Sub-Bass Resonance in Koto Performance on Tatami

We report sub-bass resonances arising from mechanical coupling between the koto (Japanese zither) and tatami flooring during performance. Recordings of a 13-string koto on three floor materials revealed strong spectral components at 29 Hz and 50 Hz on tatami, weaker and shorter-lived components on wooden flooring, and no sub-bass content on ceramic tile. These components were absent for the shamisen (Japanese three-stringed long-neck lute) and classical guitar held by the performer, suggesting that direct floor contact is essential. A mass-spring model reproduces the observed frequencies with physically reasonable parameters, supporting the proposed coupling mechanism.