Martin, D.W., Ward, W.D. (1961). Subjective evaluation of musical scale temperament in pianos. J. Acoust. Soc. Am. 33, 582-585

It is known that standard procedures used by piano tuners result in a "stretched"' scale (i.e., upper tones higher and lower tones lower than the equally tempered scale). Although there was anecdotal evidence of the desirability of this stretch (largely a natural consequence of tuning by beats) it had not been formally demonstrated. Therefore recorded tonal and chordal sequences from a small upright piano, tuned to a typical empirical stretched scale by means of a visual device, were compared with similar sequences from the same piano (1) tuned to strict equal temperament (again by means of the visual device), and (2) tuned by a factory ``fine tuner'' (by the conventional auditory method). Strict equal temperament was unequivocally rejected both by musically oriented research engineers and by music students. Sequences covering the entire piano scale have the most pronounced preferences.