## 2 Instruments and Methods  
### Spatially Resolved Acoustic Spectroscopy  
SRAS is a laser ultrasound evaluation technique that can be used to measure a variety of material properties of a sample under inspection [8]. In brief, SRAS works by generating surface acoustic waves (SAWs) on samples using a pulsed laser. The perturbation on the surface due to the SAW is measured using another laser and a detector. Where SRAS differs from other laser ultrasound techniques is in the method of generating the surface wave--a periodic grating pattern is used to image lines of the incident pulsed laser. This creates a surface acoustic wave packet, where the frequency within the envelope is determined by the distance between the pulsed laser lines and the speed of the wave. Figure 1 shows a simplified SRAS schematic. When used conventionally, the surface wave generated is a Rayleigh wave (air/solid interface with infinite half space) [11]--the wave packet generated in this regime is non-dispersive, and therefore the frequency (\(f_{s}\)) within the packet remains constant from the point of generation. Given this occurrence, the surface wave velocity (\(v_{s}\)) at the point of generation can be easily calculated using;  
\[v_{s}=f_{s}\lambda_{g} \tag{1}\]  
where \(\lambda_{g}\) is the distance (or period) between the incident pulsed laser lines, referred to as the grating wavelength. This points to one of the major strengths in using SRAS over conventional laser ultrasound techniques which measure wave velocity through time-of-flight. Detection of the waves can be made at any point along the wave propagation path, including adjacent to the generation point. This is useful for inspecting material in which heavy wave attenuation occurs. In addition to this, and considering that the measurement is made of the frequency rather than the amplitude, the influence to the measured signal is limited to the generation area, and not to material heterogeneity--this allows for strict control of the spatial resolution, without requiring multipoint measurements and post-process reconstruction. Rapid velocity measurements can be made using SRAS, with the reported systems in this paper used to raster scan samples. The generated SAW (i.e., Rayleigh) velocity is dependent on the elastic properties of the material under inspection, which can change depending on factors such as sample composition, temperature and crystallographic orientation. Using SRAS, it is possible to image a material's microstructure, by obtaining the material's elastic constants (obtained through literature) and measuring the SAW phase velocities (by measuring SAW velocities in different directions), as all are related. Practically, the process of directly determining the orientation (the inverse problem) is difficult to solve, so a fitting algorithm is used instead [12]. The outcome of the implemented algorithm can be used to create inverse pole figures--there have been a number of past publications comparing obtained crystallographic  
Figure 1: Schematic of a spatially resolved acoustic spectroscopy (SRAS) system capable of rapidly imaging surface acoustic wave (SAW) velocities on smooth flat surfaces. A grating pattern is imaged onto a sample using a pulsed IR beam, which generates a non-dispersive surface wave packet. The packet contains a frequency (\(f_{s}\)) controlled by the period of the incident pattern (\(\lambda_{g}\)) and the velocity of the surface wave at the point of generation (\(v_{s}\))—the velocity is affected by surface grain orientation or defects. The generated wave perturbs a detection beam which is detected by the knife edge detector—the two out-of-phase intensity signals are passed through a differential amplifier and captured using a fast acquisition board or oscilloscope and PC. grain orientation information using the SRAS technique against electron backscatter diffraction (EBSD) [13; 14; 15; 16; 17].