I'm looking for an portable quiet instrument that I can have fun with without disturbing the neighbors at night. And listening to flutes in recordings, I got the impression that they are relatively quiet. But looking around the net, I couldn't get a good idea about the instrument's loudness. How loud is it? As loud as a classical guitar? an acoustic guitar? a piano? a clarinet? a trumpet? Can I play it at night without disturbing others?
Loudness is only one factor.Sound works in many ways, and this will compound the answer. Listen to a band playing when you're outside the hall, and you'll hear the bass far better than guitars, drums etc. The low frequencies can travel better. So, a higher register instrument may be better. Consider the piercing aspect of a sound. In fresh air, it'll carry well - too well, maybe! However, if there is a decent brick wall separating you and the neighbours, the piercing sound tends to get absorbed by the concrete etc. A trumpet, for example, tends to be more easily heard than a flute. But, you can use a mute in a trumpet, to take lots of decibels away.An acoustic guitar is quite quiet, and if you put some foam inside it, it's even quieter.The flute is probably the best portable for you. I take one on holidays, so's not to annoy too many folks, but continue playing something.An electric piano is your best bet, with headphones. However, you did mention portable, and it ain't !
I recently started playing flute and like you thought it would be pretty quiet. This is not the case. As well as packing a surprising amount of volume, it is pretty piercing the moment you come out of the bottom octave. And especially when you're learning, you will play higher notes more loudly because playing high notes quietly is not easy (or low notes loudly).
I would say it is louder than a classical guitar for sure, and perhaps on a par with a piano. It rather depends on your house construction but my neighbours can easily hear me play my flute and they cannot hear our TV. I would certainly not play it when anyone in the house or next door was trying to sleep.
Flute can be quiet, as instruments go. But it's also quite capable of holding its own in a full symphony orchestra! I'm afraid that, if your neighbours are in a mindset to be annoyed by instrumental practice, they'll find a flute quite enough justification. Your only really safe bet is an electronic piano with headphones. And that isn't terribly portable. (And I've had neighbours who have claimed to be annoyed by the clunking of the keybed on one of those!)