Hi,
I discovered I was accidentally creating low bit rate MP3's for my basic listening evaluations on my iPhone, so I went back into Adobe Audition and found I had a crappy bitrate so I modified it to 320kbps. I also decided to investigate if lossless formats like flac could be imported in iTunes.
I found that iTunes would not support flac, but most recommendations said just convert it to "M4a" to simplify things.
 
I thought OK... I'll use Audacity to convert my WAV's, but then I did a comparison of file size from my 320 kbps Adobe Audition MP3's, and thats were I got a bit confused about standards of quality.
 
This is what I found after converting my WAV Mixdown, but I don't know if this is relevant to the quest of best quality.
 
My WAV file = 46.7Mb
The 320 MP3 = 10.6Mb
The M4a     = 6.5Mb
 
What does the files size say about the quality of the format. Does M4a always beat MP3 in quality?
 
Thanks
Adrian
 

Likely, you won't be able to hear any difference at all. For one, 320k MP3 is higher than tend to be useful. Generally 256k MP3 is pretty well regarded as transparent (meaning nobody can tell subjectively the difference from uncompressed, even on really good equipment) and for a lot of consumer uses (like earbuds on a portable) 192k sounds as good as the original.
 
Well M4a can and should contain AAC audio (I say can because it is a container format, not a compression format in and of itself) which is generally more efficient than MP3. It is newer, based on better research of acoustics and human perception and so can get more transparent sound to our ears with less bits.
 
So I'd expect that for your use, you won't notice any difference, the more efficient AAC encoded M4a files will sound just like the WAV files.

Although there is a direct relationship between file size and quality within a given format, it does not necessarily apply across unlike formats. 
 
For example, a FLAC file is going to be twice the size of a 320 kb/s MP3, but you will not hear any difference between them. Technically, the FLAC file is true to the original source and the MP3 is not, but a 320 kb/s MP3 is past the threshold below which which humans can perceive any flaws.
 
M4A is generally thought to be higher-quality than MP3, but you've got to really split hairs to make a case that anybody can hear the difference. So why doesn't everybody switch to M4A and get smaller files of higher quality? Because outside the bubble of Apple World it's not been widely adopted. MP3 assures the widest compatibility.
 
Some trivia: M4A is actually AAC in disguise. AAC is how music gets from distributors to your radio station. When you listen to FM radio, you're listening to 256 kb/s AAC-encoded files. AAC retains fidelity better at lower bitrates. 192 kb/s AAC is indistinguishable from 320 kb/s MP3, so 256 kb/s AAC is overkill.

BTW, you can get a free, fast command line AAC encoder here. It'll do up to 381 kb/s, CBR or VBR.
 
I've done a little experimenting with it, but never adopted it because my portable player doesn't support AAC so it ultimately was of no use to me. I'd use "-q 0.75" for a bitrate of 285 kb/s. That produced smaller files than a 320 kb/s MP3 but sounded exactly the same. 
 
Soundcloud, however, does support AAC, so if that's your primary window to the world for sharing your music then AAC is a viable alternative to MP3.
 

Thanks Biitflipper.
That's really interesting info. Cheers!

bitflipper
Although there is a direct relationship between file size and quality within a given format, it does not necessarily apply across unlike formats. 
 

I like to emphasize the point here that once something is transparent, you can't improve it. For some reason when it comes to audio lots of otherwise intelligent folks passionately convince themselves of stuff like, "even if I can't hear a difference, if I could hear a difference this would be better, therefore it's better".

drewfx1

bitflipper
Although there is a direct relationship between file size and quality within a given format, it does not necessarily apply across unlike formats. 
 

I like to emphasize the point here that once something is transparent, you can't improve it. For some reason when it comes to audio lots of otherwise intelligent folks passionately convince themselves of stuff like, "even if I can't hear a difference, if I could hear a difference this would be better, therefore it's better".

The truth is, even though people cannot safely tell one codec from another just by listening, there are things people can feel. I mean, in the long run, higher quality audio feels a lot better, even though it may seem undistinguishable from a lower quality audio in a AB test.

But in that case, where the listener is able to detect some very subtle difference, even when too subtle to put your finger on what that difference is, then the encoding is not "transparent". Drew's point still stands.

bitflipper
But in that case, where the listener is able to detect some very subtle difference, even when too subtle to put your finger on what that difference is, then the encoding is not "transparent". Drew's point still stands.

And vice-versa: if it's not transparent, at least technically, then the listener may be able to detect it under certain conditions. But then again, no matter how transparent, true to the source or perfectly  balanced the mix is, it's the music that we listen to, not just a bunch phase-aligned amplitudes and a conflict-free frequency spectrum. So many great tunes were recorded with gear that was far from being perfect.

SoundwiseAnd vice-versa: if it's not transparent, at least technically, then the listener may be able to detect it under certain conditions. 

If one can detect it then they can detect it. If not then they can't. In audio contexts "transparent" means the listener can't detect it - IOW, it refers to the listener's perspective, not the technical properties of the audio.