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some simple calculations
1. dB = 20 * log (V1/V2)
2. Power (dBm) = 10 * log (P1/P2)
What is this 3dB down everyone is talking about? 3dB down means 3dB down from a reference level. The reference level is usually set to 0dB and is the level where a frequency response is most flat. Sometimes this reference level is taken at 1kHz. Now 3dB down is a quantity where the power is down 1/2 from the reference level. So using equation #2 above, we have:
Power (dBm) = 10 * log ( (1/2) / 1), we are using the assumption that the reference level is set to 0dB. Solving for Power, expressed in terms of dB, we acheive 3.0103dB, or 3dB.
Now if we are assuming a "doubling of voltage," or a "gain of 2," we can use equation #1 shown above and get:
dB = 20 * log ( 2/1 ) = 6.02 dB.
When two loudspeaker drivers are using in parallel, we get a "gain of 2," or a gain of 6.02dB. Now if you are doubling the amount of power used to a particular loudspeaker, you would only get 3dB.
 