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RobusEtCeleritas t1_izsbk7v wrote

The speed of sound is the speed at which infinitesimal oscillations in the density and pressure propagate through the fluid.

If an object moving through the fluid at or above the speed of sound, there's no way for any of the fluid upstream of the object to "know" that the supersonic object is coming.

And as a result, in order for conservation of mass, momentum, and energy to be upheld, you find that there must be discontinuities in the density, pressure, etc. at some point between the fluid upstream of the object and the fluid downstream of the object. These discontinuities are called shocks, and they first start to form specifically as the object reaches the local speed of sound.

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xratedcheese t1_iztkwuz wrote

So if a submarine could reach the speed of sound in water (something like twice as fast?) there would be a sonic boom in the water?

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Game_Minds t1_izu6llu wrote

It's normally called something else in dense/incompressible fluids like water, i.e. cavitation

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zekromNLR t1_j007zm4 wrote

The speed of sound in water is nearly five times as fast as in air, about 1500 m/s. An object moving that fast through water would experience a dynamic pressure of about 1.1 GPa, compared to about 70 kPa for something moving at the speed of sound through sea-level air - in other words, not really possible outside of extreme scenarios like meteorite impacts.

But yes, there would be a sonic boom. You can with a fast enough impact even have a sonic boom in solid materials.

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nassau4 t1_izsfnsl wrote

Can you make an eli5 of that?

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RobusEtCeleritas t1_izt7f8t wrote

The speed of sound is when shock waves start to form, so that's when sonic booms start.

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DisasterousGiraffe t1_izx35l6 wrote

The speed of sound is the speed at which changes in pressure move through the air.

If a bullet is shooting through the air faster than the speed of sound it is said to be "supersonic". (The word "supersonic" literally means above, indicated by the Latin word "super", like "superior", stuck on the front of the word "sonic" meaning sound. "Sonic velocity" means at the speed of sound, and "subsonic" means below the speed of sound, Latin word "sub" meaning below stuck on the front of sonic, like submarine is below the surface of the sea.) A supersonic bullet is moving faster than the sound pressure changes move through the air. So you will not hear the bullet before it hits you.

All the energy in the sound pressure changes that are not going forward from the supersonic bullet has to go somewhere else instead. The pressure changes go into sharp changes in pressure between the air in front of the bullet and the air behind the bullet. These sharp changes in pressure are called "shocks", and they happen when the bullet reaches the speed of sound. Subsonic bullets do not create shocks because the energy can move forward in front of the bullet - you will hear a subsonic bullet before it hits you.

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Psyese t1_izuhkag wrote

What's the difference between air molecule mechanical interaction with each other normally and in a "shock"?

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cuicocha t1_izv04fo wrote

A shock is a significant discontinuity in pressure, density, and velocity. Viscosity means that nothing is truly discontinuous, so in sea-level air, "discontinuous" means a big jump in a distance of approximately microns.

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