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cabinet ratings ~vs~ max SPL

Discussion in 'Amps and Cabs [BG]' started by nil, Dec 19, 2001.


  1. more theoretical BS... ;)

    2 4x10 cabinets, with the same max SPL and sensitivity ratings.
    One cabinet 4Ohms 400w rating, the other 8Ohms 800w rating.

    All other things being equal, would there be any difference between the two if you matched the power correctly to them? ie would the 400w/4 cabinet (being fed with 400w/4ohms) sound different to the 800w/8 (being fed with 800w/8ohms)?
     
  2. MikeyD

    MikeyD

    Sep 9, 2000
    Okay - there is a contradiction here. If the cabinets have the same sensitivity (SPL @ 1w/1m), but you can feed one cabinet twice the wattage of the other, then it will ultimately have a max SPL that is 3 dB higher than the other. Theoretically, anyway.

    So - let's ignore the bit about max SPL and assume they have identical sensitivity. The impedance of the cabinets has no effect on their tone; it only affects what a particular amplifier may be capable of delivering to them in wattage. So if you can get 800 watts into the 8-ohm cabinet (using amplifier A) and only 400 watts into the 4-ohm cabinet (using amplifier B), then the 8-ohm cabinet will convert an equal percent of that 800-watt input to sound, so should be 3 dB louder than the 400-watt cabinet.

    On the other hand, if you are plugging each cabinet into the same amp, and the amp is a typical SS type, the 8-ohm cabinet is likely to receive less power than the 4-ohm cabinet, so will not be as loud *using that particular amp*. The rules are usually different with tube amps, though.
    - Mike
     
  3. Cool, I was just wondering 'cause you see a number of cabinets out there that have similar sensitivity and max SPL ratings, but different impedence or power handling...

    I guess I should've worded it as:

    Would it be worth pumping 800 watts into an 8ohm cabinet that has the same sensitivity/SPL rating as a 400w/4? Or would the gain be negligible?

    I know i've got some things arse-backwards here.

    My 8ohm 8x10 idea was quashed when I found that I had been mis-informed - they wire 'em either as a single 4ohm, or split into 2 8ohm halves. So 1200w/4ohm looks to be the option.
     
  4. Maximum SPL is not Sensitivty * Max Input Watts.

    Thiele specified the Sensitivity as typically 1w/1m, or what is called Small Signal response. The maximum SPL is a Large Signal resonse and is limited by Xmax, thermally limited, and limited by the maximum acoustic power a given driver is capable of generating. Just because the voice coil will accept 1,000 watts before melting does not mean the driver in question will automatically deliver +30dB more than 1w/1m when driven with 1024 watts.

    A driver in a sealed cabinet will run out of excursion long before the same driver does in a ported cabinet at the same input power.

    If you want to make more noise, add more cabinets.
     
  5. Yup, I understand that, but maybe i've cocked up again with my question...

    Will 800w pumped into an 800w/8ohm cabinet, with a sensitivity of 98db, and max SPL of 124db, be any different to 400w pumped into a 400w/4ohm cabinet (or 400w into a 400w/8ohm, whatever) with the same sensitivity and SPL ratings?

    Yep, this is probably not a great real-world example, but i'm from an island ya know... ;)
     
  6. geshel

    geshel

    Oct 2, 2001
    Seattle
    Yes.

    Forget the impedance for a second. If the sensetivity ratings for both cabinets are the same, then if you put twice the power in one, it'll be 3dB louder. (which isn't much!!).

    There are all kinds of things that will affect whether or not the speaker can handle that much power (what frequency? for how long? and all kinds of properties of the speaker itself), but I'm ignoring those to give you the simple answer to your question.

    bgavin:A driver in a sealed cabinet will run out of excursion long before the same driver does in a ported cabinet at the same input power

    Depends on the frequency. Not below it's tuning frequency, that's for sure! Around the tuning frequency, yes. Sufficiently above it, there's no real difference.
     
  7. Laddieo

    Laddieo

    Dec 16, 2001
    Aridzona
    How are the rules different for tube amps?
     
  8. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    The rules are the same.

    Only thing is that you can operate a tube amp beyond the specs without having the sound break up in an unpleasant way.
     
  9. The rules are different for tube amps:D. What Mike was getting at (I think) is that a tube amp will produce the same amount of power at different impedences, due to the use of an output transformer. Because of this, the actual power amp only sees the impedence of the primary side of the transformer. The caveat here is that you have to match speaker impedences to the secondary taps on the transformer. Most tube amps have switches to change output taps (4, 8, 16 ohms ,etc.). As for watts; a watt is a watt is a watt.
     
