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Thread: The myth of generic optimum room dimension ratios

  1. #1

    The myth of generic optimum room dimension ratios

    Hi all,

    in 2012 in a thread relating to room dimension ratios Amir said:

    There are no good dimensions per-se. Much if not all of the research there was relative to one speaker/position, one listening position, and ideal rectangular rooms. Change those assumptions and you invalidate them.

    From Dr. Toole:

    "None of this is wrong [research into magic dimensions], but, in sound reproduction, it is irrelevant!

    I thought I’d expand a little bit on that, so here’s a short piece I’ve prepared some time ago.

    In audiophile circles optimum room dimension ratios (height: width: length) such as 2:3:5, 1:1.6:2.5, 1.236:2:3.236 (Golden rule ratio), 1:1.4:1.9 (Louden) are recommended and used, further known are optimization criteria from (Bonello 1981) and (Walker 1996). One of the first to mention room dimension ratios was W.C. Sabine in “Collected papers on acoustics”, Harvard University Press (London) 1922:

    “Thus the most definite and often repeated statements are such as the following, that the dimensions of a room should be in the ratio 2 : 3 : 5, or according to some writers 1 : 1 : 2, and others 2 : 3 : 4; it is probable that the basis of these suggestions is the ratios of the harmonic intervals in music, but the connection is untraced and remote. Moreover, such advice is difficult to apply; should one measure the length to the back or to the front of the galleries, to the back or the front of the stage recess? Few rooms have a flat roof, where should the height be measured?”

    However, the concept of optimum dimensional ratios was originally conceived for reverberation rooms, where sound fields of mechanical devices are measured. Such devices often produce noise, i.e. the whole audible frequency spectrum, or major parts thereof, simultaneously and all the time. For measuring the sound field microphones are placed all around the device. Since the whole spectrum is constantly emitted, all of the possible room modes are excited all the time. In order to obtain useful readings from all microphones it was important to have a uniform distribution of the resonance mode frequencies on the frequency scale. Somehow this concept has migrated into home audio (Toole 2006).

    It should further be noted that all formulas for the calculation of room mode frequencies are based on the assumption that the room is empty, has perfectly reflective walls, and no wall openings. Large absorbing furniture is capable of shifting mode frequencies and lower mode levels (De Melo 2007). Large reflective furniture is capable of splitting up modes, hence generating two modes instead of one (Bork 2005). Wall openings are structural weaknesses and locations of pressure maxima and minima are shifted (Welti 2006, Toole 2008). It has further been shown, that the mode frequencies measured in real rooms may be substantially different from those calculated (Toole 2008, fig.13.8).

    In non-rectangular rooms these known “optimization criteria” do not apply anyway, and methods such as Finite Element Methods have to be used (Bolt 1939, Van Nieuwland 1979).

    The goal of all of these optimization methods is to arrange all of the modes evenly on the frequency axis. Therefore, in domestic listening rooms, in order to experience the benefits of optimum ratios, all of the modes must be excited, simultaneously and at equal levels, and the listener must be able to perceive all of them, again simultaneously and at equal levels. This is possible only when source and listener are positioned in corners. Anywhere else not all of the modes are equally energized and are equally audible (Toole 2006). When playing music, in any randomly selected position of source and listener only some of the modes will be (partially) excited and only some of those excited modes will be heard, so any ratio will be as good (or bad) as any other. None will be optimum. In my own listening room the modes are well audible at the listening position when playing pure sine tones, With music I have so far detected only three tracks where the second order width mode is, at the listening position, audibly excited. Hence, in my room of the 25 calculated modes below the Schroeder-frequency (because of the acoustical ceiling I’m not counting the tangential and oblique modes where the ceiling is involved) only 1 is actually disturbingly excited when playing music.

    In listening tests using music samples Fazenda (2005) found: “A room that scores highly on a certain metric may still suffer from problems, since the detection cue is one of stimulus–room interaction rather than one of room response per se. The modification of modal distributions by the manipulation of room aspect ratios is therefore shown to be of doubtful utility in the reduction of the unwanted subjective effects of room resonances. Controlling the problems of room modes by the consideration of expressly degenerate modal superpositions may still be regarded as appropriate at a design stage. However, the results presented here clearly suggest that overly zealous considerations of the modal distribution are likely to be rather peripheral to subjective room performance.”

