Luma
Formats & sound
Music file formats explained: FLAC, ALAC, WAV, MP3, AAC and Opus
Lossless, lossy, hi-res, bitrate: few topics attract as much half-knowledge as audio formats. Here the formats are explained: how they work, which one suits which job, and what controlled listening tests have actually shown.
In short
- For the collection you want to keep: a lossless format, so FLAC, or ALAC if you stay entirely within Apple's world. Both are bit-for-bit identical to the CD and about half the size of WAV.
- For your phone when space is tight: AAC at 256 kbps or Opus at 128 to 160 kbps. In blind tests most people can't tell either from the original.
- MP3 plays everywhere but is technically dated. There's no need to replace MP3s you have, and converting them to FLAC doesn't make them better.
- Hi-res (24-bit, 96 kHz and up) helps in the studio. For listening, controlled tests found the difference from CD quality small and mostly detectable with training.
- Over Bluetooth nothing arrives lossless, AirPods included. To hear the full difference you need a cable.
What's inside an audio file
Digital music is a very long series of numbers. A converter measures the sound wave thousands of times per second and stores each measurement as a number. The method is called PCM (pulse-code modulation), and in the end every format, MP3 or FLAC, decodes back to PCM for playback. Three values describe that data:
- Sample rate: how many measurements per second. CD uses 44,100, or 44.1 kHz. By the Nyquist–Shannon sampling theorem this captures frequencies up to half that, 22.05 kHz, exactly. Human hearing reaches about 20 kHz at best, and noticeably less in most adults.
- Bit depth: how finely each measurement is resolved. 16 bits give 65,536 steps and a dynamic range of about 96 dB, more in practice with dither. That spans the noise floor of a quiet room to the level of a rock concert. 24 bits allow 144 dB in theory, more than any real converter or ear can use.
- Bitrate: how much data per second. For CD that's 44,100 × 16 bits × 2 channels = 1,411 kbps. For compressed formats, bitrate is the main lever for size and, with lossy formats, for quality.
One more thing often causes confusion: container and codec are not the same. The codec is how the audio is encoded, the container is the wrapper around it. A file ending in .m4a can hold lossy AAC or lossless ALAC. You can't tell from the outside; the file info reveals it.
Three families: uncompressed, lossless, lossy
Every common format belongs to one of three groups. Once you know them, you can place any new format instantly.
Uncompressed: WAV and AIFF
The PCM data is simply written into the file in sequence. Maximally simple and maximally large: an hour of CD-quality audio takes about 635 MB.
Lossless compressed: FLAC and ALAC
These work like a ZIP archive specialised for audio. They predict each sample from the previous ones and store only the difference from the prediction. Playback yields exactly the same numbers as the original, without the slightest deviation. Expect 50 to 70 percent of the WAV size. Quiet music such as solo piano packs tighter, loudly mastered rock or electronic music less.
Lossy: MP3, AAC, Opus, Vorbis
Here information is discarded on purpose: the parts your hearing probably won't notice. The principle is psychoacoustics. A loud tone masks quieter tones near it, and many people don't hear very high frequencies anyway. The encoder saves data exactly there, shrinking a file to a fifth or a tenth. What's gone doesn't come back, which is why a lossy file is a poor starting point for later conversions.
The formats one by one
| Format | Type | Per hour | Tags | Good for |
|---|---|---|---|---|
| WAV | Uncompressed | ≈ 635 MB | Unreliable | Recording, editing |
| AIFF | Uncompressed | ≈ 635 MB | Limited | Production on the Mac |
| FLAC | Lossless | ≈ 300–450 MB | Excellent | Archive, everyday |
| ALAC | Lossless | ≈ 300–450 MB | Excellent | Archive in Apple's world |
| MP3 (320 kbps) | Lossy | ≈ 145 MB | Good (ID3) | Maximum compatibility |
| AAC (256 kbps) | Lossy | ≈ 115 MB | Excellent | Phone copy |
| Opus (128 kbps) | Lossy | ≈ 58 MB | Excellent | Little storage |
WAV
Microsoft and IBM's format from 1991 is the lowest common denominator of audio: every program opens it. For recording and editing that's ideal. As a format for a music collection, WAV has a real drawback: there's no consistently supported standard for tags like artist, album or artwork. Some programs write a RIFF INFO chunk, others an ID3 chunk, many read neither. Store a collection as WAV and the next change of software often wipes out all the info. And there's no quality advantage over FLAC, because both hold the same audio data.
AIFF
Apple's counterpart to WAV, also uncompressed. AIFF can carry ID3 tags, but support depends on the program. Common in Mac studios; for a collection the same applies as for WAV.
