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Omnidirectional cut-off point...

Discussion in 'Amps [BG]' started by Russell L, Dec 4, 2012.

  1. Russell L

    Russell L

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    I've heard it said, "Don't worry about the lows, they're omnidirectional" when it comes to talking about speaker radiation. Ok, I understand that, but at what frequency does it cease to be so? I realize that it's probably a gradual thing, but approximately where does it begin to not be omnidirectional?

    Thanks for any help.:bassist:
  2. Kmonk

    Kmonk

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  3. wcriley

    wcriley

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  4. lowfreq33

    lowfreq33

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    Around 80 hz.
  5. Russell L

    Russell L

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    Thanks, Kmonk, but it really didn't answer the question. Good reading, though.
  6. Russell L

    Russell L

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    Around 80Hz, huh? Interesting, and about what I expected.
  7. Russell L

    Russell L

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  8. KJung

    KJung Supporting Member

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    The Greenboy chart posted is pretty good. For a 12" driver, a bit above 1K starts to beam a bit (i.e., meaningful 'directionality'). Luckily for many bassists, not a lot of energy coming from the bass guitar up there. For those who like what is called a 'modern' tone on TB (plenty of lows, a dip in the upper mids around the 1K area, and then most of the high end coming from a tweeter), it is pretty much a non-issue.

    For those that like a lot of upper midrange grind in their tone or general upper mid/lower treble brightness, a vertical 210 cab or a cab with a mid driver will work quite well, and provide some help/improvement in spreading out that slip of frequency between the top of a driver and when a tweeter kicks in (if you have a two way cab).

    Also, remember 'side by side' configured cabs like a 410 will behave more like drivers the size of the total side by side width, but this still won't impact much below about 800hz.
  9. 1958Bassman

    1958Bassman

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    It isn't the same for all drivers, although there is similarity when a particular size is considered. The cabinet matters- if the driver is at the edge, it doesn't experience the diffraction that occurs when the distance from the driver's edge to the cabinet's edge is greater.

    Are you looking for more or less horizontal coverage at higher frequencies? If you want better coverage, narrow, vertical alignment is better (this is the reason line array speakers are used for PA systems in large venues).
  10. KJung

    KJung Supporting Member

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    If you are talking about complete omnidirectionality, that is about right (which is why it doesn't matter where you put a subwoofer in a home theatre system, or a gig for that matter). However, the stuff that can be an issue for some bassists (and a big issue for guitarists and PA cabs), depending on cone diameter and box configuration, starts at around 1K or so for the most part, and still spreads out pretty well once you get into the room.

    Big issue for guitarists, since most of their instruments' energy is in the upper mids/treble, and they don't use tweeters.
  11. countrybass007

    countrybass007

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    How about the Phil Jones cabs? Yhey are much more omnidirectioal than most cabs, and put the bass through the mix better.
  12. bgavin

    bgavin Supporting Member

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    If you want to calculate it:

    Fully Spherical = Wavelength >= 4x Piston_Diameter
    Fully Beaming = Wavelength <= Piston_Diameter

    The piston diameter and wavelength are in the same units.
    Sd is usually given in centimeters, which is used in this example.

    Worked example 3015LF:
    Speed of sound = 34630 cm/sec at 25C dry air
    Sd = 881 sq.cm
    Dia = (881/3.1416)^0.5 * 2 = 33.49 cm

    Beaming = (34630/33.49) = 1034 Hz
    Spherical = (34630/4*33.49) = 258 Hz
  13. KJung

    KJung Supporting Member

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    They are very mid present, but I don't think they 'spread' any better than a larger driver cab, due to those drivers stacked side by side.

    However, +1, in general, a cab with a bit less true low end and a higher dose of mid midrange will slot itself in many mixes nicely.
  14. Russell L

    Russell L

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    I wonder how my Markbass Traveler 151P cabs fit into the mold here?

