Room Acoustics - Large Room and Small Room

[DrWho]

Math....that's what computers are for.[]

...only after you get outta college... [6]

[mas]

Folks, for the concepts that we need, you do NOT need the math.

The math was simply there to give you a historical perspective of from where the 'numbers' come. But you will NEVER need to calculate them! (Well, we do have Mike to do it for entertainment purposes!![] )

The important thing to understand is the definition of a reverberant field defined in the Large Acoustical Space (Large Room) chapter, and how this definition is not satisfied in Small Acoustical Spaces (Small Rooms) - see the first couple of pages (p.211) in the Small Acoustical Space/Small Room chapter.

Also, the results (Only!) of the two calculations for the volume required to support a reverberant field in the LAS section should suffice to give a basis for why we are dealing with SASs! We will be focusing on small rooms - not giant auditoriums. If you happen to live in a stadium, PM me, and I will be glad to help you on the side.[]

So why did I (...math be damned...) introduce the chapter on LAS and reverberation at all?

The big issue that I am desperate to overcome is the misnomer that we are dealing with reverberant fields in a SAS. And as such, we are not going to be using RT60 measurements. There are other more useful tools at our disposal. And hopefully, before we are finished, you will understand their origin and use.

For this problem, I 'blame' the Everest "Handbook..." for creating a gross misunderstanding of reverberation in SASs by it's completely inadequate and negligent treatment of the subject. Everest alludes to the 'problem' of reverberant fierlds in SASs and then goes on to completely ignore the issue - as addressing it would have caused him extra work instead of simply forging ahead with his grossly overly simplified treatment of SAS/small room acoustics with LAS concepts. In other words, his book is effectively about LAS with pictures of SAS included. But then, his book is simply an overview and should not be construed as an in-depth text. It is just that this misunderstanding posited in Everest's book has been 'foisted' upon good sincere folks who mean well; and then this misunderstanding rears its head and then seems to repeatedly dominate subsequent conversations. ...And it is time to move beyond this fundamental misunderstanding.

So, if you will simply review the definition of the conditions required to support a reverberant field, as well as the statements that disqualify SAS from satisfying a true statistical reverberant field on page 211, then we can skip ALL of the math!

Is that acceptable? And if anyone shows up later and argues that because of 'reverb this' or 'reverb that', then I can simply tell them to "shut up" and we can move on.[]

How's that for coming across as a jerk??[]

OK, I am kind of kidding. I am always glad to discuss it (or ANY other topic!) in order to explain further, if necessary, via PMs or Skype voice chats on the side (IF someone has read the material! ) - but I will not waste more time debating it and allowing it to derail the discussion in the forum. And if someone simply shows up and wants to debate it without having first familiarized themselves with the prerequisite reading I am going to basically dismiss the subject and refer them to the text (or to offline PMs or phone conversations). I really don't want to appear a jerk, but this scenario seems to recur time after time, and I am really tired of it sidetracking the discussion - So I guess that you could say that this is my baggage. And I certainly hope that folks here understand. In the words of the illustrious Barney Fife, I am going to try to "nip it in the bud"!

(but I am always available to discuss ANY topic by phone[] ) If the truth be known (and this is just between you and me), I have a terrible bark, but very tiny teeth (unlike the Killer Rabbit of yore). And I usually wear a large grin - despite my wry wit that often comes across as (slightly) ascorbic in flat print if you can't hear the tone and inflection in my voice and you can't see my grin. So, even if I tell you to take a hike, we can still be friends over a beer. ...But you're buying! []

OK, I feel like I have restated the same idea 82 times now - much to my and everyone's chagrin... So, with a bit of luck we are all on the same page.

Oh wait! I just remembered that I DO have one more issue to address! (Oh brother![])

I don't want to hear anyone else say how they are too 'dumb' (my word!) to grasp this! If I can do it (or at least fool myself into thinking that I can grasp the basics), anyone can! Especially when the ones saying this are folks who I KNOW are completely capable of not only understanding this, but of making significant contributions!!! []

[DrWho]

Ok, so what's the ideal RT60 that we're shooting for in a home playback system? [A]

(that was entirely sarcastic btw...didn't want to be tooooo misleading)

[H]

[mas]

OK... I should have known... so we now know who is the first one to be booted out of here![][][]

...but wait...who will do the math?[]

Anyone else???[]

I was thinking a good round number like 45 seconds...

[DrWho]

Exactly [6][]

[WMcD]

mas,

I really do like this discussion. I appeciate your frustration.

I really do say with all kindness that you brought this all on yourself; quite in the spirit of correct science. That sounds snide and it is not meant that way at all.

I can see that you wanted us to read about LAS and the math for the purpose of allowing us to recognize that it does not apply, and least we be distracted by the LAS issues which are inappropiately applied to living rooms by others.

