Output Power is measured at 1% or 10% THD+N,
The most basic way to measure THD+N is by capturing a spectrum (FFT) using an analyzer and then with that captured information processing it to calculate the THD+N.
MAX98357A
Production
For improved performance, functionality, and even lower power consumption Maxim Integrated recommends the MAX98360.
The MAX98357A/MAX98357B is an easy...
Datasheet
MAX98357A on Analog.com