Experimental validation of an impact off-resonance energy harvester G. Martinez-Ayuso, M.I. Friswell, H. Haddad Khodaparast and S. Adhikari Eur. Phys. J. Special Topics, 228 7 (2019) 1635-1646 Published online: 23 August 2019 DOI: 10.1140/epjst/e2019-800155-0