Can someone please help identify the problem I'm having? I'm wondering
if I'm experiencing Comb Filtering, Room Mode Resonance, or Flutter
Echo. Can someone tell just by listening to these sound waves what is
happening? I'm too inexperienced to tell the difference between them.

The sound wave samples are he
http://www.geocities.com/lex888888/index.htm

I posted some samples of the problem there in .mp3 format. You might
need to listen through some good headphones or monitors to hear the
problem. The problem is very audible in AKG K-501's as buzzing and
amplification. Yorkville Monitors produce the same with a little less
buzzing.

I have an L shaped room caused by a closet on the right 12' section of
wall. I leave this open while recording to help scatter the sound. I
also have a small table with a few objects on it and some records
beneath it to scatter some sound around. This is situated against the
middle of
the left 12' section.


The microphone is at the bottom 10' section of the room facing in
towards the room. So it's facing down the long length of the room at
my mixing desk(diffuser sort of).


I have 4 MiniTraps(Fiber Glass panels) across the corners of the rooms
where the walls meet, and 4 where the wall meets the ceiling. These
are all spanning the corner and centered. I have one micro trap
directly over the mics and to either side of my recording position, on
stands by the walls. My mixing desk with books and cds on it is at the
top of the room, 10' section.


I'm not sure what this problem is exactly. It looks and sounds to me
to be what is described as comb filtering. My waves are very jagged
looking. I'm not sure if it's that or room modes causing extra
resonance at certain frequencies. Or perhaps this is low freq. flutter
echoes.


I don't really know. I don't have enough experience to really diagnose
it. I have two sets of headphones. One is an AKG K-501. The other is an
AKG K240-DF. The distortion is really obvious in the K-501(horrid
buzzing sound) but hardly noticeable with the
K240-DF. The sound from the monitors sounds like amplification and
buzzing on the worse
frequencies.


I thought it might be the soundcard. I set the sample rate to 96k and
32 bit. I converted to 44.1k and 16bit for the samples. I recorded at
about 18'' away with both my Aardvark DirectPro
24/96 and my Turtle Beach Santa Cruz. I get the same problem at longer
distances and lower gain settings. They both show the same problems.
I also tried listening to the playback with both cards. The Aardvark
shows the problem more clearly. The problem appears in both guitar
mics and my vocal mic.


Listening to professionally recorded music is fine, no distortion.
Taking all this into account, I'm guessing it's a room problem.

"lex" wrote in message
oups.com...

Can someone please help identify the problem I'm having? I'm wondering
if I'm experiencing Comb Filtering, Room Mode Resonance, or Flutter
Echo. Can someone tell just by listening to these sound waves what is
happening? I'm too inexperienced to tell the difference between them.

The sound wave samples are he
http://www.geocities.com/lex888888/index.htm

I listened & I can't tell from the files you posted. Sorry, I know that
doesn't do you any good.

I wonder if any one CAN tell from those files - they're fairly low level in
terms of how they'd make the room react, & therefore just simply don't seem
to have much room involvement. Man, someone's got some damn good ears if
they can tell what your problem is, based on just those examples.

Neil Henderson

Thanks for trying. I'm thinking these K-501's are highly sensitive and
therefore able to show the problem for what it is compared to the other
ones which just blend it away. Your headphones might be doing the same
thing.


Neil Henderson wrote:
"lex" wrote in message
oups.com...

Can someone please help identify the problem I'm having? I'm
wondering
if I'm experiencing Comb Filtering, Room Mode Resonance, or Flutter
Echo. Can someone tell just by listening to these sound waves what
is
happening? I'm too inexperienced to tell the difference between
them.

The sound wave samples are he
http://www.geocities.com/lex888888/index.htm

I listened & I can't tell from the files you posted. Sorry, I know
that
doesn't do you any good.

I wonder if any one CAN tell from those files - they're fairly low
level in
terms of how they'd make the room react, & therefore just simply
don't seem
to have much room involvement. Man, someone's got some damn good ears
if
they can tell what your problem is, based on just those examples.

