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Thread: Applicability of RT60 and measurement thereof to "small acoustic spaces."

  1. #11
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    Quote Originally Posted by microstrip View Post
    Amir,

    Thanks for summarizing this polemic subject - I followed parts of it in the avsforum, but I did not feel competent enough to ask questions.
    My pleasure. This is one of the reasons we created WBF .

    Here at WBF I feel more comfortable. How do you interpret Toole in page 509 of his book when he seems to say that in small rooms "the key is not the number, but the sound" or writes "Feel free to add or take materials until it sounds "right", RT numbers are not highly predictable"?
    If you read my posts you see that one has to be very careful to measure the proper range of frequencies. The general guidelines for RT don't say that. As a result, one man's 0.4 may be another's 0.6. Additionally, there is a range there for a reason. Preferences vary based on person and type of content you listen to. So to find the right balance, he is rightly suggesting to use one's ears to determine the final target.

    For me, this is a two-step process. Measure RT60 and if it is say, 1.0, then you don't need to do listening tests. Your room is too live. Likewise, if it is .1, then it is too dead. Fixing these doesn't require listening tests. But once you get it within the range of 0.2 to 0.5, then listening tests is useful to find tune your target.

    I have taken many RT60 measurements in my room, and feel happy they are good, but always feel uneasy with these statements.
    That is actually OK. This is not a precision instrument and in the degree of importance, is not that high relative to say, smooth bass response, and good speaker selection.

    Just one technical detail. I have found that some people measure RT using the main speakers in the listening position, others put two speakers in the center of the room facing opposite directions to simulate an omnidirectional excitation. What is the best technique - or better still : how do you do proceed to measure RT60?
    If you use 500 Hz as I do, your speaker will have pretty wide directivity. 500 Hz is also high enough to be outside of the very modal region of bass frequencies so the corner position of the speaker does not load it up too much. Best thing to do is turn on the Schroeder integral and see how well the estimated Topt line tracks it. If it tracks it well, then you have the right configuration.

    Omnidirectional is important if you care about high frequencies because speakers get very directional there. I see no value in measuring RT time for high frequencies. It is outside of the vocal range.

  2. #12
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    Excellent Amir, this should be made a sticky.

    --Ethan

  3. #13
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    Sticky it is....

  4. #14
    Hi Amir,

    allow me to comment on some points:

    Quote Originally Posted by amirm
    Computing the transition frequency as the above number is called, we get 352 Hz.
    In his original paper Schroeder proposed a formula which results in a tenfold modal overlap: “It is assumed that the number of simultaneously excited, independent modes is large enough to yield a Gaussian distribution of sound pressure components. Practical experience has shown that the approximation of the Gaussian distribution is good for about 10 independent components of the same order of magnitude.”

    “If the average separation of eigenfrequencies is small compared to the average half-width of the resonance response curves, then the separation of the maxima is given by the half-width since an increasing number of maxima, generated by the individual eigenfrequencies, are “screened out’.”

    * Schroeder, “Statistical parameters of the frequency response curves of large rooms”, JAES 1987, p.299 (translation of original paper (in German) published in Acustica 1954)


    In a later paper Schroeder changed that to a threefold modal overlap: “at least three resonances fall within the half-power bandwidth of one resonance”, which halfs the value of the frequency as compared to the initial equation.

    * Schroeder, “The Schroeder frequency revisited”, JASA 1996, p.3240

    However, it has been found that this transition between individual, well separated resonances to many overlapping modes occurs at frequencies well above the Schroeder frequency, so that the initial formula would better be in line with what has been observed, which would put the Schroeder frequency in the 400-600 Hz range, rather than the 200-300 Hz you mentioned.

    Baskind et al., “Sound power radiated by sources in diffuse field”, AES paper 5146

    However, Schroeder’s goal was to obtain statistical parameters of frequency response curves (i.e. the curves’ range of variation such as average rms respones fluctuation, average height of a maximum, mean separation of zeros) that are independent of the location of source and receiver, but only depend on RT60. Nothing about smoother response, sound quality, existance of a diffuse field etc.

    It has, as you’ve pointed out yourself, been found that in small rooms (but also in large rooms) the sound field is not diffuse but directional:

    * Meyer, „Definition and diffusion in rooms“, JASA 1954, p.630
    * Gover et al., “Measurements of directional properties of reverberant sound fields in rooms using a spherical microphone array”, JASA 2004, p.2138

    Since the field is not diffuse, concepts like the critical distance no longer apply:

    * Toole, “Loudspeakers and rooms for sound reproduction – a scientific review”, JAES 2006, p.451

    However, you then wrote:
    Once again we see that RT60 measurement is not only believed, but actually used to characterize how reverberant the room was – precisely what we use it for
    If that is to be understood that there is a link between RT60 and the degree of diffusivity, then the data would not totally support this:

    From the Gover paper:

    Meeting room
    102 cbm, RT60 = 0.36 s
    diffusion = 70%, anisotropy = 4 dB
    different receiver position
    diffusion = 59%, anisotropy = 7.8 dB

    Videoconferencing room
    181 cbm, RT60 = 0.4s
    diffusion = 58%, anisotropy = 7.4 dB

    Lecture theatre
    875 cbm, RT60 = 0.6 s
    diffusion 61%, anisotropy = 5.5 dB

    The data of Meyer (rooms of 300 – 15,000 cbm) show a trend of diffusivity decreasing with increasing RT60. He also found that within the same room values vary from place to place. He further found that the location of absorbing material in the room had an effect on measured diffusivity.

