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Power vs loudness

Discussion in 'Amps and Cabs [BG]' started by seanm, Feb 20, 2005.

  1. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    It is generally accepted that for bass amps you need 10x the power for twice the volume. Is the same true for guitar amps? I would think that frequency would come into play.
  2. Passinwind

    Passinwind I Know Nothing Supporting Member Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Actually,to be pedantic, it's ten times the power for double the sound pressure level, which is measured by an electromechanical device other than a human ear. Volume ratios as perceived by humans depend on what level you start and end at, what frequencies you're talking about (google "Fletcher-Munson curve"), and some other arcane stuff. The accepted rule of thumb is more or less considered to hold true for any audio source operating in the linear region though, to answer your question.
  3. Joe Beets

    Joe Beets Guest

    Nov 21, 2004
    Here is a general rule for bass wattage. If you see two of these in any combination; Marshall 100, Fender Twin, Fender Super Reverb, Mesa Triple Rectifier or Peavey XXX, then you will need at least 750 watts to hear yourself on stage. :meh:
  4. TheChariot


    Jul 6, 2004
    Boston, MA
    yep... Any guitar Tube head that's 100+Watts will call for about 400 to 500Watts of bass, depending on your genre of music of course. Then again... how often do you see a Triple Rectifier being used by a Blues Guitarist? :p

    If there's two of them suckers.... you might need s'more power, unless you've got a good arrangement of cabinets.
  5. bass-shy


    Jan 11, 2005
    I read somewhere that if you double the surface area of your speakers, your acoustic output goes up by a factor of four. Wouldn't you be farther ahead to add more speakers than more power? I also understand that speakers grow less efficient the harder they are pushed. Any truth to this?

    One more thing, 100 tube watts are roughly equal to 300 solid state watts, correct?
  6. The 0x

    The 0x

    Aug 24, 2003
    Timonium, MD

    Uh oh, you asked the question. Now there will be a HUGE debate over this.
  7. Petebass


    Dec 22, 2002
    QLD Australia
    I'm not sure what you mean by "factor of 4". Doubling your speaker area adds 3dB of sensitivity. The other way to get that extra 3dB is to double your wattage. Also, doubling your speakers can change the nominal impedance to the amp, allowing it to push more watts. Both these factors combine to make doubling your speakers a seriously good way to get more SPL.

    To double your SPL, multiply your watts by 10. This is true for all audio sources including guitar, but it assumes that all things are equal. In the real world, they often arent. Speaker frequency response and any tone shaping (EQ for example) can add or subtract apparent volume. The human ear is a complicated thing.

    It's not they get less efficient, it's that they run out of suspension. Once that happens, the speaker can't move any further so any extra watts applied will not only be wasted, but are likely to cause the speaker to first distort, and if you keep pushing, it' will blow. Guitarists love it when their speakers break up, bass players generally try to avoid it.

    Technically a watt is a watt. The difference between SS and tube lies in what happens when they start to run out of steam. SS amps clip the sound and send some nasty stuff to your speakers. Push a SS amp too hard and you'll be have to back off pretty quickly. It's start to sound horrible and may even hurt your speakers.

    Tube amps don't do that. The tubes act as power compressors and while they do distort a little, the type of distortion they produce is much more musical and usable. Therefore you can push them a lot harder and don't need to back off as early.

    That's why 100w tube amps seem louder than 100w SS amps. But it's not why a guitarist with 50w can overpower a 200w bass amp. That's because SPL is a product of both power and efficiency. Guitar speakers are very efficient, the result of not having to deal with bottom end and not having to worry about them running out of suspension travel. To make is worse, they usually use lots of speakers (I'm refering to a typical 4x12 guitar enclosure).

    The formula for calculating SPL is :-

    (10 x Log W) + E = SPL,
    where W is watts, and E if speaker efficiency in dB.
  8. TheChariot


    Jul 6, 2004
    Boston, MA
    Now that, my friends, is one educated post. How often do we discuss Logarithmics on Talkbass? :p
  9. IvanMike

    IvanMike Player Characters fear me... Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    this thread has more than enough debate on that topic.

    besides being more efficient, the other reason guitar players can get away with much less wattage is related to the anomalies of human hearing posted above. depending upon overall volume, we hear midrange freqencies better than high or low frequencies. That and the fact that the frequencies in the low end of the audio spectrum (like the fundamentals and the 1st harmonics of our bass notes) take a lot of watts to reproduce.

    as far as 10x the wattage for twice the volume, yes it's twice the spl as previously stated. which roughly sounds like twice the volume. as far as actual spl is concerned, i don't believe the actual frequency range matters much. a 10 db increase is a 10 db increase.

    "as always, i could be wrong."
  10. ravlin


    Oct 12, 2004
    thats WOAH!!! keanu reeves style from back in the bill and ted days. i thought i was a gear nerd but you guys blow me away. my hats off to you. if it helps any of you who are totally confused about all this, i switched from a single 15 to an 8x10 and had to cut my volume by half on my head and i am still much louder than i was before. surface area works!