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Forum index » Science and Technology » Physics » Acoustics
reverb in long halls / corridors
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vfunc@talktalk.net
science forum beginner


Joined: 26 Apr 2006
Posts: 4

PostPosted: Fri May 05, 2006 6:50 am    Post subject: reverb in long halls / corridors Reply with quote

I am I right in thinking that sound is like pushed in a long hall /
corridor ? The reverberations back and forth form lots of wave fronts
? How much sound can travel around a corner of a corridor ? How can
this spread of the original sound be calculated ?
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Savant
science forum beginner


Joined: 07 Jun 2005
Posts: 13

PostPosted: Fri May 05, 2006 10:54 pm    Post subject: Re: reverb in long halls / corridors Reply with quote

A good choice for prediction is the Fitzroy equation when you're
talking about long & narrow. There has also been some discussion in
this forum on the Arau-Puschades (I probably butchered the spelling on
that - my apologies if I did!) equation, though I've never used it.

Regards,

Savant

vfunc@talktalk.net wrote:
Quote:
I am I right in thinking that sound is like pushed in a long hall /
corridor ? The reverberations back and forth form lots of wave fronts
? How much sound can travel around a corner of a corridor ? How can
this spread of the original sound be calculated ?
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bert stoltenborg
science forum addict


Joined: 16 May 2005
Posts: 52

PostPosted: Fri May 05, 2006 11:03 pm    Post subject: Re: reverb in long halls / corridors Reply with quote

Eric likes the Arau approach, AFAIK, Jeff.

The question is a bit unclear.
It depends on the abosrption in the corridor, the dimensions, it's
frequency dependent as you have a Schroeder frequency etc. You have to
give more data.
And even then rooms with strange shapes are difficult to predict Smile
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Chris Whealy
science forum addict


Joined: 05 May 2005
Posts: 54

PostPosted: Mon May 08, 2006 10:11 am    Post subject: Re: reverb in long halls / corridors Reply with quote

Savant wrote:
Quote:
A good choice for prediction is the Fitzroy equation when you're
talking about long & narrow.

The Fitzroy equation accounts for what is known as "on-axis absorption".
In other words, Fitzroy assumed that a room's absorption would not be
evenly distributed on the walls, floor and ceiling (as Sabine originally
did). So, using his equation you can derive an X, Y and Z reverberation
time for each of the three axes in a room. This is useful in two
situations (that I can think of off the top of my head):

1) For rooms where the dimensions are similar, Fitzroy's equation is
able to predict flutter echo (one reverb time will be significantly
longer that the other two).
2) For understanding the reverberation in room where one dimension is
significantly larger than the other two. E.G. A corridor.

London Underground has this problem where the Tube stations are the
length of a train, but only about 8m wide/high. There are now two
distinct reverberation times - one for the two short dimensions, and
another much longer time for the long axis. This causes some nasty
problems with speech intelligibility during PA announcements.

The same principles will exist in a corridor, though on a smaller scale.

Reinhard Neubauer has modified Fitzroy's equation and introduced another
term to account for what he describes as the "almost 2 dimensional sound
field".

Quote:
There has also been some discussion in
this forum on the Arau-Puschades (I probably butchered the spelling on
that - my apologies if I did!) equation, though I've never used it.

This is the formula was developed by the Spanish acoustician Higini
Arau-Puchades. He has taken the same approach as Fitzroy, in that he
accounts for unequal absorbency in the X, Y and Z planes, but his
calculation uses a different approach.

My Control Room Calculator spreadsheet uses all the above formulae for
calculating the RT60 of a room.

You can down load this from my (temporary) website at
http://www.bobgolds.com/whealy.com/acoustics/ControlRoom.html

Regards

Chris W

--
The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.
---
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bert stoltenborg
science forum addict


Joined: 16 May 2005
Posts: 52

PostPosted: Mon May 08, 2006 12:38 pm    Post subject: Re: reverb in long halls / corridors Reply with quote

He Chris,

Not totally on topic, but maybe interesting:
I know these problems have been under investigation by a dutch speaker
company. They did tests and suggested to use High DI line-arrays using
ribbon speakers radiating sound in an open baffle, so figure of 8
radiation.
I guess this solution was to expensive :-)

Bert
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Savant
science forum beginner


Joined: 07 Jun 2005
Posts: 13

PostPosted: Wed May 10, 2006 2:09 am    Post subject: Re: reverb in long halls / corridors Reply with quote

Hey Chris! Good to see you here. I appreciate all the tools you've
made available these last few years. You're a true acoustical saint!
:-)

Chris Whealy wrote:
Quote:
1) For rooms where the dimensions are similar, Fitzroy's equation is
able to predict flutter echo (one reverb time will be significantly
longer that the other two).

