You start about modes created confusion in context.
more about the one making the statement.
i quote your previous message , snip most but the pic...
what i see is a classic interaction of many resonances. i don't see evidence that any given resonance is grounds for any given blip on the graph. ( i edit out the word chaotic, as i mean not immediately visually clear, systems like these always have explanations, and as such "chaos" does not apply in the literal definition of the term.)
finally, it's worth mentioning that this thread contains both commentary that predicted modes and peaks/dips on the graph that DO align are evidence of said peaks/dips being caused by said modes, AND commentary that modes should not be expected to occur at the proper frequency due to boundary affects.
and those two stand somewhat in contrast to one another, do they not?
it's always safe to assume, my Belgian cohort, that i mean no harm, so please explain why those two aren't in constrast, or why any given thing is as it is. And in general, things don't have to be intense or angry to be serious or well meant.
just think of me, if you like, as the "college student" that jeff referred to. i'm certainly an educable fellow...
i am not, however, convinced that in light of the sum content of this thread, other threads on the topic, experience with modes, etc. ... that any argument concretely correlating a dip on a graph with a prediction on a spreadsheet stands on solid ground.
take care,
Brian
_________________ All posted information copyright Brian Ravnaas
and, Eric, to reply to one more point in your post that i didn't previously:
"When, where, what, should you only look at fundamental axials?
What an incredible superficial answer.
Or did you just like the sound of it? "
read my comment again, and note that i meant look at the fundamental axial modes, where other modes are not nearby for purposes of trying to divine of the frequency of modes aligned with the predicted frequencies.
_________________ All posted information copyright Brian Ravnaas
Technical Director, Audio Alloy
Rod Gervais Moderator
Joined: Jun 8, 2003
Posts: 3275
Location: Central Village, CT
and if i was party #131 (prime) touting wall effects causing this shift, i wouldn't be so quick to look for a theory to explain what is just plain & simple a wildly complex situation. i'm not sold that things aren't going according to plan, so to speak.
I think you should re-read my posts........ I am not touting wall effects causing anything - although I have stated that I can see how this could effect results - as would speaker and mic placement.......... they absolutely have to be a part of the equation -
I am (in fact) in agreement with you that this is a wildly complex situation - and I also believe that presenting anything like a simple mode calculation (to a non-acoustician) is probably a waste of time and energy............
I believe that all of the various mathematical equations are very usefull tools in the right hands........... and those hands are the one's that are attached to the body of a person trained to not only understand the math - but to also understand the probabilities........... to understand those complexities that go along with all of this.
The question was raised "why use mode calculators at all" - and my belief is that with 99.999999% of the people who come here looking for information (by this I mean people who want to put together a home studio and make/record music) that this is a waste of time and energy.
Let's just look at their available space - tell them how to get the best bang for the buck - and walk away from it.
The remaining .000001% of the people - who actually want to invest the time and energy to understand acoustics can visit places like Studio-tips and learn all the theory they want.
And for professional studios - they are not being served through this process - and should be hiring professionals to design their studios.
So my suggestion is not that mode calculators are useless - just that they are useless in this venue.
Once again - as I always try to tell my questioners, stick with the tried and true - if you want to experiment - it might work - and it might not.......... you takes your chances.
Sincerely,
Rod
_________________ Rod Gervais
Acoustics Moderator Sometimes - late at night..... when the wind whips
through the trees........ and the moon shines bright in my
face......... I think deep thoughts.......... and my head hurts.
eric_desart Recording Org Pro Audio Forums
Joined: May 23, 2003
Posts: 816
Location: Belgium Antwerp
1) My picture you re-entered I entered just to show how poor this data is and to combine the data offered in this thread, to have at least a bit of overview.
2) Second, ONLY by accident by a comment of Bob it's now confirmed by Ethan that one of those graphs is in fact from his living and not related to the test lab.
3) This data shows that the measurements are made by someone who knows where the button is to start a measurement, but has little idea what to look for.
And this around 80-85 Hz is where Ethan measured in a nul of a tangential and the 157 Hz are both 95 % sure tangential.
But AGAIN, before Ethan uses this """measurement""" data to prove there aren't tangentials it can be checked.
That's what measuring is about. Not to create pictures.
One can play with reversed polarity of the speakers, position etc.
And in this thread I also stated that this data is hardly proof for whatever.
I should be ashamed to use such combination of measurements for whatever point I wanted to proof, from which one now proves to be from the living instead of the test lab.
But the Tangentials ARE there. The obvious ones, also shown by other simulations I made.
And look at the waterfall picture too.
party #256 (power of 2, beat that)'s 8:30 check in
Rod,
i think i had thought your thoughts were similar to mine, i didn't mean to present my version as conflictive. I would 1000% agree with the thought that one should measure first, ponder later.
i guess if i had any argument (and this, again, isn't really an argument with your posts) it would be that perhaps one doesn't need an explanation for why a dip is here or there, when things are such a stew of variable upon variable (overlapping modes, objects in the room, absorption, reflection/cancellation, this wall effect...)
