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Can someone help me understand this please?
Old 10th September 2012
  #1
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🎧 10 years
Can someone help me understand this please?

Hi,

Sorry for posting another thread related to theory, but I'm struggling with my Acoustics course and I don't know any other Audio Engineers besides my future self :(.

This is for a major report analysis, and I'm having trouble with the following:
Dimensions= L=2.6 W=2.06 H=2.63 (metres)

1- Please view the attached image 'All Modes plotted';
-What does the red line represent? is it the cutoff frequency (Schroeder freq), or the Center frequency? it's is labelled "f(c)" on a sample spreadsheet we were provided with.
-How do I calculate the 'Center frequency' of a room? and why is this important when looking at room modes?

2- Please view attached image 'RT'
-After calculating the 'Total Absorption' (41.6 Sabins), my room's 'Reverberation Time' calculation gives me a total of 0.05 sec... I feel that this is wrong since it's so low and therefore gives me a cutoff frequency of 124.4Hz. Do those results sound reasonable?

3- At which point exactly do I stop taking Standing waves/Modes into consideration for analysis? I know there's an average 300Hz for small rooms and 200Hz for larger rooms since beyond that point the modes have very little effect on audibility, but is there a calculation to determine an exact figure?

Sorry for the long post.. I'll greatly appreciate any help!

Cheers

Can someone help me understand this please?-all-modes-plot.jpg

Can someone help me understand this please?-rt.jpg
Old 10th September 2012
  #2
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🎧 15 years
Words

Hi Sebs, it would be useful to know your location. The US spelling of Center as opposed to the English Centre suggests something but maybe not.
Location might help in tracking down what this Center Frequency you speak of is.
I have not seen it used as an alternative to Schroeder Frequency.
I have seen it refer to the centre of a band of resonance. The peak.
Cut off may be another source of confusion. The number of reflections increases with frequency gradually, I wouldn't expect to see any sudden change, either in graphs or audibly. Schroeder transition region might be a better term.
Reverb calculations are based on a Diffuse sound field. It takes great size to be diffuse at low frequencies. So don't expect any accuracy from large room/auditorium formula in your little shoebox!

DD
Old 10th September 2012
  #3
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🎧 10 years
if you check out John Brandt's mode calculator, he has a nice graphic depicting the salient ranges. http://www.jhbrandt.net/RoomModeCalculatorSAE.xls
Attached Thumbnails
Can someone help me understand this please?-clipboard01.jpg  
Old 11th September 2012
  #4
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🎧 10 years
Thanks!
I'm in Australia and it's spelt 'centre freq' here, however I've finally figured out what the red line represents. Red line is the 'Critical Frequency' and it is calculated with the following equation:

f(c)=1.5*c/MFP

-1.5*Speed of Sound/Mean Free Path
-1.5*344/1.6m =322Hz (in my particular case)

This is an 'alternative to Schroeder frequency', however I've spent hours looking for such equation so that I can reference it but I can't find ANYTHING!

Who came up with this alternative equation and what does the 1.5 constant represent?
If my Schroeder frequency result is: 124.4Hz and this alternate result ^ is 322Hz, which is more suitable to go by for a room with these small dimensions?
Old 11th September 2012
  #5
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Nose

Interesting questions, and I for one am everything but 'sorry' to see them.
Here's a lead. Acoustics Forum • View topic - Critical frequency- Small rooms
DD
Old 11th September 2012
  #6
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akebrake's Avatar
 
🎧 5 years
Quote:
Originally Posted by sebs ➑️
...
-What does the red line represent? is it the cutoff frequency (Schroeder freq), or the Center frequency? it's is labelled "f(c)" on a sample spreadsheet we were provided with...
What literature are you supposed to have studied before this task? heh

Just wondering 'cause different authors are naming Schroeder frequencies (bolded mine) differently.

