Final answer:
To find how fast a bat must fly for a 20 kHz chirp to be barely heard, one must use the Doppler effect formula considering the bat as the source and assuming the observer is stationary. The sound of the bat's chirp would change frequency due to the motion of the bat relative to the observer.
Step-by-step explanation:
The question regarding how fast a bat would have to fly for its 20 kHz chirp to be barely audible involves understanding the Doppler effect and the properties of sound waves. The Doppler effect occurs when a source of sound moves relative to an observer, causing the observed frequency to change. Since the speed of sound in air is roughly 343 to 344 m/s (as given in the reference information) and humans can hear frequencies up to 20 kHz, we can use the formula f' = f(v + vo)/(v + vs) to calculate the required bat's speed (where f' is the observed frequency, f is the emitted frequency, v is the speed of sound in air, vo is the observer's velocity, and vs is the source's velocity).
Since we want the chirp to be just at the limit of human hearing (20 kHz), we set f' to 20 kHz. Considering that the bat is moving towards the observer (you) and assuming the observer is stationary (vo = 0), the equation simplifies to f' = f(v)/(v - vs). Solving this for vs (the bat's speed), we can determine how fast the bat must fly for a 20 kHz chirp to be barely heard. Without the exact figures from the question prompt, we can't provide a numerical answer, but the method described here is the process one would follow.