Supersonic speed motion faster than the speed of sound in a brittle material. At the beginning of the 20th century, the term "supersonic" was used as an adjective to describe sound whose frequency is above the range of normal human hearing. The modern term for this meaning is "ultrasonic". The tip of a bullwhip is thought to be the first man-made object to break the sound barrier, resulting in the telltale "crack" (actually a small sonic boom). The wave motion traveling through the bullwhip is what makes it capable of achieving supersonic speeds. Most modern fighter aircraft are supersonic aircraft, but there
Sound speed gradient creates a negative sound speed gradient. The sound wave front travels faster near the ground, so the sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source. The opposite effect happens when the ground is covered with snow, or in the morning over water, when the sound speed gradient is positive. In this case, sound waves can be refracted from the upper levels down to the surface. In underwater acoustics, speed of sound depends on pressure (hence depth), temperature, and salinity of seawater, thus leading to vertical speed gradients similar