In the presented work ultrasonically absorptive carbon-carbon ceramic was shown for the first time to delay hypersonic laminar to turbulent boundarylayer transition. Three 7° half-angle cones with nose radii between 0.1 mm and 5.0 mm and a total length of 1100 mm were tested at zero angle of attack in the High Enthalpy Shock Tunnel Göttingen (HEG) of the German Aerospace Center (DLR) at Mach 7.5. One model was equipped with an inhouse manufactured ultrasonically absorptive carbon-carbon ceramic insert with random microstructure covering one third of the model surface in circumferential direction. The remaining model surface consisted of polished steel and served as reference surface. The free-stream unit Reynolds number was varied over a range of 1.5e6 /m to 9.8e6 /m at stagnation enthalpies of 3.2 MJ/kg and a wall temperature to total temperature ratio of 0.1. The ultrasonic absorption properties of carbon-carbon ceramic (C/C) were assessed theoretically by means of the quasi-homogeneous absorber theory and experimentally by means of direct reflection coefficient measurements at varying ambient pressure levels. For the first time broadband ultrasonic sound transducers with resonance frequencies of up to 370 kHz were applied to directly cover the frequency range of interest with respect to the second mode instabilities observed on cone geometries in HEG. C/C was found to absorb up to 19% of the acoustic power transmitted towards the surface. The experimental results were found to be in good agreement with the theory. The present study revealed the second mode instability to dominate the transition process. A distinctive attenuation of instability waves and a delay of boundary layer transition on the ultrasonically absorptive carbon-carbon insert was proven by means of fast-response surface pressure measurements, high speed schlieren visualization and surface heat flux measurement.