Stationary Sound Source
Sound waves are produced at a constant frequency, and the wavefronts move away from the source at a constant speed.
The distance between wavefronts is the wavelength. All observers will hear the same frequency, which will be equal to the actual frequency of the source.
Source moving with vsource < vsound ( Mach 0.7 )
The same sound source is radiating sound waves at a constant frequency; however, now the sound source is moving to the right. The motion of the source causes the osund waves to be crowded together in front of the source and to be spread further apart behnad the source (the wavelength in front of the source is less than the wavelength behind the source). An observer in front of the source will hear a higher frequency, and an observer behind the source will hear a lower frequency.
Source moving with vsource = vsound ( Mach 1 - breaking the sound barrier )
Now the source is moving at the speed of sound. The wavefronts in front of the source are now all bunched up at the same point. As a result, an observer in front of the source will detect nothing until the source arrives. The pressure front will be quite intense (a shock wave), due to all the wavefronts adding together, and will not be heard as a pitch but as a "thump" of sound as the pressure wall passes by.
Source moving with vsource > vsound (Mach 1.4 - supersonic)
The sound source has now broken through the sound speed barrier, and is traveling at 1.4 times the speed of sound (Mach 1.4).
Notice the clear formation of the Mach cone, the angle of which depends on the ratio of source speed to sound speed. It is this intense pressure front on the Mach cone that causes the shock wave known as a sonic boom.
