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Thread: Baffle Width and BSC

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    Baffle Width and BSC

    Hi there, I always worry I am in wrong subforum, so feel free to move if needed and apologies.

    I am planning a 3 way using 2245, 2251J and a 1" yet to be determined. As it stands now, I have some 8 cubic foot ported cabs for the 2245, and am looking to put an enclosure into service for the 2251J soon. I need to determine a few things first on that. These will not necessarily be permanent and am considering what enclosure route to choose. I am not sure if I will use my DSP active system, or try to pursue a passive crossover yet, if that matters.

    Is there any benefit to the all in one large wide cabinet like the 4345 uses? I'm guessing baffle size still plays some factor in low end response? Is utilizing separates like some others and Greg Timbers uses in his Array horned system better?

    Reality is that I'll never make an exact 4345 replica for various reasons, but would like to know if it's worth pursuing a large all in one cabinet or separates? And what role of any baffle step would play in going from one large baffle to lower profile separates. Thanks in advance.

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    Administrator Robh3606's Avatar
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    Interesting question

    I have never used baffle step compensation on any DIY I have ever made. If you go through JBL schematics I have never seen it used even on the smallest systems like an L1 or L20T as an example. If you go with separate boxes depending on the size like a small cube as an example you might want to extend the baffle a bit depending on how much ripple you get in the drivers passband and to match the width of the lower cabinet.

    The only modular work I have done is woofers with separate tops where they match the lower cabinets and are otherwise full baffle width. That's an L250 Jubilee top I had an MTM and a midrange waveguide as well and a HEIL too.

    Rob
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    If I understand correctly, you're saying that a sharp transition between two different widths will cause the ripple? I noticed the angles cut down on Greg's setup, and it would make sense seeing your tops and bottoms mate. If I have my 8cuft cabs vertical, there is less width to contend with, but getting the HF at an appropriate height becomes an issue. If I place the cab on its side, things will be lower but the baffle wider and more floor bounce. I'll have to take some measurements and see where things will line up.

    It's interesting your mention of lack of BSC in most designs you've encountered. I wonder if not dealing with BSC is one way to keep sensitivity up without losing some to BSC. Then again, any X.5 way design can also compensate for baffle step, and they've used that in a few designs. Don't some of the bigger dual 15" speakers have a woofer that is crossed lower than the other?

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    Administrator Robh3606's Avatar
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    Quote Originally Posted by mortron View Post
    If I understand correctly, you're saying that a sharp transition between two different widths will cause the ripple? I noticed the angles cut down on Greg's setup, and it would make sense seeing your tops and bottoms mate. If I have my 8cuft cabs vertical, there is less width to contend with, but getting the HF at an appropriate height becomes an issue. If I place the cab on its side, things will be lower but the baffle wider and more floor bounce. I'll have to take some measurements and see where things will line up. It's interesting your mention of lack of BSC in most designs you've encountered. I wonder if not dealing with BSC is one way to keep sensitivity up without losing some to BSC. Then again, any X.5 way design can also compensate for baffle step, and they've used that in a few designs. Don't some of the bigger dual 15" speakers have a woofer that is crossed lower than the other?
    Hello Morton

    Actually a combination of step and actual box size used for the midrange. If you look around on the net you can find simulations that show the ripple and the differences between a circular vs a rectangular box. The larger the baffle width the lower in frequency it occurs so you could make the step insignificant depending on your lower crossover point vs baffle size.

    I would say you are correct on about loosing sensitivity. Both Everest 2 and the 4435 have helper woofers crossed lower, so do the Urie 813 and 815 monitors.

    Take some measurements and see what you get. Some people wouldn't think about building a speaker without BSC so it's design choice.

    Rob


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    Senior Member Eaulive's Avatar
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    I have to ask... and maybe I'll regret it but, what is BSC?
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    Sorry... Baffle Step Compensation.

    The way I understand is at a given width, a driver's low end rolls off (6db/Oct IIRC) and must be compensated for. This results in a loss of sensitivity. Less of an issue with a giant baffle or wall mounted stuff. Hopefully someone more eloquent and knowledgeable than I can chime in too.

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    Senior Member RMC's Avatar
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    Read your two lines many times to make sure i really understand what you mean by BSC, the more i read it the more i see this as being on the verge of LF Diffraction Loss from certain enclosures or their placement.

    Weems, Eargle, Murphy explain this. Simply Google John L. Murphy Diffraction Losses (he's a physicist associated with the True Audio Web site). Simply reading the first two paragraphs or so of his text you'll know what its about.

    The circular vs rectangular box, as well as a number of other cab shapes and their effect, is one of many experiments from Harry Olson (Acoustical Engineering book), shown and discussed by Eargle in his Loudspeaker Handbook.

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    I have a bit of a learning disability, so I am not very good at learning or teaching certain things.

    If you look at a frequency response graph for a given speaker driver, it's usually assuming it's placed on an infinite baffle (at least that is what I gather). This is why wall mounted speakers usually do not need BSC and have good bass. I also dabbled in open baffle (free air) speakers, so there is likely some overlap and muddying as I know in an open baffle, you want to keep the back from affecting the front, and a large baffle means it's much more difficult for the out of phase signal to be a detriment to the front waves.

    Naturally in a box speaker, this isn't an issue with out of phase signals, but the BSC thing for some reason still exists. If you take a speaker designed to be placed right against a wall, it's likely the crossover doesn't use BSC. It's also the reason why in a speaker with it, you'll see a 92db woofer in a system that's like in the mid to high 80s, as that 92db loses some to BSC. I could be way wrong tho.... Please realize that. I dunno. Like I said, I have some issues with learning. And it's not for a lack of wanting or trying.

