Two tones fuse into one unless their separation exceeds critical bandwidth

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

Categorization

• XREF: Connects to earlier chapter material on critical bandwidth introduced with Figure 2.7. — connects to: AM and FM sidebands become perceptually identical once spacing exceeds the critical bandwidth.; Frequency separation between two tones systematically shifts fusion, roughness, then resolution; Two simultaneous pure tones may fuse rather than be heard separately. (+5 more)
• corroborated: 2 independent passages support this claim — David Howard, Jamie Angus, coustics and Psychoacoustics, 1610

Broader Connections

• Dissonance depends on proportional separation falling inside the critical bandwidth, not absolute frequency difference #expands (0.85)
• Masking acts within critical bandwidths, leaving distant-frequency sounds unaffected #corroborates (0.82) — Eric J. Heller, Why You Hear What You Hear- An…, 2009
• Threshold in quiet depends on total sound intensity within one critical band only #qualifies (0.80) — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 888
• Hearing resolves close tones through beats, rough fusion, then separation bands #expands (0.80)