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  1. #61
    Senior Member 1audiohack's Avatar
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    It's a quick method to calculate axial modes out to the sixth. In a "normal" rectangular room this will get all modes out to where they are statistically spread out with insignificant energy even in a poorly dimensioned closed acoustical space.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  2. #62
    Senior Member Lee in Montreal's Avatar
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    Quote Originally Posted by 1audiohack View Post
    Multiply the speed of sound by three, divide the answer by the rooms smallest dimension, eg 344 X 3 = 1032 / 2.44m = 422.9Hz

    Divide the speed of sound by two, divide the answer by the rooms largest dimension, eg 344 / 2 = 172 / 5.8m = 29.6Hz

    That's an interesting formula. I suppose that the smallest measure is the ceiling's height, and that circa 400Hz is the usual upper range affected by the room, unless one has cathedral ceilings.

    So, to lower that frequency value, I suppose that room treatment shall begin on the ceiling. Either raise it, or dampen it...

  3. #63
    Senior Member 1audiohack's Avatar
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    Yup.

    The region where the acoustical performance of the space is mode dominated is bounded by the first mode of the space where f=.5C/RLD up to highest where f=3C/RSD. This is where RTA will lie to you, EQ can't help you, absorption is really your only weapon and your ears are your best tool.

    Above and below this region RTA is an effective and efficient tool for individual driver and channel level setting and more. Knowing it's limitations is the key to getting good results, same with EQ, especially an auto EQ system.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  4. #64
    Senior Member Mctwins's Avatar
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    Hi
    Where did you get your formulas from?

    This is what is explained in "Master Handbook of Acoustics"

    A 10x16x23,3 ft room has a volume of 3,728 cu ft and the reverberation time is 0,5 sec.

    F1=565/23,3=24,2 Hz
    F2=11,250 x sqrt(0,5/3,728)=130 Hz
    F3=4 x 130=520Hz

    Between 24,2 and 130 Hz the wave acoustical approach of modal resonances is essantial.

    Between 130 and 520 Hz is the transition region. It is a diffucult region dominated by wave lenghts often too long for rays acoustics and too short for wave acoustics.

    Above about 520 Hz the modal density is very high, statistical conditions genarally prevail, and the simpler geometrical acoustics can be used.

    This is the problems in small rooms witch we all have.

    Thanks

  5. #65
    Senior Member 1audiohack's Avatar
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    Oh right,, make me go back to the books!

    I found in my fourth edition of the Master Handbook of Acoustics the equation you correctly quoted in the Modal Resonances in Enclosed Spaces chapter.

    One thing to look out for is when formula for statistical spaces finds it's way into small room acoustics. Few of the intelectual tools of the trade for large rooms can be applied to small rooms. I am not arguing that the math is not correct, just the validity of some of it as it applies to small spaces. The 0.5 second reverb time is an indicator that this has happened.

    If you look back a couple of paragraphs it is stated;
    Region B is that region we have studied in detail in which the dimensions of the room are comparable to the wavelength of the sound being considered. It is bounded on the low frequency end by the lowest axial mode, 565/L. The upper boundry is not definite but an approximation is given by what has been called the cutoff or crossover frequency given by the equation;

    F2 = 11.250 (RT60/V) 1/2

    Where F2 = cutoff or crossover frequencyin Hz
    Where RT60 = reverberation time of the room in seconds
    Where V = volume of the room in cubic feet
    What is often overlooked in the attempted measurement of RT60 in small rooms is that the definition of RT60 has two parts, the first of which is often overlooked or ignored;

    1 RT60 is the measurement of the decay time of a well mixed reverberant sound field well beyond the Dc', or the critical distance.

    2 RT60 is the time in seconds for the reverberant sound field to decay 60 dB after the sound source is shut off.

    Since in small rooms there is no Dc', no well mixed sound field, hence no reverberation but mearly a series of early reflected energy, the measurement of RT60 becomes meaningless in these environments.

    What becomes meaningfull is the control of the early reflections because there is no reverberation to mask them.

    Finally, the formula I used comes from the Handbook for Sound Engineers, 4th edition, Small Room Acoustics chapter by Doug Jones. I have also had the good fortune to have received some formal training from him and could use a great deal more!

    All the best,
    Barry.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  6. #66
    Senior Member Mctwins's Avatar
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    Hi

    Thats why I really don't get to mutch anal about theoretichal aspect when talking about small rooms. You are absolutely right when talking about RT60, thats why RT30 is more usefull.

    It is more easier to do measurement over a particular room. You see more data how your room is rather then trying to calculate it by hand.

    That's why I have abandonded the use of full range absorption in small rooms and use broadband diffusion integrated in helmholtz principle. The decrease in reverberation is as good as in full absorption and will not over dampend the room at the same time.

    The stuff we read in varoius books is mainly for big auditorium and churches and so on and not for small rooms.

    I can read therory just to get me more understanding about acoustics but it dosen't say how to approach it in a practical solutions, construction and so on.

    If you have a good acoustical room as I have, the auto EQ will be easier to perform, but in my case I don't need it.

    Thanks

  7. #67
    Senior Member 1audiohack's Avatar
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    Hi;

    I remember the magnitude and decay charts you showed for your room and when you say it sounds good in there, I beieve it!

    You obviously read and understand the acoustics stuff, and since you are interested in it, for accuracy I would just like to share a fundamental point;

    Modal decay rates are not reverberation. Reverberation is the "time in seconds that it takes a diffuse sound field, well beyond a real critical distance, to lower in level by 60 dB when the sound source is turned off."

    Modal decay rates are dB-per second (dB/s) rate of decay for a specific frequency.

    By definition, reverberation does not exist in small rooms. RT nothing.

    Not to be picky, really, it's just a very common mistake and some of my instructors are / were quite militant about it. I guess it rubbed off. OK last on that from me.

    All the best and enjoy!
    Barry.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  8. #68
    JBL 4645
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    Jonas_h

    What video source components are you running in your home cinema?

    Betmax
    VHS
    S/VHS
    Laserdisc
    DVD
    HD-DVD
    Bluray

  9. #69
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    Quote Originally Posted by JBL 4645 View Post
    Jonas_h

    What video source components are you running in your home cinema?

    Betmax
    VHS
    S/VHS
    Laserdisc
    DVD
    HD-DVD
    Bluray
    A computer with a bluray drive. All movies are stored on a NAS.

  10. #70
    JBL 4645
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    Quote Originally Posted by Jonas_h View Post
    A computer with a bluray drive. All movies are stored on a NAS.
    Interesting! Let’s see the current new pictures of the set-up and around the room.

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