We provide a simple but robust bound on the primordial curvature perturbation in the range 10 ^ { 4 } { Mpc } ^ { -1 } < k < 10 ^ { 5 } { Mpc } ^ { -1 } , which has not been constrained so far unlike low wavenumber modes .
Perturbations on these scales dissipate the energy of their acoustic oscillations by the Silk damping after primordial nucleosynthesis but before the redshift z \sim 2 \times 10 ^ { 6 } and reheat the photon bath without invoking CMB distortions .
This acoustic reheating results in the decrease of the baryon-photon ratio .
By combining independent measurements probing the nucleosynthesis era and around the recombination epoch , we find an upper bound on the amplitude of the curvature perturbation over the above wavenumber range as { \cal P } _ { \zeta } < 0.06 .
Implications for super massive black holes are also discussed .