Why do some rockets make a smooth, continuous “woosh” sound while others make a rougher popping/rumbling sound?
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Your question is a little vague. Can you give some examples of what you’re talking about? Maybe 2 different YouTube videos?
It depends on the type of engine the rocket has. A continuous feed liquid fuel or solid fuel rocket will produce the “whoosh” sound while a pulse engine (such as in the V1 “Buzz bomb”) will produce a popping or buzz sound.
The pitch of the rocket sound depends on the fuel feed and the size of the nozzle. Larger rockets produce a deeper sound.
Generally rockets with solid rocket boosters sound a lot rougher and poppier than rockets that only use liquid fuelled engines.
That is because SRBs use propellant that is made up of grains in a binder and as each grain burns it creates a flare of noise which overlaps with all the other grains burning.
In a liquid propellant engine there are droplets of fuel burning but they are much smaller than the grains in solid propellant so the noise is smoother.
Rockets with big SRBs such as the Space Shuttle and SLS create more noise than those with smaller SRBs such as Atlas V and Vulcan. The quietest rockets for their thrust are the Falcon 9 and Falcon Heavy but they are still plenty loud.
Aerospace engineer here with a background in rocket propulsion, I’m also doing my thesis on acoustic load on launch vehicles.
The overall sound power level of a rocket can be calculated approximately by using W = 0.005*n*g*v*Isp
Where n is the number of exhaust nozzles, g is the gravitational acceleration, v is the exit velocity of the exhaust, and Isp is the specific impulse of the engine.
Larger rockets have higher exhaust velocities giving a higher overall sound power level. Converting to decibels will give a rocket like the Saturn V, which has an exhaust velocity of 2607 m/s, around 203-206 dB, which would literally kill you if you are within 3 km of the launch.
Small rockets like a hobby rocket have vastly lower exit velocities for their exhaust, on the range of 150-200 m/s. Their db value would be around 90 dB, less than a concert.
Plus the duration of burning is much different. This affects what frequency range is more prevalent. Larger rockets peak at very low frequencies, <200 Hz, while small rockets are around 6-8k Hz.
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