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Information on sound wave propagation
The non dimensional parameter "M" is called the
Mach number, which is defined by:
| where "U" is the speed of the source of sound such as an aircraft, and
"a" is the speed of sound |
M = 0 |
|
Stationary:
Mach Number =
0 |
When the source of sound is stationary (its speed is zero), waves of air pressure
spread uniformly, and they form a circle (2-dimensional case) or a sphere (3-
dimensional). |
M = 0.4 |
M = 0.7 |
| Subsonic speed: Mach Number <
0.3 |
When the source of sound moves (or the aircraft flies) at a speed slower than the
speed of sound, air pressure waves move ahead of the aircraft because
the waves travel at the speed of sound, faster than the source. |
M = 1 |
|
| Transonic speed: Mach Number ~
0.3-1 |
As the aircraft speed approaches the speed of sound, the pressure waves are
"piled up" ahead of the aircraft. These dense air pressure waves form a shock
wave, shown as a red line. |
M = 1.4 |
M = 2.0 |
| Supersonic speed; Mach Number >
1 |
As the aircraft moves faster then the speed of sound, it breaks through the
shock wave. The shock wave angles back behind the aircraft, reaching the
ground as a sonic boom. (You may be caught off-guard because you do not hear
any noise until the aircraft has passed overhead, often at high altitude.) |
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