Neil Henderson

lex wrote:
Can someone please help identify the problem I'm having? I'm wondering
if I'm experiencing Comb Filtering, Room Mode Resonance, or Flutter
Echo. Can someone tell just by listening to these sound waves what is
happening? I'm too inexperienced to tell the difference between them.

The sound wave samples are he
http://www.geocities.com/lex888888/index.htm

OK, I got the second one now.

I'm hearing a couple of honkin' room resonances in the ~100 Hz region.

Not knowing the guitar, mic, or preamp your using I can't give you much more detail.

Oh, and the tuning is still bjorked.

On 1 Jan 2005 20:25:21 -0800, "lex" wrote:


Can someone please help identify the problem I'm having? I'm wondering
if I'm experiencing Comb Filtering, Room Mode Resonance, or Flutter
Echo. Can someone tell just by listening to these sound waves what is
happening? I'm too inexperienced to tell the difference between them.

The sound wave samples are he
http://www.geocities.com/lex888888/index.htm snip

I tried, but can't access the files, for some reason. Out of
bandwidth allotment?

I'll try again tomorrow.

dB

On 1 Jan 2005 20:25:21 -0800, "lex" wrote:


Can someone please help identify the problem I'm having? I'm wondering
if I'm experiencing Comb Filtering, Room Mode Resonance, or Flutter
Echo. Can someone tell just by listening to these sound waves what is
happening? I'm too inexperienced to tell the difference between them.

The sound wave samples are he
http://www.geocities.com/lex888888/index.htm

I posted some samples of the problem there in .mp3 format. You might
need to listen through some good headphones or monitors to hear the
problem. The problem is very audible in AKG K-501's as buzzing and
amplification. Yorkville Monitors produce the same with a little less
buzzing.

Can you make another file, not of the guitar, but of a hand clap - as
sharp as possible. That is the best way to find a flutter echo. do it
several times in different positions in the room - stay fairly close
to the mic each time though.

d


Pearce Consulting
http://www.pearce.uk.com

In article .com writes:

Can someone please help identify the problem I'm having? I'm wondering
if I'm experiencing Comb Filtering, Room Mode Resonance, or Flutter
Echo.

Comb filtering is a result of room mode resonance, and it sounds
different at every listening point. The way to tell this is to move
around while music is playing. Flutter echo is most easily heard by
clapping your hands, or having someone else clap hands while you
listen.

It's not clear (or maybe I forgot) whether you're trying to solve a
problem with your monitors - where the source is in the same place all
the time and your listening position is within a fairly well defined
area - or if you're trying to solve a problem in the room where the
musicians and microphones are - where the positions of the source and
microphone could be anywhere.


--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

On 2 Jan 2005 10:05:30 -0500, (Mike Rivers) wrote:

Comb filtering is a result of room mode resonance, and it sounds
different at every listening point. The way to tell this is to move
around while music is playing. Flutter echo is most easily heard by
clapping your hands, or having someone else clap hands while you
listen.

It's not clear (or maybe I forgot) whether you're trying to solve a
problem with your monitors - where the source is in the same place all
the time and your listening position is within a fairly well defined
area - or if you're trying to solve a problem in the room where the
musicians and microphones are - where the positions of the source and
microphone could be anywhere.

Comb filtering has nothing to do with room resonances. It is caused by
a signal arriving at a point by two (or more) different paths -
usually one direct and one reflected. Depending on whether they arrive
in or out of phase the signal will be boosted or cancelled. The comb
reference is because if you look at a frequency spectrum, you will see
alternate cancellation and reinforcement through the band.
Flutter echoes are associated with standing wave modes, and are caused
by two facing flat surfaces rebounding the wave several times. The
flutter rate depends on the distance between the surfaces.

d
Pearce Consulting
http://www.pearce.uk.com

On 1 Jan 2005 20:25:21 -0800, "lex" wrote:


Can someone please help identify the problem I'm having? snip

Yes. Try tuning your guitar!

dB

lex wrote:
Can someone please help identify the problem I'm having? I'm wondering
if I'm experiencing Comb Filtering, Room Mode Resonance, or Flutter
Echo. Can someone tell just by listening to these sound waves what is
happening? I'm too inexperienced to tell the difference between them.

