A Detailed Look at the Importance of Proper Loudspeaker Cabinet Bracing

Discussion in 'Loudspeakers' started by admin, Jan 2, 2012.

  1. admin Audioholics Robot Staff Member

    admin
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    A recent Audioholics article about loudspeaker cabinet bracing posited that one measure of quality is in a well-braced cabinet. Poorly or improperly braced loudspeaker cabinets vibrate unduly, coloring the overall acoustic output and lowering fidelity. In this article, we will explore the reasons that a stiff cabinet is a desirable feature of a well-designed loudspeaker based on established engineering mechanics using Finite Element Analysis to illustrate modal behavior of a cabinet under load with and without bracing. We will also discuss how improper understanding of this subject matter and a limited amount of measurement of a test mockup could falsely lead one to believe that using fewer braces, thereby lowering the panel resonance of a cabinet, is desirable. Simply adding more braces to a cabinet may not be the most effective way to reduce vibration IF those braces are not stiff enough to force higher modal behavior in the panel under time varying loads, such as music. As you will see in this analysis, the stiffer the cabinet, the lower the resonances become which will greatly reduce unwanted colorations of sound.
    [​IMG]

    Discuss "A Detailed Look at the Importance of Proper Loudspeaker Cabinet Bracing" here. Read the article.
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  2. lsiberian Audioholic Overlord

    lsiberian
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    A very thorough analysis.
  3. agarwalro Audioholic Ninja

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    I'll say something intelligent in 4 years (after I enroll and compete an undergrad in something science).
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  4. haraldo Audioholic Spartan

    haraldo
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    Perhaps Richard Vandersteen should read this too...... :D
  5. Swerd Audioholic Ninja

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    I looked this over twice with the same effect - my eyes glazed over. There was an a lot of math and computerized analysis that made the startling conclusion that cabinet braces are good and that more is better.

    I didn't see any attempt to correlate the physics of speaker cabinet vibration modes with audible results. Just how audible is the lack of bracing, and how much of a negative impact does it have on the speaker's sound? How much might bracing improve things? And finally, how much cabinet bracing is good enough and how much is over kill?

    The computer simulations were nice for predictions and certainly provided a large visual gee-whiz effect. But if the author had gone to the next level and demonstrated with real examples how all this actually audibly affects a speaker, the article would have been much better.
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  6. gene Audioholics Master Chief Administrator

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    The point of this article was to show the modal effects of panels under various loading and bracing conditions using the industry's most sophisticated modeling tool (Finite Element Analysis) used by Structural Engineers which David Waratuke is. It makes no pretense on audibility, that wasn't the point. Audibility depends on a lot of factors such as panel size, bandwidth of the drivers, how the cabinet couples to the floor, etc.

    Modeling tools are useful prior to prototyping an actual design and when building actual skyscrapers it is often the primary tool to use since once cannot build a 100 story mock up to test.

    WE have already done several articles on cabinet bracing showing REAL examples of speaker cabinets as can be seen here:

    Identifying Legitimately High Fidelity Loudspeakers: Myths & Facts about Cabinets — Reviews and News from Audioholics

    Lowering Mechanical Noise Floor in Speakers — Reviews and News from Audioholics

    Lowering Mechanical Noise Floor in Speakers Pt 2 — Reviews and News from Audioholics

    I would hope its clear to the reader that the stiffer the cabinet is, the better and that using the least amount of bracing to lower cabinet resonance is pure hogwash based on poor understanding of the subject matter and/or a deliberate attempt to justify cost reducing a product. The end result of a poorly braced cabinet, especially a subwoofer, is a boomy, hollow and colored sound in the bass.

    Years ago I had two identical speakers, one made from MDF and one made of a granite type of material. The speaker enclosure made of granite had a much snappier bass and lively feel to it. At the time I didn't really understand why since the wooden enclosure was quite inert and well braced too. The time we spent examining this subject matter has given me better insights since then however. Stiffness is a good thing that's not just limited to speaker cabinets ;)
    gene,
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  7. DavidW Audioholics Contributing Writer

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    Actually, I did address audibility of sound from the cabinet in the article, amongst the eye-glazing math, as referenced from Sound and Structural Vibration:

    I just didn't solve the equation for results from my particular cabinet panel, because as Gene mentioned, this was not the main point of the article, which was to look at what the cabinet panel would be doing with different amounts of bracing. It was taken for granted within the article, albeit with a few statements to reiterate, that the correlation between a greater amplitude of cabinet movement and a greater amplitude of air movement, and hence sound, was understood.

    Also, as Gene mentioned, the effect on audibility for any specific cabinet design would depend on the materials and geometry of the cabinet, braces, and other components particular to that loudspeaker.

    An investigation of the fluid-structure interaction to generate actual SPL results would be more than another article in itself. But there is a direct correlation between resonant modal peaks in the cabinet and sound transmission, and these same relationships govern why we don't want modal behavior in the driver diaphragms themselves within the operational frequency range of the drivers: modal resonances make noise.

