Octave?

Discussion in 'Archived Threads 2001-2004' started by gomez_a, Feb 20, 2002.

  1. gomez_a

    gomez_a Stunt Coordinator

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    Hello,

    Can someone please explain (or provide a link) what the octave freq's are, why they were chosen, and why significant?

    Thanks
     
  2. John Kotches

    John Kotches Cinematographer

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    Why were the frequencies chosen?
    That depends on the tuning method.
    The most common used to be the Just Tempered scale, which was based on ratios like 2:1, 3:2 etc etc etc. This type of tuning is good for 1 key at a time, arggggh.
    In practice to day is the "even tempered scale". The distance between each semi-tone is the 12th root of two. The base frequency is A4 @ 440Hz.
    Here's a handy little link that goes into this with some detail and example frequencies for one octave:
    http://www.phy.mtu.edu/~suits/scales.html
    Hope this helps, and post back here if you'd like to discuss this further.
    Regards,
     
  3. Dan Hine

    Dan Hine Screenwriter

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    Gomez,
    Maybe I'm misunderstanding the question (and I'm sorry if I am) but when I see
     
  4. John Kotches

    John Kotches Cinematographer

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    Dan,
    I was trying not to overwhelm with too much information [​IMG]
    Heck, most people don't even realize there are multiple tuning strategies (Just and Even tempered are two of them).
    Bach's "Well Tempered Clavier" was also built around yet another tuning standard (maybe, it is up for argument by a number of musicologists).
    So anyway, thanks for expounding on my original post.
    Regards,
     
  5. Keith Mickunas

    Keith Mickunas Cinematographer

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    My brain hurts.

    Boy I hope someone does a comprehensive FAQ on speaker/sub calibration at some point. Its overwhelming at times.
     
  6. gomez_a

    gomez_a Stunt Coordinator

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    Guys,
    Thanks for the replies, the main thing I got out of the repllies is that
    1-Octave freq's are not set in stone
    2-and that an octave is the range from one frequency to another that is twice that of the orignial frequency (when going up) and half the orignial when going down.
    These are the answers I was looking for! [​IMG]
    Now another related question . . . my recvr onk898 has a rated octave slope at the LFE @80Hz (don't what it is, have to look it up in the manual specs) - what is a generally acknowledged good slope to have at @80Hz (for the LFE)?
    Obviously, the steeper the slppe, the quicker the drop off from 80Hz, and what is considred as a good slope can depend on what you are using for spkrs, ie, Towers w/built in sub, book shelf spkrs, Sub-less towers, etc, and whether you set you spkrs to SMALL or LARGE, etc,
    Thanks!
     
  7. StephenL

    StephenL Second Unit

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    The THX crossover has a high-pass filter frequency response of 12 dB per octave below the -3 dB point of 80 Hz and a low-pass (subwoofer output) frequency response of 24 dB per octave above the -6 dB point of 80 Hz. The THX crossover is designed to work with main speakers that are also down 3 dB at 80 Hz and rolling off at 12 dB per octave. The combined response of the high-pass filter and main speakers is 24 dB per octave below the -6 dB point of 80 Hz, so the high-pass and low-pass response is symmetrical. In theory, this design keeps the main speakers and subwoofer in phase and provides a relatively flat frequency response. These articles explain the THX crossover:
    http://www.soundandvisionmag.com/Buy...p?ArticleID=66
    http://www.hometheaterhifi.com/volum...tspeakers.html
     
  8. John Kotches

    John Kotches Cinematographer

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    Gomez,
    I'm not sure you'll understand, so forgive me if you already realize this is the case....
    The frequency relationships between the same note in different octaves is a 2:1 (geometric) progression, regardless of the tuning standard.
    From tuning std A4 of 440Hz, to the next A "up" the keyboard, is 880Hz, to the next A "down" the keyboard is 220Hz. All of the A's on a piano keyboard are:
    A0 = 27.5 Hz
    A1 = 55 Hz (same as A string on the Acoustic and Electric Bass)
    A2 = 110Hz
    A3 = 220Hz
    A4 = 440Hz * Typical tuning reference
    A5 = 880Hz
    A6 = 1760 Hz
    A7 = 3520 Hz
    I think that's the number of "As" on a piano keyboard, sorry I don't have one downstairs to check, and I was a saxophonist [​IMG]
    When we talk about tuning conventions like Just Tempered, Even Tempered and Well Tempered, we are discussing the relationships between the intervals within an octave.
    As far as crossover slopes are concerned, the typical crossover implemented at 80Hz is a THX one. This equates to 12dB/octave (I forget the type) high-pass filter (to small speakers) and a 24dB/octave low-pass filter (to the subwoofer).
    What are the implications then for things around the crossover region? Taking the above crossovers, and ignoring harmonics, let's see.
    Assumptions: Target SPL is 85dB, 1 speaker + 1 subwoofer, no room effects, speaker has flat response 2 octaves past crossover (not likely for both sides).
    Freq.....Speaker......Subwoofer
    320Hz....84++dB.......37dB (99.+%, way less than 1% sub)
    160Hz....84+dB........61dB (99+% speaker, less than 1% sub)
    80Hz.....82dB.........82dB (drivers are equal at crossover)
    40Hz.....70dB.........84+dB (roughly 94% sub vs 6% speaker)
    40Hz.....58dB.........84++dB (99+% from Sub)
    You can see how quickly the other speaker gets out of the way, especially when you consider the way we hear, which is such that the louder sound will win out.....
    The THX spec works across a broad range of speakers quite well, and the reason the high pass is less steep, is that it expects a natural rolloff in conjunction with the filter. I almost always use the THX spec as a starting point, which works well for most things.
    For DVD-Audio, I use the Outlaw Audio ICBM for Bass Management, and crossover at 60Hz for front + surrounds (both good to 35Hz) and 80Hz for CC (good to 45Hz).
    I hope this helps you out some more.
    Regards,
     
  9. gomez_a

    gomez_a Stunt Coordinator

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    John, Stephen,
    It's incredible the amount of know-how on this forum. Thanks for all the info - I need to print this out and study it - (ie, it's a "keeper" )[​IMG]
    I have an engineering background, but certainly hot in the area of harmonics
    [​IMG]
     
  10. John Kotches

    John Kotches Cinematographer

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    Gomez,
    I was a Music major, and you can probably figure out that I minored in a subject or two that wasn't fine arts [​IMG]
    Regards,
     
  11. Sebastian

    Sebastian Second Unit

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    DP
     

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