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The space shuttle's SRBs each produced ~2x the thrust of the three main engines. Which source produced the majority of the sound volume at liftoff?

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    $\begingroup$ No source, but unquestionably the SRBs. $\endgroup$
    – Russell Borogove
    May 26, 2021 at 3:53
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    $\begingroup$ Personal experience, what Russell said. Just watch any liftoff film. The main engines run for ~6 seconds before liftoff. $\endgroup$
    – Organic Marble
    May 26, 2021 at 3:54
  • $\begingroup$ I know which was brighter though that's (probably) not directly related. $\endgroup$
    – uhoh
    May 26, 2021 at 4:47
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    $\begingroup$ A rando on nasaspaceflight.com claims solid rockets tend to be louder than liquid per unit thrust. $\endgroup$
    – Russell Borogove
    May 26, 2021 at 4:51
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    $\begingroup$ To add to what @RussellBorogove said if solids are louder per unit thrust AND the SRBs had a lot more thrust, gut feeling says that SRBs were much louder. SRBs are also a lot rougher ride (lots of astronauts have confirmed) and it stands to reason that the same "rough energy" would generate "rougher soundwaves ®" $\endgroup$
    – Carlos N
    May 26, 2021 at 12:45

2 Answers 2

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Submitted as a somewhat subjective answer - you never know what sound editors did.

These are two clips from the early shuttle IMAX file The Dream is Alive. This agrees fairly well with my experience of hearing the more white-noisy main engines followed by the louder crackly, snappy rumbling of the solids.

There are significant differences between the sound in the two clips, perhaps due to microphone placement and characteristics, or ?

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    $\begingroup$ Funny how the countdown man skips 14 in the second video :-) $\endgroup$
    – JohnEye
    May 27, 2021 at 11:02
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(not an answer yet, I have yet to find SPL data for the SRBs)

Thanks to the discussion and videos it does seem that the SRBs are louder. However, I had a vague notion that turbulence was important, and Reynolds number scales with jet velocity.

The paper "Arenas, Jorge P., and Ravi N. Margasahayam. "Noise and vibration of spacecraft structures." Ingeniare. Revista chilena de ingeniería 14.3 (2006): 251-264." appears to be an excellent review of the principles involved.

A quote:

"The total acoustic power radiated by a jet exhaust is proportional to the eight power of the jet efflux velocity (between the cube and the fifth power of the jet efflux velocity for rockets, see figure 3), and the square of the jet diameter."

Figure 3 shows that the jet power to sound power efficiency is, surprisingly, actually higher for the SSMEs:

enter image description here

For sub-sonic flows this is apparently called "Lighthill's eighth power law".

SRB: $242 s * 9.8 \text{m/s}^2=2371\text{m/s}$ SSME: $366 s * 9.8 \text{m/s}^2=3586\text{m/s}$

Given the diameters of 0.26 m vs 3.8 m, and a pessimistic cube scaling for both engines,

$14.4 * 2371^3 \approx 191 $ GW sound power $0.0676 * 3586^3 \approx 3 $ GW sound power

So indeed, despite the much faster jet velocity, in terms of total power SRBs could win by a factor of ~20 or so.

This answer doesn't make sense however because the total power at liftoff is only about 12 GW.

It would be interesting to see how this normalizes to thrust.

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