So I've read various articles about the sound emitted from the Perseus Cluster black hole, and I understand that the way NASA was able to detect the drone 57 octaves below middle C being emitted by the black hole was by studying the effects on the surrounding gas clouds, and by converting the x-rays emitted as a sound we can interpret; so in other words it was all indirect measurement.
However the Gizmodo article stated that "Sound waves can only travel through a medium if the length of the wave is longer than the average distance between the particles." The thing I don't really understand here, and isn't really addressed in any of the info I've found online about this is:
If the length of the wave needs to be longer than the distance between each particle in the medium, why can't incredibly low frequency infrasound travel through deep space or regions where density of particles is say 1 atom per cubic metre or more? According to the Wikipedia page about the Perseus Cluster, is says "No human will actually hear the note, because its time period between oscillations is 9.6 million years" So this would mean the hertz of this wave must be incredibly small, and thus the wave must be very long allowing it to transfer between even very spread out particles in space?
So essentially I'm asking why can't infrasound travel though the imperfect vacuum of space where particles are very spread out, but are still present meaning the space isn't completely empty? Why is it that the sound waves will stop traveling at the edge of the cluster, and not continue through these very spread out particles between galaxies/clusters/superclusters?
Is it that the wave has lost it's energy long before it reached earth? At what point is it decided that infrasound can no longer travel through gas? Because if it can travel through the gas cloud in the cluster, where does it suddenly stop and decide that it can propagate no longer.