If you look at the Austrian Audio - The Composer it has:
And Dan Clark Audio - Noire X has:
So one is dBspl/V and the other is dB/mW.
Is that the same? Or how can I compare the specifications?
I am bit confused 
THX
Torben
Try this!
Even more confused 
I must be doing something wrong:
Austrian Audio - The Composer
Power Sensitivity: 95.4 dB SPL at 1 mW
Voltage Sensitivity: 112.0 dB SPL at 1 Vrms
Dan Clark Audio - Noire X
Power Sensitivity: 94.0 dB SPL at 1 mW
Voltage Sensitivity: 112.9 dB SPL at 1 Vrms
Almost the same. But this can’t be: Composer is very easy to drive and Dan Clark harder to drive.
Torben
Easy/hard for whom? 
At 13 ohms in order to reach 1 Vrms and put out those 112 dB, the Noire X requires 77 mA of current, while the 22 ohm Composer only needs 45 mA. A source that’s not designed with IEMs/low impedances in mind might have trouble putting out the higher current between these two.
Yeah, looks like most headphone amps, even portable ones that aren’t the cheapest USB dongles should have no issues driving either of these headphones.
Hmm? Not sure how you can tell from that page. Manufacturers usually don’t quote the maximum current their units are capable of, only the maximum voltage, as if we were still in the era of 150-300-600 ohm headphones dominating the market. Even the KA3 you pointed to is only advertised to handle 16+ ohm headphones well, so FiiO are low-key (without putting a mA number on it) admitting it might have trouble with the Noire X at 13 ohms.
shrug if you say so… it can do 130mw at 32ohms and 120mw at 16ohms, probably doesn’t have an issue supplying 7.9mw for 103db? (more than loud enough?)
The point I was trying to get across is that neither of the two headphones he asked about are difficult to drive. I’d personally get something with more headroom than the little dongle for EQ purposes.
103 db might be “more than loud enough” for outliers that listen all quiet-like, such as myself, but most people probably fall within NwAvGuy’s region of recommendations of 105-115 dB needed for peaks. So you’ve got quite the crowd out there who will agree the Noire X is insufficiently powered by the KA3, probably saw one such example up-thread.
That’s reallllly loud, but yeah. Add some headroom for EQ and you’d probably want something more powerful than the second cheapest dongle Fiio sells.
Anyway, not difficult for most amps/dongles out there to drive these headphones to unreasonably loud levels.
I should point out, 115dB peak would imply that either A, you listen at a level which is likely to cause hearing loss if sustained for more than an hour a day, or B, your music has a crest factor above 30, which is…rare.
Yeah, well NwAvGuy was relating that to some massively dynamic classical music(?), while 105 he said would correspond to modern music. That proposed level of 103 in the screenshot satisfies neither.
Damn, I almost got away with it too!!! 
It is funny (maybe funny-sad would be more appropriate) that we can’t even agree on basic stuff, like how loud a headphone needs to be.
I usually go by the THX spec, which calls for 85 dB SPL + 20 dB headroom in the midrange. And a little more headroom (say another +5 db) for the lower frequencies. That will put you at THX reference levels, which is pretty loud. (Think how loud a well-configured movie theater can get during FX and explosions.)
That’s probably loud enough for most folks. As abm0 and Blaine’s posts suggest though, the type of music you listen to makes some difference.
This may seem a bit counter-intuitive at first, but if you listen to music which has had its dynamic range more compressed to increase its loudness (which is commonplace with popular music), then you can probably listen comfortably at a somewhat lower volume level than if you listen to mostly full dynamic range recordings, where there are larger differences between the louder and quieter passages in the music. You need more volume (and headroom) for full dynamic range recordings to make the quieter passages in the music more listenable.
Audiophiles, who sometimes enjoy listening at louder than reference levels, may also want another +5 to +10 dB on top of what’s mentioned above. You are potentially putting your hearing at risk though at those kinds of levels, as Blaine says… unless you’re doing it with a very quiet recording.
These figures are for in-room listening btw, but they also translate to headphones fairly well.
A typical listening level would probably be closer to 75 dB SPL + 20 dB headroom in the midrange, which is only 95 dB SPL total (or ~100 dB SPL if you also factor in the low frequency levels).
Popular music is generally compressed to make it sound better (and louder) on poorer performing gear with less volume/dynamic range. Some examples would be the speakers on a phone, computer, TV, or desktop. And also some lower-cost bookshelf and floor-standing speakers.
The problem that prevents agreements a lot even in the science enjoyer camp is that human limits as determined by experiments known so far are too often treated as commandments handed down from the heavens, like the population vs. ability graph stops at brick walls like -100 dBFS or 20 kHz, when it does nothing of the kind - it trails off with fewer and fewer people preferring or being able to hear xyz sonic characteristic. Even the canonized “0 dB SPL” threshold of hearing isn’t the absolute threshold of hearing for everyone everywhere - at the ear’s resonance frequency some (many?) people can hear even quieter levels, which now have to be represented as negative-dB.
These things should always be discussed in terms of “how much of the population are we leaving out / leaving unsatisfied if we stop at level x”, because there’s always a tail you’re cutting off the end of the graph. I prefer to say “there will be few/many/some people who will dislike this”, not “whoever’s looking for performance beyond this point is an idiot” - the latter is the ASR way. Let us not follow the ASR way. 
But anyway, to get back to what I thought was the point before the loudness detour, the takeaway from me is that both sensitivity and impedance have to be scrutinized to get a conclusion on headphone “driveability”. Sensitivity characterizes the headphone itself in terms of how much its membrane will move in response to standard audio-signal voltage oscillations, it doesn’t also cover the electrical matching between it and a given amplifier - for that you need to consider impedance as a separate topic. There you’ll be looking at an amplifier’s ability to create those standard voltage oscillations over this or that impedance, and you can run into limitations of either voltage output over very high impedances, or current output (without which you can’t raise the voltage to standard audio levels) into very low impedances. So in order to find all the limits of “driveability” you have to ask:
You can’t just look at the sensitivity and close the case.
Re: 0dBSPL, I actually think it’s kind of shocking how close 20 micropascals comes to being the threshold of hearing (at 1khz or so) for an average human, given how ancient the research that produced that standard was and how poor much of the work at that time was.
So neither dB/mW or dB/V will work for this?
Alone. Neither will work alone.
dB/V separates electromechanical transduction sensitivity from power delivery concerns so you will get clear information on one of the two aspects of driveability but tells you nothing about the other - this is how much sound I will get for each volt applied to the headphones, OK, but how many volts can this amp apply to them? Still need impedance for that.
dB/(m)W tries to combine sensitivity and power matching into one, but since watts are an indistinct blend of voltage and current information, you still don’t know what an amp will be able to put through this pair of headphones until you ask what is its driver impedance and where does it sit on the amp’s performance graph (manufacturers who resolved a while back to stop being trash at this have started giving us at least 3 points from that graph, like power into 16, 32 and 300/600 ohms, but that still leaves us guessing as to what happens outside of those limits; ideally we should get a proper graph and be able to find any impedance of existing market products on it).
Okay. I think I’m with you on most of that, abm0.
If headphone A is 10 dB/mW more sensitive than headphone B though, then doesn’t that mean headphone B needs more power to reach the same volume as headphone A, regardless of the load impedances? And wouldn’t that be essentially the same as saying A is easier to drive than B, since loudness/volume is related to power?
I apologize in advance if these are dumb questions btw. But I’m somewhat easily confused when the subject of volts, watts, and amperes arises.
If A and B both have the same impedance yes, if they don’t that’s not necessarily true.
Volume is Voltage.
Everything has to follow Ohms Law V=IR , power is I*V, and R is the headphone impedance, if the headphone has a high impedance, the amplifier might struggle to output enough voltage, if it has a low impedance it might struggle with current, which will cause the delivered voltage to drop.
Amplifiers can’t output infinite voltage or current and if they exceed their limits for either the other will drop off to obey Ohms law. To put it differently, amplifiers will have an impedance they output their maximum power into, as you get away from that power falls off.
The other issue is that for Dynamic drivers, impedance is a function of frequency, and it’s generally just quoted at a single frequency, so while say a HD600 might be 300 Ohms at 1000Hz, it’s closer to 600Ohms at 50Hz.
Generally though sensitivity in either form is a good ballpark indicator of easy to drive, unless your using a very weak headphone amp, or the headphone is moderately insensitive, and has either excessively low or high impedance.
Howdy Polygonhell. And thanks for the reply.
The subject of headphone “drivability” is an interesting and confusing one, because it’s really a subjective term, with no specific scientific definition I’m aware of. So it can be very easy to get lost in the minutiae.
Let’s take the idea of “volume is voltage”, for example. If you’re referring to an amplifier, then it’s usually correct to say that the knob labeled “volume” adjusts the output voltage. I think (or hope) we can all generally agree on that.
When I think about “volume” in the context of a headphone’s drivability though, what I’m really thinking about is the headphone’s sound pressure level in decibels. And unless I’m mistaken, what produces that SPL at your eardrum is actually power.
You need power to drive your headphones or loudspeakers. And that power can come in the form or both voltage and current. And it’s measured in watts or milliwatts, rather than volts.
The more sensitive your headphones (or speakers) are, the more easily and efficiently they can convert power in watts into sound in dB SPL. And the less power you need to drive them to a specific volume or loudness (in dB SPL). So they are (by this definition) easier to drive.
Note I haven’t mentioned impedance at all so far. So where does that come in? Well it can effect a headphone’s ability to convert watts into decibels, and therefore also its drivability. But that’s generally already factored into its sensitivity.
The Beyerdynamic DT770 comes in three different impedances and sensitivities, for example. In this case, the impedance (and sensitivity) of the headphone is determined by the thickness of the voice coil. The thicker the coil, the lower the impedance, and also the higher the sensitivity. So the different impedances are factored into the sensitivity ratings of the headphones in dB/mW.
Impedance effects drivability. But you can see that difference right in the headphone’s sensitivity spec.
Will the relationship of impedance to drivability always be this simple? The answer is no, because there can be headphones with low impedance that are also lower in sensitivity, and possibly also the reverse in some cases (ie higher impedance with higher sensitivity). Generally speaking though, a higher impedance usually implies lower sensitivity, and therefore also a headphone that is harder to drive when compared to a similar design with lower impedance. Sensitivity in dB/mW is the spec that will tell you which one converts power more efficiently into sound though. And which is easier to drive.
Perhaps this is a bit overly simplistic, but this is the way I sort of look at the question of drivability in a headphone. Your mileage may vary of course. 