FLAC
FLAC (Free Lossless Audio Codec) has been the standard for lossless music since 2001. It's open and royalty-free, and since December 2024 it's specified by the IETF as RFC 9639, on the same footing as many internet protocols. Nearly every store selling lossless downloads delivers FLAC. Tags are stored as Vorbis comments, a simple, flexible scheme, and artwork as an embedded picture. An often overlooked detail: every FLAC file carries an MD5 checksum of its audio data, so you can verify at any time that a file is still intact. More on that in the article on building a digital music archive.
The compression level (0 to 8) only affects file size and encoding time, never quality. Level 5 is the usual compromise; level 8 typically saves just a few percent more.
ALAC (Apple Lossless)
Apple's lossless format, technically equivalent to FLAC and usually a few percent larger. Apple open-sourced the codec in 2011. ALAC lives in an M4A container and uses the same tags as AAC, which makes it convenient across the Music app, Finder and iPhone. Outside Apple's world FLAC is more common. Because both are lossless, you can convert between them at any time without any loss.
MP3
MP3 changed how we listened in the 1990s, and it still plays on practically every device. The patents have expired; Fraunhofer ended its licensing programme in 2017. Technically, though, MP3 is the oldest and least efficient lossy format: it needs more bitrate than AAC or Opus for the same quality. Good encoders like LAME still produce results at 320 kbps, or with the variable setting V0 (around 245 kbps on average), that hardly anyone can tell from the original. One catch for albums that flow from track to track: MP3 adds a little silence at the start and end for technical reasons. Gapless playback works only if the encoder stored that information in the header, which LAME does.
AAC
AAC (Advanced Audio Coding) is MP3's successor and sounds noticeably better at the same bitrate, especially at low bitrates. Apple has sold AAC at 256 kbps without copy protection in the iTunes Store since 2009, and AAC is the codec iPhone and AirPods use over Bluetooth. For a space-saving copy of your collection on an iPhone, AAC at 256 kbps is the natural choice.
Opus
The youngest codec in the field, standardised by the IETF in 2012 as RFC 6716 and royalty-free. At low and medium bitrates Opus is the most efficient format. In a large public listening test run by the Hydrogenaudio community, Opus came out ahead of AAC and Vorbis at 96 kbps. At 128 to 160 kbps Opus is considered practically transparent for music. The downside: not every player supports it, and support across Apple's apps is patchy.
Ogg Vorbis
The open predecessor of Opus and long the format Spotify streamed in. For new files there's little reason to choose Vorbis over Opus today.
Outliers: DSD, MQA, WMA, APE
- DSD is the format of the Super Audio CD. Instead of 16 or 24 bits it stores 1 bit, 2.8 million times per second. Files are huge and support is limited to specialised players and converters.
- MQA was a heavily marketed proprietary hi-res scheme. The company behind it went into administration in 2023, and Tidal replaced its MQA catalogue with FLAC in 2024. A good illustration of why open formats are the safer choice for a collection.
- WMA (Windows Media Audio) and APE (Monkey's Audio) are formats from the 2000s that few programs support today. WMA Lossless and APE convert to FLAC without loss, and that's worth doing.
How much space does each take?
This table turns bitrates into storage. For the lossless formats it assumes typical compression (around 60 percent at CD quality, around 65 percent for hi-res). Real values vary with the music.
| Format | Bitrate | Album (45 min) | 1,000 songs |
|---|---|---|---|
| WAV 24-bit / 192 kHz | 9,216 kbps | ≈ 3.1 GB | ≈ 277 GB |
| FLAC 24-bit / 96 kHz | ≈ 3,000 kbps | ≈ 1 GB | ≈ 90 GB |
| WAV 16-bit / 44.1 kHz (CD) | 1,411 kbps | ≈ 475 MB | ≈ 42 GB |
| FLAC / ALAC 16-bit / 44.1 kHz | ≈ 850 kbps | ≈ 285 MB | ≈ 25 GB |
| MP3 320 kbps | 320 kbps | ≈ 108 MB | ≈ 9.6 GB |
| AAC 256 kbps | 256 kbps | ≈ 86 MB | ≈ 7.7 GB |
| Opus 128 kbps | 128 kbps | ≈ 43 MB | ≈ 3.8 GB |
A simple rule of thumb follows: lossless CD-quality music needs about three to four times the space of good AAC, and 96 kHz hi-res roughly three times more again. For an archive on a hard drive that hardly matters; a 2 TB drive holds about 80,000 songs in FLAC. On a 128 GB iPhone that also holds photos and apps, it matters a lot. How to combine both sensibly is covered in storage and tags in your music library.
Can you actually hear a difference?