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    Senior Member RMC's Avatar
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    Unfortunately i don't have the time to "hold you by the hand" at every step, i.e. explain this all over in details.

    RE it's usually assuming it's placed on an infinite baffle. Speaker design software and LF specs often assume cab 2Pi boundary condition, i.e. box on the floor, flush mounted in a large wall or Ground plane.

    RE in a box speaker, this isn't an issue with out of phase signals, but the BSC thing for some reason still exists. Since you refer to "have good bass" i can see a reason why it would still exist, you call it BSC and i call it LF diffraction loss like John Murphy, its related to speaker enclosures placement, most often in a room, even with a closed back or sealed box.

    Speakers on a base or on stands are not 2Pi, not being directly on floor, in wall or on the ground. Therefore you have some low bass loss, i.e. not getting as much LF reinforcement from the room for example. An image given to me a while back to describe that was the omnidirectional "bass wraps around the enclosure". The lower level bass in such situation is the reason why a well known place will frequently pad the mid and/or tweeter in a system to avoid having too loud MF/HF vs reduced LF level. This being done even if the woofer and tweeter for example, have the same sensitivity rating!

    That's the time i had for this issue now.

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    Baffle step compensation is the crossover compensating for the low end roll off you're describing, no? Ones the symptom and the other is the treatment, no?

    How the roll off happens, like I said, I cannot explain it as I have a bit of an issue. I grasp it, and while i cannot explain it, I don't believe I asked for any hand holding. Just stated that as I don't want my word taken as gospel here as I know my ability to explain things isn't always clear or lacking in detail.

    I will look at those writings you mentioned when I get a chance tho, thanks.

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    Senior Member Eaulive's Avatar
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    Quote Originally Posted by mortron View Post
    Sorry... Baffle Step Compensation.

    The way I understand is at a given width, a driver's low end rolls off (6db/Oct IIRC) and must be compensated for. This results in a loss of sensitivity. Less of an issue with a giant baffle or wall mounted stuff. Hopefully someone more eloquent and knowledgeable than I can chime in too.
    Ok, gotcha. You're referring to half space, quarter space, eighth space speaker placement? I never heard about a crossover compensation for that except on some amplified studio monitors, but usually this is done in the final EQ after setup is finished.
    I don't know about baffle itself because at low frequencies the wavelength is usually long enough not to be bothered with a couple of inches.

    Then again, this is way above my pay grade
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    RMC is probably the one to listen to, as he understands a lot more than I do, and I feel I've just muddied things.

    I understand that it happens, but my ability to explain is severely limited. I just know over on the other audio forums, it's usually a consideration to compensate for it when designing a crossover.

    Am just trying to figure out what I need to consider if I were to make a new cabinet for my 2245, and whether that would affect crossover design much. Was hoping to use the 4345 woofer filters for a starting point.

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    The short version of BSC is how much does the surface of the baffle add to the speaker output by supporting the wavelength being produced. For the likely highest frequency of interest in an 18" woofer, say 250hz the wavelength would be 4 feet. You'd have to build a fairly large enclosure for BSC to be of interest. Don't sweat it.


    Quote Originally Posted by mortron View Post
    ...

    Am just trying to figure out what I need to consider if I were to make a new cabinet for my 2245, and whether that would affect crossover design much. Was hoping to use the 4345 woofer filters for a starting point.

  14. #14
    Senior Member RMC's Avatar
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    Might have found an illustration of what the OP means or has in mind, when talking about cab width, in a pair of EMES OWL monitor speakers with side panels. The pair of tweets do seem like an Owl staring at you!

    Eargle describes and explains that monitor. Shortly, its fairly complicated in terms of electronic processing (patented), one box with two sides and a divider between woofers, it radiates diagonally outward toward each side of the room. I'm guessing the side panels here might be used to reduce LF diffraction loss/BSC. Since Eargle's material is copyrighted i'm giving a link to a "pair" of OWLs for sale.

    I don't necessarily agree that i'm the one to listen to, i've been wrong before and will be in the future, though i try to check my stuff before posting. That said I agree with both Eaulive and Riley.

    The "crossover compensation for that except on some amplified studio monitors", (note the OWL is two independant two-way biamped monitors), plus their mentions about wavelength.

    When you think of it why go through all the motions of a potentially complex crossover if simply reducing MF/HF output balances things or even adding removable or folding side panels if its really felt to be necessary, might do the trick. The ones on the OWL seem to be plexiglass or similar.

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    Administrator Robh3606's Avatar
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    Quote Originally Posted by mortron View Post
    RMC is probably the one to listen to, as he understands a lot more than I do, and I feel I've just muddied things.

    I understand that it happens, but my ability to explain is severely limited. I just know over on the other audio forums, it's usually a consideration to compensate for it when designing a crossover.

    Am just trying to figure out what I need to consider if I were to make a new cabinet for my 2245, and whether that would affect crossover design much. Was hoping to use the 4345 woofer filters for a starting point.

    For an 18" I wouldn't worry about it. That's what I was saying when I said some would not even consider a crossover without incorporating BSC. Usually it's using smaller drivers with narrow baffles. You should be fine using the 4345 crossover as a starting point. There is no BSC in that circuit just a zobel for the impedance rise.

    Rob
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