I have downloaded various MP3s from the Internet. I also buy vinyl, which I record into Steinberg Wavelab and convert to MP3. The Wavelab MP3s sound different to the downloaded ones and are generally quieter. Are there different MP3 types and how can they differ sonically?
Firstly, the MPEG Layer 3 format (MP3 for short) supports different levels of data compression. Perhaps the most common is 128kbps (kilobits per second), but 48, 56, 64, 96, 192 and more are all in use. The lower the bit-rate, the more extreme the compression, and the more obvious its audible consequences.
Secondly, there are two types of MP3 coding — constant and variable bit-rate. In the former, the data compression is applied 'evenly' to an entire audio file, so the compressed version will use the same amount of data to encode each 'frame' of the file. In the latter, the limited amount of data available is used more efficiently. Some parts of an audio file will be more complex than others and will require more data to encode without audible side-effects, so variable bit-rate encoding 'saves up' data from less demanding passages to code these more accurately. As a result, a variable bit-rate MP3 usually sounds better than a constant bit-rate one for a given amount of data reduction.
Thirdly, and most fundamentally, different encoders can produce different results. The basic function of an MP3 encoder is to take an audio file and output a data file that conforms to certain requirements. A decoder does the reverse — it takes a data file and 'reconstitutes' it as audio. However, the MP3 format doesn't specify exactly how the encoding should take place, and programmers have developed a number of different encoders, which make different decisions about what parts of the audio to discard when creating an MP3 file. The original 'Fraunhofer codec' is one of the most widely used, but there are numerous others, and you will certainly notice the difference between them even on MP3s coded at the same bit-rate.
For more detail, take a look at www.mp3-converter.com/mp3codec/implementation.htm.