A computer implemented method of determining the detectability of a defect within a supplied part out of which a final component is to be machined, the method comprising the steps of: defining a domain of interest representative of the geometry of the supplied part having a simulated defect; defining a plurality of virtual particles, which correspond to spaced points in the domain of interest; defining a plurality of master particles spaced within the domain, the master particles being representative of field variables including at least the position, magnitude and direction of propagation of a wave front originating from a wave source of an energy wave at the location of the respective master particle; defining a supporting domain Ω m for each master particle, the supporting domain defining a region of the domain of interest in which the magnitude of the master particle is to be distributed to the virtual particles, said virtual particle being representative of the magnitude of a field variable of the wave at the location of the respective virtual particle; computing field variables including the position, magnitude and direction of propagation of each master particle at a predetermined time step; calculating a magnitude u v of the field variable of each virtual particle at the time of interest by applying the formula: u v = ∑ m = 1 N u m f q 1 × g q 2 Where v ∈ Ω m where N is the total number of master particles, u m is the magnitude of a respective master particle, f ( q 1) is an envelope function relating the magnitude of the virtual particle to the distance from the master particle in the direction of propagation of the master particle, and g ( q 2) is a smoothing function relating the magnitude of the virtual particle to the distance from the master particle normal to the direction of propagation of the master particle; determining the magnitude of a field variable of a virtual particle located at a receiver location within the domain of interest at a predetermined time; wherein the defect is determined to be detectable where virtual particle located at a receiver location within the domain of interest at a predetermined time has a field variable magnitude greater than a predetermined amount. defining a domain of interest representative of the geometry of the supplied part having a simulated defect; defining a plurality of virtual particles, which correspond to spaced points in the domain of interest; defining a plurality of master particles spaced within the domain, the master particles being representative of field variables including at least the position, magnitude and direction of propagation of a wave front originating from a wave source of an energy wave at the location of the respective master particle; defining a supporting domain Ω m for each master particle, the supporting domain defining a region of the domain of interest in which the magnitude of the master particle is to be distributed to the virtual particles, said virtual particle being representative of the magnitude of a field variable of the wave at the location of the respective virtual particle; computing field variables including the position, magnitude and direction of propagation of each master particle at a predetermined time step; calculating a magnitude u v of the field variable of each virtual particle at the time of interest by applying the formula: u v = ∑ m = 1 N u m f q 1 × g q 2 Where v ∈ Ω m where N is the total number of master particles, u m is the magnitude of a respective master particle, f ( q 1) is an envelope function relating the magnitude of the virtual particle to the distance from the master particle in the direction of propagation of the master particle, and g ( q 2) is a smoothing function relating the magnitude of the virtual particle to the distance from the master particle normal to the direction of propagation of the master particle; determining the magnitude of a field variable of a virtual particle located at a receiver location within the domain of interest at a predetermined time; wherein the defect is determined to be detectable where virtual particle located at a receiver location within the domain of interest at a predetermined time has a field variable magnitude greater than a predetermined amount.