Sorry, I haven't got the bandwidth at the moment to listen to your sound
files, but it sounds like you are using some terminology without fully
understanding what it means. Allow me to explain the terminology, then
you will be better placed to solve your own problem.

_Comb Filtering_
Comb filtering is the name give to a specific type of interference
created when a signal from a sound source reaches the listener via two
paths of slightly different length. The difference in path length
causes the sound travelling along the longer path to arrive at the
listener with a slight delay. It is the delayed copy of the original
sound that causes the interference pattern known as comb filtering. The
path length need only differ by as little as 12" (causing a delay of
approx 1ms) before comb filtering becomes apparent.

Comb filtering gets its name from the sound's appearance in the
frequency domain. Imagine you are looking at a frequency domain display
of a broadband signal. When comb filtering is absent, you will see a
roughly smooth line indicating that sound is present at all frequencies.
However, when comb filtering is present, sharp peaks and troughs can
be seen in the display which gives the appearance of the teeth of a
comb. As the difference in the path length changes, so the density of
the peaks and troughs changes. If the time delay (i.e. the path length
difference) is very small, then you will see a high number of narrow
bandwidth peaks and troughs. As the delay increases, so the peaks and
troughs become more widely spaced (and the effect becomes more noticeable).

_Room Mode Resonance_
Assuming you are listening to sound in a room that is roughly
rectilinear, then the dimensions of your room will cause certain
frequencies to resonate. Resonance will occur for frequencies having
half wavelength multiples of the room dimensions.

All rooms resonate to some degree or another. Resonance can be
controlled by applying absorbency to the room's inner surfaces as either
passive absorption (such as Rockwool) or active absorption (such as
membrane resonators or Helmholtz absorbers).

The degree of resonance within a room is one of the room's fundamental
acoustic properties. This is measured by playing a loud sound in the
room (such as a hand clap or bursting a balloon), and then measuring the
length of time it takes for the sound level to drop by a predetermined
amount (usually 25dB or 40dB). This time is then extrapolated to
determine the time taken for the energy of the sound field to drop by
60dB. This value is then quoted as the room's reverberation time
(better known as the RT60).

Reverberation times are frequency specific, and should be measured using
at least the standard six octaves of 125Hz, 250Hz, 500Hz, 1KHz, 2KHz and
4KHz. Other frequencies can be used, but the six quoted above are the
standard ones. By convention, the RT60 value at 500Hz is taken as the
representative reverberation time for the room. So if someone says
their room has an RT60 of 1.3s, then they mean that when a 500Hz sound
source is switched off, the energy of the sound field will take 1.3
seconds to decay by 60dB.

The term "mode" simply mean the frequency that will naturally resonate
within a room. Modes come in harmonically related sets. Modes are
considered undesirable if their presence colours the sound being played
in the room. This typically happens when a group of modes all occur
within a narrow frequency range, thus making their contribution noticeable.

If possible, the geometry of a room should adjusted so that the modes
are as evenly spaced as possible. From a theoretical point of view, the
dimensions of a room can be checked to see whether they will produce
these bunched sets of modes that will colour sound.

_Flutter Echo_
Flutter echo is typically caused by an unequal distribution of
absorbency within a room. Consider the behaviour of a reverberant sound
field within the three pairs of parallel surfaces in the room: the floor
and ceiling, the two side walls and the front and back wall.

Due to the different distances between these pairs of surfaces and their
differing absorption characteristics, it is entirely possible for the
room to possess three different reverberation times: one for each axis
of the room.

Now consider what would happen if you applied lots of absorbency to the
walls, but left the floor and ceiling reflective. The reverberation of
sound in the two horizontal axes would very short because of the
absorbent walls, but the reflective floor and ceiling would cause the
reverberation time in the vertical axis to be significantly higher.
This means that the overall energy level in the sound field would die
off quickly in the two horizontal axes, but persist in the vertical
axis. This is the cause of the very audible (and very irritating!)
artefact known as flutter echo.

