This contribution demonstrates that a one-end free capsular fibre Bragg grating, well documented as a strain-free temperature sensor head to discriminate temperature effect from a fibre Bragg grating strain sensor, successfully functions as an acousto-ultrasonic sensor. It implies that both built-in strain and temperature compensation sensors can be utilized for acousto-ultrasonic damage detection. The wave sensing principle of the capsular fibre Bragg grating is explained on the basis of the existence of a fibre-guided wave. Furthermore, it is reported that the capsular fibre Bragg grating is a resonant acousto-ultrasonic sensor through frequency analysis of the sensor responses. Finally, we present a method capable of controlling the resonance frequency of the capsular fibre Bragg grating.