A method of processing measurements of an acoustic wave, the acoustic wave being within a member and caused, at least in part, by an impact to the member, the member being made of a composite material, the method comprising: analysing the measurements of the acoustic wave to detect an initial acoustic wave feature in the measured acoustic wave; analysing the measurements of the acoustic wave to determine whether or not there is one or more further acoustic wave features present in the measured acoustic wave, each of the one or more further acoustic wave features occurring in the acoustic wave after the occurrence of the initial acoustic wave feature; if there are no further acoustic wave features present in the measured acoustic wave, determining that the impact to the member has not damaged the structure of that member; if there is one or more further acoustic wave features present in the measured acoustic wave, determining a value of a function of one or more of the further acoustic wave features; if the determined value satisfies one or more criteria, determining that the impact to the member has damaged the structure of that member; and if the determined value does not satisfy the one or more criteria, determining that the impact to the member has not damaged the structure of that member, wherein the measurements of the acoustic wave are filtered such that the initial acoustic wave feature is a feature of the filtered measurements and each of the further acoustic wave features is a feature of the filtered measurements, and wherein the function of one or more of the further acoustic wave features is the ratio between an amplitude of the initial acoustic wave feature and an amplitude of a further acoustic wave feature. analysing the measurements of the acoustic wave to detect an initial acoustic wave feature in the measured acoustic wave; analysing the measurements of the acoustic wave to determine whether or not there is one or more further acoustic wave features present in the measured acoustic wave, each of the one or more further acoustic wave features occurring in the acoustic wave after the occurrence of the initial acoustic wave feature; if there are no further acoustic wave features present in the measured acoustic wave, determining that the impact to the member has not damaged the structure of that member; if there is one or more further acoustic wave features present in the measured acoustic wave, determining a value of a function of one or more of the further acoustic wave features; if the determined value satisfies one or more criteria, determining that the impact to the member has damaged the structure of that member; and if the determined value does not satisfy the one or more criteria, determining that the impact to the member has not damaged the structure of that member, wherein the measurements of the acoustic wave are filtered such that the initial acoustic wave feature is a feature of the filtered measurements and each of the further acoustic wave features is a feature of the filtered measurements, and wherein the function of one or more of the further acoustic wave features is the ratio between an amplitude of the initial acoustic wave feature and an amplitude of a further acoustic wave feature.