A geophone lying on the floor records the tiny tremor each step sends through the structure. No camera, no microphone, nothing worn. Press play and listen to people walking.
Each strip is the full recording. The shaded box is the stretch shown and played below.
A single step is over in well under half a second. The heel strikes and the floor answers with a sharp jolt, then it keeps ringing and slowly dies away.
The wavelet pictures below the trace show the same step as time against frequency. The impact is a short vertical streak reaching high frequencies; the ringing is a horizontal band that fades out.
That ringing belongs as much to the floor as to the person. Separating the person from the floor is the hard part of the research.
Cuts the signal into equal slices of time and measures the frequencies in each. One slice length for everything: either the sharp impact is smeared in time, or the slow ringing is smeared in frequency.
Compares the signal with short wave packets of many sizes. Short packets catch high-frequency detail at the exact moment it happens; long packets measure low tones precisely. That suits a footstep: a sudden impact followed by slow ringing. Frequency runs on a log scale from 5 to 400 Hz.
Most footstep energy sits between 10 and 100 Hz, the bottom edge of human hearing. Laptop and room speakers barely reproduce it, so “Untouched” needs a subwoofer.
Playing the recording faster lifts every frequency by the same factor, into the range our ears like. The price is time: a walk becomes a drum roll.
Each detected step is sped up on its own but played at the moment it really happened. You hear the true walking rhythm with a floor that sounds like a drum.