Yes, RESONATOR is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, S 1, O 1, N 1, A 1, T 1, O 1, R 1.

Is it a valid word?

RESONATOR is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a hollow chamber whose dimensions allow the resonant oscillation of electromagnetic or acoustic waves
an electrical circuit that combines capacitance and inductance in such a way that a periodic electric oscillation will reach maximum amplitude
any system that resonates

Also: cavity resonator, resonating chamber, resonant circuit.