  We achieve a comprehensive understanding of the magnetic excitations observed
by electric spin resonance and far-infrared spectroscopy in a frustrated spin
gap system ${\rm SrCu_2(BO_3)_2}$ by considering the effects of magnetoelectric
couplings and Dzyaloshinskii-Moriya interactions in the Shastry-Sutherland
model. The transitions from the dimer singlet ground state to the triplon and
the bound states of two triplons are electroactive through the magnetoelectric
couplings, even in the Shastry-Sutherland Heisenberg model. The results
indicate that an electro-triplon and a novel electroactive magnetic excitation,
as with an electromagnon in the multiferroics, can be realized in the
conventional spin-gapped singlet and anomalous spin-liquid states. Clarifying
these electroactive magnetic excitations in various spin systems will help
analyze a broad range of quantum magnets, {\it e.g.}, quantum spin liquids and
spin nematics.