    1958Bassman, no, I'm not looking for anything in particular. It's just a thought I had in my quest to understand things better.
  15. KJung

    KJung Supporting Member

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    Shouldn't be a big deal. However, with that 15 starting to beam pretty well above 1K, and the little piezo tweeter only covering the very highest frequencies (i.e., once the tweeter kicks in, the beaming pretty well stops, and many tweeters start around 3K or so... not that little piezo though), you will notice a bit of darkening of the upper midrange/lower treble (i.e., that grind or 'bright' tonal area) if you move significantly to the side of the cab on the stage. Once you get out into the room, since the beaming is relatively gradual as you increase frequency, you will be relatively even through the range of that cab. Probably not much above 2K coming out of that driver anyway, so again, not a huge deal.... narrow strip of frequencies impacted.

    If you are a P Bass with flats guy, you would hardly notice the more directional lower treble. If you have a lot of that 'Steve Harris' grind in your tone, the people close to and on the extreme sides of the stage will hear less grind than those more in the center or further back in the room. If you are a 'roundwound J' type guy, not a huge deal, since many players cut the frequencies around 1K anyway.

    IMO and lot of IME on this one:)
  16. Russell L

    Russell L

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    Thanks for that, Ken. Actually, I play flats on my Jazz Bass Special (often using just the P pickup) and my regular Jazz Bass. Interestingly, the regular Jazz can sound fuller than the Special with the P pickup due to bigger string guages. I just noticed that the other night. Maybe the pickups have an effect as well, dunno. Another thing that throws a wrench in my eq is that I play with a bit of fingernail on my plucking hand. That fact makes me wanna turn down my high mids often to quell the clack some, but that's also a range that sounds good without fingernails, so I have to compromise some.
  17. KJung

    KJung Supporting Member

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    That 'fingernail plucking tone' is in the range of frequencies that doesn't spread nearfield quite as well as the lower freqs, or the higher freq's coming out of the tweeter. However, since you are turning down the high mids anyway, it, as I posted above, becomes a relatively minor deal in live performance, especially out in the room.
  18. Russell L

    Russell L

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    Yes, I have noticed that while in a mix I can leave all the eq knobs at noon and not ever hear my fingernails, unless I REALLY try to. Good to know that it doesn't come out too much out front. I use them only because I also play guitar.

    I love when I ask question on gig day. I'll get to roll this all around tonight at the jam.
  19. will33

    will33

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    Does the baffle and/or box size/dimensions come into play here as part of a radiating surface or something....this pushing the frequency lower?


    I ask because when reading through Bill's stuff on PA system setup/integration, subwoofer placement, etc., 100hz is almost a standard corner frequency between subs and tops and almost never higher than 120, and then only if needed because the tops can't play low enough or other placement issues that requires the subs and tops to be placed closer together than is "optimal". The idea being for the subs that "a blind man couldn't find them", meaning omnidirectional, or beginning to become a bit directional around that 120hz mark.



    @ the OP, yeah, it's a gradual thing where things become incrementally more directional the higher you go. It doesn't instantly become directional. Actually, the directionality isn't too much of a concern until you get inside of a 90 degree horizontal pattern. Some designers may try to negate the very wide dispersion of woofers in the lower frequencies that spread beyond 90 degrees for example to better integrate with the dispersion of the higher frequency components of the system. Purposely having narrow vertical dispersion can help with floor/ceiling interference. Some subwoofers try to design in such a way as to create a beam where there normally wouldn't be one and try to steer it so as to focus the subs energy toward the audience where you need it and not waste half it's output playing to a backstage area at an outdoor concert for example. (omnidirectional).

    And all sorts of other cool stuff I don't fully understand.:D
  20. bgavin

    bgavin Supporting Member

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    The baffle step is an additional influence, but is beyond the scope of the OP question.
    The calculation above shows the math for a simple point source piston.

    Subwoofer steering requires a number of boxes to make it work.
    Reference: McCarthy, Sound Systems Design and Optimation.

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