Sound is the wave of air molecules caused by the motion of an object. So, no
movement means no sound. If object moves faster and it vibrates more, it means
higher frequency and louder the sound. Sound travels at different rates in
different media. In dry air, sound travels at 331.6 meter/sec at 0 C.
The most common sound unit is decibel (dB), it's 1/10 of bel (B). The other
sound unit which is rarely used is neper (Np).