Two tones presented together generate an audible combination (difference) tone not physically present.

Evidence

The difference tone of two frequencies, fdiff = f2 −f1, can be perceived; for example, frequencies f2 = 1.1 kHz and f2 = 1.0 kHz produce the perception of a faint, low-frequency tone at 100 Hz. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 773

Categorization

• XREF: The ‘Tartini tone’ or missing fundamental phenomenon — ties to pitch perception literature that audio designers rely on.
• CANDY: A fun, concrete demo: playing 1.0 and 1.1 kHz tones makes listeners hear a phantom 100 Hz tone.
• corroborated: 2 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 773