When we plan to create an operational amplifier for headphone applications (the last choice is the OPA1622 operational amplifier), the first problem we need to solve is to determine the power required for the headphones.
Think of headphones and speakers as converters that convert input power to audible output power. Analogous to all processes, the electroacoustic conversion process has a related efficiency problem. As you might have thought, the efficiency of different headphones is different. Generally speaking, the efficiency of the ear type earphone is lower than that of the ear earphone.
Headset manufacturers usually use the sound pressure level ([SPL], decibels) of specific input power (usually 1mW) to indicate the efficiency of their products. For example, an earphone maker may mark the efficiency of their earphones as 100dB/wM, and you should read this value as “100dB at 1mW”.