Can anybody list the main audio artefacts and try to describe them.

Artifacts:  (by David McIntyre)

A definition of artifacts is very difficult without audio examples (and I don't have time to make some) and again, different people hear different things, but here are the most common:

chirps: high-frequency distortions added to the audio, will sound like short chirps, whistles, sometimes can shwoosh up like a high-pitched glissando (slide).

pre-echo: the killer artifact that everyone has trouble with and the whole TNS structure in AAC was designed to handle but didn't work as well as expected (there's sentence). To hear this find a track with very precise sharp hits and no background noise (the best is a castanets track we use and is part of the SQAM testing disc). When you code it the attacks will be smeared - if you listen closely or look in Cool Edit you will see that there is an attack before the real attack, thus "Pre-echo". This is a huge problem for MP3. The reason no one has completely solved it is basic filter theory - as you improve time resolution, you lose harmonic resolution and/or vice versa. Codecs that have little pre-echo tend to sound less warm and more brittle.

Imaging: (my worst, so my description may be bad) listen to the placement, in the stereo field, of all the parts. Also, get a sense for how "big" the stereo field seems. When you listen to compressed audio you will find that parts get moved towards the center or pushed out to the extreme edges. Also, you will find that smooth pans from side to side seem to get quantized into steps instead of a smooth continuum. The other thing is a simple, general, loss of "width".

Pumping: pumping is a general term that can apply to frequency, amplitude, imaging, etc.. This simply means something, lets say left channel amplitude is going up and down fast enough to be bothersome and unatural, but slow enough to be really noticeable. Maybe 2-4 times per second. Amplitude pumping is the most common type and has been solved in most codecs, but can be a problem.

Harmonic content/depth: simply, does it sound shallow or brittle (PAC's biggest problem), or does it sound warm and ful.

Noise: has noise (static, hum, etc.) been added or removed. Is it centered on one particular event or frequency range.
Other time resolution problems: other than pre-echo the most noticeable is the watery sound many codecs get, particularly on voice. This has to do with insufficient time resolution on the rich harmonic content in the vocal spectrum.

Phasing: to kill AAC, or the QDesign Music Codec, trickling water, or rain on a sheet metal roof with other sounds in the background will be very difficult. In particular, the definition of the water will be lost and it will start to sound like frying oil and not water drops. This is due to quantization of the phase information in the audio. With all these water drop hits - a series of like sounds close together - you get a lot of signals that are similar and slightly out of phase with one another adding up to make a noise-like sound. Usually, a codec will screw up these delicate phase differences and tend to make it sound more like white noise (I don't understand the details, but I know this is THE KILLER SIGNAL for AAC).