A site dedicated to the review and analysis of potential sasquatch vocalizations, Sasquatch Bioacoustic combines techniques from the domains of intelligence collection, audio analysis and bioacoustic studies to examine the evidence of sasquatch through their vocalizations. ~Monongahela

Friday, April 1, 2011

Differentiating quadrupedal and bipedal footsteps in spectrograms

I had a couple of comments about the nature of footfalls in a clip shared on March 31st, so I thought I'd try to illustrate the differences between bipedal and quadrupedal walking in audio recordings. First thing to remember is not every leafy sound you hear in a recording like this is a step. Sometimes shuffling feet will cause extra leaf noise that obscures a step, or make a sound like multiple steps. Second thing to know is steps in a spectrogram appear as tall vertical bars with each foot impact.

The big difference between bipedal and quadrupedal steps is the pace or timing with which the feet strike the ground. Healthy bipedal steps (with no limp) are regularly spaced one from the other. They really only change their relationship when the walker's pace quickens or slows. Here's an example of bipedal steps in a spectrogram (my own steps in fact). Notice how evenly spaced the numbered steps are:

Quadrupedal steps on the other hand are rarely ever so evenly spaced. Instead you'll find a walking quadrupeds steps tend to group into tight pairs, 1-2, 3-4. And when they do step evenly its not a pace that's maintained for long, the quadrupedal "lope" quickly returns. Here's a spectrogram of a quadruped walking past my recorder in the night. Note the pairing of steps:

So with that in mind, take a look at the spectrogram of the foot falls as they approach in the recording shared on March 31st. The vertical bars are fainter because the walker is farther from the mic. But notice how evenly spaced they are, as opposed to grouped up in pairs: