The basics

Scientific definition

Wattage. The “watt” is a unit of power and is what you measure when somebody talks about the power of an amplifier. There is no subjective leeway in this definition; power is calculated by multiplying current with voltage. Or maybe you’d want to use resistance multipled by current-squared. Or maybe voltage-squared divided by resistance.

“Power” is a basic physics concept and one can easily determine the power of an amplifier with easily obtainable measurements and manufacturer-provided spec sheets, provided they don’t fudge the numbers.

impedance, power and volume

This is a question that I’ve been getting a lot so this is certainly a very useful section of this article, even if you may not agree with my final conclusions.

You’ve probably seen amplifier spec sheets citing their power ratings in units of “mW @ ohms”. So there we go, we’ve found the magic Watts number so that’s our power right? Then what’s with the “ohms” part at the back?

That’s when the impedance value of your IEMs come into play. The real power output of your amplifier depends on its load, in this case being the IEM. Usually amp manufacturers cite their power ratings at a load of 16 ohms, so if your IEM just so happens to be at exactly 16 ohms, that’s the amount of MAXIMUM power that the amplifier is able to send into it.

But of course, the world is not perfect. Your IEM’s impedance is probably slightly different, so that number the amplifier manufacturer gave is not going to be exact for your situation. If your IEM’s impedance is lower than the load used in the manufacturer’s measurement, then you can expect even MORE power flowing into your IEM AT MAXIMUM. If it’s higher, then you can expect less power. At MAXIMUM.

Wait, what about volume? Well wouldn’t you know it, even the IEM guys have the numbers you need. You can probably find a spec sheet for your IEMs if they aren’t a shady bunch (I’m looking at you, Spiral Ears) and so the magic metric you’d be looking for in this case is “Efficiency”. Stated as “dB SPL/mW”.

There we go, the magic Watt number again. SPL is pretty self explanatory if you’ve been in this hobby enough or have been hanging around construction sites; it’s just how loud your IEM goes. So the efficiency of an IEM is measured as how loud it can go when you put in a single milliwatt of power through it. And in case you need a decibel-to-exposure scale: