The problem with low frequency sound is the wavelengths- at low frequencies - can be 36' long!
(Thats feet, not inches!)
Whatever one places into the path of that wave has to affect some even portion of the wave: usually a 1/4 wavelength is accepted as a minimum.
That would mean your windowplug should be at least 9' thick!
So you can see why it's so difficult to block low frequency sound by much. (That doesn't mean "SOMETHING" won't be effective at all, just somewhat effective).
This is a extremely simple explanation to keep the mumbo-jumbo out of it! Don't all you acousticians jump on me!
BJ Nash Super Soundproofing CO