again taken from wiki about mp3 quality

 

When creating an MP3 file, there is a trade-off between the amount of space used and the sound quality of the result. Typically, the user is allowed to set a bit rate which specifies how many kilobits the file may use per second of audio. The lower the bit rate used, the lower will be the audio quality. Likewise, the higher the bitrate used, the higher quality the resulting MP3 will be.

MP3 files encoded with a lower bit rate will generally play back at a lower quality. With too low a bit rate, "compression artifacts" (i.e., sounds that were not present in the original recording) may be audible in the reproduction. A good demonstration of compression artifacts is provided by the sound of applause: it is hard to compress because of its randomness and sharp attacks. Therefore compression artifacts can be heard as ringing or pre-echo.

As well as the bit rate of the encoded file, the quality of MP3 files depends on the quality of the encoder and the difficulty of the signal being encoded. As the MP3 standard allows quite a bit of freedom with encoding algorithms, different encoders may feature quite different quality, even when targeting similar bit rates. As an example, in a public listening test featuring two different MP3 encoders at about 128kbps^[2], one scored 3.66 on a 1?5 scale, while the other scored only 2.22.

Quality is heavily dependent on the choice of encoder and encoding parameters. While quality around 128kbps was somewhere between annoying and acceptable with older encoders, modern MP3 encoders can provide very good quality at those bitrates ^[3] (01/2006), not statistically different from quality provided by AAC, the technical successor of MP3. However, in 1998, MP3 at 128kbps was only providing quality equivalent to AAC-LC at 96kbps and MP2 at 192kbps^[4].

The transparency threshold of MP3 can be estimated to be at about 128k with good encoders on typical music as evidenced by its strong performance in the above test, however some particularly difficult material can require 192k or higher. As with all lossy formats, some samples can not be encoded to be transparent for all users.

The simplest type of MP3 file uses one bit rate for the entire file - this is known as Constant Bit Rate (CBR) encoding. Using a constant bit rate makes encoding simpler and faster. However, it is also possible to create files where the bit rate changes throughout the file. These are known as Variable bit rate (VBR) files. The idea behind this is that, in any piece of audio, some parts will be much easier to compress, such as silence or music containing only a few instruments, while others will be more difficult to compress. So, the overall quality of the file may be increased by using a lower bit rate for the less complex passages and a higher one for the more complex parts. With some encoders, it is possible to specify a given quality, and the encoder will vary the bitrate accordingly. Users who know a particular "quality setting" which is transparent to their ears can use this value when encoding all of their music, and not need to worry about performing personal listening tests on each piece of music to determine the correct settings.

In listening tests, MP3 encoders have performed significantly worse than those using more modern compression methods (such as AAC) at low bit rates. In a 2004 public listening test at 32 kbit/s^[5], the LAME MP3 encoder scored only 1.79/5 - behind all modern encoders - with Nero Digital HE AAC scoring 3.30/5.

It is also important to note that perceived quality can be influenced by listening environment (ambient noise), listener attention, and listener training.