The No.1 Website for Pro Audio
SBIR
Old 8th January 2018 | Show parent
  #451
Size Matters

Seems to me it is all about relative size and spectrum.
Imagine an extremely large LF source. Something hypothetically large, equal to the lowest wavelength being reproduced. Very directional.

As I see or imagine it, HF bounces off the boundary reflection area in a specular ray like manner. But LF wavefront is an expanding sphere, so when it virtually comes up from beneath the boundary, there or not, it is a vague thing in terms of timing. Which brings me to perception of pitch and blending of delay into the direct. No on topic but in case you haven't come across this guy, great writer, great clarity of understanding and sharing it. How We Hear Pitch |

DD
Old 9th January 2018 | Show parent
  #452
Gear Maniac
Quote:
Originally Posted by DanDan View Post
As I, or the mic, sees it, the path from every image speaker is longer than the direct. So I can't see how removing any of them, by removing or making absorbent the boundary, could be said to introduce a delay.
Diffraction. Take the ideal case of infinite floor and loudspeaker & listener at same height. Make a small area at the midpoint completely non-reflective. The edges will now diffract some of the LF sound towards the listener and crucially the new diffracted path is longer than the direct reflection path.
Old 9th January 2018 | Show parent
  #453
Mission Impossible

Creative, and interesting but I am not laying on the floor listening to a point source , nor am I likely to suspend over a missing floor. I see the thought processes now, but am not seeing any practical aspect.

DD
Old 9th January 2018 | Show parent
  #454
Gear Addict
 
ReDRuMx's Avatar
Quote:
Originally Posted by DanDan View Post
Creative, and interesting but I am not laying on the floor listening to a point source , nor am I likely to suspend over a missing floor.
I would like to point out (again) that this applies to other surfaces other than the floor. I.e., side walls and ceiling. Common locations of early reflection treatment.
Old 9th January 2018 | Show parent
  #455
Practical

Indeed Red. But what practical difference? Currently in domestic or prosumer scenarios, the manufacturers and some others recommend using fairly thin traps for side reflections and ceiling. This will remove the higher frequencies from the reflection, leaving the lower alone. i.e. nothing has happened at LF.
Your geometry is simplified, idealised to make a point. Interesting but..... But the issue of the spreading of the LF wavefront vs the rays of HF has not been diagrammed or theorised. Basically this wavefront is rising up from the whole ground under the speaker and listener. Ground being solid, much faster speed of sound, so this LF can well hit the listener BEFORE the direct air path.
Now, to pro world, side absorption deep enough to be full range, a la Newell. Now the LF could be said to be delayed in your hypothesis, and accelerated through the floor in the other one. Ultimately looking at the distances and frequencies involved, and the fact that there is nothing to be done about this, Que?

If I remember correctly, the purpose of this thread was to examine SBIR, particularly FW, with the intention of determining are there two effects at play. i.e. Cancellation at the Speaker and cancellation at the listener. If so do they combine or are there two different frequencies? This might affect our choice of trapping and where to put it.

DD
Old 9th January 2018 | Show parent
  #456
Gear Addict
 
ReDRuMx's Avatar
Quote:
Originally Posted by DanDan View Post
Indeed Red. But what practical difference?
Heh Dan, I think I explained the purpose of my interest in this specific topic a couple of times already. Ditto the practical difference. I'm not sure I can do it better than I already did.

No one is forcing you to apply the same qualification of importance to the conclusion of the discussion. I can live with you saying "not important", so perhaps agree to disagree and we leave it at that?


Quote:
Originally Posted by DanDan View Post
Basically this wavefront is rising up from the whole ground under the speaker and listener. Ground being solid, much faster speed of sound, so this LF can well hit the listener BEFORE the direct air path.
You are describing structure-borne transmission. Not a factor in any of my examples. Just wanted to make it clear for anyone reading. Doesn't change your point, whatever it is (not perfectly clear to me), I just wanted to make clear that this is not something I was describing.
Old 10th January 2018 | Show parent
  #457
Gear Maniac
Quote:
Originally Posted by DanDan View Post
Creative, and interesting but I am not laying on the floor listening to a point source , nor am I likely to suspend over a missing floor. I see the thought processes now, but am not seeing any practical aspect.
You wanted an explanation for the delay. I provided an explanation for the delay.

Any single explanation is only going to be valid in simple cases, such as a large plane with a hole (aka ceiling or side wall with acoustic tile on it). With complex geometries (such as your typical living room or small studio) apparent delay can be the result of multiple propagation paths (with and without diffraction) interfering constructively and destructively in different ways once absorption is added to some particular point. IOW, the question is only really answerable for the simple cases.