In my view, you should have said, LAS is orchestra hall (lots of math and difficult theory). Your living room is the SAS men's room at orchestra hall (no math, easy time domain theory , measurable with moderate priced test equipment).

This HAS come through.

So don't worry. We all love your efforts.

Smile,

Gil

[mas]

Thanks Gil...[:$]

But why do I feel like a cat wrangler trying to get this herd of cats into a straight line? [][]

Gotta get me one of them new rechargeable cattle prods![6]

[WMcD]

mas,

I'm glad you took my comments in the spirit I meant them.

My guess is that some of us will eventually get to using the testing equipment and displayng data. Then people will flock to the project. You can't prod cats to a destination, but the opportunity of tinkering and fooling with computers will be like catnip.

This happy place (forum) has not yet gotten to the state of subject headings devoted to specific DIY projects. E.g. the home built K-Horn, the home built Jubilee, etc. We're somewhat splintered and threads wander.

But I believe your efforts will reach fruition. Dare I say, "Space, small acoustic space, the final frontier."

Smile,

Gil

[colterphoto1]

MAS, hope you haven't given up on your students. Doc is coming over today and we plan on discussing the class. We both have been unable to decipher the downloaded assignments due to clarity, so I'm thinking of buying the book for us.

So hang in there with us ok? We do want to learn more about this. Just really need the handbook.

Michael

[gaspr]

Hey Michael

I know what you mean about the clarity of the homework

assignment. I found that if I printed it out, it was much easier

to read. Still a bit fuzzy, but much better than trying to

read it off the screen. Hope this helps.

[mas]

We'll definately keep going...I was simply trying to give you guys a chance to 'catch up' as I know many have been busy (as have I [:$] ).

Now, if I could just engender the same interest as ...hey, I know!

Imagine a small room on a conveyor....the sound is travelling at

[][]

Mark

[gaspr]

Hi Mark

We need something to chew on here. Any chance you can throw us a bone or two?

[mas]

Since we began this, some questions has arisen from a few regarding topics. One of those topics is superposition.

As this topic is fundamental to acoustics, we may as well tackle it now, as you will be asked to understand this concept over and over as we move along.

In this light I have assembled a bit of information along with a few linked sites where you can play with some tools and hopefully get a feel for the concept.

As usual, we are trying to make this as math free as possible, as at this point we are trying to focus on the basic concepts.

Also, may I ask folks to try to minimize the tangents as much as possible. But please don't mistake my intent - I do NOT want to quell any legitimate exploration! Quite the contrary! As we get this cranking up we will start moving forward and if you can make a list of some of your speculative questions I suspect we will get to them - but I don't want to get off on tangents at the risk of losing some of the folks who are not as jaded as some of us 'old farts' who have lived with the technology for a bit longer than others!

So for now we are trying to get everyone as much as possible comfortable with some common terminology and concepts.

My hope is that after we have covered some of the basics that we will have plenty of time to chew the fat and to explore even more interesting topics - but I have this fantasy of doing so along with those who are currently new to some of the ideas.

So here is a request: If you have questions or disagreements regarding the content, please PM me first. If I have made an error, I want to know so that I can make corrections! Also it will help me in incorporating and /or clarifying concepts as we move along. And if you are shy about asking a question for clarification, I often find that it is easier to chat rather than to have an interactive discussion in an awkward form over days - so PM me on the site as opposed to via email (I find them faster!) and we can chat over Skype and via phone as well. I hope that this does not sound like I am trying to minimize discussion. I am not! But if you are confused, I want to try to zero in on the issue and get it clarified quickly! THEN we can discuss the ramifications of the topic itself.

Also, in order to allow for editing and corrections, I will start posting the attachments on the following post, as the reattachment process becomes a bit of a pain - especially as I can't get the spell checker to work! [:$][]

Oh, and if I don't have a chance elsewhere, may I wish everyone a wonderful and safe holiday season!

Note!: Skype is offering a deal for free long distance PC to Phone calls anywhere in the US and Canada for the year 2007 for only $14.95 until January 31, 2007. Their service is excellent, and you can't beat the price! After January 31 it goes up to $24.95.

[mas]

Here is a bit of information that may help some to better understand the concept of superposition - the fancy name for how waveforms combine.

Be sure to check out the included link for comb filtering and also the second listed waveform tool - and the sublinks on the site.

[mas]

Here is another generally related topic regarding viewing measurements in various domains, most notably time and frequency.

It is not intended to be a discussion of the differences between time based measurement gear and frequency based gear. With all due respect, I don't think enough folks are familiar enough with some of the gear nor with the the nature of the relationship between the two domains, let alone the larger domain relationship picture to approach that topic at this point. And there is always time to do that in the future as we catch everyone up.

