I tried to approach a pretty specific side of the problem with little knowledge and no actual measurement gear here(I'm so good at selling myself ^_^):
https://www.head-fi.org/threads/wha...out-a-dap-hissing-i-cant-hear-a-thing.777273/
all device will generate some noises, the lowest stuff we can expect is thermal noise that we can hardly hope to avoid, but in practice we always end up with much louder noises coming from other origins. some noises will increase with the music(like the background noise recorded on the album), but other noises will stay at fairly stable amplitudes no matter how loud you listen to the music. for that second kind of noises, how loud it will be in your headphone is directly function of the headphone's sensitivity, which is often given in dB for one mW, but can be turned around to give dB for 1volt. so in general, if you could know the voltage of the noise coming out of a device, you'd be able to estimate how loud that will be in your headphone. sadly there are several factors adding trouble to our estimations:
- the headphone's sensitivity is given for a signal at 1khz. so depending on the noise spectrum and the headphone's own frequency response, you could end up with something much louder than anticipated.+10dB is perceived as about twice as loud, on a headphone it's not rare to have that much boost around 3khz compared to 1khz. but the opposite could also happen, the noise measured might be high only in the trebles or only in the sub bass where you won't be as sensitive to it. but because you'd get a vague value that doesn't detail the noises repartitions over the audible range, you might think something will be very audible when it isn't.
also some weighting will be applied, there is one that does a good job of following our hearing curve so we'd like to have that, but you're out of luck. most data will be given with A weighting instead because it makes the numbers look better.
-SNR values and what they really measure. SNR is signal to noise ratio, so on principle it's the very thing we're interested in. in practice... no so clear. the value is obtained by measuring a given signal at a given amplitude, and then measure the same circuit without the music, so that what's measured is only self generated noise. the big issue here is that many manufacturers will not specify the signal amplitude correctly, nor the weighting applied. so we have a ratio between 2 signals we really know nothing about. professionals have some agreed upon standards that most will follow, but for consumer gears... even when they do, they often end up writing down the value with incomplete nomenclature so we can only hope that they did it the way we think they did... and of course many count on that for us to misinterpret some incomplete spec in their favor. I believe the most common signal amplitudes used as references are 0dBu(0.775Vrms) and +4dBu for pro stuff. for us and our consumer gears, it can be that too, or 1Vrms or 2Vrms, or simply the maxed out output so that they can write down the most impressive number. so if they don't clearly specify how they got their SNR value, I don't have a clue what it means. and this happens way more often than I care for, making prediction very difficult. I've been a few times in situation where a device showing the better SNR was in practice clearly noisier.
for the DACport I have to give them a cookie, the specs givea clear
Noise Floor 7 µV RMS (A-weight), max gain
so you can look at your headphone's sensitivity and calculate how loud that will be. but of course that's the noise they measure in what I imagine to be great conditions for power supply, and load. plus the A weighting could hide some stuff up or down. so maybe this is not the noise you're haring in practice. but it's at least a start to estimate something.
so sadly in practice the answer is often that you have to try yourself with your gear, or have somebody try with an IEM or headphone you know, and hope that he's a hiss maniac and not one of those guys who doesn't notice background noise on vinyls. I am a little obsessed with background noise, and aside from trying myself, for years I relied on guys like
@shigzeo to mention if a device was prone to hissing or not. not very technical, but hey, if it works, it ain't stupid ^_^.
I try to avoid very sensitive IEMs, because obviously they will make any signal louder in your ear. I also tend to avoid those with crazy low impedance for various reasons but in this specific case, because they're usually sensitive, but also because on occasion low impedance loads make the amplifier worst(higher current, more noise...). and if I really want that one IEM with crazy high sensitivity specs because it just sounds so good, then I have my portable amp which has a fairly clean background, so things usually work out just fine. but that does force me to carry an extra brick which in itself is a little annoying IMO.
also as a very vague and general idea, really high gain on the amplifier is more likely to be noisy. running after mad power output when your headphone doesn't need it at all, can be one way to end up with background noise.