    By looking only at the eigenfrequencies of a room, the relative excitation of each mode by a real source at a particular position in the room is not accounted for. Equally, the sound pressure resulting in a particular listening position, which pressure varies greatly, is not accounted for. For instance, Bonello’s approach is moderately useful with a single source in a corner, and has reduced usefulness when the source is not in a corner (Welti 2009).

    All of those optimization methods are hence, inherently, designed to obtain optimum conditions for room corners only. If possible, ratios where one dimension is a multiple of another (square, cube) should be avoided, but even in this case, the result is not necessarily worse (Fazenda 2005, Wankling et al. 2009).

    “So it is not that the idea of optimum room ratios is wrong, it is simply that, as originally conceived, it is irrelevant in our business of sound reproduction.” (Toole 2006).

    The only possibility to employ the concept of optimum dimensional ratios is to know in advance the exact location of loudspeakers and listener. This in turn means that the benefits of such a ratio are experienced only in one single location in that room. In any other location a listener will experience different bass. For this listener (or listeners) the energy in the corresponding resonances must be attenuated, by absorption, equalization, or mode cancellation by use of multiple subwoofers (Welti 2002). In non-rectangular and asymmetrical rooms additional signal processing in the feeds to the subwoofers is necessary (Welti 2003, 2006).

    In a conference paper from 1990 Toole says: „It has long been puzzling that music and speech can sound as natural as they do in rooms that are horrendously flawed by numerous resonances. The explanation seems to be that room modes are generally medium- to high-Q phenomena. In steady-state measurements, such as these, modes are very much in evidence. However, when excited by the sounds of speech and music, which are mostly transient or discontinuous events, they are not always as apparent to the ear as the measurements suggest.”



    References

    Bolt, “Normal modes of vibration in room acoustics: experimental investigations in nonrectangular enclosures”, J. of Acoust. Soc. of America 1939, vol. 11, p.184

    Bonello, „A new criterion for the distribution of normal room modes“, J. of the Audio Engineering Society 1981, p.597

    Bork, „Modal analysis of standing waves (in German)“, Progress of Acoustics, DAGA ’05, 31st Annual Convention of Acoustics. (German Society of Acoustics), Munich 2005

    Fazenda et al., “Perception of modal distribution metrics in critical listening spaces - Dependence on room aspect ratios”, J. of Audio Engineering Society 2005, p.1128
    ¬
    Louden, „Dimension-ratios of rectangular rooms with good distribution of eigentones”, Acustica 1971, vol. 24, S.103

    De Melo et al., “Sound absorption at low frequencies: room contents as obstacles”, J. of Building Acoustics 2007, vol. 14, no. 2, p.143

    Toole, “Loudspeakers and rooms for stereophonic sound reproduction”, Audio Engineering Society 8th International Conference 1990: The Sound of Audio

    Toole, “Loudspeakers and rooms for sound reproduction – a scientific review”, J. of
    the Audio Engineering Society 2006, p.451

    Toole, „Sound reproduction - Loudspeakers and rooms”, Focal Press 2008

    Van Nieuwland , “Eigenmodes in non-rectangular reverberation rooms”, Noise control engineering 1979, Nov., p.112

    Walker, “Optimum dimension ratios for small rooms”, Audio Eng. Soc. preprint 4191 (1996)

    Wankling et al., “Subjective validity of figures of merit for room aspect ratio designs”, Audio Eng. Soc. Preprint 7746 (2009)

    Welti, “How many subwoofers are enough”, Audio Eng. Soc. preprint 5602 (2002)

    Welti, “In-room low frequency optimization”, Audio Eng. Soc. preprint 5942 (2003)

    Welti, “Low-frequency optimization using multiple subwoofers”, J. of Audio Eng. Soc. 2006, p.347

    Welti, „Investigation of Bonello criterion for use in small room acoustics“, Audio Eng. Soc. Preprint 7849 (2009)

  2. #2
    Member Sponsor [WBF Founding Member] FrantzM's Avatar
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    Hi

    Excellent well researched post. I had an intuition to that but this anchor my point of view. Thanks.
    Last edited by FrantzM; 02-22-2013 at 08:36 AM.
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    VIP/Donor [VIP/Donor] microstrip's Avatar
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    Analysis of room dimensions is mostly a question of common sense , together with some knowledge of acoustics - it can help you trace existing problems and if you are lucky enough and you have one room that pleases one of the several different criteria you pick this one as the best existing work in acoustics and create a big expectation bias in your room!

