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!)
No musical instrument is quiet. Pretty sounding flute can annoy even the player. Try to practice when no one will be bothered..and play recitals at night. The quietest instrument out there is probably an oversized plywood top, low priced dreadnaught style guitar. I had one, and was it quiet..this on the other hand may annoy teachers and band mates.
According to what appears to be a popular enough channel on how to play the flute...
...Recorder, Guitar, and Viola are the only instruments that the flute is louder than.
That said, I've been practicing flute in my apartment and haven't gotten any complaints (at least not yet). I'm a beginner who's trying to get better at long tones, so being able to maintaining integrity on the newer notes (like G4) is still a challenge. I'm definitely NOT able to play loudly like the professionals do. OTOH, I'm fairly sure I got handed a beginner flute (used by someone when they were learning flute in 5th grade), and those aren't known to be as loud.
Last but not least, I can sort of gauge how loud I am with my own ears, compare it to how loud my computer is when I play a typical YouTube video, and they seem about equivalent (although I haven't factored for variances in how different sounds would travel).
And my neighbors (at least a few years ago) were definitely listening, as I got complaints by way of my apartment's manager when practicing on euphonium after hours (I've played that at normal dynamics). I've since then use a practice mute.
If you're in a single family home, you should be fine. In an apartment/condo/townhouse, that'll depend on proximity to adjacent neighbors (if you're in a side unit, playing on the side away from everyone else could help), and how thin the walls are.