That's the real question, and opinions can't answer it, only tests can. The most reliable method is an ABX test: you hear version A and version B, then an unknown version X, and say whether X is A or B. Score well above half over many rounds and you hear a difference; land around 50 percent and you're guessing. Crucially, both versions must be exactly level-matched, because even one decibel louder is often perceived as "better".
Lossless versus well-compressed
At low bitrates the difference is clearly audible. MP3 at 128 kbps often smears cymbals, applause or reverb, sometimes with a warbling in the treble. As the bitrate rises, these artefacts disappear. The Hydrogenaudio community has run public blind tests for years. Its 2014 multiformat test ran at 96 kbps, deliberately low so that differences would show at all. Opus came first, AAC close behind, and both beat an MP3 at around 130 kbps. At that low bitrate, though, far from every sample was transparent to trained listeners.
At 256 kbps AAC, 320 kbps MP3 or 160 kbps Opus, experienced testers consider the formats transparent for nearly all music, meaning indistinguishable from the original. There are exceptions, so-called killer samples: castanets, harpsichord or very transient-rich electronic music, where trained ears on good headphones can catch pre-echo. In everyday listening they barely matter.
CD quality versus hi-res
Here the research gets more interesting. In 2007 E. Brad Meyer and David Moran published a study in the Journal of the Audio Engineering Society that ran for over a year: high-resolution recordings from SACD and DVD-Audio were played either directly or through a CD-quality bottleneck (16-bit, 44.1 kHz). The listeners, including recording engineers, recording students and dedicated audiophiles, could not detect a difference at normal to loud listening levels. Only at extremely high gain did the noise floor of the 16-bit signal become audible during silent passages.
In 2016 Joshua Reiss of Queen Mary University of London pooled 18 studies with more than 400 participants and over 12,000 trials in a meta-analysis. The result: a small but statistically significant ability to discriminate hi-res. Without training it was barely above chance; with dedicated training the hit rate rose to about 60 percent. So hi-res isn't provably inaudible, but the difference is tiny compared with everything else that shapes the sound.
A practical reason hi-res versions sometimes do sound better: they occasionally come from a different, more careful master with more dynamics. Then you're hearing the mastering, not the sample rate. Conversely, some "hi-res" downloads are simply upsampled from CD material and bring nothing but larger files.
What matters far more
If you want better sound, you'll gain much more elsewhere than in the format:
- Mastering: many albums since the late 1990s are mastered so loud that the dynamics are gone. A well-mastered MP3 can sound better than an over-compressed FLAC. Older pressings or remasters with more dynamics are often the better pick.
- Headphones and speakers: the difference between two pairs of headphones is orders of magnitude larger than between FLAC and 256 kbps AAC.
- Room and listening position: with speakers, the room shapes the bass more than any format.
- Equal loudness: songs at different levels can't be compared fairly. Volume normalisation, which Luma uses by default, helps in everyday listening too.
Bluetooth, AirPods and cables
A point often missing from debates about lossless music: Bluetooth always re-compresses the signal. Apple states plainly in its support article on lossless audio that Bluetooth connections aren't lossless. AirPods and most Beats headphones use Apple's AAC codec. Whether the file on the iPhone is FLAC or AAC, the headphones receive an AAC signal either way.
The exceptions are few. AirPods 4, AirPods Pro 3 and AirPods Pro 2 with the USB-C case play lossless, but only with Apple Vision Pro. AirPods Max with USB-C have played lossless 24-bit/48 kHz since a 2025 update, when connected with the USB-C cable. On Android there are higher-bandwidth codecs like LDAC (up to 990 kbps, but still lossy) and, on some devices, aptX Lossless. The iPhone supports neither.
According to Apple, the iPhone also needs an external digital-to-analogue converter (DAC) for sample rates above 48 kHz. So if you play hi-res files over AirPods, you never hear them in hi-res.
Does that make lossless files pointless if you only use Bluetooth? No. They're still right for an archive, because every other format can be made from them at any time without generation loss. And on wired headphones, a hi-fi system or a Mac they play to their strength. For an iPhone short on space, an AAC copy is a sensible choice that you'll practically never hear over Bluetooth. Every codec from SBC to LDAC, the AirPods special cases and the wired options are covered in Bluetooth and AirPods.
Common mistakes
Converting MP3 to FLAC
Converting a lossy file doesn't make it lossless. The missing information stays missing; the file just grows three to four times larger. Such "fake FLACs" turn up on file-sharing sites and occasionally in dubious stores. A spectrogram exposes them, for example with the free program Spek: depending on bitrate, MP3 encoders cut the treble above roughly 16 to 20 kHz, while a genuine CD recording usually extends to 22 kHz.
