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Calculating how much power I need

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  • Calculating how much power I need

    I had a few questions regarding how to calculate the amount of power you'll need to drive a set of speakers.

    So, let's say that you get speakers that are rated at 91dB at 1Watt of power, 1 meter away from the speaker.
    From what I've read, you have to double the power to get 3 more dB out of that speaker.
    91db @ 1 watt
    94db @ 2 watts
    97db @ 4 watts
    100db @ 8 watts
    103db @ 16 watts
    106db @ 32 watts
    109db @ 64 watts
    then for a true 100 watt amp, at max volume, you'll be getting around 110db or 111 db at 1 meter, right?

    Well, that's only 1 meter away, and I have no idea if full power means "max power output before the amp starts distorting."

    So:
    • Typically, do respected amps start distorting at a certain % of the max output? (I'm talking about denon, harmon kardon, emotiva, krell, etc: not off-branded crap you'd find in a white van like Sorny or Fillips, Panasonique)
    • How much in terms of loudness do these figures get reduced as you add distance from the speakers. Say at 2 meters through 4 meters.
    • Does the fact that there are 2 front speakers add another 3db to the figures above, because you're double the power of just 1 channel?
    • (How) Does speaker placement in regards to walls/other objects affect the loudness level like it does for subwoofer placement. (for the sake of the topic, I'm not really interested in echoing, just loudness.)
    • Is max input power on a speaker specification a easy way of saying "Max volume:" In which case, a 200 max Watt speaker shouldn't be pushed past around 112-113 dBs?


    :scratchchin:


    Thanks for helping a new guy out...

  • #2
    "Respected" amps don't distort much unless you try to push them past their rated output. A quick copy/paste from the LPA1 page at Emotiva's site, to use an example you mentioned:
    • Rated Power Output: Channels 1-5 (All Channels Driven at 0.04%THD, 120vAC/ 60HZ supply) 8 ohm: 125 watts / 4 ohm: 225 watts

    • Channels 6-7 (All Channels Driven at 0.05%THD, 120vAC/ 60HZ supply) 8 ohm: 50 watts / 4 ohm: 95 watts

    So you can see the distortion (THD) remains very low all the way up to the rated maximum power output.

    The volume will decrease by 6db each time you double the distance, and yes, you can add 3db because you have two speakers. So, if your listening position is at 4m, your chart will look something like this, not taking into account any sort of room gain/strange circumstances:

    82db @ 1 watt
    85db @ 2 watts
    88db @ 4 watts
    91db @ 8 watts
    94db @ 16 watts
    97db @ 32 watts
    100db @ 64 watts


    And SPL is a tricky subject all around. Doubling the power does not double the perceived volume--you need 10 times the power to reach twice the volume, which is a 10db increase (you can see from this example that 1watt=82db and 10watt=~92db). So by doubling your speakers (from one to two) and then moving from 1 meter away to 4 meters away, you basically lost half your volume. Still, 100db is frickin' loud.

    Comment


    • #3
      Originally posted by zworykin
      "Respected" amps don't distort much unless you try to push them past their rated output. A quick copy/paste from the LPA1 page at Emotiva's site, to use an example you mentioned:
      • Rated Power Output: Channels 1-5 (All Channels Driven at 0.04%THD, 120vAC/ 60HZ supply) 8 ohm: 125 watts / 4 ohm: 225 watts

      • Channels 6-7 (All Channels Driven at 0.05%THD, 120vAC/ 60HZ supply) 8 ohm: 50 watts / 4 ohm: 95 watts

      So you can see the distortion (THD) remains very low all the way up to the rated maximum power output.

      The volume will decrease by 6db each time you double the distance, and yes, you can add 3db because you have two speakers. So, if your listening position is at 4m, your chart will look something like this, not taking into account any sort of room gain/strange circumstances:

      82db @ 1 watt
      85db @ 2 watts
      88db @ 4 watts
      91db @ 8 watts
      94db @ 16 watts
      97db @ 32 watts
      100db @ 64 watts


      And SPL is a tricky subject all around. Doubling the power does not double the perceived volume--you need 10 times the power to reach twice the volume, which is a 10db increase (you can see from this example that 1watt=82db and 10watt=~92db). So by doubling your speakers (from one to two) and then moving from 1 meter away to 4 meters away, you basically lost half your volume. Still, 100db is frickin' loud.
      VERY informative, thank you.

      Comment


      • #4
        two speakers playing the same signal adds closer to 6 db if they are in phase, but I would work without that addition if you have discrete sources like stereo or multichanel movie soundtracks. BTW 4 meters is pretty far back, in most rooms you will be closer to 3 (I know the math is harder :))

        Comment


        • #5
          Originally posted by m-fine
          two speakers playing the same signal adds closer to 6 db if they are in phase, but I would work without that addition if you have discrete sources like stereo or multichanel movie soundtracks. BTW 4 meters is pretty far back, in most rooms you will be closer to 3 (I know the math is harder :))
          4 meters is right about where my sweet spot is from my fronts, so the math stays easy for me.

          Comment


          • #6
            Here is some interesting info on the topic of how much power you need.
            http://www.polkaudio.com/forums/showthread.php?t=67972

            engtaz
            engtaz

            I love how music can brighten up a bad day.

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            • #7
              Welcome micky0604. why such short post??

              engtaz
              engtaz

              I love how music can brighten up a bad day.

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