Audio crew, how is audio power measured?

Discussion in 'OT Technology' started by Jerboy, Jan 3, 2006.

  1. Jerboy

    Jerboy Guest

    I know nameless crap simply over inflate their ratings, but I came across a Panasonic with ratings that do not make sense.

    "340W total output"

    On the back, it says "AC 120V 238W"

    How is the wattage rated?
     
  2. 7960

    7960 New Member

    Joined:
    Oct 17, 2004
    Messages:
    60,415
    Likes Received:
    0
    Location:
    New England
    might have better luck here
     
  3. amill94

    amill94 OT Supporter

    Joined:
    Oct 5, 2003
    Messages:
    31,218
    Likes Received:
    346
    Total output is the amount of power output from all channels. So if the receiver is 5 channels...divide 340 by 5..and you get 68 watts...which could be true.

    Is this a home theater in a box setup?
     
  4. Tills

    Tills Lets Go Flyers

    Joined:
    Dec 8, 2001
    Messages:
    12,684
    Likes Received:
    0
    Location:
    Los Angeles
    Voltage – Voltage is the “pressure” that the electricity is “under”.
    Current – Is the volume of electricity.

    Think of a garden hose. The water pressure is the voltage and the current is the amount (volume) of water that is going through the hose.

    Watt - A watt is a unit measurement of power. The equation is W=V*I. W=Wattage, V=Volt, I=Amperage.

    Wattage(power) is your voltage(pressure) multiplied by your current(volume). If you turn up the water pressure the water shoots out of the hose farther. If you increase the amount of water going through at the same pressure the water shoots farther as well.

    An easy way to understand this is to think of a lightbulb. A 100 watt light bulb is using 120V (This is a household constant) and ~.8 amps of current to generate the light. A 60 Watt light bulb uses 120V and .5 amps of current.

    OK now we know how the wattage is determined. Let’s look at your question.

    First we have 2 indicators here "340W total output" and “AC 120V 238W”. I am going to take the 2nd indicator first.

    Not seeing the receiver myself I am assuming that the “AC 120V 238W” rating is one of 2 things. It is either the rating of an electrical outlet or 2 on the back of the receiver (if you wanted to plug in another device) or it is the max power supply rating (how much power the device can draw) when in use.

    The 340W total output is probably the total amount of power that can be applied to the speakers. This brings us to our next lesson. Peak power vs. RMS (Root Mean Square) power (I am not going to debate the validity of RMS here).

    Let’s take the example above. 340Wpeak/5 Speakers = 68 peakWatts per speaker.

    So the receiver can supply a peak power of 68 watts per channel. Do you want to run 68 watts per channel? Hell No! This would damage our speakers and probably our receiver. There is a term called RMS wattage. For this discussion RMS wattage is the continuous power that both your amp and your speaker can handle safely. You want to have your receiver capable of providing MORE RMS wattage then your speaker can handle. The reason for this is you want a clean signal to the speaker as distortion can ruin a speaker even if it is in the “safe” RMS wattage range.

    There is also a power measurement called the decibel which I can go into if you want me too. It is the measurement of the sound in the room, not the electrical portion of it.
     
  5. Jerboy

    Jerboy Guest

    It is a box setup, a bit smaller than a dorm room fridge with detached speakers, a class above boomboxes.

    The "120V AC 238W" is the input rating printed on the back of the stereo unit. In an honest rating, there's just no way the output is 340W while the input is only 238W, or even anywhere near 238W, because the device itself uses power and the amplifier is far from being 100% efficient.

    Neither mentioned peak or avg power(instantaneous power integrated over time).

    I'm not sure about the calculation or the definition they use in their system. A reputable company like Panasonic wouldn't just BS the ratings and I believe there is a specific methods used to get the rating.

    In conventional sense,

    A 120ohm resistor across 120Vrms AC would draw 1Arms and has a peak power of (120 x sqrt2)v *(1* sqrt2)A = 240W-pk at crest assuming sinewave. Power at any given point changes from 0-240 continuously and that is "instantaneous watt".


    The average power is obtained by Wavg = ∫(instantaneous power,x, x=a, x=b)
    If it's a clean sinewave, it's simply watts x amp, but if its a comlicated waveform like music, then it has to be integrated and furthermore, the result depends on limits of integration.
     
    Last edited by a moderator: Jan 3, 2006
  6. Tills

    Tills Lets Go Flyers

    Joined:
    Dec 8, 2001
    Messages:
    12,684
    Likes Received:
    0
    Location:
    Los Angeles
    Actually there is. 238 is represented as an RMS value and 340 is the peak. I guess I should have pointed this out.

    To find RMS you take the peak voltage (represented by a sine wave on an oscilloscope) and multiply this by .707 to get RMSVoltage. You can take this number and then multiply the measured current and get the RMS Wattage rating.

    Since I know what my peak wattage is already I can multiply this by .707 and get RMS wattage.

    340Wpeak * .707 = 240.38 Wrms

    That is how you have your numbers.
     
  7. Jerboy

    Jerboy Guest

    238W is the input power at the plug, which accounts for the display, mechanical drives, etc and amplifier dissipation. 238W avg input != 340W peak amplifier output.
     

Share This Page