# Can the ISS boost maneuver engine be heard by the astronauts on board?

In the extremely cool demonstration video Demonstration of Acceleration Inside the International Space Station During a Reboost ISS Expedition 22 Commander Jeffry Williams says near the end at 04:48 that he could hear that the boost burn was complete.

I am not sure which engine was used for these burns in 2010, but how does it manage to make enough noise throughout the ISS to be heard over the drone of the ventilation system and all of the other equipment?

I here a faint "ping-pong" (sounds like the 7-11 automatic doors where I live) just before Williams speaks, but I'm assuming that's a video camera being turned off.

video cued at 04:48

• @GittingGud I can speculate and imagine as well as anyone else, but here I'm looking for an authoritative and sourced answer. – uhoh May 16 '19 at 6:56
• The nasa publication Acoustics and Noise Control in Space Crew Compartments might have an answer for you but it might take some time to find the information n the 500+ pages.(ISS starts at page 256) – GittingGud May 16 '19 at 7:04

Human hearing is fantastically sensitive in dynamic range, and able to precisely discriminate different frequency ranges.

The Zvezda module's integrated thrusters are sometimes used for reboost; this is a pair of S5.79 rocket engines with 3kN thrust each. I can't find a video of such an engine in action, but here's a test of a Chinese engine of similar size and, for comparison, a test of the much smaller (400N) SpaceX Draco. The alternative to using Zvezda’s thrusters is to use thrusters on a Progress resupply ship. Those thrusters are of generally similar scale.

You can hear that the sound of the engine has a lot of high-frequency hiss. That's sound transmitted through the air, of course, and the frequency spectrum of sound conducted from the engine to the body of the space station would likely be rather different, but I imagine that a fair amount of hiss would still be audible inside the station, which might be a contrast to the lower-frequency "rumble" of the station's air system.

I can't hear a difference on the video between the during-burn and post-burn soundscape, but that's sound going into a cheap mic, encoded to low-bandwidth video, and reproduced on cheap laptop speakers, so I'm not surprised. I imagine that after being on the ISS for a while, you learn to tune out the steady-state ventilation sounds, and the sound of the boost engines themselves stands out as distinct and unusual.

• The Zvezda module's thrusters are only rarely used for reboost. Zvezda never used its engines for reboost from 2001 to 2006. – David Hammen May 24 '20 at 19:32
• Will edit, thanks. – Russell Borogove May 24 '20 at 20:09