Comb filtering, Room Modes, or Flutter Echo?

Discussion in 'Pro Audio' started by lex, Jan 2, 2005.

  1. lex

    lex Guest

    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 here:

    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

    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

    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

    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, Jan 2, 2005
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  2. 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
    Neil Henderson, Jan 2, 2005
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  3. lex

    lex Guest

    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
    lex, Jan 2, 2005
  4. 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.
    Kurt Albershardt, Jan 2, 2005
  5. lex

    DeserTBoB Guest

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

    I'll try again tomorrow.

    DeserTBoB, Jan 2, 2005
  6. lex

    Mike Rivers Guest

    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

    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 here: double-m-eleven-double-zero at yahoo
    Mike Rivers, Jan 2, 2005
  7. lex

    DeserTBoB Guest

    Yes. Try tuning your guitar!

    DeserTBoB, Jan 2, 2005
  8. lex

    Chris Whealy Guest

    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.

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


    Chris W
    Chris Whealy, Jan 3, 2005
  9. lex

    Chris Whealy Guest

    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
    Chris Whealy, Jan 4, 2005
  10. 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 Hornbeck, Jan 4, 2005
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