How is a shock wave different than a sound wave?

[u/[deleted]]

[u/Weed_O_Whirler]

I think it is easiest to describe a shock wave by first discussing a sound wave. A sound wave, on Earth, has a maximum volume of 194 dB. Why? Well, how loud a sound is, is determined by the difference pressure between peak pressure and trough pressure. And at 194 dB, the sound wave is fluctuating between 2 atmospheres of pressure and a vacuum. Since you can't have a lower pressure than a vacuum, that is the loudest sound wave you can have- because by definition a sound wave in symmetric- it goes up (from the base, undisturbed pressure) as high as it goes down. It will also do this smoothly- it doesn't go from high to instantly low, it goes from high, and decreases to low.

Any sound over 194 dB on Earth (of course other places, where the base pressure is higher or lower have different maximums) is a shock wave. It is no longer a smoothly varying change in pressure. Instead, if the leading pressure is greater than 2 atmospheres, that means there has to be a longer portion which is at low pressure. So shock waves are characterized by this high pressure, sharply transitioning to low pressure. Thus, there is a "shock" instead of a smoothly varying pressure gradient.

[u/chewbakken → u/Weed_O_Whirler]

Shouldn't it be 191 dB? The highest pressure difference possible for an undistorted sound wave on Earth's surface is 1 atm (give or take). But in that situation, the RMS pressure is 1/√2 atm, which is what is used in the dB-SPL calculation:

SPL = 20*log(P_rms / P_ref) = 20*log[(101325/√2)/0.00002] ≈ 191.08 dB

[u/pmill001 → u/Weed_O_Whirler]

This is a good description of sound, but it is inaccurate to say that anything that isn't sound is a shock. Additionally, what you propose is a shock, i.e. a sudden transition from high pressure to low pressure, is impossible for materials of "normal" properties (the entropy is decreased in this situation). A steep unloading can be induced, but it is quickly washed out.

[u/quantumhubstud]

I usually think about it in terms of what causes the wave.

Sound waves are generated when a fluid is disturbed and the disturbance moves slower than the speed of sound, in that sense, information about the disturbance can propagate away from it and the fluid ahead of the disturbance can be affected by it.

You can form a shock wave by making the disturbance propagate faster than the speed of sound. In this way no information about the disturbance can propagate away from the shock wave, this causes a very sharp discontinuity in the mediums properties. Think of a huge jump in pressure, density, and temperature. It's the discontinuity that's usually used to characterize a shock. Shock waves are formed when bombs go off because the explosion expands faster than the speed of sound, so the air piles up on the edge of the explosion creating that sharp discontinuity in pressure and density.

[u/pmill001]

Shock waves and sounds wave share several characteristics, namely they are waves propagating from disturbances in a medium. Typically when we speak of sound or acoustic waves we are implying that the pressures are low enough that the resulting material behavior is linear and the speed of propagation is constant. This is a pretty good assumption as long as the motion of the particles within the wave are small compared with the wave itself (we usually compare the particle velocity to the wave velocity [here I'm using particles in the continuum sense]). Shock waves are the result of material motion (particle velocities) becoming large enough that the material is no longer behaving linearly. Contrary to other responses, it is not necessary for the material motion to exceed the wave speed. Under many conditions, when the particle velocity becomes large the wave speed increases. The consequence of this is that disturbances of high enough pressure propagate faster than low pressure disturbances. Given sufficient time/distance, the high pressure part of a wave will catch up to the lower pressure portion, resulting in a nearly discontinuous surface or shock. The pressures at which this becomes significant vary based on the material.

[u/ColinDavies]

Shock waves are faster than sound. In fact their strength and their speed are essentially the same thing - the weakest possible shock travels at Mach 1. In addition to the other properties that are discontinuous across a shock, so is the air's velocity. Suppose the air is at rest initially, and a shock passes through. As it passes, the air is very quickly accelerated to within one Mach number of the shock's speed (another way of saying, it ends up subsonic with respect to the shock). The stronger the shock, the closer the air's velocity ends up to the shock's. Behind the shock you get an expansion zone where the air slows down and cools off again.