RMS

[Mukhalled]

Hi,
A speaker marked with 80W RMS. What does RMS (Root Mean Square) means here?

Thanks

 

[logan_dslasher]

hi Mukhalled!

from http://www.pcmech.com/show/multimedia/738/:

...there are usually two wattage ratings provided: normal (peak output under perfect conditions) and RMS (Root Mean Square). Note that the RMS wattage is always lower than the advertised ‘normal’ wattage. RMS wattage is what to consider when buying speakers, as it measures how much power the speakers can safely handle for long periods of time.

 

[Mukhalled]

Thanks man  ;D

 

[logan_dslasher]

no probs! 

 

[Kevin Weddle]

Mathematically, you have to remove the voltage offset to calculate RMS. Also, RMS is kind of like the Average, but it stinks mathematically. The equation get's squared, which creates a totally different equation. Then you find the area, then you take the square root. The only thing I know is that it produces a single voltage. Apply this voltage to a resistor and it dissipates a certain amount of energy in the given time frame. Apply the signal to the same resistor and the energy will be the same in the given time frame. 

 

[audioguru2]

RMS power does the same amount of heating in a resistor as DC power.
"Peak" power is measured with square waves where the output of the amplifier is clipping very badly (producing horrible distortion) and therefore its peak voltage is 1.414 times its RMS voltage which causes its peak current to also be 1.414 times its RMS current. Power= Volts times amps so 1.414 times 1.414= 2.0 times as much power.

 

[windoze killa]

Thats not quite correct. Unless otherwise specified all power measurements (RMS or Peak) are referenced to a sinusoidal waveform. PMPO is about the only power measurement that doesn't. It is what you find on PC speaker systems. Little tiny speakers that cost $30 and can handle 600W PMPO. I am beginning to believe PMPO is latin for "Peak power just prior to putting speaker coil into the wall".

 

[audioguru2]

When audio amps or speakers are rated in phoney power I call it Whats instead of Watts.

Most IC amps are rated at 10% distortion where the output is heavily into clipping, since with an output level just below clipping the distortion is very low.
TI rate the power of their Class-D amp ICs with the input pre-clipped into a square-wave.

Speaker ratings are affected by frequency and by duration, both are usually not mentioned in the ratings for cheap speakers, good speakers have their RMS power rated over their full frequency range and continuously.

 

[Kevin Weddle]

Peak power is instantaneous. RMS power takes the peak, the low, everything in between, and gives you one value. It is, one value, because the RMS voltage is, one value. Square the RMS voltage and divide by the resistance.