- The series of wave fronts form a disturbance envelope given by a straight line which is tangent to the family of circles. It will be seen that all the disturbance waves lie within a cone (shown in fig.) having a vertex/apex at the body at time considered. The locus of all the leading surfaces of the waves of this cone is known as
**Mach cone**.

- Whenever there is any disturbance in a compressible fluid, the disturbance is propagated in all the directions with a velocity of sound. The nature of propagation depends on the Mach number.

**Mach angle**:- It is defined as half of the angle of Mach cone. It is denoted by 'a'.

**Zone of action**:- When the Mach number is greater than 1 (M>1), the effect of disturbance is full only in the region inside the Mach cone. This region is known as the zone of action.

**Zone of silence**:- When the Mach number is greater than '1.0' (M>1), the effect of disturbance is not full in the region outside the Mach cone. This region which is outside the Mach cone is known as zone of silence.