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Anudder Q about loudness and SPL...

Discussion in 'Amps and Cabs [BG]' started by GeorgiaHonk, Jun 15, 2004.


  1. Okay guys, I’ve done a few searches and have been reading a few threads concerning loudness and max SPLs, and the relationship with power and sensitivity. Thanks to Petebass and a couple other Tbers, I now understand a bit more about the math involved with determining the max SPL from the sensitivity and the power. But my question applies to multiple cab combinations. If I have one cab with a max SPL of 117 dB, and another with a max SPL of 126 dB (at the same power level,) what would be the SPL of the two combined? I know intuitively that it can’t be 243 dB. So how do you calculate the loudness expected from two cabinets rated at different sensitivities?

    Also, how does the cone area of the respective cabs enter into the loudness equation? Surely the fact that more speakers would "move more air" must have an effect on percieved loudness, above what the calculated max SPL is.

    Thanks in advance,
    Roger
     
  2. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    If you have two coherent sound sources (such as two loudspeaker cabinets putting out the same sound in the same acoustic space) and one is 9 dB louder than the other, the net SPL on-axis would be about 1 dB louder than with just the louder one alone.

    If the sources are non-coherent (e.g., each producing different sounds), the increase over just the louder one alone would be about 0.5 dB.

    The one that is 9 dB softer is radiating about 1/8 the acoustic power to the listening point that the louder one is. Therefore, the net acoustical power increase is 1.125. Coherent sounds sum on-axis at 6 dB per doubling, as opposed to 3 dB for non-coherent sounds. But coherent sounds will start to cancel as you move off-axis.
     
  3. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Bob is correct.

    BTW, the coherent sounds MUST cancel elsewhere, since the 6dB apparent increase for equal coherent sounds would imply a power increase of 4 times. But you only increased power by a factor of 2, which should be 3 dB.

    Therefore, the apparent 6dB is a "directivity" increase, meaning that some areas get more power than before, and some must receive less. *
    This generally works best when the two speakers are set next to one another and pointed in the same direction.


    *Techie detail:
    The total power "integrated" over a surface surrounding both speakers must increase only 3 dB.
     
  4. As I reported in the thread about mixing Acme cabs with others I felt the sound was "bigger". Not very scientific, but that was my perception both on stage and 40 ft out front.
     
  5. geshel

    geshel

    Oct 2, 2001
    Seattle
    Thank you! I should have had this concept in my head by now, but for some reason I didn't.
     
  6. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Until I got the directivity part some years ago, it didn't make sense to me, either!
     
  7. fleetwood

    fleetwood

    Aug 29, 2001
    Swansea UK
    Hi Guys
    I've just come in on this thread to ask a question that may relate to this subject.
    I have a Gallien- Krueger MB150E and the 112 extension cab that matches it. For maximum loudness, should I place them side by side, pointing in the same direction. Or side by side in angled direction. Or wherever?
     
  8. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Side-by-side in same direction should usually give the maximum directivity and SPL out front.

    Angling outwards will increase dispersion, but cost a bit of the SPL increase.

    A slight angle inwards *may* improve directivity, depending on the speaker.

    In general, you will put them where they fit unless the bar stage is bigger than most.........so it goes as it goes......!
     
  9. fleetwood

    fleetwood

    Aug 29, 2001
    Swansea UK
    Thanks for that. It helps to know the best placing.
     
  10. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    What Bob and AI said! In plainer English, the louder one usually buries/overpowers the quieter one, so your overall increase in SPL is barely noticable.

    There are situations where mis-matched output levels can be an advantage. For example if the quieter driver is a tweeter and the other one is a woofer, it could save you the trouble of attenuating the tweeter (in practice this is a very unlikely scenario). Or if you find yourself on a massive stage with the quieter speaker right beside you and the other one some distance away....... but generally speaking you're better off trying to match sensitivities if possible.