  The behavior of hairy black hole solutions, obtained by the gravitational
decoupling (GD) method, is investigated under scalar perturbations. The
quasinormal mode frequencies of such solutions are regulated by GD hair. The
numerically generated wave solutions are derived for a range of values for the
GD hairy black hole parameters, with higher-frequency modes very sensitive to
them. The results are confronted with the corresponding ones for the
Schwarzschild solution, whose deviations from it demonstrate a unique physical
identification of GD hairy black holes. The method here presented comprises the
first steps towards the obtainment of the observable signature of GD hairy
black holes at ground-based detectors, emitted from coalescing binary systems
of GD hairy black hole mergers in the ringdown phase.