    ... once you add absorption to get the RT down to recommended level, in this case in the 0.4 range ...
    The only source that mentions research w.r.t reverberation times in listening rooms is

    * Gilford, “The acoustic design of talk studios and listening rooms”, JAES 1979, p.17

    where an investigation by the BBC is described on half a page. The result then was 0.4 seconds. I wonder on what research are based the figures one usually encounters in recommendations such as EBU 3276, or such as the range of 0.2 – 0.5 seconds you mention.

    This range however is not computed but based on industry's collective experience of what the value should be based on countless in field experiences in home listening spaces. That experience has been formed from typical home spaces, not a small box like this or an auditorium.
    There are a few papers where RT in actual homes has been measured. I’ve made a graph showing the EBU 3276 requirements and the data of those measurements. Some of the figures fit within the recommendation as per EBU, some don’t. None go as low as 0.2 seconds.

    Name:  RT60.jpg
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    The question is, at what value does RT begin to disturb, is 0.2 in studio control rooms really necessary, has this ever been investigated?


    So as you see, RT60 is not useless at all.
    The problem I see is that a single figure doesn’t tell you a lot: a RT60 of two seconds in a concert hall will be fine, the same figure in your living will be a disaster. My room is 100 cbm with a calculated (Sabine equation) RT of about 0.4 seconds. If I go to half that volume, will 0.4 seconds be too much? If I go to twice that volume, will 0.4 seconds be to little? Does 0.4 seconds feel the same everywhere, regardless of room size?

    Klaus

  5. #15
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    Big spaces :

    I was in the amsterdam concertgebouw today , also quite a famous hall for its acoustics , beautifull interiour too with its balconies .
    i asked also about the reveberation time , the big hall : 2,2 seconds full with people , 2,8 in the empty hall.
    small hall 1 second and lastly the smaller choir room in the basement well below 1 second .

    i asked also what made the hall acoustics famous :
    according to them no concrete anywhere , its made of bricks and a poreus plaster cover and wood off course .
    absorption: wool covert seats and a fair amount of wool curtens .
    its not a deep hall , it has irregular shaped floor walls ceiling so that would act as diffusion.
    and lastly the peoples clothes in it act as absorption as well
    Last edited by andromedaaudio; 06-23-2013 at 12:03 PM.

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  7. #17
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    I d sure be interested i ve sent a mail , thanks

    greetz hj

  8. #18
    Quote Originally Posted by KlausR. View Post
    The question is, at what value does RT begin to disturb, is 0.2 in studio control rooms really necessary, has this ever been investigated?
    I'm pretty sure you know a higher Tm than recommended can be nice. I think Bob Walker was the head of the group that developed the EBU 3276 and he designs studios with high Tm so we can keep 0,2 as a common recommendation, not a dogma.
    As you can read, my English isn't good. Corrections will be appreciated.

  9. #19

    Measuring RT60 in Room EQ Wizard

    Quote Originally Posted by amirm View Post
    If you use 500 Hz as I do, your speaker will have pretty wide directivity. 500 Hz is also high enough to be outside of the very modal region of bass frequencies so the corner position of the speaker does not load it up too much. Best thing to do is turn on the Schroeder integral and see how well the estimated Topt line tracks it. If it tracks it well, then you have the right configuration
    .
    Hi Amir, I'm new to the forum and have found your tutorials and explanations to be very useful and informative. Just needed you to clarify something: I have a 7.1 HT configuration for movies and music. For RT60, does one use a sweep going from 125hz to 2,000Hz ? Do these values have to be entered into REW before generating the sweep ? Furthermore, do you use just one of your front speakers for the RT60 ? i.e. - the left, center or right front speaker ?

  10. #20
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    Quote Originally Posted by Sam Ash View Post
    Hi Amir, I'm new to the forum and have found your tutorials and explanations to be very useful and informative. Just needed you to clarify something: I have a 7.1 HT configuration for movies and music. For RT60, does one use a sweep going from 125hz to 2,000Hz ? Do these values have to be entered into REW before generating the sweep ? Furthermore, do you use just one of your front speakers for the RT60 ? i.e. - the left, center or right front speaker ?
    Hi Sam. Thanks for the kind words.

    With REW you do one sweep and it generates all the other measurements from that one. Make it full bandwidth (20 to 20000) for completeness. But with respect to RT60 pay attention to the value shown for 500 Hz. It is this value for which we have the rules of thumb for RT60:



    As to which speaker to use, you can use any of them as this is a measure of overall reverberations in the room. For the sake of specificity, use the center channel since the above rules of thumb are for speech intelligibility and that is where the voices are typically in movies.

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