What does "compression" like?

Agree with the above posts. A good example of what compression sounds like would be if you listen to a classical FM station on your tuner for a big orchestral piece like a Mahler symphony. At the climaxes where the whole orchestra is playing full blast, you'll notice how the volume drops dramatically. For a contemporary pop recording, as a general rule that won't happen because the recording itself has had its dynamic range compressed, so the difference between loud and soft parts is minimal.

Compression comes in two forms. Digital file compression, such as MP3s. It discards information that for portable, uncritical listening at low levels won't be disturbing. However, today, when you can hold gigabits of data on a USB drive the size of a fingernail, there's no longer any need for file compression.

The other compression is dynamic compression. This also is no longer necessary for most digital recordings. 24bit provides a 144 dB dynamic range—that goes from just barely audible to painfully loud and hearing damage. There's no reason for music recordings to be compressed, because, for example, the range of sounds you'll hear from a symphony orchestra, from the softest solo instrument to the loudest passages with all the brass blaring and the percussion going full force, spans about 60 dB. Compression kills the expressive dynamics that musicians painstakingly put into their music. It takes away the transient spikes that give percussion its punch.

How it sounds is difficult to describe, because it doesn't produce any gross distortion that would make you cringe.

Here's a link that has both a visual and audio example of dynamic compression, plus a couple of additional links.

Made an error in the url, try this dynamic compression.

If it always sounds loud, no matter the volume of playback, it is compressed.

This is not an answer to your question, Rebbi, rather just a side-note. We all hate the compression used to affect dynamics for ipod and radio play.

Compression is also used in the recording studio and not in the negative ways that have been mentioned. A compressor would typically be used in recording Rock music to shape the sound of an instrument. For example, by applying compression to a bass drum or tom tom, you can affect the decay of the sound; if it is too boomy or has a long decay, adding a little compression can change the sound of the drum.

This type of compression along with other effects have been used creatively in the studio for many years and is a tool used by the producer and engineer. Usually you would never even know that it was being used.