Eric,
it's always a pleasure catching you in a thought exchange, never such a pleasure when things must get personal.
it seems from your last post perhaps we are in agreement that to conclude for either "case" here is shaky based upon a few measurements, and perhaps (?) you'd agree that this should be extended to lining up a blip on the spreadsheet map with a blip on the room measurement as well as for cases when the blips don't line up.
as for a null at the higher frequencies being 95% sure, i stand politely, but firmly unconvinced that anything with so many overlapping factors is sure.
and i again offer anybody a chance to prove that my assertion is wrong, i'm a pretty reasonable chap at heart.
take care Eric,
Brian
_________________ All posted information copyright Brian Ravnaas
Technical Director, Audio Alloy
David French Respected Past Moderator
Joined: Jun 19, 2002
Posts: 2852
Location: Indiana
3 room modes within a hz or two or four... do we know they are all 3 nulled at this point in the room, hence explaining the dip in the graph? or do we know that they are all 3 peaked? or that their sum should create this null at this given point in this given room with this given speaker location, etc.?
if you respond "yes" to that, i'd ask that you substantiate it and not just say "yes" and let your expertise in this area be the proof.
I thought the jist of this thread was to
a) measure the room (ETF5) to get peaks and dips
b) run a modal calc to get predicted frequencies
c) compare them
At this point, even if the frequencies match between (a) and (b) you don't know that the measured peaks and dips are in fact what the prediction says they are. It's merely darn likely at this point.
This is the point that David French has gotten to with his nice bit of work in the related thread he just mentioned. (Good show David!)
But, if the prediction says that peak is axial (1,0,0) then what you do next is
d) fill the two small walls with 4" of absorbtion spaced 4" off the wall (I call this the Paul Woodlock damit-I-want-my-speakers-right-here method). EOR change the phase and/or position of speakers/microphone into nulls at that mode which is cheaper and probably faster.
e) measure the room (ETF5) to get peaks and dips
and if that peak/dip disappears then you know that it's the predicted axial.
In other words, the room modal calculation is there to save you time in predicting where to place treatment to get rid of a troublesome mode.
There are three sets of surfaces. Sure every time you have any kind of a problem one could cover all six surfaces like an anechoic chamber and you'd be likely to treat the mysterious unknown mode. But with room modal prediction at least which room surface pair you likely have to treat is identified, saving you 2/3 of the treatment.
The alternative is to try to follow the sound around the room -- which I think means that you have to use an infinite number of gate times at an infinite number of microphone positions and then start comparing graphs -- which might take a while.
In otherwords, room modal prediction software is a tool to assist. And considering how long it takes to run, even if it's off a bit, it's worth the bother.
Quote:
one should measure first, ponder later
A room mode calculator is a ponder aid
z60611 Recording Org Pro Audio Forums
Joined: Dec 29, 2003
Posts: 820
Location: Ontario, Canada
Obviously I've been rather pro calculators in this thread.
Nevertheless I also note that I've never been able to get room mode spreadsheets and RPG Room Sizer and RPG Room Optimizer to agree on a room dimensions. i.e.
(room mode spreadsheets vs Room Sizer) = different rooms are best
(Room mode spreadsheets vs Room Optimizer) = different rooms are best
(Room Sizer vs Room Optimizer) = different rooms are best
Yet my belief is still that rules are:
for the before you build, try to predict the room first -- but build it for your needs, a bigger room is usually better than a 'perfect' shape. And if needs say a given shape, build it anyway and treat it later.
And for the after you build, someone I know had great success using Room Optimizer to place thier subwoofer and listening position.
Beyond that, room modes calculators can help the pondering.
Novice stereo option: don't precalc nor postcalc, just use four corner treatment and first reflections and be done with it.
Ethan Winer Respected Past Moderator
Joined: Mar 19, 2001
Posts: 3209
Location: New Milford, CT USA
> "If a room surface absorbs sound ... This explains why, very often, it will be found that the measured resonances do not exactly correspond with the calculated ones." <
Yes, but when walls absorb I'm pretty sure they shift the frequencies down, not up as appears to be the case in all the test rooms that measured different than predicted. This point was made early on.
But, if the prediction says that peak is axial (1,0,0) then what you do next is
d) fill the two small walls with 4" of absorbtion spaced 4" off the wall (I call this the Paul Woodlock damit-I-want-my-speakers-right-here method). EOR change the phase and/or position of speakers/microphone into nulls at that mode which is cheaper and probably faster.
e) measure the room (ETF5) to get peaks and dips
....
hehe
Being soffit mounted, putting the sound source at the room boundary, my speakers will excite ALL Modes right?!
3 room modes within a hz or two or four... do we know they are all 3 nulled at this point in the room, hence explaining the dip in the graph? or do we know that they are all 3 peaked? or that their sum should create this null at this given point in this given room with this given speaker location, etc.?
if you respond "yes" to that, i'd ask that you substantiate it and not just say "yes" and let your expertise in this area be the proof.