Flutter echo is easily fixed by applying absorption to one of the
surfaces forming a reflective pair. Normally, control rooms have
reflective floors, and absorbent ceilings.

If you wish to take a more technical look at the acoustic properties of
your room, please download a free spreadsheet that I have created. This
spreadsheet allows you to experiment with different materials on the
floor, walls and ceiling, and then it will predict many different
acoustic properties of the room, including the reverberation time.

http://www.rmmpnet.org/members/ChrisW/index.html

Follow the "Control Room Calculator" link and then please read the
instructions on the download page *carefully*!

Regards

Chris W

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

In article writes:

Comb filtering has nothing to do with room resonances. It is caused by
a signal arriving at a point by two (or more) different paths -
usually one direct and one reflected.

To someone who doens't know what "room resonance" is, these are the
same thing. What causes "room resonance" other than sound reflecting
off a boundary? Reflections cause comb filtering. OK, so they're not
the same. If there are reflections, there will be more than one, and
there will be comb filtering someplace.

This is irrelevant to the original poster's problem though.


--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

On 2 Jan 2005 21:41:27 -0500, (Mike Rivers) wrote:


In article writes:

Comb filtering has nothing to do with room resonances. It is caused by
a signal arriving at a point by two (or more) different paths -
usually one direct and one reflected.

To someone who doens't know what "room resonance" is, these are the
same thing. What causes "room resonance" other than sound reflecting
off a boundary? Reflections cause comb filtering. OK, so they're not
the same. If there are reflections, there will be more than one, and
there will be comb filtering someplace.

This is irrelevant to the original poster's problem though.

This is just a touch patronising to the original poster, who actually
asked what the difference was between these phenomena, and how to tell
the difference. To just say that because he doesn't know, then you can
just give him some crappo nonsense just isn't that helpful. In another
post, I have told him specifically how to identify flutter echo.

d


Pearce Consulting
http://www.pearce.uk.com

Chris,

Comb filtering gets its name from the sound's appearance in the frequency
domain.

Great explanation. Just to add a little, here's a frequency response graph
of the comb filtering as measured 20 inches away from a wall:

http://www.realtraps.com/art_spaces2.gif

--Ethan

Ethan Winer wrote:
Great explanation. Just to add a little, here's a frequency response graph
of the comb filtering as measured 20 inches away from a wall:

http://www.realtraps.com/art_spaces2.gif

Ethan, that's a good plot. One suggestion though; the effect of comb
filtering could be better visualised if you showed the original signal
over top of the comb filtered signal.

Also, if you showed several plots showing varying time delays laid over
the original signal, it would visually demonstrate the effect that
variation in path length causes to the Q of the peaks and troughs in the
comb filtered signal. The longer delay, the lower the Q.

Chris W

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

On Tue, 04 Jan 2005 02:23:25 +0000, Chris Whealy
wrote:

Also, if you showed several plots showing varying time delays laid over
the original signal, it would visually demonstrate the effect that
variation in path length causes to the Q of the peaks and troughs in the
comb filtered signal. The longer delay, the lower the Q.

That's an interesting point that on first blush would seem
to conflict with received wisdom; that is, that room Q is
independent of measuring point.

Are you sure that we're not discussing a measurement artifact?

Thanks for your thoughts,

Chris Hornbeck
"They'd meet at the Tout Va Bien, a cafe just off the highway."
-JLG, _Bande a part_, 1964

Chris,

the effect of comb filtering could be better visualised if you showed the
original signal over top of the comb filtered signal.

That graph is from my June 2004 article in EQ magazine. In the article it's
explained that this is the frequency response as measured by the ETF
software. For anyone who doesn't get EQ, the full article is now on my
company's web site:

http://www.realtraps.com/art_spaces.htm

variation in path length causes to the Q of the peaks and troughs in the
comb filtered signal. The longer delay, the lower the Q.

I didn't know that. But why does the path length / delay time affect the Q?
Isn't each "multiple cycle" exactly the same, only instead of 360 degrees of
shift it's 720 then 1080 etc?

Thanks.

--Ethan