Amplifier output power will change by adjusting the gain, and it is good to verify the output even if the amp is pre-biased to a desired level. By calculating your amplifiers output power, you can ensure the speaker that is being operated is within its power rating. There are many different ways of calculating this, not only mathematically, but by the use of software or panel meters. There are also many other circuit variations and waveform patterns which can complicate analysis.
In this basic example below, we will use a sine wave commonly found from a class A or A/B amplifier output. In this test circuit using 3-Watt Soberton Inc. WSP-4508-7 speaker, we are able to determine if this amplifier is operating the speaker within the speakers power rating.
Oscilloscope waveform across the speaker leads:
Since this is a sine wave (AC environment), we will use RMS values to determine power. Keep in mind RMS values would not apply for saw, square, or any other non-sinusoidal waveform.
First we need to calculate the RMS voltage:
Vrms = Vp*.707=
Calculating RMS power:
2.04W= RMS power
This is within 3W power rating of speaker. It is good to leave some headroom as in most audio applications the frequency, duty cycle, amplitude, and waveform will change and this may increase the power output. Make sure the frequency range of the amplifier is within that of the speaker to avoid speaker damage, this gets more critical as power going to the speaker is increased.