I am not sure I understand what you're getting at here. A flutter echo
is a transient wave phenomenon dependent on the distance between two or
more surfaces. Do you mean to say the Fitzroy equation will *imply* a
flutter echo by calculating a higher RT for one direction when the
absorption is concentrated in the other(s)? Or do you mean to say the
flutter echo itself would somehow be predicted mathematically?

Quote:
The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.

Sorry for being so loud! :-D

---Savant---
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Chris Whealy
science forum addict


Joined: 05 May 2005
Posts: 54

PostPosted: Wed May 10, 2006 7:39 am    Post subject: Re: reverb in long halls / corridors Reply with quote

Savant wrote:
Quote:
Hey Chris! Good to see you here. I appreciate all the tools you've
made available these last few years. You're a true acoustical saint!
Smile

Aww gee, I've gone all shy....

(Adjusts halo) :-P

Quote:
I am not sure I understand what you're getting at here. A flutter echo
is a transient wave phenomenon dependent on the distance between two or
more surfaces. Do you mean to say the Fitzroy equation will *imply* a
flutter echo by calculating a higher RT for one direction when the
absorption is concentrated in the other(s)?

Yes. On my Control Room Calculator spreadsheet (sheet "Initial
Values"), there's an RT60 graph that shows the on-axis reverberation
time using Fitzroy's equation.

If this graph shows that the reverberation time on one particular axis
will be greater than the other two axes, then this should be recognised
as a warning sign that a flutter echo will probably occur.

Quote:
Or do you mean to say the flutter echo itself would somehow be predicted mathematically?

Well that's sort of what will happen given the explanation above. It
would be more accurate to say that the on-axis reverberation times given
by Fitzroy's equation can be used as early warning indicators of the
likelihood of a flutter echo.

Without performing further calculations, I don't think the word
"predict" is helpful here.

I haven't examined this point in much detail, but I wonder what time
difference there needs to be for the RT on one axis to be perceived as a
flutter. 10% longer, 20% longer?

Answers on a post card...

Quote:
The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.

Sorry for being so loud! :-D


Sorry for being so quiet!! :-P

Chris W

--
The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.
---
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Savant
science forum beginner


Joined: 07 Jun 2005
Posts: 13

PostPosted: Fri May 12, 2006 1:11 am    Post subject: Re: reverb in long halls / corridors Reply with quote

Chris,

I understand. And that is a very good application of Fitzroy's
equation. Was it in his original paper? (It's been a long time since I
read it.) Thanks for clarifying.

Also...how best to put this? I think flutter and RT are distant
acoustical cousins, but not siblings. Before I start getting to
philisophical on the subject, I should probably refer you to Maa's JASA
paper, "The Flutter Echoes" (October 1941). In it, he gives the
equation(s) to calculate flutter echoes. You might also check out this
thread, if you haven't already seen it:

http://forum.studiotips.com/viewtopic.php?t=1441&highlight=flutter+echo

Thanks again, Chris! :-)

---Savant---
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Chris Whealy
science forum addict


Joined: 05 May 2005
Posts: 54

PostPosted: Fri May 12, 2006 9:12 am    Post subject: Re: reverb in long halls / corridors Reply with quote

Savant wrote:
Quote:
I understand. And that is a very good application of Fitzroy's
equation. Was it in his original paper? (It's been a long time since I
read it.) Thanks for clarifying.

To be honest, I don't know. I haven't read read Fitzroy's original
paper. However, I have read Reinhard Neubauer's comments on Fitzroy's
equations and also how he believes the original equation can be improved.

See http://www.ib-neubauer.com/Literatur/ISSEM_99_Gdansk.pdf for details

Quote:
Also...how best to put this? I think flutter and RT are distant
acoustical cousins, but not siblings. Before I start getting to
philisophical on the subject, I should probably refer you to Maa's JASA
paper, "The Flutter Echoes" (October 1941). In it, he gives the
equation(s) to calculate flutter echoes.

I've just downloaded this article, but haven't read it yet. It looks
like something I can get my teeth into!

Quote:
You might also check out this thread, if you haven't already seen it:

http://forum.studiotips.com/viewtopic.php?t=1441&highlight=flutter+echo

Ok, I've pulled off the references quoted in the thread and will take a
look at them when I get some time (maybe in the next year or so!! :-O )

Chris W

--
The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.
---
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