Two simultaneous pure tones may fuse rather than be heard separately.

Evidence

When two pure tones are played simultaneously, they are not always per­ ceived as two separate pure tones. The discussion relating to Figure 2.7 introducing critical bandwidth in Chapter 2 provides a first example of sounds which in some sense deceive the ear. These two pure tones are only perceived as separate pure tones when their frequency difference is greater than the critical bandwidth. Otherwise they are perceived as a single fused tone which is “rough” or as beats depending on the frequency difference between the two pure tones. — David Howard, Jamie Angus, coustics and Psychoacoustics, 1610 (#expands, via linked claim)

This section concerns sounds that in some sense could be said to “deceive” the ear. Such sounds have a psychoacoustic realization that is not what might be expected from knowledge of their acoustic components. In other words, the subjective and objective realizations of sounds cannot be always directly matched up. While some of the examples given may be of no obvious musical use to the performer or composer, they may in the future find musical application in electronically produced music for particular musical effects where control over the acoustic components of the output sound is exact. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1568 (#expands, via linked claim)

Categorization

• XREF: connects to: Slight mistuning among voices fuses into a single pleasant tone; Two tones fuse into one unless their separation exceeds critical bandwidth — David Howard, Jamie Angus, coustics and Psychoacoustics, 1610; What the ear perceives can diverge from sound’s physical components. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1568 (+4 more)

Broader Connections

• Two tones fuse into one unless their separation exceeds critical bandwidth #expands (0.95) — David Howard, Jamie Angus, coustics and Psychoacoustics, 1610
• Hearing resolves close tones through beats, rough fusion, then separation bands #expands (0.85)
• What the ear perceives can diverge from sound’s physical components. #expands (0.75) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1568
• Simultaneous tones become harder to perceive due to masking effects #qualifies (0.72) — David Howard, Jamie Angus, coustics and Psychoacoustics, 1628