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Mach number
Intermediate
Speed relative to the speed of sound.
M = v/c_sM = v/c_s measures speed relative to the sound speed. M < 1 subsonic, M > 1 supersonic, M > 5 hypersonic.
A jet at 680 m/s at altitude (c_s = 295 m/s): M ≈ 2.3.
Sound speed varies with temperature, so M for the same speed changes.
In more detail
The Mach number is the ratio of an object's speed to the local speed of sound. Mach 1 is the speed of sound, and above Mach 1 shock waves form.
Real-world uses
- Aircraft speeds are quoted in Mach numbers.
- Rocket and jet engine designers consider Mach number for shock effects.
- Aircraft speeds are quoted as Mach numbers, which show how fast they travel compared with the speed of sound.
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Frequently asked questions
Does the speed of sound depend on altitude?
Yes. It depends on temperature, so the Mach value changes with altitude.
What is a subsonic speed?
A speed below Mach 1.
What is the speed of sound at sea level?
About 340 metres per second in dry air at 15 degrees Celsius, which varies with temperature.
Related topics in Waves & Sound
Doppler with moving observer · Doppler in light · Shock waves and sonic boom · Energy transport in waves · Wave packets and group velocity · Dispersion