I remember reading somewhere that since sound travels way faster through the ground as compared to air, soldiers back in the day used wooden pillows that were hard. So if the enemy started marching in a surprise attack at night. The sound would reach them in their sleep. The vibrations in the ground would shake the pillow.

Is there any evidence for anything like this?

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    I'm doubting that soldiers who were moving in for surprise attack at night would be marching.
    – Steve Bird
    Feb 10 at 16:37
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    @SteveBird Yeah I might be completely wrong here. I just thought I would post this in a few places because I have a strong feeling I read it in some authoritative text. Other than the fact a sneak attack at night would have the enemy soldiers moving very quietly. I am not sure the small speed difference between the sound traveling through air and the ground would warrant adoption of such a ritual. taiwantoday.tw/….
    – Aditya P
    Feb 10 at 16:49
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    @AdityaP: Your comment of "the small speed difference between the sound traveling through air and the ground" is woefully uninformed. Sound travels through the air at about 300 m/s, and through the ground at speeds ranging from (for P-waves, or pressure waves) 5 km/s to 13 km/s: i.e. anywhere from 15 to 40 times faster. P-waves, being pressure waves, also are capable of travelling through loose topsoil. S-waves, shear-waves, can only travel through solid rock, but that's not relevant in this context. Feb 11 at 4:33
  • @PieterGeerkens oh ok... I didn't do the math on that. thanks for correcting me. Then for a large distance this might mean precious time to get prepared for a confrontation.
    – Aditya P
    Feb 11 at 4:53
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    @AdityaP It's not just the time, but also the distance. Sounds travel farther in the ground as well.
    – Ryan_L
    Feb 11 at 19:32

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