Ground transmission won't really have effect for this since the re-radiation efficiency is going to be so low as to make the amplitude small compared to any remaining reflections / diffraction.
Old 10th January 2018 | Show parent
  #458
Gear Addict
 
ReDRuMx's Avatar
Quote:
Originally Posted by Antti H View Post
The edges will now diffract some of the LF sound towards the listener and crucially the new diffracted path is longer than the direct reflection path.
Although the crucial factor here is not an edge as such, as there's no diffraction happening in the classical sense -> sound isn't bending around anything, not even the edge. For my example to work, the edge doesn't have to have any depth, we can visualize a simple 1D floor, with an equally 1D gap. The "longer" path is simply a result of a part of the floor being gone, as is evident from my illustration.

With that said...

Quote:
Originally Posted by Antti H View Post

...Any single explanation is only going to be valid in simple cases...
I agree!!! To be honest -> Diffraction, omni-directivity, the Huygens-Fresnel principle -> Are all different expressions of the same thing. So please don't take what I wrote above as a correction, it was more an addition

This reminds me of a quote I read somewhere by Eric Desart (he was paraphrasing Einstein):

"Things should be made as simple as possible, but not any simpler, because then they become wrong"

Old 10th January 2018 | Show parent
  #459
Gear Maniac
Quote:
Originally Posted by ReDRuMx View Post
Although the crucial factor here is not an edge as such, as there's no diffraction happening in the classical sense -> sound isn't bending around anything, not even the edge. For my example to work, the edge doesn't have to have any depth, we can visualize a simple 1D floor, with an equally 1D gap. The "longer" path is simply a result of a part of the floor being gone, as is evident from my illustration.
Yes. It's enough for there to be any sharp discontinuity, which a panel "conveniently" provides at LF. Of course in real world small rooms there's diffraction and reflection from all over the place, so things do get much more complicated. As evidenced in my living room listening position which has mysterious upper bass nulls (reflection, not modal) that I can't seem to get rid of by placing any single panel...
Old 10th January 2018 | Show parent
  #460
Ground Transmission

Although cloudy some posts back, as stated, the hypothesis has been clarified. So no need to provide other explanations or ways of stating it, thank you.
You are dismissing Ground Transmission based on an opinion that is either doesn't exist or isn't important.
There was another discussion on a different matter, where this issue became quite real.

The discussion was about flush mounting while isolated from the wall, speaker stands, isolating transmission from speaker to stand/wall to floor and onwards.
Several posters referred to measurements of actual radiation resulting from the ground path. Even back to Heyser afaik.
It happens unless one prevents it.

Now this hypothesised 'delayed' sound as a result of absorption or partial removal of boundaries. Any examples of this actually happening? Trust that I am in no way devaluating the interesting nature of the hypothesis. But I have not seen nor can visualise it actually happening in any realistic scenario. But then I never thought about the conundrum before either...... Do you have a practical scenario where this might change our treatment, location, or other choices?

DD

Last edited by DanDan; 10th January 2018 at 08:42 PM..
Old 10th January 2018 | Show parent
  #461
Gear Addict
 
ReDRuMx's Avatar
Quote:
Originally Posted by DanDan View Post
You are dismissing Ground Transmission based on an opinion that is either doesn't exist or isn't important.
Not sure who you are addressing, but I wasn't dismissing ground transmission. I just wasn't talking about it in the same way I wasn't talking about, e.g., cabinet edge diffraction - or any other feature or anomaly of sound which might influence the frequency response or decay or...


Quote:
Originally Posted by DanDan View Post

Now this hypothesised 'delayed' sound as a result of absorption or partial removal of boundaries. Any examples of this actually happening? Trust that I am in no way devaluating the interesting nature of the hypothesis. But I have not seen nor can visualise it actually happening in any realistic scenario. But then I never thought about the conundrum before either...... Do you have a practical scenario where this might change our treatment, location, or other choices?

DD
Not sure you are looking at it from the correct angle -> This is not something which "occurs", and is obvious as a delay which will happen and then you will notice it in a measurement or listening test.

This describes why notches have "width", and it describes why EXACTLY you need large patches of absorbent to fix a SBIR problem - even if the reflection is coming from one surface only.

