Sources

Every study and reference cited on the site, grouped by topic, with the pages that use it.

Music theory on this site (intervals, chords, scales, keys, staff positions) is standard and generated by tested code. Claims about perfect pitch, relative pitch, pitch memory and learning come from the peer-reviewed studies below; each page states sample sizes and limits next to the claim.

A sound behind the research

Most of these studies use notes like this one, A4 at 440 Hz, the standard tuning reference:

Absolute (perfect) pitch

Van Hedger, S. C., Heald, S. L. M. & Nusbaum, H. C. (2019). Absolute pitch can be learned by some adults. PLOS ONE, 14(9), e0223047. Six adults, about 32 hours of training over 8 weeks; two reached accuracy comparable to people with absolute pitch, one passed every test; most improved only partly. Cited on: Perfect pitch, Relative pitch, Perfect pitch test.
Van Hedger, S. C., Heald, S. L. M., Koch, R. & Nusbaum, H. C. (2015). Auditory working memory predicts individual differences in absolute pitch learning. Cognition, 140. Adults who held sounds better in auditory working memory learned note categories better. Cited on: Perfect pitch.
Wong, Y. K., Lui, K. F. H., Yip, K. H. M. & Wong, A. C.-N. (2020). Is it impossible to acquire absolute pitch in adulthood? Attention, Perception, & Psychophysics, 82, 1407–1430. Training studies in adults; in a 12-hour experiment one of ten participants met the strict absolute-pitch criterion. Cited on: Perfect pitch.
Deutsch, D., Henthorn, T., Marvin, E. & Xu, H. (2006). Absolute pitch among American and Chinese conservatory students: prevalence differences, and evidence for a speech-related critical period. Journal of the Acoustical Society of America, 119(2), 719–722. Among students who began training at ages 4–5, about 60% of Mandarin-speaking students in Beijing met the criterion versus about 14% of US students who speak a non-tone language. Cited on: Perfect pitch, Perfect pitch test.
Takeuchi, A. H. & Hulse, S. H. (1993). Absolute pitch. Psychological Bulletin, 113(2), 345–361. Review of the research on absolute pitch: definition, prevalence estimates, early training. Cited on: Perfect pitch, Perfect pitch test.

Pitch memory

Levitin, D. J. (1994). Absolute memory for musical pitch: evidence from the production of learned melodies. Perception & Psychophysics, 56(4), 414–423. 46 people sang popular songs from memory; 40% hit the recording's exact starting pitch on at least one of two trials, 12% on both. Cited on: Perfect pitch.
Frieler, K. et al. (2013). Absolute memory for pitch: a comparative replication of Levitin's 1994 study in six European labs. Musicae Scientiae. 277 participants; 25% sang the exact pitch of at least one song and 4% of both: a smaller effect than the original study. Cited on: Perfect pitch.

Relative pitch

Plantinga, J. & Trainor, L. J. (2005). Memory for melody: infants use a relative pitch code. Cognition, 98(1), 1–11. Six-month-olds recognised a familiar melody by its relative pitch, whatever key it was played in. Cited on: Relative pitch.
Micheyl, C., Delhommeau, K., Perrot, X. & Oxenham, A. J. (2006). Influence of musical and psychoacoustical training on pitch discrimination. Hearing Research, 219(1–2), 36–47. Untrained listeners needed 4 to 8 hours of practice to reach the pitch-discrimination thresholds of classical musicians. Cited on: Relative pitch.
Little, D. F., Cheng, H. H. & Wright, B. A. (2019). Inducing musical-interval learning by combining task practice with periods of stimulus exposure alone. Attention, Perception, & Psychophysics, 81. Musical-interval discrimination improved with training that mixed practice and passive listening, and transferred to interval identification. Cited on: Relative pitch.

Music theory and history

Tritone (Wikipedia): history of the term diabolus in musica. No medieval source for the nickname is known; stories of church punishment are considered fanciful. Cited on: Tritone.
Scientific pitch notation (Wikipedia): octave numbers, C4 = middle C, A4 = 440 Hz Cited on: Piano notes.
ISO 16:1975, Acoustics: standard tuning frequency (A4 = 440 Hz) Cited on: Methodology, Piano notes.
Solfège (Wikipedia): Guido of Arezzo, Ut queant laxis, movable and fixed do Cited on: Solfège.

Methods

de Cheveigné, A. & Kawahara, H. (2002). YIN, a fundamental frequency estimator for speech and music. Journal of the Acoustical Society of America, 111(4), 1917–1930. The pitch-detection method used by the microphone tools. Cited on: Methodology, Pitch matching, Vocal range test, Pitch detector.