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Room Modes / Comb Filtering / Flutter Echoes
Old 25th May 2012
  #1
Gear interested
 

Thread Starter
Room Modes / Comb Filtering / Flutter Echoes

I have two questions. The first:

If I understand correctly, both room modes and comb filtering are caused by constructive/destructive interference as a result of room dimensions and wavelengths. Are they not the result of the same phenomenon? I could be totally wrong, I am getting a little confused here.

Secondly. I understand so far that flutter echoes are the result of interfering reflections too. Can they be understood as a result of / a facet of comb filtering? Or is comb filtering strictly a description of the comb like waveform which results from a particular type of phase interference? Are flutter echoes another phenomenon entirely?

Also, thanks to all the contributors on this forum, I have read (and I think) learned a lot so far. Any answers to my questions much appreciated.
Old 25th May 2012
  #2
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boggy's Avatar
Quote:
Originally Posted by Archipelagos View Post
I have two questions. The first:

If I understand correctly, both room modes and comb filtering are caused by constructive/destructive interference as a result of room dimensions and wavelengths. Are they not the result of the same phenomenon? I could be totally wrong, I am getting a little confused here.
Yes, they are both acoustical interferences caused by boundaries, where "room modes" are resonant, and comb filtering, SBIR, etc. aren't (resonant).
Quote:
Originally Posted by Archipelagos View Post
Secondly. I understand so far that flutter echoes are the result of interfering reflections too. Can they be understood as a result of / a facet of comb filtering? Or is comb filtering strictly a description of the comb like waveform which results from a particular type of phase interference? Are flutter echoes another phenomenon entirely?
Flutter echo is again resonant acoustical interference between parallel boundaries in the room, which characteristics are higher working frequency (than similar interferences we called room modes) and noticeable longer decay times.
Comb filtering isn't resonant acoustical interference.

Cheers
Old 25th May 2012
  #3
Gear Guru
 
DanDan's Avatar
Repeating

IMHO, It would be useful to have your location in your visible profile.
(Language, local materials and climate)
Just for clarity, BIR is generally viewed as one reflection and it's effects.
Of course the reflection can travel to the opposing boundary and get reflected itself. If there is little absorption at the boundaries or in the space between, this ping pong can go on for quite a while. One sees decays of the order of
1- 2S in small untreated rooms.

So in simple terms, resonant=repeating.

Flutter echo is most audible on transients or impulsive sounds. A handclap in a space between two parallel walls is a classic example. In untreated rooms there can be so many reflections going on that flutter is inaudible. Introducing treatment can reveal it.

DD
Old 25th May 2012
  #4
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
Just for clarity, BIR is generally viewed as one reflection and it's effects.
So any reflection after the first arriving, is not considered SBIR or what? In order to understand SBIR and its effects, you need to take into account all contributing reflections or the prediction (in terms of fc of cancellation etc.) will be incorrect. Needless to say, it´s hard to predict SBIR unless simple (non-complex impedance) boundary conditions (not often the case).

https://www.gearslutz.com/board/7443518-post250.html
https://www.gearslutz.com/board/7444136-post254.html
Old 25th May 2012
  #5
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DanDan's Avatar
I predict a Riot

Quote:
BIR is generally viewed as one reflection and it's effects.
generally - definition of generally by the Free Online Dictionary, Thesaurus and Encyclopedia.

I will assume understanding of the preceding phrase 'Just for clarity' Although I could bold JUST for the obstinately obtuse.

Predicting? Work away, but that is not in the discussion I am having with the OP and Boggy.

Although all boundaries contribute in some small way to everything everywhere across the millennia, specific BIR nulls can be associated more strongly with specific single boundaries. Usefully.
This is clearly shown by changes of frequency specific to particular changes of direction. e.g. the vivid example of BWBIR here

http://recording.org/studio-construc...en-matter.html

DD
Old 25th May 2012
  #6
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
generally - definition of generally by the Free Online Dictionary, Thesaurus and Encyclopedia.

I will assume understanding of the preceding phrase 'Just for clarity' Although I could bold JUST for the obstinately obtuse.

Predicting? Work away, but that is not in the discussion I am having with the OP and Boggy.

Although all boundaries contribute in some small way to everything everywhere across the millennia, specific BIR nulls can be associated more strongly with specific single boundaries. Usefully.
This is clearly shown by changes of frequency specific to particular changes of direction. e.g. the vivid example of BWBIR here

http://recording.org/studio-construc...en-matter.html

DD
You are free to believe whatever you want to believe ...


EDIT:

Using your own logic; reflections from nearby boundaries (assuming global boundary condition) should affect the total response more than for example the rear wall since they are stronger in level (and which they naturally do unless absorbed, but the effect might not be very troublesome due the long wavelength that might sums them more or less in phase in some situations). Regardless, you need to include all reflections that contribute to make a qualified estimation of cancelation etc. You simplified view of SBIR might work to some extent in some situations, but to draw general conclusions form them is disastrous.


EDIT 2:

And just to be extra clear; you cannot use one single example of a room with unknown boundary conditions to prove anything.
Old 25th May 2012
  #7
Gear Guru
 
DanDan's Avatar
Fact

I am not predicting or making a qualified estimation. I am referring to events that occur daily in real life.
In two different tests in different rooms. Furthermore we see it every time we measure and move speakers or listener position.

Here we have a null which drops frequency with distance from the Front Wall.
One could suspect a link. Cause and effect.
A trap was placed between the boundary and speaker, blocking the suspected cause. The effect disappeared.
I would have difficulty not believing the obvious here.
Quote:
Room Modes / Comb Filtering / Flutter Echoes-screen-shot-2012-05-25-16.25.36.jpg
I believe it was Hannes who suggested that FWBIR should, by level, have a strong effect on Frequency Response, while BWBIR should be more effective on modes. Logically, I went along with this.
Until I saw Ethan's test and repeated it myself.

In both tests, FWBIR and BWBIR were easily singled out by associating the nulls directly with distance. But BWBIR seems to have an unexpectedly very large influence.
I have yet to see a hypothesis as to why this is occurring.
I would welcome such, in the appropriate thread.
SBIR

DD
Old 25th May 2012
  #8
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Dange's Avatar
 

OP's questions in this thread were answered by Boggy in the second post. Let's just leave it as it is shall we and wait for OP to respond? The later posts are moving further away from being useful to the OP on an exponential curve
Old 26th May 2012
  #9
Gear Guru
 
DanDan's Avatar
Clean

Thanks for cleaning that up Jay.
I see no need for the two extraneous posts, maybe the posters would oblige?
I think it worth repeating that any or all of that OT stuff could/should have been done in PM.

Speaking of topic. Here's a neat little calculator which calculates likely BIR nulls. Like any of these calculators it presumes idealised conditions.
Useful nevertheless. SBIR & Floor Bounce calculator
Another one.
Thomas BareFoot has a 2D Wall Bounce Calculator.
John Sayers' Recording Studio Design Forum • View topic - Wall Bounce Calculator 2D

DD
Old 26th May 2012
  #10
Gear interested
 

Thread Starter
Thanks to everyone for their replies. This really helps clear things up for me.

Best,
A.
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