the earliest and strongest reflections is intuitively valid and hardly imaginary. Of course ETC or even what is called the Impulse Response graph in FM clearly shows these strong reflections and their death after treatment. But I kinda knew that

The 'haircut' is my humorous reference to the ETC graph in an earlier post. The point being there are as many spikes as in a buzz cut and how can you tell which one is the ceiling for instance? Oh of course the measuring tape and calculator........

Despite my irony and more here Lupo, you will find that I am open and appreciative if you would take the trouble to explain how you use and find ETC useful.
DD

[SAC]

The points of incidence for a 2nd order reflection will NOT share its first point of incidence with a first order reflection.

[johndykstra]

Originally Posted by SAC

First, we address the energy content in reflections, we are not trying to EQ a reflection! A waterfall is simply a frequency plot. It provides NO information about any component reflection of which a given sound field might be comprised. Precisely why we address such issues in the time domain and NOT the frequency domain.

Sorry to beat a dead horse.

I'm not rationalizing the eq'ing of reflections, rather looking at the frequency of the particular reflection to ensure that the treatment is sufficient to handle the content. I realize as well, that these areas of reflection are not necessarily a spot, but a series of spots...a zone if you will.

Given the etc plot shows spikes at points in time... and this particular point in time points us to the directions of where these are coming; could we not superimpose that same point in time on the waterfall, and deduce the frequency content of the reflection(s)? For example, it would tell us how thick the absorption needs to be at this particular reflection, or if a combination of absorption...again how thick, and what depth our diffusor on top of the absorption should be to achieve successfully diffusing the reflection.

If this isn't a clarification of my intent I apologize, but perhaps I need a dumbed down explanation of why I may be wrong.

[DanDan]

dyk, I like your questions, I hope you don't mind me answering them, although they may have been directed elsewhere. We have other readers, and we all may learn something here.

Certainly when it comes to dumbed down explanations, I may be able to help

My (somewhat limited) understanding of Waterfall plots comes from FM and REW manuals and personal discussions with both authors. I am under the impression that those 'slices' in the waterfall should not be taken literally time wise. They are not intended for absolute decay measurements, e.g. reading the length of a mode off the time axis. They provide EDT, T30 and such for that job.
I believe each slice is is an average of what went on between it and it's predecessor.
I don't see how a spike in an ETC can have directional information, except of course by intervention in the path, proving where it came from. Again this reverse thing, but could be useful.
I am open to further illumination on this.

And each spike corresponds to a unique IDENTIFIABLE refolection of which its individual characteristics are knowable.

SAC, honestly without rancour, how does one identify a particular reflection?
Is it by tape measure and calculator to determine the expected time?
Or is it by intervention with a panel? Are you also used to 'continuous' ETC display, as in Real Time or close to?


dyk. In terms of 'frequency' of a reflection, I would rather view that as a spectrum of reflected frequencies. A hard massive concrete surface will reflect all frequencies pretty much equally. Thus the strong modes in a concrete room. Sheetrock on the other hand resonates in a typical stud frame at say around 125 or so, absorbing some of this frequency area, thus not reflecting it fully.
Thus the frequency varying absorbent coefficients of different materials, including their mounting methods.
e.g. a Hard wood floor, suspended will hardly reflect bass very well.
Thus empirical assessment and experience leads to choice of thickness, gap.

Lupo, you mentioned 'continuous' ETC measurement. I have not seen an instrument capable of this. Smarrt? The commonly used software around here is FM, REW, ETF which take 'snapshots' Please elaborate.

DD

[PaulP]

Originally Posted by DanDan

I really don't know why you are banging on about Impulse Responses and ETC.

The ETC is useful to identify early reflections so you can get rid of them.
Strong reflections occuring within the first 20ms or so will mess up your
sound and the ETC is the only way to track them down (the ETC can be
used for other stuff as well).

G.E. is probably the first member here to (at least publicly) use the ETC to
locate reflections, see his great thread on diffusors starting around this post
http://www.gearslutz.com/board/5041294-post200.html though the good stuff is currently right at the end
of the thread.

Paul P

[johndykstra]

Originally Posted by DanDan

Thus empirical assessment and experience leads to choice of thickness, gap.

Fair enough, but given that we know that a concrete wall will reflect more bass than a stud wall, and so forth and so on... isn't it reasonable to expect the waterfall to show this? And if the waterfall does show this can't the data be interpreted without experience or empirical knowledge?

Keep in mind that there is a lot of floating around the use of the word "we". In SAC's case we means physicists. In Dan's case, "we" means people with a lot of experience, ... or whatever. You get the point.

