Wattage vs. Decibels?

Discussion in 'Amps and Cabs [BG]' started by jaymeister99, Oct 23, 2005.

  1. jaymeister99


    Aug 2, 2005
    Ive heard the general rule of twice the watts to make a 3 decibel increase.

    But then how many decibels does a 10w amp put out at say 5 feet? How about a 100w?

    Or a 500w at 5, 10, or 20 feet?

    If I had a 3,000w PA system running, would an increase to 6,000w still only add 3 decibels?
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    True. But it's not a general rule, it is an absolute.
    Watts and decibels do not have a direct correlation, so there is no answer.
    Same answer as above. Knowing only the wattage tells you nothing about the dB level. It's like asking how fast 300 horsepower will go. That depends. Is the 300HP in a Ferrari? Or a dumptruck?
  3. FireBug


    Sep 18, 2005
    This is where speaker efficiency (sensitivity) comes into play.
  4. Petebass


    Dec 22, 2002
    QLD Australia
    SPL = (Log(Wattage) x 10) + Sensitivity.

    So lets say we're using a 3,000W PA that has a sensitivity of 100dB (nice round figures).

    SPL = (Log 3,000) x 10) + 100
    SPL = 134.77dB

    Now lets see what happens when we use 6000w with the same speakers

    SPL = (Log 6,000) x 10) + 100
    SPl = 137.78

    The difference is 3dB.

    Now this formula isn't the be-all and end-all because sensitivity specs are usually given at 1K. Performance a 1K doesn't really tell us much about performance as a bass speaker.

    There is also a formula for calculating decrease in SPL with distance, but I don't know it off the top of my head and I'm about to charge out the door so I can't look it up now. If I get time I'll do it later, or you can try google.
  5. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    A rule of thumb I have heard is that doubling the wattage gets you about a 10% increase in perceived loudness. Obviously this is not as precise or objective as the watt/dB formula, but it may be more useful when you're trying to figure out how much wattage you want to pay for or lug around.
  6. jaymeister99


    Aug 2, 2005
    Im missing something here. This will probably sound dumb but bear with me.

    You have log 3,000 X 10, wouldnt that be 30,000? Then + 100 would add out to 30,100. I know that isnt right, how did you come to 134.77db?
  7. Snarf


    Jan 23, 2005
    New York, NY
    Log is an operation, you were leaving that out when you got 30,100.
  8. jaymeister99


    Aug 2, 2005
    so what is the operation, how does the full formula go?
  9. Thor

    Thor Moderator Staff Member Gold Supporting Member


    log 3000 is 3.477

    Try saying it this way.

    10 raised to the what power is = 3000
    10 ^3.477 = 3000

    Also, see here


    Section 3.3 Decibels
  10. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    Dang-it, Bill! You made me cackle out-loud at work!

    I s'posed to be writing a report...

  11. Thor

    Thor Moderator Staff Member Gold Supporting Member

    Bastage does that to me all the time! I start snarfing and then start choking on the coffee trying to exit thru
    my nostrils ...
  12. get a calculator, see the Log button, and the In button, they are to do with logorythms (log being standard exponential and In being natural log? i cant really remember)

    When you are doing maths when your about 15 you'll know what it is . . .
  13. jaymeister99


    Aug 2, 2005
    I got it now. Im now 34, I did do loads of math back when I was 15, regents sequence 1 to 3. But we didnt do logorithms till college. It was useless for me then, and still is now.

    WAIT, almost useless. Damm, if I could still remember all that stuff I would have one use for all of it now!!!
  14. fretlessrock

    fretlessrock Supporting Member

    Aug 8, 2002
    I was out to confirm my memory that SPL dropped at twice the distance increase and I found this:

    -6 dB
    The amount of loss in SPL as you double the distance away from a sound source.

    from the prestigious DJ University and there are some other fine(er) sources. Check them out.

    So, twice the distance, half the power. 100db@1W@1M would be 94db at 2m, and 88db at 4m...

    Just for fun, (round numbers, don't make me come over there!) you take that 100db@1w@1m sensitivity and add 3db for each doubling from 1W, and you hit the cab with 500W, and you get 127db, only 27% more SPL, not anywhere near the 500x increase in applied wattage.
  15. As has been said many times on similar threads, additional wattage in not only about volume, but quality sound and headroom in the lowest frequencies. A 100 watt amp can be hugely loud if you don't mind distortion and limited bass response (all other things being equal... which of course they never are). It's not about average increased volume, but more about IMO clean, forceful low end. Isn't that why, for example, Acme speakers take so much power, even though even with all that power they typically aren't as loud as other speakers with much less power.... it's because of all that low end coming out... the tuning of the cab makes it much less efficient, but results in a big low end for a cab its size. Same thing with a more efficient cab.... when you turn up the bass control and really start driving an amp to high volumes, additional wattage can really make a difference in the low end.... not just the 'average' volume of the cab.
  16. I have a dB calculator in my spread sheet. Plug in the SPL value of your driver, and read the dB at various distances and power levels.

    The sheet assumes no power compression or any real-world variables. It is simply a -6dB drop per doubling of distance, and +3dB increase per doubling of input power.
  17. ESP-LTD


    Sep 9, 2001
    Of course, this discussion has been about modern solid state amps, and does not apply to <br>
    Tube Decibels<br>
    because of their growly ohmishness. :)
  18. jaymeister99


    Aug 2, 2005
    which file on there is the db calculator?
  19. theshadow2001


    Jun 17, 2004


    Growly omishness should be made into an acutal engineering phrase
  20. jaymeister99


    Aug 2, 2005
    I found the spreadsheet with the calculator. Thats awesome! How do you find the sensitivity for a given amp. I looked on the Fender website, didnt say anything about the sensitivity on the amps.