    The big issue is not the calculus - existing computing power could simulate with large accuracy and resolution the response of all small rooms with small pitch. The main question is what defines a good small room. There are always compromises and the way we weight them will return very different results. As you say, Toole considers that the importance of room dimensions are overestimated. In part perhaps because his perspective is mainly focused on home theater systems, that most of us accept have different bass requirements from stereo listening.

    The optimum dimensions have cost me some trouble and money. My listening space was originally a long room - length is more than twice the width. When I found that building a wall could separate it in a room having the golden rule ratio dimensions and a good independent stowage space I decided to have it built. It was a disastrous decision, only corrected a few months later by removing it ...

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    [WBF Founding Member] Addicted to Best! JackD201's Avatar
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    It is over estimated, without a doubt because the construction is just as important. I wouldn't go as far as saying however that they are irrelevant. From a cost perspective, starting off with good dimensions makes the job of the acoustician much easier. Perhaps Toole was referring to very small rooms and systems with more typical bandwidth. Half wave will trump quarter wave. I don't think that there will be any debate about that. If he's saying, any dimension can be dealt with rendering the ratios irrelevant, in an absolute sense that may be true. It does not mean however that it will be just as easy.
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    I don't think it's completely irrelevent. What if you have a cube room... 10' x 10' x 10' Wouldn't that not work at all?

    So if you know the speaker/listener location and could build a room around it, wouldn't the inverse be true? You could take the dimensions of the room and know exactly where the speakers/listener "should" be? Where's the calculation for that?
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    [WBF Founding Member] Addicted to Best! JackD201's Avatar
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    Quote Originally Posted by Bruce B View Post
    I don't think it's completely irrelevent. What if you have a cube room... 10' x 10' x 10' Wouldn't that not work at all?

    So if you know the speaker/listener location and could build a room around it, wouldn't the inverse be true? You could take the dimensions of the room and know exactly where the speakers/listener "should" be? Where's the calculation for that?
    How does one deal with a cube? Build into it so it's no longer a cube. Sounds like a joke but it isn't. Perfect example Bruce.
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    Lightbulb

    Quote Originally Posted by KlausR. View Post
    I thought I’d expand a little bit on that, so here’s a short piece I’ve prepared some time ago.
    Nice post Klaus. As Bruce explained, room ratios are not irrelevant. But I agree their importance is often overstated. One key point that many overlook is that even with "ideal" dimensions in a suitably large room, you still have flutter echo, peaks and deep nulls, comb filtering, and modal ringing. the notion that a purpose-built room doesn't need bass traps and other acoustic consideration is misguided.

    --Ethan

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    Member Sponsor [WBF Founding Member] FrantzM's Avatar
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    Quote Originally Posted by Bruce B View Post
    I don't think it's completely irrelevent. What if you have a cube room... 10' x 10' x 10' Wouldn't that not work at all?

    So if you know the speaker/listener location and could build a room around it, wouldn't the inverse be true? You could take the dimensions of the room and know exactly where the speakers/listener "should" be? Where's the calculation for that?
    Hi
    If you read carefully, this is covered ..
    All of those optimization methods are hence, inherently, designed to obtain optimum conditions for room corners only. If possible, ratios where one dimension is a multiple of another (square, cube) should be avoided, but even in this case, the result is not necessarily worse (Fazenda 2005, Wankling et al. 2009).
    I am reminded of one gary L. Koh installation in a round room. According to Gary it worked. I was skeptical and would always avoid a round or cubic room but ... The other point that is worth mentioning is the position of the listener and the speakers in the room. The nulls and peaks relative positions and levels are also a function of those. You move a sub or add another and the level, distribution and frequencies of the modes change.
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    — E. F. Schumacher
    (mis-attributed to A. Einstein)

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    VIP/Donor [VIP/Donor] microstrip's Avatar
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    My relative inexperience in these matters has shown me that one of the biggest problems in small rooms are nulls caused by cancellation due to reflections. I am not aware that the optimum dimensions often referred consider this aspect.

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    -- Great post Klaus. ...Thank you for having taken the time.
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