Converting lossy to lossy
Every lossy encode throws information away, and each further one throws away different information. MP3 to AAC, or AAC to Opus, adds up the errors. At high bitrates that often stays inaudible, at medium ones it doesn't. The rule: always make lossy copies from the lossless source.
WAV as a collection format
The sound is identical to FLAC, but tags get lost easily and files are about 1.7 times larger. If you have WAV files, convert them to FLAC once and check the tags afterwards.
Hi-res made from CD material
A 24-bit/96 kHz file upsampled from a CD contains no more information than the CD. When buying hi-res, look for details on the source and, when in doubt, take the cheaper CD-quality version.
Which format for what?
| Situation | Recommendation |
|---|---|
| Ripping CDs for your archive | FLAC, with a program that verifies the rip against a database (such as XLD on the Mac or Exact Audio Copy on Windows), see ripping CDs |
| Apple devices and the Music app only | ALAC, lossless and best integrated there |
| Buying music | CD-quality FLAC is enough. Hi-res only if it costs little more or comes from a better master |
| iPhone with little storage | AAC at 256 kbps, made from the lossless source |
| Saving as much as possible | Opus at 128 to 160 kbps, if your player supports it |
| Old devices, car stereo, USB stick | MP3 at 320 kbps or V0 |
| Recording, editing, DJ software | WAV or AIFF while working, then FLAC for the archive |
The key rule runs through every row: keep a lossless original and make smaller copies from it when needed. That prepares you for any future device and any future codec.
Which formats Luma plays
Luma plays FLAC, ALAC, WAV, AIFF, CAF, MP3, AAC (including M4B audiobooks) and Opus on iPhone, iPad and Mac. With the equalizer, volume normalisation and cross-fade off, Luma outputs the audio data unchanged, and consecutive tracks play gaplessly. Tags you edit in Luma are written straight into MP3, M4A and FLAC files without touching the audio. With WAV and AIFF, edits stay in the app, because these formats have no reliable tags.
Ogg Vorbis, WMA, APE and DSD aren't supported. Lossless files in those formats (WMA Lossless, APE) are best converted losslessly to FLAC. All details are in the help center under Formats and sound quality and Equalizer, volume and cross-fade.
Frequently asked questions
Is FLAC better than MP3?
FLAC is lossless and contains exactly the audio data of the source; MP3 discards information. At 320 kbps most people still can't hear a difference in blind tests. FLAC's biggest advantage is that you can make any other format from it at any time without further loss.
Which is better, FLAC or ALAC?
They sound identical; both are lossless. FLAC is more widespread and usually a little smaller, ALAC is best integrated with Apple's apps. You can convert between them at any time without loss.
Is hi-res worth it with AirPods?
No. Bluetooth isn't lossless, and AirPods always receive an AAC signal from an iPhone. The iPhone also needs an external converter for sample rates above 48 kHz. Hi-res files bring no audible benefit over AirPods, only larger files.
Does converting MP3 to FLAC improve quality?
No. What was discarded when the MP3 was encoded can't be recovered. The FLAC sounds exactly like the MP3 and is just three to four times larger.
Which bitrate should I choose for AAC or MP3?
For AAC, 256 kbps is a proven setting that's transparent for nearly all listeners. For MP3, 320 kbps or the variable V0 setting make sense. Opus reaches comparable quality at 128 to 160 kbps.
Is 24-bit better than 16-bit?
For recording and editing, yes, because 24 bits leave more headroom for levels and effects. For listening, 16 bits with about 96 dB of dynamic range are enough for practically all music and listening situations. Studies found only small differences, detected mainly by trained listeners.
Sources
- E. Brad Meyer, David R. Moran: Audibility of a CD-Standard A/D/A Loop Inserted into High-Resolution Audio Playback. Journal of the Audio Engineering Society, 2007. aes2.org/publications/elibrary-page/?id=14195
- Joshua D. Reiss: A Meta-Analysis of High Resolution Audio Perceptual Evaluation. Journal of the Audio Engineering Society 64 (6), 2016. qmro.qmul.ac.uk/xmlui/handle/123456789/13493
- Hydrogenaudio: overview of public listening tests, including the 2014 multiformat test at 96 kbps. wiki.hydrogenaudio.org/index.php/Hydrogenaudio_Listening_Tests
- Apple Support: About lossless audio in Apple Music. support.apple.com/en-us/118295
- IETF: RFC 9639, Free Lossless Audio Codec (FLAC), December 2024. rfc-editor.org/rfc/rfc9639.html
- IETF: RFC 6716, Definition of the Opus Audio Codec, 2012. rfc-editor.org/rfc/rfc6716.html
- Tidal replaces MQA with FLAC (July 2024). headphonesty.com/2024/06/tidal-officially-dumps-mqa
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