Take a look, and I hope it helps you put a few things into perspective, and, who knows, maybe it will make you think of a few things you haven't gotten around to pondering before.

Again, the attachment is in the next post....

[mas]

Here it is...

Please PM me if anything is confusing and we can straighten it out quickly (and ignore anything Coytee might tell you! [] I mean that literally! As if you listen to him I will not be able to deny it- well, not with a very straight face!)

[Coytee]

Ok, that does it... Chloe has been RELEASED! My guess is you have about a 4 day head start before she'll make it down there for her rendezvous! Be forewarned that she'll be relentless. She's pretty vicious too!

[Z].....................................[&]

[Coytee]

That item you posted, has a circle on the bottom page of it. When I was a kid, I could draw things like that with my Spira-gyra too!!

Now, in seriousness, what do the various 'circles' (if I may call them that) mean? How does one interpret what is being seen?

Did you make that with the DELUXE Spyro-Gyra set?

[]

[mas]

Gee Richard, I was rather hoping that you would be ready to start explaining them to me!

Actually, the circle thingies are polar responses. They provide an indication of how the sound pressure wave radiates out from the speaker. In this case they are indicative of the horizontal plane - meaning that they illustrate the dispersion of the wave from 'left to right' relative to the speaker (rather than up and down - which would be the vertical polar pattern.)

To help you visualize these patterns, may I suggest you look at the EASE generated 3D volumetric balloon plots. If you take a horizontal cross sectional 'slice' across the horizontal center, you will see a similar pattern. In the spaced woofers and comb filter example, the view is that of looking down from above. If you take a few minutes to look at the polar plots and to compare them with the 3D balloon plots generated with EASE in the superposition attachment, I think you will see that all of the polar plots pretty well correspond to this diagram.

OK, let's pretend that my awkward description above makes sense...(hey, stop laughing!), here's how it translates into the real world.

Say you are in a large auditorium. As you walk from the left side of the room to the right side of the room, you will effectively walk through the regions - the lobing- indicated by the diagram. As you entered a region where the lobes exist, that frequency would be present. As you left that region and entered into a region where the lobes were not present, the sound would lack those frequencies. It would sound like the signal had been equalized and certain frequencies removed. And indeed those frequencies are 'gone'. Due to destructive superposition, the signal at those frequencies would have combined at 180 degree 'out of phase' and the signals would cancel. At other locations, the intensity (gain) of certain frequencies would be greater or less depending upon the relative phase of the combined signals.

So the character of the signal varies, often quite considerably as you move across the room. The degree that this occurs can be quite startling. It is rather interesting that few people who attend concerts really notice this, but considering the program material often playing before a show, and the fact that most people remain relatively stationary during the show I guess rather minimizes one's notice of this effect.

And without getting into this too deeply now, the traditional 'solution' (if only it worked!!!) would be for the FOH mix position to EQ the signal in a valiant, but fruitless effort to fix the problem. The irony is that he may have seemingly fixed a problem that he noticed in his mix position. The reality is that the best he could realistically expect is that due to introducing additional phase delay into the signal by virtue of the LRC filters in the EQ (the L & C - inductance and capacitance, respectively, causing phase shift), he has simply caused the polar distribution of the particular frequency range focused upon, to be shifted slightly by a few feet - thus fixing the problem in his mix position but 'dumping' the problem onto someone else in another listening position. And in doing so he moves the entire spectral composition as well - simply moving the problems but fixing none of them.

There, it seems that I may have helped a little in one area and really confused others in another area! Talk about "moving problems around"! I don't think EQ will help me either! []

What we need to do is to perform the experiment with 2 Auratone 3" speakers stacked atop each other vertically and fed with the same pink noise signal. If everyone is seated in the audience in front of them, as someone slowly moves the top 'cube' slowly back, the developing offset will cause the sound to appear to revolve about the listening area much like Leslie cabinet induces - or like that of a rotating directional horn like that often used for air raids or emergencies. As the offset, the distance and hence time relationship between the two signals changes, the lobing changes as well. It very clearly illustrates the polar lobing that is exhibited by spaced signals or reflections and it is a quite dramatic and rather 'startling' demonstration.

I'll be waiting for Chloe! Make sure she brings the Jubilees!!!!

[artto]

I'VE HELD MY TONGUE LONG ENOUGH, SO HERE GOES:

Folks, for the concepts that we need, you do NOT need the math. THEN WHY DO YOU KEEP POSTING SO MUCH OF OF IT? I FOR ONE, CERTAINLY DON'T GIVE A DAMN ABOUT YOUR MATHEMATICAL APTITUDE, ESPECIALLY IF IT'S NOT REQUIRED TO UNDERSTAND THE CONCEPTS.

So why did I (...math be damned...) introduce the chapter on LAS and reverberation at all?

