One of the most visible ways that HydrogenAudio members have shown their enthusiasm for audio quality and their respect for scientific principles over the years is by organizing and participating in public blind listening tests of lossy codecs and presenting statistical analyses of the results. These results show the advantages and disadvantages of different formats and encoders and help identify which are "state of the art."

Private listening tests also provide useful data, but the public HydrogenAudio tests are a rarity in having enough participants to both average out variations among individual listeners and still have statistically significant results.

Note that a listening test cannot really compare formats or encoder "brands." Instead, it compares particular versions of particular encoders at particular bitrates. Results will differ with different encoders, newer versions of the same encoders, or different bitrates. Use caution when extrapolating, and use the most recent results when extrapolating to the newest encoder versions.

Note: There would be no point in listening tests of lossless encoders; the decoded results are bit-identical to the original uncompressed recordings. Lossless encoder comparisons instead consider technical aspects such as compression ratio and encode/decode speed.

All tests are sorted by date, newest first. All dates are in ISO8601 format.

Multiformat Tests

Date	Title Samples	Performed by	Codecs	Links
2011-04-10	64-kbps General	igorc	Vorbis aoTuV, Quicktime HE-AAC, Opus	HA Thread Complete Result
2007-08-16	64-kbps General	Sebastian Mares	Vorbis aoTuV, Nero AAC, WMA Pro	HA Thread Complete Result
2006-11-22	48-kbps General	Sebastian Mares	Vorbis aoTuV, Nero AAC, WMA Pro, WMA Standard	HA Thread Complete Result
2006-01-13	128-kbps General	Sebastian Mares	iTunes & Nero AAC, LAME MP3, Vorbis aoTuV, WMA Pro	HA Thread Complete Result

Single-format Tests

AAC

Date	Title Samples	Performed by	Codecs	Links
2011-08-20	96-kbps General	igorc	CT, Fraunhofer IIS, Nero, QuickTime TVBR & CVBR	HA Thread Complete result
2006-02-20	48-kbps HE-AAC General	Gabriel	3gpp (v1), CT (v1 & v2), Nero (v1 & v2)	HA Thread Complete Result
2006-02-05	48-kbps HE-AAC Classical	Ivan Dimkovic	Nero HE-AAC v1 & v2	HA Thread

MP3

Date	Title Samples	Performed by	Codecs	Links
2008-10-17	130-kbps General	Sebastian Mares	LAME 3.97, LAME 3.98.2, iTunes, FhG, Helix	HA Thread Complete Result

External Tests

Other people have held listening tests in venues outside of Hydrogenaudio (although the organizers of the tests tend to be HA members :). The results of these tests are found below:

ff123's site on audio testing and listening tests (most recent test 2002-07-27)
Roberto's Public Listening Tests (most recent test 2004-07-11)
Sebastian Mares' Listening Tests (most recent test updated 2008-11-22)
SoundExpert Listening Tests (continually updated)