Spotify deletes most of a song's data. Your ears let it get away with this

Take any carriage of the last Metro out of Rajiv Chowk. It is nearly midnight, the doors sigh, the rails hum a low, steady note beneath everything. A woman by the window has closed her eyes, one earphone in, a song she has loved since college folded into the rattle of the train. The song reaching her is mostly absent.

Somewhere between a studio and that earphone, a machine went through the song moment by moment and removed most of the information a CD would have carried, so carefully that she would never miss it.

In this instalment of Science of Sound, we find out what was removed, and why her own ears let it get away with it.THE FILE THAT SHRINKS

A CD stores sound as 16-bit samples, taken 44,100 times a second, in two channels. That adds up to 1,411 kilobits per second, or 1,411,000 tiny pieces of information every second. Engineers call this the bitrate. Spotify streams to free listeners at about 160 kilobits per second, and to Premium listeners at up to 320.

Do the arithmetic. At 320, a song carries about 23 per cent of a CD's information. At 160, barely 11. Between 77 and 89 per cent of the data is gone.

Spotify is not alone. Google's support page says YouTube Music Premium members can stream at up to 256 kilobits per second, using codecs called AAC and Opus, while the default setting tops out at 128. Spotify streams to free listeners at about 160 kilobits per second, barely 11 per cent of the 1,411 kilobits a CD carries. (Photo: Unsplash) An hour of CD-quality music takes about 635 megabytes. At 320, it takes about 144. For a service with hundreds of millions of listeners, that difference is the whole business.

This is lossy compression. Compression means the file is made smaller, and lossy means something is lost along the way. Its opposite, lossless compression, is like a zipped folder, which shrinks and then restores everything exactly.

Amrit Srivastava, a musicologist based in Rome, puts it plainly. You make the bitrate smaller to get a smaller file, he told India Today Digital, and what you are doing is squeezing out some parts of the sound.

The astonishing part is that the deletion is chosen, so that it falls only on what the ear would have missed anyway. How can a machine know that?WHEN ONE SOUND BLINDS ANOTHER

The answer is masking, and everyone has experienced it. Stand beside a roadside drill and try to hear a friend whisper. The whisper still travels through the air. The ear simply cannot register it under the roar.

Hearing science measures this. A loud sound raises the level a quieter sound must reach to be heard, and the effect is strongest for sounds close to it in pitch. Pitch is how high or low a sound is, counted in vibrations per second, or hertz. This is frequency masking. The ear sorts pitch into about 24 bands, called critical bands, a division first set out by Eberhard Zwicker in a 1961 paper in the Journal of the Acoustical Society of America. Spotify Premium streams at up to 320 kilobits per second, so an hour of music takes about 144 megabytes against 635 for a CD. (Photo: Unsplash)

Masking also works across time. After a loud burst, the ear stays partly deaf to quieter sounds for a fraction of a second. This is temporal masking. A technical report on the theory behind MP3 describes how both effects are used to decide what a compressed file can drop.

So, when a cymbal crashes, a soft violin note at a similar pitch in that instant is still in the air, yet effectively gone from hearing.THE EAR'S OWN BLIND SPOTS

Masking is not the only gap. The ear is also unevenly sensitive. It hears best between about 2,000 and 5,000 hertz, and grows steadily deafer towards deep bass and very high pitches, a pattern first mapped by Harvey Fletcher and Wilden Munson in 1933 and known as the equal-loudness contours.

The software that does this is called a codec, short for coder and decoder.

It slices the song into frames, each lasting a tiny fraction of a second. For every frame, it splits the sound into frequency bands that mirror the ear's own. It finds the loudest components and calculates how much each one masks the sounds beside it. The result is an invisible line called the masking threshold. Spotify's apps use Ogg Vorbis, an open, patent-free format that discards sound the ear is unlikely to miss. (Photo: Unsplash)

Everything above the line is kept. Everything below it is judged inaudible and is not stored.

