Since the frequency is ultrasonic, it can't be heard by humans.
b. Sound travels faster in solids than gases. So when we put our ear on railway track, we can hear the sound of approaching train through the steel track. The sound wave travelling through air takes very long time to reach us as it travels at much lower speed. Sometimes it doesn't reach us and we can't hear the sound as the energy in the wave is dissipated in the air.