Speakers in terms of Watts.

Discussion in 'General Electronics Chat' started by cjdelphi, Jan 19, 2010.

  1. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
    I bought one of them plug in the wall things and then you plug your device into the socket, a little LCD screen then shows how many amps/watts/kwh your device(s) consume.

    So I got wondering, the speakers are rated "120watts" 4 20 watt speakers and then 1 sub which is rated 40 watts, anyway whatever it is, it's rated all up at 120watts...

    but the Amp / Watt monitor says even when turned up to full volume, never goes past 30 watts, i see it fluctuate and spike with deep base noises but, this is so underrated not even close to 120 watts..

    so does "120watts" of sound power not actually mean 120 watts of electricity? or does it just mean a cheap amplifier? a true sound system rated 120 watts consumes 120 watts of electricity?
  2. JDT

    Well-Known Member

    Feb 12, 2009
    This is normal. For most music, the average power is much less than the peak power. Speakers need the peak power capacity only for peaks in the music. Continuous 120W is seriously loud!
  3. Darren Holdstock

    Active Member

    Feb 10, 2009
    Don't get me started. Oh, too late...

    Watts is watts, same as seconds is seconds, no music, peak or rms about it. However, convenient untruths are often employed.

    rms watts really means a (reasonably) undistorted sine wave driven into a stated load. Here the rms voltage of the sine wave is used in the equation P=V^2/R. This is the least deceptive of all the power ratings, as it reveals the available headroom of the amplifier.

    Peak power is usually the peak (positive or negative) amplifier output voltage, fed into a stated load. This is only really useful for those who like to listen to loud squarewaves.

    Music power is manipulated BS, as it's pretty much peak power with a favourable tailwind, i.e. peak voltage into a minimum impedance load, all the speakers summed together, only a few peaks measured before the fuse blows etc. Its purpose is to sell cheap hi-fi.

    Your mains wattmeter will be taking some sort of rolling average - bear in mind that the quiet spots in the music will significantly lower the average reading here. As a rough rule of thumb, most domestic amps are about 50% efficient (in terms of mains power input to electrical power delivered to the speakers), so that 30 W is only 15 W by the time it drives the business end. You can increase the power you see at your wattmeter by playing music that has been more heavily compressed, e.g. acoustic guitar and vocals played at an undistorted volume will contain less energy than, say, the last Metallica album, which was so heavily compressed it sounded painful to even the most die-hard fans (google "loudness wars"). This is also the reason TV adverts are louder than the shows - more compression fits greater power into the same available headroom.

    A useful term here is crest factor, which is defined as the peak of a waveform divided by the rms.

    I've worked on professional PA systems, and it's not unusual for a pro band to have 1000 W amps just for the stage monitor speakers. A moderately sized club would have a 40 kW* PA system, and if it's a good 'un it won't sound excessively loud. But it will make you just as deaf. 120 W, however, would be a bit much for a living room.

    * A sign of the times. By comparison, at the 1969 Isle of Wight Festival, The Who played through the biggest PA system yet assembled, at 2.5 kW. They felt obliged to put up warning notices.
  4. Audioguru


    Dec 20, 2007
    Many speakers and amplifiers are rated in Whats, not in Watts.

    Many car radios are rated at 250W. They have 14 Watts in each of its 4 amplifiers which is really only 56 Watts. Computer speakers are rated at 480 Whats. But their little power supply is rated at only 8 Watts.
    A What is usually a Watt that is multiplied by the age of your grandmother.

    The lousy manufacturers lie about power so that the number is the same or higher than the good quality competition.
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    Don't know how the measurements are done now, but amps used to be rated in terms of "music power" back in the 1970's. That was the power level short of clipping that was defined by one cycle of 1 KHz sine wave signal. Not very realistic.
  6. Audioguru


    Dec 20, 2007
    Peak music power is simply double the phoney music power number.