When is this practically important? -> Again, not the only importnat question for me. You can observe this effect EVERY TIME you absorb a reflection from a mirror point. Let's say you absorbed that frequency 100% from that patch of floor or wall - But if the frequency is low enough to "diffract" from beyond the patch, it will reach the listening position quieter AND delayed. Let's say this reflection originally caused a dip. Now the dip is smaller because of BOTH of these contributions - quieter and delayed. If it was only quieter, the dip wouldn't be reduced as much, because it would still be at 180 degrees out of phase. Bit if it's both quieter and 200 degrees out of phase, then the dip reduced a bit more.

So the point is - Saying "mirror point absorption makes SBIR dips smaller because it attenuates the reflection" is a very useful heuristic, but it is not 100% true. It is better to say that, given a low enough frequency, "mirror point absorption makes SBIR dips smaller because it attenuates and delays the reflection".

So again, this clarifies something you've been aware of all the time. At least for me.

It makes me understand how sound works in a bit more detail, it makes me understand why large patches of absorbent are important, it makes me understand why and how non-mirror point reflections can "reach" the listening position...

Understanding how things work is better than not, no?

So along with everything I already explained, it is enough for me.
Old 11th January 2018 | Show parent
  #462
Specific

I was addressing Annti's ground plane idealised scenario.

Again, I am not at all convinced about this delay theory.
Again I refer to a ballooning wave front. Imagine it seeming coming from behind the side wall. An absorbent patch centred on it. There is just as much of that wavefront at any side of the patch. One of those sides is a lot closer to the listener than the other say 3. So now, let's say that different areas of this LF wavefront arrive at the ear at different times. Whether one calls a selected one delayed or earlier is a matter of what one compares them too. Essentially Plane or Spherical wavefronts don't really seem to have a time of arrival in the sense that Specular do.
DD
Old 11th January 2018 | Show parent
  #463
Gear Addict
 
ReDRuMx's Avatar
Quote:
Originally Posted by DanDan View Post
I was addressing Annti's ground plane idealised scenario.

Again, I am not at all convinced about this delay theory.
Again I refer to a ballooning wave front. Imagine it seeming coming from behind the side wall. An absorbent patch centred on it. There is just as much of that wavefront at any side of the patch. One of those sides is a lot closer to the listener than the other say 3. So now, let's say that different areas of this LF wavefront arrive at the ear at different times. Whether one calls a selected one delayed or earlier is a matter of what one compares them too. Essentially Plane or Spherical wavefronts don't really seem to have a time of arrival in the sense that Specular do.
DD

I really don't understand how the situation you described proves or disproves anything. I have nothing new to add, sorry. Also, sorry to have to say it like this, but this is not a matter of "convincing", but "understanding".

My illustration in post 446 pretty much proves the basic concept. In this situation, the reflected signal arriving "later" is FACT.
Old 11th January 2018 | Show parent
  #464
Fantasy

Quote:
this is not a matter of "convincing", but "understanding".
Neither. There is no fact in your proposal, and a major missing behaviour of sound.

Inverted, a concrete bandstand outdoors, I can see your hypothesis working. You have your solid surface, abruptly terminated, which should result in LF time smearing. Horizontally this might have sonic consequences, and perhaps this is part of the reason why LF speakers are often placed outside the shell, on the ground.

In our typical Prosumer CR scenario we have an IMO way too small and too thin absorptive patch.
This will do nothing at LF, no absorption, no delay, nada.
This is because of it's size and depth compared to the wavelengths of LF passing through and around it.

Now, let's make it significantly large. Say the middle half of the side wall, between speakers and listener. Let's make it 60cm, decent.
Let's take the journey from Speaker to listener. I won't do air paths, fibre paths and angles etc. etc. crap at Geometry and numbers. I have just one pertinent point. Sound travels slower through fibre than air. Let's call it 80 M/S at 50Hz. So just taking a shot at the numerical result, twice through, 45 degrees, the fibre causes the reflection to reappear 5mS late? This would be the strongest LF reflection, hopefully though utterly inaudible due to the absorption.

The first quarter of the room, by the speakers, is untreated, so virtual half images radiate LF on the slightly longer journey. On the way it would appear to have to pass through the big fibre barrier too. Additional Delay inevitable.

Perhaps someone can put some numbers on how much LF level would occur in such a somewhat reasonably realistic scenario?

Certainly though, the often stated advice that thin traps are sufficient for Cloud and Side Reflections is just daft. Clearly also side wall and cloud should go all the way to the Front Wall, ideally increasing in depth to fill the corner. Flush to the speaker seems advisable. Newell remains right. Flush Mount has advantages and No Reflection at any Frequency is the only pure transmission path.

DD
Old 11th January 2018 | Show parent
  #465
Gear Addict
 
ReDRuMx's Avatar
Quote:
Originally Posted by DanDan View Post
Neither. There is no fact in your proposal, and a major missing behaviour of sound. ...

