How does mach 2 relate to sound

As an aircraft moves through the air, the air molecules near the aircraft are disturbed and move around the aircraft. If the aircraft passes at a low speed, typically less than mph, it is observed that the density of the air remains constant. For higher speeds, some of the energy of the aircraft goes into compressing the air and locally changing the density of the air. This compressibility effect alters the amount of resulting force on the aircraft since the aerodynamic force depends on the air density.

The effect becomes more important as speed increases. Sharp disturbances generate shock waves that affect both the lift and drag of the aircraft, and the flow conditions downstream of the shock wave. On this slide, we will investigate the dependence of the density change on the Mach number of the flow. The Mach number is the ratio of the speed of the aircraft, or the speed of the gas, to the speed of sound in the gas.

And I make those last two qualifiers on it because the speed of sound is not always the same. It has a different speed depending on whether it's traveling in air, or water, or even depending on what makes up the air. And even if it is only traveling in a certain type of air, or a certain makeup, a mix of gases, it'll also change depending on the temperature.

So if you're at sea level, at roughly about 20 degrees Celsius, the speed of sound is , so sea level-- do a different color, maybe blue for sea level-- so if you're at sea level, the speed of sound is about meters per second. This is at 20 degrees Celsius, so it's a nice comfortable temperature at sea level, which is roughly about miles per hour. Now if the temperature were to drop, so if temperature goes down, so does the speed of sound. And likewise, if the temperature goes up, the speed of sound goes up.

So when someone tells you that something is traveling at Mach 2, they're saying its traveling at two times the speed of sound for that medium. Usually they're talking about something traveling through air. And for the speed of sound at the temperature that they're going through. So if they're at some super-high altitude, and normally to travel at these types of speeds, you have to be at a fairly high altitude where the air is less dense.

You're also traveling in an environment where the temperature is significantly lower than 20 degrees Celsius. So you might say, hey, does that mean if someone's traveling at Mach 2, does that mean that they're definitely traveling at meters per second, or at miles per hour? And of course, whenever someone gives an air speed, it's relative to the air. And the answer is no. If the aircraft passes at a low speed, typically less than mph, the density of the air remains constant.

But for higher speeds, some of the energy of the aircraft goes into compressing the air and locally changing the density of the air. This compressibility effect alters the amount of resulting force on the aircraft.

The effect becomes more important as speed increases. But a sharp disturbance generates a shock wave that affects both the lift and drag of an aircraft. The ratio of the speed of the aircraft to the speed of sound in the gas determines the magnitude of many of the compressibility effects. Because of the importance of this speed ratio, aerodynamicists have designated it with a special parameter called the Mach number in honor of Ernst Mach , a late 19th century physicist who studied gas dynamics.

The Mach number M allows us to define flight regimes in which compressibility effects vary. There is no upstream influence in a supersonic flow ; disturbances are only transmitted downstream. The Mach number appears as a similarity parameter in many of the equations for compressible flows , shock waves , and expansions.