Lossy codecs try to find an approximate signal that sounds as close as possible, but is easier to compress. They drop parts of the signal entirely, or reduce their resolution, based on models of human hearing to identify the parts that are least likely to be noticeable if they are missing. E.g. not all frequencies can be heard equally well, so quality is dropped on the ones that are heard less clearly anyways. And loud signals on one frequency can make signals on another frequency or following quickly after harder to perceive.

Most such audio codecs are based to some degree on variants of fourier transforms, so this modification is done by dropping or reducing resolution of parts of the output.

When you listen to music there's a lot of "fine detail" that you can't really hear that gets buried by louder sounds.

CD audio is perfectly lossless - it encodes the signal that you put in by measuring a voltage 44100 times per second and recording that exactly. When you play it back you get exactly the same signal back out. The only problem is, this takes up a lot of space, roughly 10MB per minute for stereo audio.

MP3 audio is lossy in that rather than storing the exact values of a waveform, it stores a description of how short segments of the waveform change. The higher the bitrate, the better the description, and the more detailed the reproduction. A low bitrate MP3 is like trying to redraw the original waveform from a vague description with a paint roller, a high bitrate MP3 is like trying to draw it with a mapping pen from a really detailed description.

FLAC audio is lossless because it takes the precise values of the audio, and uses a technique similar to zip files to find similar-looking blocks of data. Think in terms of having a one-second silence recorded as "Zero, then 44099 more of 'em" rather than "zero zero zero zero zero..." and so on 44100 times.

> What is preserved when it is "LOSSLESS"

Just like any general purpose compression: everything - flac could be used just like zip/zlib/gzip as a general purpose compressor it just wouldn’t compress as well on most data that isn’t audio.

> dropped when "LOSS" occurs?

Lossy compression generally employs some type of perceptual coding where data is reorganized such that the signal data is sorted or localized according to its perceptual importance. This does partly involve removing or reducing the density of signal in the higher frequencies but it also exploits things such as masking both in time - inability to perceive signals occurring close in time of similar frequency. And frequency masking - our inability to hear quieter signals that are close in frequency to a louder one.

If you feed an .EXE file into the FLAC encoder, you'll get a file with white noise, but you can always restore the original .EXE by decoding the FLAC.

You cannot do the same with MP3, AAC, OGG, Opus or any other lossy codec.

The key point is masking: if a sound at a given frequency is loud enough, you are less likely to perceive weaker sounds at other frequencies. So there is little point in wasting bits on those frequencies during the time intervals where they are being swamped.

The exact frequency/amplitude relationships where masking effects come into play were studied by the telecoms early on (meaning in the 1950s-1960s era), and are still a key part of most lossy encoding models these days.

See https://en.wikipedia.org/wiki/Critical_band and https://en.wikipedia.org/wiki/Psychoacoustics as an introduction to the principles.

From my understanding (I took two relevant undergrad courses, definitely not an expert) lossy compression can involve either losing certain frequencies (like those above/below human hearing), or losing the accuracy of the reproduction (like a certain frequency component of the sound could be the smallest bit louder/quieter or higher/lower than it was originally).

usually frequencies too high to be heard.