This is the idea behind MP3. The audio standard it belongs to, MPEG-1, was published in 1993, and its engineers Karlheinz Brandenburg and Gerhard Stoll described the scheme in a 1994 paper in the Journal of the Audio Engineering Society. Spotify's apps use a relative called Ogg Vorbis, an open, patent-free format built on the same principle.

Brandenburg has recalled how he tuned his encoder on an unaccompanied recording of Suzanne Vega singing Tom's Diner. He knew it would be nearly impossible to compress that warm a cappella voice. A lone voice leaves nothing to hide behind, which made it the hardest possible test.WHAT YOU LOSE, AND WHO NOTICES

What disappears is subtlety. Srivastava compares the effect to hanging four carpets on your wall when the neighbours are having a party. The party is still there, but its finer textures are muffled.

Whether you notice, he says, depends on how you listen. The difference shows only if you listen to a lot of good, well-mixed music on great headphones or a hi-fi. A casual listener has long been used to it. Spotify added a lossless tier for Premium listeners in September 2025, streaming FLAC at 24-bit and 44.1 kilohertz with nothing deleted. (Photo: Unsplash)

Apple makes a similar point. Its support page says the difference between AAC, its compressed format, and lossless is virtually indistinguishable, though it still offers lossless as an option for those who want it.

Srivastava's advice for anyone who wants the subtleties back is simple: a good pair of speakers at home, a soundbar for a television, and better headphones on the move.WHERE THE TRICK STRUGGLES

Masking works beautifully inside a dense rock chorus, where plenty of loud sound is available to hide the deletions. It has less cover when a single voice or a single flute holds a note.

Indian music asks a particular question here. A gamakam is an ornament, a continuous glide or oscillation around a note, and it is the soul of a raga. Professor M. Ramanathan of IIT Madras, who works on teaching machines to recognise ragas, says Indian music is difficult for machines because of its gamakams and the variations within them. Even on Spotify's lossless tier, a Bluetooth earphone compresses the sound again before it reaches the ear. (Photo: Unsplash)

We asked him two questions. Could compression make gamakams harder to catch? And might compression built mainly for simpler, Western-style music handle such variations badly? He said he would have to test both before answering properly.

His possible short answers, he told India Today Digital, were these. To the first: "No, we don't usually compress." To the second: "Maybe yes."

It is a tentative answer, and an honest one. The question for the next experiment is whether a method tuned to hide deletions in dense music treats a gliding note with the same care.THE LOSSLESS TWIST

The industry has an answer of its own. After announcing changes to its Premium plans in August 2025, Spotify added a lossless tier in September, streaming Free Lossless Audio Codec (FLAC) at 24-bit and 44.1 kilohertz, with nothing deleted.

It arrived late. Apple's support page says Apple Music offers lossless audio in a format called ALAC (Apple Lossless Audio Codec), from CD quality up to 24-bit and 192 kilohertz, and Apple announced it in 2021. In the same month, Amazon Music said its HD tier would stream lossless audio at CD quality, with Ultra HD going up to 24-bit and 192 kilohertz, at no extra cost to Unlimited subscribers.

There is one more catch, and Apple states it itself. AirPods and Beats headphones use Apple's AAC Bluetooth codec, and Bluetooth connections are not lossless. So, even on a lossless stream, the last few metres to a wireless earphone are compressed again.

Every song on a phone is a quiet collaboration between the recording and the limits of the person listening. The ear has blind spots, and a machine learnt to hide inside them.

Music has always been finished by the listener. Streaming has simply learnt where the listener stops.- Ends

Radifah Kabir tries to make the universe slightly less mysterious as Senior Sub-Editor at India Today’s Science Desk. A lifelong lover of physics and mathematics, she has spent four years (and counting) translating complex physics papers into English, chasing scientists for interviews, and writing explainers that don’t put readers to sleep. Off-duty, her heart belongs to Lana Del Rey's melodies, Quentin Tarantino's films, poetry, the Star Wars universe, good English movies, series and books, and one very demanding cat.