Sound that is generated by the unsteady motion of a gas and its interaction with surrounding surfaces. Aerodynamic sound or noise may be pleasant, such as the sound generated by a flute, or unpleasant, such as the noise of an aircraft on landing or takeoff, or the impulsive noise of a helicopter in descending or forward flight. The typical human ear can detect a great range of sounds. A logarithmic scale, called the decibel (dB) scale, is used to describe the sound pressure level (SPL). The sound pressure level is given by the equation below,