Cheers

Old 11th September 2012
  #7
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Definition

It does seem odd that people are using different words for essentially the same thing. However, I can see why. We know that Schroeder and Reverb formulae only work well in diffuse fields, which do not exist in small rooms.
In this study, http://journal.telfor.rs/Published/No4/No04_P06_fin.pdf the authors come up with a more accurate approach to small rooms. Their smallest room result shows a +/- 50% variation from the classical predictions.
So, IMO, the variety of terms are various attempts to describe, tweak, deal with, warn, the fact that prediction in small rooms using large room formulae is pretty much a waste of time.

DD
Old 11th September 2012 | Show parent
  #8
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avare's Avatar
 
🎧 15 years
Quote:
Originally Posted by sebs ➑️
Thanks!
I'm in Australia and it's spelt 'centre freq' here, however I've finally figured out what the red line represents. Red line is the 'Critical Frequency' and it is calculated with the following equation:

f(c)=1.5*c/MFP

-1.5*Speed of Sound/Mean Free Path
-1.5*344/1.6m =322Hz (in my particular case)

This is an 'alternative to Schroeder frequency', however I've spent hours looking for such equation so that I can reference it but I can't find ANYTHING!
The bolded part makes no sense. The Schreoder frequency is based on modal bandwith and overlap. The equation you wroe does not address that at all.

Andre
Old 12th September 2012
  #9
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🎧 10 years
I understand this is probably a useless calculation in this scenario, however I must do it regardless for the purpose of a comparative report that looks at theoretical modal analysis, and practical IR measurement using Fuzz Measure Pro.

Thanks for the great sources DD! Very informative

Quote:
The bolded part makes no sense. The Schreoder frequency is based on modal bandwith and overlap. The equation you wroe does not address that at all.
The equation I wrote is simply one that was used by our lecturer to estimate the cutoff frequency, since at that point in the analysis process we shouldn't have the Reverberation Decay time of the room to calculate Schroeder's method.
These two approaches are discussed in the first link provided by DD.

So, what exactly doesn't make sense? I'm here to learn so please be more constructive with your posts, instead of 'that doesn't make sense' because that's useless to me, mate'.

By 'Schroeder frequency' I mean the estimated cutoff frequency which is given by the equation: f(c)=2000?RT60/V.
We are taught through a variety of different sources at SAE, though 'Master Handbook of Acoustics 5th Ed' is the one we focus our studies on.

Cheers!
Old 12th September 2012 | Show parent
  #10
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avare's Avatar
 
🎧 15 years
Quote:
Originally Posted by sebs ➑️
The equation I wrote is simply one that was used by our lecturer to estimate the cutoff frequency, since at that point in the analysis process we shouldn't have the Reverberation Decay time of the room to calculate Schroeder's method.
These two approaches are discussed in the first link provided by DD.

So, what exactly doesn't make sense? I'm here to learn so please be more constructive with your posts, instead of 'that doesn't make sense' because that's useless to me, mate'.
I am agreeing with your post #4! Without development, the equation makes no sense. For more on Shroeder Frequency, and room modes see the attached documents.

FYI at least onE highly respected acoustician considers the Shroeder equation and other simplifications, such as the Davis Frequency to be misealding because they do not take into account the effect of rooms with large dimensional ratios.

Good luck with your research!

Andre
Attached Files
File Type: pdf MS_Schroeder_Frequency_Pres.pdf (243.5 KB, 578 views) File Type: pdf RD 1992-08.pdf (1.87 MB, 310 views)
Old 12th September 2012
  #11
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Is this one of those questions posed by a professor in order to lead the student to the discovery that the analysis does not work in a small room?
Old 12th September 2012 | Show parent
  #12
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🎧 15 years
Quote:
Originally Posted by 2manyrocks ➑️
Is this one of those questions posed by a professor in order to lead the student to the discovery that the analysis does not work in a small room?
The OP started an interesting thread. The Shroeder Frequency et al are specifically for small rooms!