The big issue that I am desperate to overcome is the misnomer that we are dealing with reverberant fields in a SAS. And as such, we are not going to be using RT60 measurements. There are other more useful tools at our disposal. And hopefully, before we are finished, you will understand their origin and use.

For this problem, I 'blame' the Everest "Handbook..." for creating a gross misunderstanding of reverberation in SASs by it's completely inadequate and negligent treatment of the subject. Everest alludes to the 'problem' of reverberant fierlds in SASs and then goes on to completely ignore the issue - as addressing it would have caused him extra work instead of simply forging ahead with his grossly overly simplified treatment of SAS/small room acoustics with LAS concepts. YEAH, RIGHT. SO YOU KNOW MORE THAN ALTON. PERSONALLY, I DON'T THINK SO. WHAT DO YOU HAVE TO SHOW FOR ALL OF YOUR KNOWLEDGE? ALL I'VE SEEN SO FAR IS A BUNCH OF POSTS WITH ATTACHMENTS COMPILED BY SOMEONE ELSE.

So, if you will simply review the definition of the conditions required to support a reverberant field, as well as the statements that disqualify SAS from satisfying a true statistical reverberant field on page 211, then we can skip ALL of the math!

Is that acceptable? NO And if anyone shows up later and argues that because of 'reverb this' or 'reverb that', then I can simply tell them to "shut up" and we can move on.[] UH, NO, YOU SHUTUP

How's that for coming across as a jerk??[] PISSOFF

OK, I am kind of kidding. I DON'T BELIEVE YOU. I am always glad to discuss it (or ANY other topic!) in order to explain further, if necessary, via PMs or Skype voice chats on the side (IF someone has read the material! ) - but I will not waste more time debating it and allowing it to derail the discussion in the forum. ARE YOU TRYING TO TELL ME THIS IS A "DISCUSSION"? LMFAO. I COULD HAVE SWORN YOU WERE FEEDING US "A LITTLE BIT AT A TIME" SO WE COULD HAVE ENOUGH TIME TO UNDERSTAND IT AND NOT OVERWHELM OUR ITTY BITTY BRAINS. OF COURSE YOU WILL DETERMINE WHAT THAT PACE SHALL BE, TEACHER. And if someone simply shows up and wants to debate it without having first familiarized themselves with the prerequisite reading I am going to basically dismiss the subject and refer them to the text (or to offline PMs or phone conversations). I really don't want to appear a jerk (YOU FAILED), but this scenario seems to recur time after time (I WONDER WHY?), and I am really tired of it sidetracking the discussion - So I guess that you could say that this is my baggage UNFORTUNATELY IT APPEARS YOUR BAGGAGE HAS A LOT OF STUFF IN IT THAT WE DON'T NEED. And I certainly hope that folks here understand WHAT'S TO UNDERSTAND? WE DON'T NEED THE MATH. In the words of the illustrious Barney Fife, I am going to try to "nip it in the bud"! BARNEY WAS NEVER ABLE TO "NIP IT IN THE BUD" AND QUITE FRANKLY ITS TOO LATE FOR THAT ANYWAY.

(but I am always available to discuss ANY topic by phone[] ) If the truth be known (and this is just between you and me), I have a terrible bark GIVE ME A BREAK, YOU THINK YOU HAVE A TERRIBLE BARK? [:'(] but very tiny teeth (unlike the Killer Rabbit of yore). And I usually wear a large grin - despite my wry wit that often comes across as (slightly) ascorbic in flat print if you can't hear the tone and inflection in my voice and you can't see my grin. So, even if I tell you to take a hike, we can still be friends over a beer. ...But you're buying! [] I DON'T THINK WE'RE GOING TO BE FRIENDS IF I'M BUYING.

OK, I feel like I have restated the same idea 82 times now - much to my and everyone's chagrin... So, with a bit of luck we are all on the same page. LUCK IS FOR RABBITS.

Oh wait! I just remembered that I DO have one more issue to address! (Oh brother![])

I don't want to hear anyone else say how they are too 'dumb' (my word!) to grasp this! WHAT'S TO GRASP? SOME CALCULUS, A FEW DIFFERENTIAL EQUATIONS, FOURIER TRANSFORMATIONS. HELL, ANYBODY CAN DO THAT STUFF. OH YEAH, THATS RIGHT, WE DON'T NEED TO DO THE MATH THAT KEEPS GETTING POSTED. If I can do it (or at least fool myself into thinking that I can grasp the basics), anyone can! Especially when the ones saying this are folks who I KNOW are completely capable of not only understanding this, but of making significant contributions!!! [] ACTION SPEAKS LOUDER THAN WORDS. WHAT DO YOU HAVE TO SHOW ME?

NO, I'm not having a bad day, I took my meds, exercised, good diet. Let's see something REAL.