A true difference tone is audible even from inharmonic partials separated by 200 Hz

Evidence

For example, suppose f1 = 1000 Hz, f2 = 800 Hz. These beat at 200 Hz, and perhaps therefore we will hear a 200 Hz tone. But not so fast—200 Hz is also the residue pitch. We should examine instead, say, 1042 and 842 Hz. These are now inharmonic partials, but they differ by 200 Hz. Indeed, a 200 Hz difference tone—a true phantom tone—is clearly heard, weaker than the two generators but still quite distinct. — Eric J. Heller, Why You Hear What You Hear- An…, 2300

Categorization

• QUESTION: The distinction between residue pitch and a true phantom difference tone is subtle — worth pinning down exactly how they differ physiologically.