Understanding Amplifiers and Speaker Power ratings

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Understanding Amplifiers and Speaker Power ratings

#1 Post by LelandCrooks » Tue May 09, 2006 11:31 am

Amplifier power and it's relationship to volume is commonly misunderstood. The relationship between between output and power is not complex, but it is a logarithmic scale not linear. For example in a Tuba 24 HL10c loaded:
  • 1 watt at 1 meter spl: 102db.

    10watts at 1 meter spl: 112db.
    • 10 times the power, a 10db gain.
      Spl has doubled. A 10db gain is doubling of volume
    100watts at 1 meter spl:122db.
    • 10 times the power again, another 10 db gain.
    200watts at 1 meter spl: 125db
    • Double the power, only a 3db gain.
      3db is about the least audible change you can hear.
    400watts at 1 meter spl:128db.
    • Again double the power, 3db gain.

    At 400 watts you are over the limits of the driver design.

    800 watts at 1meter spl:
    • Theoretically it should be 131db. In reality it's the same as the 400w level. You've reached the limits of the box and driver, and all the excess power is turning into heat, cooking your speaker.
Why use 1.5-2x speaker rating on your amplifiers? Headroom and clipping. When an amplifier clips, distortion rises. Clipping takes out tweeters and midranges.

So to double your volume, you have to increase power 10 times. For a 3db increase, you have to increase power by 2 times. Diminishing returns very quickly once you exceed the 500-600 watt level, at least with horn loaded designs. Inefficient speakers require greater power, because more is converted to heat, and they are designed for it, but the same rules still apply.

WARNING:Math coming up :!:

You can calculate and test your amplifiers to limit them, if you don't have some kind of limiter in your PA rig. V=squareroot(W x R). To find the AC voltage for the maximum output you require from your amp, take the watts, say 400, multiply by the resistance of your speakers, say 8 ohms, then take the square root of the result.
V=sqrt(400x8). V=56volts. That would be 400watts into an 8ohm load. Heres a link to calculators to do this in various ways. http://www.csgnetwork.com/ohmslaw2.html

Most amplifiers are rated with with a 1k signal. Get a signal generator for your computer. This is what I used: http://www.natch.co.uk/downloads/SigJenny/SigJenny.html Hook it up to your pa. Unhook your speakers. Drive the pa with the 1k tone up to the 0db level on your meters. Put a voltmeter set to read AC on the amp, neg on neg, pos on positive. It will read the output voltage of your amplifier. Adjust the sensitivity gain on the amp to limit the voltage to your calculated speaker rating. This can also be done with a limiter on the PA, which is probably better, but requires more adjustments to gain to make everything match. This will give you some headroom, as the lower frequencies will be putting out more voltage, but it should still be in the range of what the speaker can handle.

Leland 8)
Last edited by LelandCrooks on Fri Jan 12, 2007 1:47 pm, edited 1 time in total.
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#2 Post by bgavin » Tue May 30, 2006 12:31 pm

Tuba Values for 1W/1M:

4 ohms = 2.00 volts

6 ohms = 2.44 volts

8 ohms = 2.83 volts

12 ohms = 3.464 volts