DD

You have provided multiple scenarios, with multiple varying factors, but the point of what you are trying to say is not clear to me. I fell you are muddying the waters (not on purpose) by throwing in random variables, which disable us from discussing the matter in a focused manner.

I can tell you what the scenario I am describing is:

We have a speaker, a reflective surface (floor or side wall), and the usual listening spot. A 100 % absorbent patch placed at the mirror point.

The reflected signal path is now longer.


This is what I'm claiming, and nothing you said made me see things differently.
Old 11th January 2018 | Show parent
  #466
Grandstanding

Red, I ask again for a halt to repeating and rephrasing the same thing. We all get it. A small portion of the reflected signal is delayed. This portion is related directly to the area and depth of the absorbent patch vs the whole wall.
I think we are 100% agreed that in most, almost all, cases in our world here, these patches are too small to have any significant effect at real LF. Result poor BIR and Width Modal Control. With a side order of delay.
Your idealised construct, is an already known failed treatment, for other simple reasons. The delay thing is by now I expect fully understood, but remains of no practical significance except perhaps in those outdoor bandshell scenarios I described. With poor Width BIR and Modes, the delay is the least of our problems. When the 100% absorptive patch is large enough, your issue disappears right? I am posting other views on the basis that you expressed interest in fuller understandings and your little anomaly kinda restarted the motor here. The other aspects that have big significance IMO. e.g. Slow sound in fibre changes the Acoustic dimension quite a bit. But overall the Topic of this Thread was intended to be SBIR. It is very long and probably too polluted to be of any real use anymore. But still for archival purposes, as the instigator I do try to keep it On Topic and Significant. If you feel disabled or moderated in some way here, you can easily start a focussed thread on this particular nuance which particularly interests you.
DD

Last edited by DanDan; 11th January 2018 at 11:33 PM..
Old 11th January 2018 | Show parent
  #467
Gear Addict
 
ReDRuMx's Avatar
Quote:
Originally Posted by DanDan View Post
Red, there is really no need or point to keep repeating and rephrasing the same thing. A small portion of the reflected signal is seemingly delayed. The proportion is related directly to the area and depth of the absorbent patch vs the whole wall.
This is why I keep repeating the basic claim. Because what you wrote above is not following from it.

No "small portion" is delayed. Literally NO PART of the reflected signal is arriving at the original "mirror point delay time".


Quote:
Originally Posted by DanDan View Post
When the 100% absorptive patch is large enough, your issue disappears right?
If the 100% absorptive patch is now covering the whole floor, then yes, the "issue" disappears because there's no floor, and consequently - no reflections at all.

If you start with a small patch covering the mirror point, then you have NO SIGNAL reaching the listening spot at the "mirror point reflection time", and as you increase the size of the patch, this first reflection arrival time becomes longer and longer.

Note that in order for these claims to hold true, the scenario must be the one I described in mssg # 465.

Quote:
Originally Posted by DanDan View Post
remains of no practical significance
Not the point. I don't think we can discuss the practical significance, as I don't think you fully understand what my claim is - or implies (or at least not fully).

Quote:
Originally Posted by DanDan View Post
If you feel disabled or moderated in some way here, you can easily start a focussed thread on this particular nuance which particularly interests you.
DD
All good. If you feel this is messing up the thread, we can stop (as I suggested some messages ago).
Old 11th January 2018 | Show parent
  #468
Messing

Quote:
No "small portion" is delayed. Literally NO PART of the reflected signal is arriving at the original "mirror point delay time".
You are introducing new terms and strictures in order to appear correct.
'Mirror point delay time' Really?
Typically a LF wavefront will be way larger than a typical absorbent patch.
Typically a LF wavefront will cover the whole wall even at that distance, due to low Q. It will reach and continue along all boundaries around that patch, before, after, above, and below. Only the relatively small absorbed portion is removed from this/these large time smeared reflection/s.

Do start your own thread though, your thought processes here and in threads you have started, are IMO provocative of further thought, interesting, and welcome.

DD
Old 11th January 2018 | Show parent
  #469
Gear Addict
 
ReDRuMx's Avatar
Quote:
Originally Posted by DanDan View Post
You are introducing new terms and strictures in order to appear correct.
No I'm not. Feel free to point me to something I wrote which contradicts this "new" claim.

Quote:
Originally Posted by DanDan View Post
'Mirror point delay time' Really?
Really what? By this I simply meant that the mirror point reflection arrives after a certain time has past, when compared to the original sound. Not sure what's weird.

