SRB camera mounts and SOUND AFTER MACH 1

[TeeGee]

Hi guys/gals!

Quick observation, here is a video from a shuttle launch:

WHY is there ANY sound from the SRB and SSME engines after mach 1? Isn't the spacecraft outrunning the sound waves produced by the engines? If so, shouldn't there be no sound in front of the engines? The only explanation I have is: 1) The audio is mixed in from ground crews 2) The camera is IN the SRB tank, and picks up the vibrations from the rocket motor ONLY.

Does anyone have an explanation for this?

[R0cketC0der]

The microphone is mounted on the booster and therefore picks up any vibrations from it. Why would it have to be in the tank to do that?

[PlonioFludrasco]

I think it propagates through the structure, but I'm not sure.

Also, inside the Mach cone the flux is subsonic, so the air surrounding the booster is moving below Mach 1.

Plus, there's the aerodynamic noise from the front.

[Camacha]

Plus, there's the aerodynamic noise from the front.

Good point, just the air at those speeds will generate a huge amount of noise.

[xenomorph555]

I heard a space kraken/whale in that video, it was creepy...

[TeeGee]

Good point, just the air at those speeds will generate a huge amount of noise.

Aahh. Cool but the noise still sounds like a rocket motor.

[TeeGee]

The microphone is mounted on the booster and therefore picks up any vibrations from it. Why would it have to be in the tank to do that?

I meant that the microphone is not exposed to the air outside the rocket. What it's picking up are the vibrations going up and down the SRB structure that shakes the bubble the camera is inside.

[Camacha]

Aahh. Cool but the noise still sounds like a rocket motor.

I understand the clipping in recordings comes from supersonic shockwave interaction. I am not knowledgeable enough when it comes to aerodynamics to say anything definite, but I can imagine the interactions being similar to those of exhaust gasses.

[TeeGee]

So basically the sounds are from inside or within the booster structure itself... not from the rocket motor sounds outside the craft, but vibrations traveling along the booster tube.

[softweir]

Exactly so!

[TeeGee]

[K^2]

Supersonic flows are complicated. If a rocket has a blunt nose, there is going to be a normal shock area just ahead of the rocket, which means that flow immediately around the rocket is likely to be subsonic. But most of the sound is probably from the structure, as people suggested.

[Camacha]

Supersonic flows are complicated. If a rocket has a blunt nose, there is going to be a normal shock area just ahead of the rocket, which means that flow immediately around the rocket is likely to be subsonic. But most of the sound is probably from the structure, as people suggested.

Which would also explain nicely why the sound barely seems to change when going supersonic - most of the noise was not coming from the air anyway.