    From another reference on the topic, Computational Aspects of Structural Acoustics and Vibration:

    [​IMG]

    I hope this clears some things up.

    David

    Attached Files:

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  8. kevon27 Annoying Poster

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    Hey.. What do you think about what Krell and some other companies are doing using 100% aluminium for speaker building? I think this way you don't have to worry about cabinet flexing.[​IMG]
  9. haraldo Audioholic Spartan

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    It's a much more expensive way of building a resonant cabinet, it flexes too but in other ways... Not even sure it's better than wood...

    Swedish Mårten design makes cabinets in Carbon fibre with the reasoning that resonances will be at a much higher frequency and as such easier to deal with, but it never eliminates the need for bracing...

    The Krell speakers seem so overpriced it's beyond measure.....

    Vandersteen killed resonances close to 100% in his new model 7, going to extreme measures with CLD, but still lots of bracing.....
    Last edited: Jan 4, 2012
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  10. Rickster71 Audioholic Spartan

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    [​IMG]

    I wonder what the reasoning is (or difference) behind so many horizontal braces, instead of one full length vertical brace?
  11. Adam Audioholic Jedi

    Adam
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    David - sorry if I missed you mentioning this, but did you verify that your results were insensitive to your mesh density?
    Adam,
  12. haraldo Audioholic Spartan

    haraldo
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    It brings resonances much higher up in frequency... where it will not make so much importance... as it will not be that audible... if we talk about a subwoofer, yes?
  13. gene Audioholics Master Chief Administrator

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    b/c you are effectively making one big panel infinitely smaller by adding more bracing, thus raising the resonance frequency. The RBH method is overkill but its for a $50k pair of speakers so one would expect them to over design it.

    I've written about this before and here is an excerpt:
    gene,
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  14. lsiberian Audioholic Overlord

    lsiberian
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    Speak for yourself. No amount of bracing is overkill. :p Especially when the tornadoes come.
  15. DavidW Audioholics Contributing Writer

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    Doubling the mesh to 0.25 inches changes the fundamental frequency of the unbraced 3/4 inch panel from 114.05 Hz to 114.13 Hz, but at a substantial penalty to the run time. The difference in the results are less than 0.1%.
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  16. Adam Audioholic Jedi

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    Thank you - looks like you nailed it! Did you check any of the braced cases, as well? I'm not trying to be insulting at all - I don't do this work, but I know enough to be dangerous...well, or annoying. :eek: :D
    Adam,
  17. DavidW Audioholics Contributing Writer

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    The use of a stiffer material could help, but would not necessarily make the vibration problem go away. It all depends on the cabinet geometry.

    The natural material stiffness, known as the modulus of elasticity, of aluminum is significantly greater than for MDF. Aluminum has an E = 10,000 ksi while MDF has an E = 580 ksi, so it is 17.2 times stiffer.

    However, the stiffness of an object is also a function of geometry, not just the material.

    For a plate, the rigidity term is D:

    [​IMG]

    For a beam, the rigidity term is EI, where I:

    [​IMG]

    In both cases, E is a first order term, while the thickness of the plate or the depth of the brace, transverse to the load, is a cubic term. Assuming the same spacing of braces, an equivalent wall thick to 3/4" MDF would be 0.297 inches based on plate rigidity and Poisson's ratio of 0.25 and 0.35 for MDF and aluminum, respectively.

    The other component in this is mass. Aluminum has a unit weight of 170 pcf and MDF is 50 pcf, so the mass is about 3.4 times the MDF. So at the same stiffness, 3.3 times more mass will change the frequency by sqrt(1/3.4) = 0.54. For the hypothetical unbraced panel in the article with a fundamental mode at 114.05 Hz, the frequency reduces to 61.85 Hz.

    If the Krell cabinet is 0.3 inches thick, all other things being equal, then MDF would still perform better than the aluminum at attenuating resonant vibrations.

    David
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  18. mtrycrafts Audioholic Slumlord

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    Now we are getting somewhere;):D
  19. Nuance AH Audioholic General

    Nuance AH
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    Here is a quote (edited the first part out because it doesn't apply to this topic) from Dr. Sean Olive that I found applicable to this thread.

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  20. haraldo Audioholic Spartan

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    It is a lot of details in here and hard to understand all details

    So it gives us a clue why experienced designers like late Jim Thiel would spend years only to perfectionize the cabinets of the CS 3.7

    I can't even start to think how the following will make the calculations more complex...
    - How about using several layers of constrained layer damping using different materials that improve stiffness and damping, will you be able to still perform calculations?

    - How will a carbon fibre cabinet change this? there are several options where you can get pre-preg materials, like used in advanced sailing boats or airplanes.... you roll this out and put it into the oven and it's being baked to an incredible strength and stiffness, carbon fibre also having a much better strength to weight ratio, less weight means quicker damping, yes?

    - The application of asphalt or other damping on the inside will have an impact, resonances will change and damping improves, how can this be catered for in this model?

    There's probably just about a million other things too.......

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