In the first case the observer experiences a reduced audibility and in the second case he observes enhanced
audibility
Sound waves have rectilinear propagation only if the speed of sound is constant in space (ie., under no wind conditions).
Since the sound speed depend on temperature which in particular varies with height, the speed of sound is almost never constant in space. In a medium with varying speed of sound, the wave would no longer propagate in a rectilinear fashion. They get more refracted, leading to propagation along curved lines.
Reduced audibility:
The speed of sound decreases with height, since temperature decreases with altitude. As the sound propagates against the wind,
the sound waves are refracted upward. Thus, as a consequence, an acoustical shadow is observed into which sound energy
cannot penetrate directly. Only by diffraction and scattering, the sound energy can enter into the acoustical shadow. However,
the intensity of sound remains noticeably less noisy in a shadow zone.
Enhanced audibility:
The speed of sound increases with height, since the temperature increases with altitude. As the sound propagates with the wind,
the sound waves are refracted downwards towards the ground. At the ground surface the sound waves ate subject to reflection.
The reflected sound is again refracted downward. A possible consequence is multiple reflection which is favorable to the sound
propagation near the surface over large distances. The intensity of sound remains quite audible.
A guitar string can vibrate with fundamental frequency and corresponding harmonics
a) Natural frequency and Harmonics:
When an object is forced into resonance vibrations at one of its natural frequencies, it vibrates in a manner such that a
standing wave pattern is formed within the object. Whether it be a guitar sting, or the air column enclosed within a trombone,
the vibrating medium vibrates in such a way that a standing wave pattern results. Each natural frequency which an object or
instrument produces has its own characteristic vibrational mode or standing wave pattern. These patterns are only created within
the object or instrument at specific frequencies of vibration; these frequencies are known as harmonic frequencies, or merely
harmonics. The harmonic frequencies are related to each other by simple whole number ratios. This is part of
the reason why such instruments sound pleasant.
A trajectory representing the interaction of two vibrational modes of a molecule is caught in a strongly
interacting resonance
b) Resonance:
Resonant frequency is the frequency at which an object oscillates, and if you bombard an object with sound waves that are at
the same frequency as the resonant frequency of the object, the object will shatter and implode (i.e., the object will oscillate
or vibrate with larger amplitudes).