From the vantage point of the uninformed... is using the combination of the two tools as I have presented a better way than trial and error?

[SAC]

('We' means knowledgeable folks who have taken the time to study the concepts as well as to do EXTENSIVE applied research and experimentation in the process of validating theory with respect to practical objectively verifiable results! )



The waterfall is not truly a time domain measurement. Let me try to explain…The cumulative spectral decay uses a left-slope window to successively remove the early part of the impulse response at a user determined time interval.

In other words, each slice of the CSD shows the spectrum of the portion of the impulse response that remains after application of the window. It is useful for observing portions of the spectral response that persist–namely resonances. Since the end of the time window is fixed, frequency resolution is lost with each successive sectioning (in other words, the window gets smaller as the leading edge is moved.) This reduction in resolution is apparent at low frequencies as the resolution seems to be cut off in an arc curving outward and increasing with time – with the low frequencies appearing to be cut off or tapered as the persistence in time increases, deleting initial time data.

And it makes no sense to try to determine the frequency at a point in time due to the inherent resolution limits imposed on the reciprocal time/frequency relationship (f=1/t).

Bottomline, the waterfall does not show the frequency response of a particular reflection. And the persistence or resonance at a particular frequency is not a reflection.

We are talking 2 different things here, for two different purposes. You cannot combine them as if they are the same.

The waterfall tells you nothing about the specular reflections. Just as the specular reflections ‘in’ the ETC tell you nothing about modes.

Essentially a reflection is a redirection of the acoustic energy identical to the source. Now, the acoustic impedance of a reflecting surface can alter this somewhat, but for all intents and purposes we are employing broadband reflectors and absorbing surfaces.

The reflections are rays – vectors. They have magnitude and direction.

Each peak in an ETC represents the sum of the imaginary and real energies in the system from both the impulse and doublet response. (For more on this we will need to explore the Heyser spiral – the rotating phasor diagram that contains the full information of the system under review – as imaginary by no means implies unreal! – a truly unfortunate and misleading choice of terms).

As each peak represents energy arriving at some point in time relative to the direct signal, we are presented with an overview of all of the arriving reflections from all surfaces at any specified point in space. Thus we are necessarily dealing with vector quantities relative to the measuring position.

In any case, the magnitude is specified as a measured gain relative either to some time or distance. We also know that each vector/ray exhibits an orientation angle indicative of the reflection point(s) of incidence and likewise to the signal source.

Identifying a particular reflection is easy. It appears as a discrete spike in the ETC response. Depending upon the measurement platform you are using, various information is automatically generated by moving the cursor to correspond with the peak of each reflection. At its most basic, time is displayed. A simple augmentation displays its analog travel distance as well as corresponding gain indicated by the y-axis.

Given this information, as well as knowing the location of the source and the signals reception, it becomes a trivial exercise to extrapolate the travel path, via whatever means is determined to be easiest by the user – depending in large measure on how sophisticated the platform is in automatically calculating such information and also with regards to the physical layout and accessibility of the physical space one is in.

If you are using REW or Fuzz, your choices are reduced, as they apparently only display the time, and you must perform the trivial calculation to determine the normalized time of travel (relative to the direct signal arrival time without any machine or system propagation delays). Many big words meaning that you may have to subtract the direct signal arrival time from the reflection arrival time for each reflection of interest. And then you will have to calculate the distance traveled by each reflection by multiplying the time of travel with the speed of sound. Both trivial ‘busywork’ calculations. This would be a very simple and nice feature to see added to both programs.

Typically, there will be one unique path for each reflection that is incident to boundary surfaces within the room between source and microphone receiver.

Initially you may want to use a string long enough to accommodate the farthest reflection distance (and marked in feet and perhaps in inched in the segments where you know you have reflections) until you gain a bit of pattern recognition and confidence to do this reliably, and thereafter you will quickly acquire sufficient pattern recognition to be able to carefully repeat the sweeps such that intercepting the path with a small piece of absorber intercepts the path causing the reflection to be damped in the measurement. A few auto-repeated sweeps will then allow you to quickly and accurately ‘walk’ the absorbent material along the incident pathway back to the precise spot on the boundary. In the process you move from clocking a relatively large region when near the mic to a progressively smaller and more fine tuned location the further away one moves from the mic. This is actually a generally quick and easy process.

Most platforms allow for the auto-repetition of the measurements to be performed. If REW and Fuzz do not, you will simply have to press the button a few more times.

