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Thread: Why do many Acousticians recommend absorption for non-bass freq: Valid OR prefer dead room & Dogma?

  1. #11
    WBF Technical Expert (Speaker & Acoustics) [Technical Expert] Mark Seaton's Avatar
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    May 2010
    Chicago, IL
    Quote Originally Posted by caesar View Post
    Gentlemen, thank you very much for great responses!

    What I am still a bit unclear about is that every time I have heard diffusion, it adds spaciousness, while absorption - when used in spots where diffusion could be used - kills the room. There is even a video somewhere of Ethan Winer playing a guitar near diffusion panels and you can tell - even on youtube - that diffusion allows one to hear a more spacious sound. But then, as a seeming contradiction, when one looks at Ethan Winer's room, it has so many absorption panels, it is probablly deader than disco...

    So if diffusion positively adds to the experience in terms of hearing the spaciousness of the venue, while absorption takes away from that experience (other than in lower regions), why don't most people use it?

    Is there a room measurement that can help explain this? Or is it just preference? Thank you!
    The short answer is to not confuse sound creation with sound reproduction. If you have too much contribution of the room, every recording sounds like it's in that space and gives up the ability to convey the characteristics of recorded space.

    There are measurements to indicate what effect different reflections will have. Unfortunately there are many qualities and we will never have one single metric or graph to see all acoustic qualities of the room. The thread you started about speaker distance from walls comes into play as well, just as the simplified graphic I linked showing the general effect of early reflections with respect to time of arrival and relative level vs the direct sound.

    So far as preference for using diffusion vs absorption, it goes back to my last post about ease of deployment. Size of the room also matters. With larger dimensions, you greatly reduce the % of early reflections, where it becomes easier to attenuate the excessive early reflections with diffusion, and the higher order reflections (= reflections hitting multiple surfaces before arriving at the listener), quickly loose their strong directional qualities. As rooms get smaller, there are more reflections within a shorter time period, and you have to be more concerned with where the diffused energy goes and what happens at the next surface it hits. Some degree of absorption will often make it easier to get a desirable result in these smaller rooms.

    Diffusion is also not a magic bullet, and there are some pitfalls. The shape and physical depth of a diffusor determines how low in frequency it has effect, below which it simply reflects sound. All diffusors are not equal in their scattering qualities, and many trade off different qualities in different dimensions or frequency ranges. Some diffusors also create patterns of scattering if multiples are lined up which can result in undesired hot and cold spots as you move across the diffusor in the dimension of the array. Simply put, it's best to have some understanding of the devices being used, else you might get very different effects from leaning back in your chair vs sitting forward. As is often the case, execution is everything.
    Mark Seaton

    Seaton Sound, Inc.

  2. #12
    Quote Originally Posted by caesar View Post
    Why do so many acousticians recommend absorption for everything in the room, vs. just using absorption for the bass portion of the space?

    Are guys just unquestioningly going back to some research paper someone wrote a century ago? Have acousticians become accustomed to dead sounding spaces and everything else sounds wrong? Or are there valid reasons?
    Interesting thread insofar as a you've had a few great answers thus far.

    To answer your question as to absorption of 'the non-bass portion' the higher frequencies often require absorbition or alternatively diffusion in order to prevent getting a reflected signal.
    Another way to look at it is that the mid-higher frequencies, particularly around 300hz is at a level the ear is veyr sensitive to - unlike bass - higher frequencies are more directional than bass. Now if this comes out of your mid and treble drivers there is a lot of it coming off in a relatively wide field - if this bounces off side walls and ceiling it gets to you after that coming directly from the tweeter and mid - this means that there will be 'time- smears' of the reflected sound. This means the image becomes more diffuse and loses focus. Likewise timing becomes smeared as well (think of echoes and how the reverberant (repeated) sound of the echo remains in existence whilst a subsequent sound comes out)
    Can I just add that tall bass absorption towers are likely to also damp higher frequencies too by reason of placement.

    Have a read on the cards website about room acoustics as a good starting point. Good Audio Engineers don't always agree on a perfect room BTW - and that's among the really good ones out there - so try not beat yourself up too much so to speak.

  3. #13
    Senior Member Bjorn's Avatar
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    Oct 2010
    Many areas that come into play here. Especially the size of the room and distance from ears to the surfaces. Diffusion lower in frequency requires distance. Which in many cases aren't met in several small rooms. Using primarily high frequency diffusion will only treat part of the specular reflection, thus altering the spectral content. A diffuser is also less accurate than an absorber in the way that it doesn't treat the different frequencies as equal as an absorber.

    While taste and acoustic principle will matter - generally speaking; using broadband absorption to treat early arriving reflections will be a more accurate way of treating a room. And using mainly diffusion for late arrival reflections.

    If the room is large enough and built from scratch, it's possible to make it both very accurate without any absorption above the schröder frequency and with a lot of diffused energy. Psychoacoustically this might be the preferred way, but it has requirements that are not easily met.
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