Quote:
Originally Posted by DanDan View Post
Only the relatively small absorbed portion is removed from this/these large time smeared reflection/s.
It is very clear to me now that you don't understand what my claim is. But I really feel no need to pursue this further unless you ask me a question, or misrepresent my claims.
Old 12th January 2018 | Show parent
  #470
Red, your Mirror Point Reflection Time is a newly coined term. It's only purpose is a self serving attempt to define energy from that small patch as 100% of interest. Circular thinking, 'Here be dragons'.

Again you are protesting loudly and repeatedly boringly that you are misunderstood. I assure you that are understood fully. Perhaps a bit more than you can ever realise. If you wish to continue pushing your particular agenda, please do start your own thread. Your thread, your house, anyone entering is a guest. There I will show you the courtesy of not accusing you of arcane obscurity, repetition. I hope that once you have gotten a few under your belt, experienced some clutter and grandstanding, you will understand how diffusion dilutes.

The first post in this thread defines the Topic. i.e. Are there two SBIR's, one at the speaker, another at the listener, and can they be considered separately or are they intrinsically combined?

DD

Last edited by Northward; 14th January 2018 at 10:30 AM.. Reason: Watch your language and attitude
Old 12th January 2018 | Show parent
  #471
Gear Addict
 
ReDRuMx's Avatar
Quote:
Originally Posted by DanDan View Post
Red, your Mirror Point Reflection Time is a newly coined term. It's only purpose is a self serving attempt to define energy from that small patch as 100% of interest. Circular thinking, 'Here be dragons'.

Again you are protesting loudly and repeatedly boringly that you are misunderstood. I assure you that are understood fully. Perhaps a bit more than you can ever realise. If you wish to continue pushing your particular agenda, please do start your own thread. There I will show you the courtesy of not accusing you of arcane obscurity, repetition. I hope that once you have gotten a few under your belt, experienced some clutter and grandstanding, you will understand how diffusion dilutes.

DD
Okay Dan. Sorry for the confusion. Cheers.
Old 12th January 2018 | Show parent
  #472
No worries at all. I would still and always welcome deep and interesting thoughts on that question though.

i.e. Are there two (or many) SBIR's, one at the speaker, another at the listener, etc. and can they be detected and considered separately or are they intrinsically combined? Thus can we do something priority targeted (on the biggest one) or are we stuck with just mandatory deep absorption at all possible boundaries? That is probably self answering, but perhaps we can do better.

I particularly enjoyed your thread, where we were free to trot out suggested theories, many wrong, but without conflict, ultimately leading to the answer. Two speakers = +6dB = twice electrical power?

DD

Last edited by Northward; 14th January 2018 at 10:31 AM..
Old 24th March 2018
  #473
Gear Addict
Great info and I read the entire thread. However, and perhaps I missed it, with regard to DanDan's test and the predicted effects, how close were they in the end?
Old 9th September 2018 | Show parent
  #474
Here for the gear
 

Grateful

Quote:
Originally Posted by DanDan View Post
But overall the Topic of this Thread was intended to be SBIR. It is very long and probably too polluted to be of any real use anymore. But still for archival purposes, as the instigator I do try to keep it On Topic and Significant. DD
Agreed. This thread is extremely long and polluted beyond belief at several points along the way. However, it is also eminently useful.

I'm in the process of testing and treating only the third room I've built. So I know just enough to get lost quickly, at which point things come to a screeching halt! SBIR was one of those points (as it seems to have been for more than a few of the experts posting here).

I'm still not sure of the solution I will choose, but at least i'm better informed and can move ahead. For that I thank you, and everyone who contributed to what became a fascinating and educational discussion.

Regards,
Wayne
Old 22nd October 2018
  #475
Here for the gear
 

Still researching methods, testing my room as I go.

Found this useful online SBIR Calculator. Enter your room and speaker dimensions and get nulls and peaks for Front Wall, Rear Wall, Side Wall, Opposite Side Wall, Floor and Ceiling. as well as Speaker Baffle Step data.

SBIR calculator
πŸ“ Reply

Similar Threads

Thread / Thread Starter Replies / Views Last Post
replies: 115 views: 9764
Avatar for poshook
poshook 2nd July 2009
replies: 405 views: 127452
Avatar for JblKid95
JblKid95 16th December 2019
replies: 44 views: 8694
Avatar for mmplisskin
mmplisskin 27th March 2011
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
πŸ–¨οΈ Show Printable Version
βœ‰οΈ Email this Page
πŸ” Search thread
πŸŽ™οΈ View mentioned gear
Forum Jump
Forum Jump