There are 3 resonance search methods: transmissibility, first transmissibility derivative and Q-factor. The QST ‘s engineers used the last two.
The first transmissibility derivative is calculated numerically, any change of derivative sign from plus to minus is considered a resonance.
Q-factor is calculated as the ratio of the center frequency of the resonance to its half-power bandwidth and determines how sharp the resonance is, and, therefore, how much damage it can do.
Image: Transmissibility line and derivative lines before and after the resonance