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I wonder when I should play FLAC instead of 320 Kbps: size of room, type of music, etc. I saw plenty of discussions about 320 Kbps MP3 vs. FLAC on Internet but none with reliable source. Are there any scientific studies (e.g. blind experiments) or at least expert opinions (e.g. by some famous DJ) on that matter?

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Depends on your sound system and by system I mean extremely high-end hifi systems. Then on some systems you can identify a slight dithering artifact. But your gut feeling is right; people mostly make up stuff that is either not audible or doesn't exist on their average sets. –  percusse Feb 23 at 22:22
    
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Well, the thing I found requiring surprisingly large MP3 bit rates was encoding my accordion's tremolo registers. Imagine three reeds sounding with lots of harmonics, almost equal volume, and slightly different pitches, hand-tuned so that all of the respective slow beatings combine into a pleasant sound. That's actually a somewhat unusual combination since high-quality accordions tend to have one reed in a tone chamber, giving it a different volume and/or using only two beating reeds, while folksier accordions use rather heavy beatings in order to get a piercing rather than pleasant sound.

At any rate, a high quality recording of something played slowly with that sound reencoded at 128kbs does not sound "slightly lower quality, discernible by experts" but totally awfully crappy. Like coal grain microphone crappy. With 192kbs, you start getting tolerable. Tolerable!

Ogg/Vorbis faired decidedly better regarding the quality/bitrate ratio. So this kind of thing caught me on the wrong foot.

As a rule, I'd stay away from any lossy encoding until you are finished mastering and listening. You never really know what kind of actually-not-all-that-relevant audio component will end up hogging the bitrate.

Some music styles might be less touchy here. I think it is mostly the high frequency content: in my case it was getting too unmusical, but it is similarly distracting when fine-grained noise (like a hihat) is getting too musical, having discernible and wobbling tones in the noise (actually, "musical noise" is a well-known compression artifact).

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overal rule of thumb: never consider any lossy compression for archival. also, here's a nice test you can do re ogg - transcode ogg to m4a. even if you up the bitrate, you'll hear that m4a's psychoacoustic algos favour information that the vorbis codec has discarded. so the transcoded files end up sounding much worse than if you transcode mp3>mp3 or mp3>m4a. –  georgi Feb 24 at 14:59
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"lossless" means you get your waveforms reconstructed exactly, bit for bit, identical to the original.

The biggest problem of MP3, and people keep ignoring this as if it really doesn't matter, is that it smears transients. internally, MP3 is split into frames of fixed size. The same math is used to reconstruct the entire frame. Even though the frames are small (1152 samples = 1/40th of a second), the duration of one frame is sufficient for the ear to detect that some sounds have been "altered".

320kbit still means you keep 1/4th of data only. Come transient time (snares, percussion, hihats, etc) that loss of data shows and what may be a beautifully crunchy attack is sacrificed because the encoder math optimised the data to cover the entire frame's needs, not necessarily those few important milliseconds.

I've never found audible difference between 320kbit and 256kbit mp3. As long as mpeg audio is a bunch of matrices through which data is passed back and forth, adjusting the bitrate primarily affects how much data you're going to discard at the entry point.

If your ears can't detect this, stick with MP3. If they can, go for FLAC. My favourite is ALAC - less open, but more supported on devices I actually use.

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FLAC as its clear is : Free Lossless Audio Codec , that means the compression applied on the sound file does not affect on the sound quality... but we have compressions in 320 Kbps that we will lose some frequencies over 16 KHz to have a lower file size , but is that make a real huge difference? as a music producer and a sound engineer I can say yes! it will make a real difference for me and its because I'm very accurate on sounds because of my work , experience and also my equipments (I mean accurate monitoring speakers and more...) , but for peoples there is no difference even on a high-end hifi system they can't sense it, I also asked this time over time to my listeners and costumers but all of them say no! there is no difference! but I also know some frequencies are lost , but if they can't recognize that, so its ok , you have less size files on your disk. but in professional music production ,all DAWs are using lossless audio to make a good output quality and when you are going to export as mp3 you need to apply a dithering on it if you don't want to have mp3 compression artifacts (sometimes dithering applies automatically embedded in DAWs).

(thanks to Alex Basson)

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"Lossless" doesn't mean there isn't any compression done to the sound file. It simply means that the encoded FLAC file can be decoded to the original format with 100% fidelity to the source. "Compression" in the codec sense is different from compression in the audio sense—it just means that the encoded file size is smaller than the original file. It's quite possible for a codec to compress the file size while retaining fidelity to the original source file. –  Alex Basson Feb 24 at 1:37
    
oh you are right about that , sorry I was so tired last night and confused a little , yes I will edit my post –  Mohammad Rafigh Feb 24 at 9:12
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I've done a few A-B tests between WAV and 320kbps mp3 and have yet to notice an audible difference. I've only tested rock and electronic music and not classical or jazz but for what it's worth my ears couldn't hear any discernible loss in quality.

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do a zero sum, compressing original wav to mp3, then decompressing that, inverting its polarity, adjusting to same sample (since the encoder may introduce an offset) and summing with original. this will give you just the difference between original and encoded. you can't unhear it afterwards. it's not for the faint hearted... –  georgi Mar 8 at 20:02
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Listen to music with good bass speakers (sub-woofer)...MP3 totally cuts the lower frequencies. It's true that we can't hear those, but we can FEEL the boom vibration. Good -professional or really hifi- sub-woofers (e.g. Bose 320) make rooms tremble from a CD or FLAC (or live music), but input MP3 and that vibration is gone.

I'm a volunteer sound technician at a 300 member church, with Bose 820 and 320, like 3000W sound total, with good mixing tables and professional amplifiers (Bose too) and crossovers cutting the high frequencies for the 320s, and cutting the low frequencies for the 820s. Every time somebody brings MP3s or we play Spotify, the booming is gone.

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Thanks. Does MP3 totally cuts the lower frequencies even when encoded at 320 Kbps? Spotify is 128 Kbps and I guess most people bring 128 Kbps too. And I do feel the difference between 128 and 320 for most electronic tracks (if good sound system + mastering). –  Franck Dernoncourt Mar 8 at 18:31
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