{If one is truly serious, and especially if you generally encounter more complex spaces where access to ceiling and boundary surfaces can be difficult, you might want to investigate the polar ETC capabilities of various platforms whereby the software package can generate the 3 space coordinates suitable for one to replace the mic with a laser pointer in a transit suitable for identifying the precise points on the boundary surfaces. Additionally, you may want to investigate the means by which you can do this yourself. But this process is currently FAR beyond the scope of this thread and this forum at this point. I mention this simply to provide an example of what is possible, but admittedly overkill for what one might be doing here. But I will also say that once you become used to it, it is quite easy to become very spoiled.}

In this manner you are able to not only specifically identify each reflection of interest in the display based upon gain and/or arrival time, but you are able to specifically identify the precise path it traverses. And this ability will pay great dividends in treating the room – as it will afford you a great deal more leverage than simply identifying early reflections to be absorbed.

It is going to afford you much knowledge, understanding, and leverage over the manipulation of the entire sound field, which is a significantly important process that is generally overlooked here. It is THIS aspect of tuning a room where the intelligibility, imaging, and tonal qualities of the signal response will be adjusted. And the ETC response will form a invaluable tool for doing this.

So, with this you have the basic tools to investigate, acquire and manipulate the data.


But this is not enough. You then need to understand what needs to be done with the tools -You need a defined goal that will determine what criterion need to be met in order to turn a space into a comprehensive tuned space according to current best practices relative to the particular acoustic model you choose.

[johndykstra]

Thank you for the explanation. It makes sense...mostly. Allow me to fade into the background and continue listening.

[Seamus TM]

I apologize, but since the beginning of this thread, myself, my wife and my 2 year old have all gotten sick.
I haven't had a chance to really go through all of this info, yet.
From what I can tell, it has matured into a conversation, which is outstanding.

Forgive me if this has no basis in what is being discussed, but it reminded me of a graph in "Acoustic absorbers and diffusers: theory, design, and application" By Trevor J. Cox, Peter D'Antonio.
It's on page 34 in the chapter Application and principles of Diffusers.

Acoustic absorbers and diffusers ... - Google Books

Is this graph kind of what we are talking about, or is this only showing first reflections?

When I am back at work and have the time to go through everything, I'll be able to contribute something more to the thread, I hope.

Thanks,
Seamus

[DanDan]

Paul, thanks, I am interested but that link shows only one post. Could you give it another go. It seems to suggest that the ETC indicated another mystery path which normal faculties nor intuition revealed. If that is the case I am very interested.
If someone can show the tool to be useful I would be delighted. From the one post I note the measuring tape calculator again. So once again we use organic methods to identify a certainly destructive reflection, then use ETC to view it amidst a forest of pampas grass? I have repeatedly asked why one would do this. What is the advantage of the ETC. What can be seen through this lens that the simpler views don't have.
I look forward to seeing that thread or another contribution from Lupo, but till then we have zilch here. SAC, Mark, foxfyr, Mac, (Is he Eric Desart) as ever, has not delivered anything.

In the typical small rooms which this forum deals with, pretty much all reflections are typically under 20mS. Unless you have over a 16 foot ceiling for instance. So, the reflective surfaces certain to do damage are close at hand, literally in many case, and all entirely visible to the eye. There is no mystery here. Anything within 10ft is dangerous. A simple audio recording of a click (sophisticated handclap) shows them quite clearly. So does an Impulse response graph. The log squared and ETC variations seem to me much more cluttered than either of the aforementioned. The normal RFZ treatment with possibly some rear and/ or overhead diffusion eliminates these at the listening position. So, all the closest loudest reflections are killed, leading to no second bounce (I am not talking of second order, just the understandable pool shot that I and Lupo refer to). What's left to investigate? Is there really another way?
Perhaps, talk to Ethan about Diffusion at close quarters, try some Space Couplers. Newish and uncertain but that is a route forward.

('We' means knowledgeable folks who have taken the time to study the concepts as well as to do EXTENSIVE applied research and experimentation in the process of validating theory with respect to practical objectively verifiable results! )

Or is this case, we= multiple personality disorder, with delusions of adequacy, coupled with pomposity of regal dimensions. Is there no beginning to this man's talents?

dyk.
Do you have FM or REW? The REW manual is very illuminating. I have made the observation before that software, contrary to people's expectations, does not directly answer the common questions.
How is my room, what should I do about it, where to put treatment, and how much of it?
That would be like asking Big Ben for directions to Dubai.