Andre
Old 12th September 2012
  #13
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akebrake's Avatar
 
🎧 5 years
More fuel

sebs,

this subject was discussed in a long thread on this Forum some time ago.

Hopefully some more light...

Best
Old 12th September 2012
  #14
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🎧 15 years
Revealed

Quote:
Is this one of those questions posed by a professor in order to lead the student to the discovery that the analysis does not work in a small room?
I reckon so. All those seemingly different words meaning pretty much the same thing......
Or maybe the lesson is that one needs to understand the limitations of such calculations.

DD
Old 13th September 2012
  #15
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🎧 10 years
Thanks for being so helpful and resourceful guys!

Avere, sorry for the misunderstanding & thanks for the files !

Apparently the formula used is just to find a 'critical frequency', at which beyond this point, the concept of wave acoustics is irrelevant. Since my research is focused on low-end response, this approximate 'critical frequency' is needed to illustrate the frequency point in which we stop considering the effects of modes.

To my understanding, that's exactly what the 'Schroeder frequency' or 'cutoff frequency' is measuring... However, this approach doesn't require you to calculate the reverberation time of the room.

I had never heard of this method before and i'm unsure of it's accuracy. It took me HOURS to find the damn thing, but I finally found a reference in 'Acoustics and Psychoacoustics' By David Howard, Jamie Angu.

I took a shot of it in case anyone was interested.


Can someone help me understand this please?-screen-shot-2012-09-13-12.24.10-pm.png





Cheers
Old 13th September 2012
  #16
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Sense

It makes sense to attempt analysis using factors other than RT.
However, like many LF formulae, I see nothing here to distinguish between acoustically hard and soft (at LF) boundaries. There is a very big difference between concrete and studded plasterboard or wooden walls. The latter can have very significant LF absorption. e.g. (from memory) Alpha of 0.3 in the 100Hz octave. Such a vast variation in LF absorption will greatly diminish the strength and length of modes. Changing everything.

DD
Old 13th September 2012
  #17
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I think it is a way of determining where in the spectrum interference is omnipresent. And going above that frequency you can find other discrete zones of interference.
Old 14th September 2012
  #18
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Hey!

It seems answers have been achieved, but I figured I'd jump in as I just did the acoustics subject through JMC Sydney and came across the "critical frequency" as well. And I think you'll find the critical frequency formula concept is just used as a basic introduction to room analysis, and creating a rough "turning point" based solely on the dimensions of the room. So that's where that red line/Fc comes from.

Also, on your RT calculations did you subtract the area of the door/window from the wall's total area?
Old 14th September 2012
  #19
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Oh! Don't worry, just noticed the different SA in your calculations, I retract my above question.
Old 14th September 2012 | Show parent
  #20
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Dange's Avatar
 
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Quote:
Originally Posted by sebs ➑️
It took me HOURS to find the damn thing
Isn't that kind of the point of study?

I've found that all that reading around would have been useful because you're bound to have covered something that's useful in the future

I do share your frustration though. I found that academic course material is normally based on a few sources so you can use that fact to narrow your search.

The lecture notes might have sources listed but might not so you have to read through things. What they should provide is a reading list, generally the info can be found from these sources or the sources the reading list items use.... if that makes sense?

Room Acoustics by Kutruff would be a good source to look at.... although you'd most likely want to get this from a library as it's costly to buy ~Β£100
Old 14th September 2012 | Show parent
  #21
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avare's Avatar
 
🎧 15 years
Quote:
Originally Posted by Matt0o1 ➑️
Hey!

It seems answers have been achieved, but I figured I'd jump in as I just did the acoustics subject through JMC Sydney and came across the "critical frequency" as well. And I think you'll find the critical frequency formula concept is just used as a basic introduction to room analysis, and creating a rough "turning point" based solely on the dimensions of the room. So that's where that red line/Fc comes from.
+1!

Andre
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