  10. Doesn't it take TEN times the wattage to add 3dB? I always get confused in this stuff.
     
  11. Not quite. Here's the scoop:

    2x wattage = 3dB: noticeable difference in volume.
    10x wattage = 10dB: apparent doubling of volume.
     
  12. Thanks! Impedence and such often leads me down the garden path... :D
     
  13. Nope, sorry. Take a look at a large signal plot and you will see steadily increasing excursion as the frequency goes lower.

    The curve is very similar to a photographic response curve. The shoulder/heel accelerate very slowly where the midline accelerates in a linear fashion. The response rolls off steadily below Fc at a 12 dB rate, and this is where the excursion acceleration begins to slow. If you add EQ, it accelerates very quickly. Sealed boxes require drivers with large Xmax to perform well in the low ranges.

    http://www.ofgb.org/reference/Music/Carvin/PS10 Sealed Excursion at 75 Watts.bmp

    This plot shows the large signal cone excursion for the Carvin PS10 driver used in the RL210 cabinets. 75 watts input is used, which is conservative, but still too much power for this driver.

    The horizontal red line is the Xmax limit for this driver.

    http://www.ofgb.org/reference/Music/Carvin/PS10 Vented Excursion at 75 Watts.bmp

    This plot is the PS10 driver at 75 watts in the RL210 cabinet and tuning. Note the sealed environment is actually harsher on the driver than the ported one. The sealed environment exceeds Xmax from 80 Hz and downward. The vented enclosure exceeds Xmax below 40 Hz.
     
  14. geshel

    geshel

    Oct 2, 2001
    Seattle
    Maybe you didn't understand what I was saying. The cone excursion in a vented box, below the tuning frequency, greatly exceeds that of the sealed box at the same frequency. In the pictures you gave, the tuning frequency of the vented box is 45Hz. Below that frequency, there is a rapid rise in the excursion, reaching a maximum which is double that in the sealed box.

    For that driver, in those boxes, at that power, if all you require is 45Hz and above, yes the vented box has lower cone excursion and hence higher power handling. Depending on all of these factors, the "best" design from an excursion standpoint may be either sealed or vented. Not to mention, the same driver might only work well in one type or the other, so it's hard to compare.

    My point was that your statment, without mentioning frequency, was not accurate. (if it wasn't clear, I was quoting you in my post)
     
  15. geshel

    geshel

    Oct 2, 2001
    Seattle
    P.S. check my profile - for a while this year you were exactly twice my age! :)

    [edit] OK, duh, on 11/4 of 2000 you were "exactly" twice my age.
     
  16. geshel

    geshel

    Oct 2, 2001
    Seattle
    Actually, I may have misunderstood you: by "long before", did you mean with decreasing frequency, or increasing power? Sounded like the latter to me, but now I see otherwise.
     
  17. A picture is always worth a thousand words.

    The vented plot shows why the damage potential is so high below the tuning frequency. I was taken to task here recently for discouraging operating a cab below the tuning frequency with any power applied. Despite the arguments given in this forum about tone, sounds good to me, etc, etc, the excursion facts stand.

    http://www.ofgb.org/reference/Music/Carvin/PS10 Vented Excursion at 75 Watts.bmp

    The PS10 example here clearly demonstrates how notes lower than low E at 75 watts will drive the PS10 beyond its rated Xmax. Due to the vented design, the lower the note, the faster the over excursion accelerates.

    http://www.ofgb.org/reference/Music/Carvin/PS10 Vented Excursion at 14 Watts.bmp

    This plot shows how the PS10 in the RL210 cabinet can be driven into over excursion at low B with as little as 14 watts input power.

    It comes down to the PS10 is just a cheap driver and just isn't designed for any more than 75 watts, and for notes lower than a 4 string.
     
  18. geshel

    geshel

    Oct 2, 2001
    Seattle
    . . . .so I think we're in agreement?
     
  19. MikeyD

    MikeyD

    Sep 9, 2000
    I note that when driving one channel of a Carvin R600 head into a 4-ohm cabinet, 75 watts is only about 5.2 dB below the max. output of 250 watts. The other thing is (reference Joris's thread on spectral content of bass guitar signals), I would assume that the 75 watts is strictly power at the fundamental frequency: that's a fair amount of power at one frequency. It would be interesting to see what the actual power is of the fundamental vs. harmonics when the amp is driven to its 250 watts (steady) maximum with a typical bass guitar note. I wonder how many of those watts are actually going to the fundamental itself.
    - Mike
     
  20. geshel

    geshel

    Oct 2, 2001
    Seattle
    I should be able to figure that out ... getting rusty :)

    But my guess is definitely less than half. In fact there's probably more power in each of the 2nd and 3rd harmonics than there is in the fundamental. The number might even be as low as 10%.