If I remember correctly, the research that lead to these foam inserts was that low frequency vibrations "echoed" in the tire air chamber - and adding foam greatly reduced the "echo." Further, low frequencies tended to set off various components in the car - and those vibrations were difficult to silence.
Higher frequencies got damped out by the tire itself and even if transmitted to the car were lost in the other vibrations the car experiences. If I also remember correctly, car manufacturers try to set up the vehicle so that different components vibrated at different frequencies, thereby confusing the occupants - where low frequency vibrations were easily discernible.
Put a different way, a lot of vibrational energy spread out over a large frequency range is difficult for humans to perceive, but humans easily perceive low frequencies.
Higher frequencies got damped out by the tire itself and even if transmitted to the car were lost in the other vibrations the car experiences. If I also remember correctly, car manufacturers try to set up the vehicle so that different components vibrated at different frequencies, thereby confusing the occupants - where low frequency vibrations were easily discernible.
Put a different way, a lot of vibrational energy spread out over a large frequency range is difficult for humans to perceive, but humans easily perceive low frequencies.
