Final answer:
Beat frequencies are created when adjacent piano keys are struck, with differences of 21 Hz, 22 Hz, and 43 Hz for this particular discordant combination. To distinguish between high- and low-pitched instruments visually, one should consider the size and structure of the instruments, with larger ones typically producing lower pitches.
Step-by-step explanation:
When three adjacent keys on a piano are struck simultaneously, specifically F (349 Hz), F-sharp (370 Hz), and G (392 Hz), they produce combinations that create beat frequencies. The beat frequency is the difference in frequencies between two notes played together. In the case of F and F-sharp, the beat frequency is 370 Hz - 349 Hz, which equals 21 Hz. Similarly, the beat between F-sharp and G is 392 Hz - 370 Hz, resulting in a beat frequency of 22 Hz. Finally, though less commonly heard because they are not adjacent, the beat frequency between F and G would be 392 Hz - 349 Hz, which is 43 Hz.
In regards to non-acoustic methods, to differentiate between two unknown musical instruments where one plays high-pitch sounds and the other low-pitch sounds, one approach would be to look at the physical characteristics of the instruments. Generally, larger instruments with longer, larger strings or air columns produce lower pitches, while smaller instruments with shorter, thinner strings or smaller air columns tend to produce higher pitches.