What Makes an Audio Format Lossy?

A Look at Lossy Audio Compression and How It Affects Digital Music

Unhappy girl listening to the music in the city
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The word lossy is used in digital audio to describe a type of compression used to store sound data. The algorithm used in a lossy audio format compresses sound data in a way that discards some information in order to minimize file size. This signal loss means that the encoded audio isn't identical to the original.

What Makes an Audio Format Lossy?

When you create a series of MP3 files by ripping one of your music CDs, some of the detail from the original recording will be lost—hence the term lossy. This type of compression isn't just restricted to just audio either. Image files in the JPEG format, for example, are also compressed in a lossy way.

Incidentally, this method is the opposite to lossless audio compression used for formats such as FLAC, ALAC, and others. The audio, in this case, is compressed in a way that doesn't discard any data at all. The audio is therefore identical to the original source.

How Does Lossy Compression Work?

Lossy compression makes certain assumptions about frequencies that the human ear is unlikely to detect. The technical term for the study of sound perception is called psychoacoustics.

When you convert a song to a lossy audio format such as AAC, the algorithm analyses all the frequencies. It then discards ones that the human ear shouldn't be able to detect. For very low frequencies, these are usually filtered out or converted to mono signals that take up less space.

Another technique discards very quiet sounds that the listener is unlikely to notice, especially in a louder part of a song. This approach reduces the size of the audio file while limiting the effect on audio quality.

How Does Lossy Compression Affect Audio Quality?

The problem with lossy compression is that it can introduce artifacts. These artifacts represent undesirable sounds that aren't in the original recording but are by-products of compression. This noise degrades the quality of the audio and can be particularly noticeable when you convert music files using low bitrates.

Different types of artifacts affect the quality of a recording. Distortions are one of the most common ones that you will likely come across. A distortion makes drums, for example, sound weak—without any real punch. Voices in a song can also be affected, leading to vocals that sound coarse and lack detail.

In many cases, ordinary listeners cannot detect the difference between a lossy and a lossless encoding algorithm, although some audiophiles using very expensive equipment claim to hear a difference. The quality difference becomes noticeable only when very low bitrates or very aggressive compression algorithms come into play.

Why Compress Audio at All?

Most digital audio formats employ some sort of compression to store sound in an efficient way. But without compression, file sizes would be very large.

For example, a typical 3-minute song stored as an MP3 file consumes 4 MB to 5 MB of space. Using the WAV format to store this same song in an uncompressed way would result in a file size of approximately 30 MB—that's at least six times larger. Far fewer songs fit on your smartphone or hard drive when you favor uncompressed audio formats.