A watt is the measure of electrical power in a speaker. This also corresponds with the decibel number, which is where the real loudness of the speaker comes from and is measured.
When you have a speaker that is 15 watts and a speaker that is 30 watts, the latter one is going to have double the power, not necessarily double the sound. When you double the power, you only increase the decibel by 3.
That 30 watt speaker will still sound louder than the 15 watt, but in order for the sound to be truly doubled, you'd need to figure out the equivalent of doubling the decibels.
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