IEM driver vs sensitivity vs frequency

Spec are mostly trivial (while frequently misleading) and should never be relied on for judging sound quality. Read review instead, a lot of them in that matter..

Welcome to Head-fi, and sorry about your wallet.

Mmm, I learnt the hard way, many years ago, that specs are mostly useless.

Let's start by frequency range. First of all, you don't know if the frequency response is flat or not, you just get two numbers. You may think that those numbers indicate the lower and higher frequency, respectively, that the phone can reproduce. This is wrong. You don't know how these numbers were measured, and there are many ways. For example, the numbers may indicate the frequencies at which the emitted sound pressure halves. Or they may indicate the point at which the sound pressure is below the usual hearing floor. Or they may be just invented.

Without knowing the exact procedure the manufacturer followed to get those numbers, they're meaningless. The procedure should be standard so the numbers gave some meaning. But even in that case, they won't tell you anything about the sound signature. What if a phone can reproduce from 8Hz to 26kHz? It may have a "hole" in the middle and crappy voice reproduction. And I don't know about you, but I cannot hear above 18kHz if I remember correctly, so a phone with an upper limit of 26kHz won't be "better" than one with just 20k to my ears.

But again: the numbers doesn't describe the sound signature so... Impossible to compare properly a pair of phones by that number.

Another number is the sensitivity. It should be measured according to a set of standards, but you cannot be sure, so we go back to the first problem: unless you know how it was measured, it is useless. Usually, it is measured as the sound pressure level produced when you apply 1mW of power to the phones, using some predefined waveform. Please note, that's milliwatts (power), not millivolts (voltage).

So, sensitivity only gives you an approximate idea of how loud are the phones. But since manufacturers may measure this differently, you can have a phone with a sensitivity of 100dB/mW which sounds louder than another with 110dB/mW. And the manufacturer may tend to stretch the specifications and procedures so their phone gets a high number. You know, people tend to thing that the bigger, the better.

Sensitivity, again, doesn't tell you anything about sound signature, so you may buy a phone with a big sensitivity (let's say you like loud phones) and a signature you hate. 

Last, another common value is THD. Total Harmonic Distortion. Again, if the procedure was standard and could be enforced, then this number may have some interest, as it would tell you how prone the phone is to distortion. Well, not exactly, but more or less it would be that.

Anyway, THD doesn't tell much about sound. The THD usually is measured applying a known waveform to the phones, at some predefined power, and then the phone response is measured, and the produced waveform compared to the original. Then, you get a percentage, indicating how the original and produced waveforms compare.

THD is meaningless, too, unless the manufacturer admits it's very high, around 1%. The manufacturer won't do that, so... I don't think there are phones with a THD greater than 0.5%, as they would sound like crap due to distortion. Don't worry about that number, either, a phone with a THD of 0.05% (not unusual) won't sound worse than another with THD of 0,01%.

Summing up, I'm with ClieOS. The only way of knowing what are you buying and if it is better than another model is reading reviews. And still, this is not enough, as reviews are written by people with particular preferences in sound (the best case) or even bias (the worst case. I cannot tell, honestly, I'm bias free). Reading many reviews may give you a general idea of the sound signature, and that may be the base to decide if you will like the phones or not. But deciding about quality only with reviews is difficult, so imagine doing it by looking at some numbers...