DD

[PaulP]

Originally Posted by Mumblesound

Forgive me if this has no basis in what is being discussed, but it reminded me of a graph in "Acoustic absorbers and diffusers: theory, design, and application" By Trevor J. Cox, Peter D'Antonio.
It's on page 34 in the chapter Application and principles of Diffusers.

Acoustic absorbers and diffusers ... - Google Books

Is this graph kind of what we are talking about, or is this only showing first reflections?

Yes, except that the room in question is far from the typical small room most
of us are stuck with. It's a purpose-built large control room with no parallel
walls.

You'll have a hard time getting any kind of gap like that.

Paul P

[PaulP]

Originally Posted by DanDan

Paul, thanks, I am interested but that link shows only one post. Could you give it another go.

Just click on the "Thread:..." link in the top right corner of the page.

Paul P

[Seamus TM]

Originally Posted by PaulP

Yes, except that the room in question is far from the typical small room most
of us are stuck with. It's a purpose-built large control room with no parallel
walls.

You'll have a hard time getting any kind of gap like that.

Paul P

Understood.

Thanks, Paul.

[PaulP]

Originally Posted by DanDan

From the one post I note the measuring tape calculator again. So once again we use organic methods to identify a certainly destructive reflection, then use ETC to view it amidst a forest of pampas grass? I have repeatedly asked why one would do this. What is the advantage of the ETC. What can be seen through this lens that the simpler views don't have.

The thing is, without the ETC you won't know that you have a harmful
reflection, or at least you can't say exactly when it's happening unless you
have ears calibrated in milliseconds. Once you see it on the graph and
figure the distance the reflection traveled, you can go about figuring out
which path it took and then treat it. The ETC is then used again to see
the improvement. Repeat until everything within the first 20ms is 10-20db
down.

In the typical small rooms which this forum deals with, pretty much all reflections are typically under 20mS. Unless you have over a 16 foot ceiling for instance. So, the reflective surfaces certain to do damage are close at hand, literally in many case, and all entirely visible to the eye.

In general I agree with this but as G.E. has shown, some reflections are
a bit less obvious. He has one that effectively goes around his side and
ceiling aborbers.

Paul P

[DanDan]

Thank you Paul, looks like a good workout there. Plus some very interesting side issues. e.g. the varied impedance corner arrangement. I will take some time to it later. However I did go straight to the good stuff as you suggested. There is indeed a 'mystery' path which would not be obvious to the eye or intuition. The path would be of great interest to a Snooker pro . However, it is a third bounce, likely to be of very low level, and it doesn't end up in the mix area. I reckon such 'mysterys' are stretching things a bit, and I can't agree that we need calibrated ears to detect dangerous surfaces/reflections closer than 10 feet.
Those graphs mumblesound linked to are simple and clear. Not at all like the IMHO difficult to view 'haircuts' we see in our typical hovels.

DD

[Seamus TM]

Originally Posted by DanDan

Thank you Paul, looks like a good workout there. Plus some very interesting side issues. e.g. the varied impedance corner arrangement. I will take some time to it later. However I did go straight to the good stuff as you suggested. There is indeed a 'mystery' path which would not be obvious to the eye or intuition. The path would be of great interest to a Snooker pro . However, it is a third bounce, likely to be of very low level, and it doesn't end up in the mix area.
Those graphs mumblesound linked to are simple and clear. Not at all like the IMHO difficult to view 'haircuts' we see in our typical hovels.

DD

Yes, they are simple and clear, but that is by design. In a space retro-fit, like most of our spaces, it ain't so clear.
My room is not tiny (2,500 cubic feet) but it is narrow. It's almost as tall as it is wide. It's a weird room. I am fairly confident that there are reflections at work that would be very hard to find with a mirror.

There is one thing about this that puzzles me.
We keep talking about first, second, third reflections and how early or late they are arriving to the listening position. In all the tests that I know of, there is a single impulse that is used to determine when they arrive and at what levels, correct?
Music is far from a single impulse. It's pretty relentless in that way. Who's to say that the third reflections aren't effecting the second or first reflections, even if they are lower in level by themselves.. in a vacuum.

This is going to sound stupid, but.....
You know how the song "Row, Row, Row Your Boat" can be sung by an infinite number of people all starting out at different points in the song? Imagine 4 or 5 people in an anechoic room singing that. Depending on where they are standing in the anechoic room, the 3rd person that starts the song could trample all over the first or second person.
That is for 4 or 5 people singing the same thing, just starting at different times.
Now replace all of those people with 4 or 5 identical bands (a band like a musical group). Drums, bass, a couple of guitars and a vocal or 10. 4 or 5 of these bands in the same room, all playing the same song, but starting at different times.
Things could get pretty messy.

I get the Haas effect and RFZ, but it seems like something is missing from the big picture when we base all of our facts about how reflections effect a room (and, ultimately, the original signal) when we are just using a gun shot or a sine sweep.

I'm on Dayquil and I can't breath out of my left nostril, so if I'm making an ass of myself, just ignore me.

[Seamus TM]

I'm making an ass of myself.
That's cool.
Maybe I should read the rest of the thread after I take a nap.

[DanDan]

mumblesound, the wheel is simple. These attempts at analysis begin with very simple concepts, not realistic but useful nevertheless. Take the ray approach, begins with a mirror and a friend, or even an eyeball guesstimate. Escalate to several stick-on mirrors and a laser pointer and it gets busy. Apply a computer program and it starts to get close to reality. Similarly the single impulse is not realistic but an extremely useful tool nevertheless. Note we often drive just one speaker. IMHO we should drive two AND record the IR using a dummy head or binaural mic. Closer to reality. I hope you get well soon.
DD

[SAC]

Apparently a few find this all of this all to be nonsensical and of limited use. As after all, they would eliminate all reflections within 10 feet, which for a small space would conceivably mean ALL reflections.

Not only that, but you don’t even need an ETC to identify the reflections as you can hear them – except for the fact that the Haas Effect renders that claim spurious as our ear-brain does not detect signals arriving within that time window as distinct reflections. But no matter, a small technicality like that won’t slow some down.

And the proper way to measure this environment is with both speakers driven and measurements at both ears using a "dummy head". No, that one is simply too easy....

Well, it’s funny one should say that. Especially as the same people who find the ETC response FUNDAMENTAL were the same ones who 25 years ago were performing interaural cross-correlation measurements with polar ETCs, only not using a SASS mic (which ironically was developed by Techron/Crown as a direct result of their efforts!), but they were using Knowles and the Meade Killion developed ITE (in the ear) mics inserted into the pressure zone of the eardrum, thus maintaining ALL of the phase information our ear ‘sees’!

(And we won’t even mention the literally thousands of pinnae measurements of folks whose hearing response was measured in the pressure zone of the eardrum by Don and Carolyn Davis specifically to study and quantify the norm as well as the optimal pinnae response for exactly such measurements. (...how many of us were taught the lesson of not keeping on moving!)

Note the near ‘reverse’ R-L symmetry. And you will also note that we 'effectively' already have both responses in each.


And don’t make the connection that ever since Beranek the ITD/ISD – initial time/signal delay gap properly defining the anechoic RFZ zone is in fact what Beranek labeled the Interaural Time Delay gap corresponding to exactly that which some say is apparently lacking, but were(are) unaware of this small fact.

Oh, and it best practice to measure each speakers response, driven one speaker at a time.
(But as some still have trouble figuring out what a single ETC means and how to interpret it, can you imagine trying to sort out a 'summed' ETC of two ears with both speakers playing!?!?!?! LOL! So, just how comfortable are you with multi-variant calculus? So, if: 5(abcd)= 10, what is the numerical value of b? Haha!)

For you see, others have already been there and done that. And THEY are the same ones who posit the practices that some here decry as unnecessary and limited.

But then, those folks who actually did the research also rejected the notion of simply eliminating all reflections within 10 feet and then essentially being satisfied that they had done a full days work.. Instead, their substantial research yielded quite a few other results which form a strong foundation utilized by the best in the industry today that specifically rejects the aforementioned notion.

And what is even more humorous is that we can't get by the practical function of an ETC! And it is simply a small tool used in the process of developing a MUCH larger acoustical model. This is akin to watching workers charged with building the Shanghai Tower standing about arguing over the effectiveness of a hammer.

But hey, don’t take my word, as you have already heard that I know not of that which I speak.

But do take a look at a page illustrating a snippet of exactly that work performed by P. D’Antonio, John Konnert, Ferrell Becker and Charles Bilello. Oh, and a few of you who have them will note that this is from page 13 of the ‘15.1’ document you received. And this was done, oh…..25 years ago! My time has flown! If only awareness and understanding followed more closely.

To paraphrase Don Davis, ‘it’s indeed funny how the ancients keep stealing all of our inventions.’

One wonders just what marvels we could achieve if we were simply aware of what has ALREADY been done. And who says the Luddites won’t take over the world. OK, well, perhaps not the world, but they are well on their way in the consumer world of audio.

It is said that “those who are ignorant of history are doomed to repeat it”. One can only hope that 'they' hurry up. As at this rate I suspect the phrase may be proven wrong.