I thought the jist of this thread was to
a) measure the room (ETF5) to get peaks and dips
b) run a modal calc to get predicted frequencies
c) compare them
At this point, even if the frequencies match between (a) and (b) you don't know that the measured peaks and dips are in fact what the prediction says they are. It's merely darn likely at this point.
This is the point that David French has gotten to with his nice bit of work in the related thread he just mentioned. (Good show David!)
But, if the prediction says that peak is axial (1,0,0) then what you do next is
d) fill the two small walls with 4" of absorbtion spaced 4" off the wall (I call this the Paul Woodlock damit-I-want-my-speakers-right-here method). EOR change the phase and/or position of speakers/microphone into nulls at that mode which is cheaper and probably faster.
e) measure the room (ETF5) to get peaks and dips
and if that peak/dip disappears then you know that it's the predicted axial.
In other words, the room modal calculation is there to save you time in predicting where to place treatment to get rid of a troublesome mode.
There are three sets of surfaces. Sure every time you have any kind of a problem one could cover all six surfaces like an anechoic chamber and you'd be likely to treat the mysterious unknown mode. But with room modal prediction at least which room surface pair you likely have to treat is identified, saving you 2/3 of the treatment.
The alternative is to try to follow the sound around the room -- which I think means that you have to use an infinite number of gate times at an infinite number of microphone positions and then start comparing graphs -- which might take a while.
In otherwords, room modal prediction software is a tool to assist. And considering how long it takes to run, even if it's off a bit, it's worth the bother.
Quote:
one should measure first, ponder later
A room mode calculator is a ponder aid
hi Z, i'm not sure what thoughts i can offer to that, but your hypothetical experiment process is about what i was trying to offer as (sorry if this is the wrong word) necessary if you want to be 105% sure that what you see as a blip on some graph is, in fact, due to X, Y, Z or some combination thereof.
caution in conclusion is usually prudent would be a nice 6 word sum of my posts yesterday, i guess.
but i think i have to wander back to the world of walls and TL, which are roughly 1,089.4 (not prime) times more intriguing, and away from this fallout zone in which i seem to have a unique ability to cause a ruckus...
_________________ All posted information copyright Brian Ravnaas
Technical Director, Audio Alloy
z60611 Recording Org Pro Audio Forums
Joined: Dec 29, 2003
Posts: 820
Location: Ontario, Canada
Looking up references to answer a question posed over at Ethan's EQ forum, I happened to take a fresh look at Gilford's 1979 AES paper, The Acoustic Design of Talks Studios and Listening Room. (This paper was originally presented for the Radio and Telecommunication Section of the IEEE.) I wanted to quote a few sections that have relevance here:
Gilford wrote:
There will be room modes...in all parts of the audible spectrum, but whether or not they will be appreciable as colorations depends on the following factors:
(1) the bandwidth of the mode
(2) the degree of excitation of the mode
(3) its separation from neighboring strongly excited modes
(4) the positions of the sound source and microphone with respect to standing-wave systems
(5) the frequency content of the source.
What's interesting is that the above was detailed in Section 2.1, Formation of Simple Modes in Small Rooms, that discussed only axial modes.
He goes on (from Section 2.3 The Relative Importance of Axial, Tangential, and Oblique Modes):
Gilford wrote:
For the case of a typical small studio...calculation shows that no frequency is likely to become prominent unless it (has) a high early intensity and a long decay. This condition* is satisfied only by the axial modes, which are therefore the only ones likely to become individually significant. An exception to this rule is that a few tangential or oblique modes of low frequency may possibly be audible, owing to their high initial intensities or wide spacings.
Emphasis on the last sentence was added by me.
{*Gilford is explaining an image-source method of calculation. I omitted references to it to keep things simple here.}
Finally, I believe I may have discovered another alternative for Ethan to consider for his axial mode calculator:
Gilford (context: from Section 3.4 Design Precautions for Avoidance of Colorations) wrote:
A simple calculation...enables a list of all the axial modes for all three dimensions to be written down in order of frequency. It will be unnecessary to continue the list beyond, say, 350 Hz because...the axial modes in a well-designed talks studio will not be prominent above that frequency. The list must next be examined to find modes, or groups of modes with almost the same frequency, which are separated from their nearest neighbors on either side by intervals appreciably larger than their bandwidths. In practice the minimum separation for audibility appears to be about 20 Hz.
Modes or groups separated from their neighbors by greater intervals than this should be noted, and...attempts should be made to alter the groupings by changes in the proposed room dimensions.
IMO, the above gives Ethan the option of adding some sort of mode spacing evaluator. Spacings wider than 20 Hz can be called to the attention of the user and steps can be taken then to change the "proposed room dimensions" to give a better spread of axial modes.
The science makes sense. And it takes the "need" for calculating "non-axials" out of the picture.
Whaddyathink, Ethan?
{Note: Posting same message in other thread.}
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