knock doorThis is the frequency response when knocking the door. It produces sound mainly in three frequency ranges: 250~350 Hz, 450-550 Hz, and 700-900 Hz. Since these frequency ranges make pretty recognizable pattern comparing with the sound caused by closing door, they are used for feature extraction in later machine learning.
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Differences in Energy |
Differences in ZCR |
The two graphs show the energy and zero-crossing rate of each activities. The zero-crossing rate(ZCR) is the rate of sign-changes along a signal, i.e., the rate at which the signal changes from positive to negative or back. This feature is heavily used to speech recognition.