The human hearing range tops out around 20kHz (20,000Hz). So a 20,001Hz bell is just beyond what humans can detect. Meanwhile, this character is bleeding from the ears, clearly experiencing severe trauma, yet insists "I didn't hear no bell" - technically correct in the most painful way possible. It's what happens when you combine Rocky's determination with a physics textbook. Your eardrums might be shattered, but your scientific pedantry remains intact.