Recent techniques that allow comparison between time-resolved predicted and recorded sound
fields include time-resolved comparisons of acoustic analogies with DNS calculations [1], semi-empirical sound calculations from time-resolved PIV data [2,3],Kirchhoff surfacem projection methods [4], and real-time projections to simplified source model swith analy ticalsolutions [5,6]. Ultimately,the comparisons are made between either spectral or time-domain signal shapes,
finding the maximum deviation or determining qualitatively if they are ‘similar’. A quantitative similarity metric would be helpful in evaluating these techniques, and an optimal metric would be sensitive to errors indicating poor model agreement,while remaining relatively invariant to random noise and small errors arising from unmodelled phenomena.This work considers four metrics from this perspective.A Structural Similarity Metric(SSIM) originating from image compression
analysis has been success fully applied recently to time–frequency representations of biological neurogram signals [7] and speech signals [8]'prompting the current study in to its use for aeracoustics. For images,the optimal compression stores an acceptable representation of the image as compactly as possible,ignoring extraneous detail and retaining only the information most important for human interpretation (i.e.structuralinformation). The aeroacoustic analogue is that sound predictions cannot be expected to predict all the rich detail of the sound
emissions,but they must capture the mechanics of the dominant sound emissions to be useful.