What Makes an Audio Format Lossy?

Save disk space with lossy audio files

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. This signal loss means that the encoded audio isn't identical to the original and is technically lower quality sound.

However, the tradeoff for the reduced audio quality is a reduced file size. This means you can store lots of lossy audio files without taking up as much space as the alternative compression method.

Lossy compression is also called irreversible compression because it's impossible to rebuild the data that's been stripped away.

Lossy vs Lossless

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 restricted to just audio; image files in the JPEG format, for example, are also lossily compressed.

Incidentally, this method is the opposite of 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.

When it comes to compatibility, lossily compressed files are definitely at an advantage. While some devices and software programs absolutely support a wide variety of audio formats, lossy formatted files like MP3s are sure to work anywhere and everywhere.

Examples of Lossy & Lossless Audio File Types
Lossy Lossless

How Does Lossy Compression Work?

Lossy compression makes certain assumptions about frequencies that the human ear is unlikely to detect.

When you convert a song to a lossy audio format such as AAC, the algorithm analyses all the frequencies and then discards ones that the 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 disk 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 retaining as much audio quality as possible.

How Lossy Compression Affects 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 bit rates.

Different types of artifacts affect the quality of a recording. Distortions are one of the most common artifacts that you'll 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 bit rates or aggressive compression algorithms come into play.

Why Compress Audio Files?

Most digital audio formats employ some sort of compression to store sound in an efficient way. 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, but uncompressed results in a file size of approximately 30 MB — at least six times larger. Far fewer songs fit on your smartphone or hard drive when you favor uncompressed audio formats.

How to Compress Audio Files

There are lots of ways to turn a lossless audio file into a lossy one because any program that can convert to a lossy format already includes the necessary tools to make the lossy audio file.

For example, this list of free audio file converters includes several options for converting various audio file formats to MP3 and other lossy formats.