## 4 Results
### CFRP\({}_{\text{FBH}}\) Coupon
The reason why TSA gave low DBR values lays in the fact that TSA's background amplitude (\(\pm\)270 DL in the TSA output and \(\pm\)0.25 after normalization) was not zero-mean, and was of the same order as a defect's amplitude (\(\pm\)550 DL for \(\text{FBH}_{\text{A}}\) in the TSA output, and 1 after normalization). FDT's \(\text{maxigram}^{\text{F}}\) and ASBI, on the other hand, obtained very high DBR values, which indicates that these defects were well detectable and that these techniques obtained an almost zero-reference background (see Figure 5d,f). Additionally, here, their DBR values came closer together with increasing defect depth, as was already discussed for the CNR results. Figure 6 presents the cross-sections A-A' and B-B' (indicated in Figure 5a) with the normalized output of the post-processing techniques, i.e., DTT's \(\text{maxigram}^{\text{T}}\), TSA, FDT's \(\text{maxigram}^{\text{F}}\) and ASBI. Notice that TSA did not obtain a zero-mean background since it is not a contrast quantity, which makes a comparison difficult. The normalization was therefore done by subtracting the minimum value in the \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multicolumn{6}{c}{**Context-to-Noise Ratio (dB)**} \\ \hline  & \multicolumn{3}{c}{**Time-Domain**} & \multicolumn{3}{c}{**Frequency-Domain**} \\ \hline  & **Depth** & **DTT** & **TSA** & **FDT** & \\  & **(mm)** & \(\text{maxigram}^{\text{T}}\) & **(0–50 s)** & \(\text{maxigram}^{\text{F}}\) & **ASBI** \\ \hline \(\text{FBH}_{\text{A}}\) & 0.85 & 25.8 & 37.1 & 45.6 & 73.8 \\ \(\text{FBH}_{\text{B}}\) & 1.64 & 21.34 & 35.9 & 42.7 & 59.0 \\ \(\text{FBH}_{\text{C}}\) & 2.47 & 14.3 & 30.7 & 38.0 & 47.5 \\ \(\text{FBH}_{\text{D}}\) & 3.68 & \(-\)0.7 & 19.7 & 28.8 & 30.7 \\ \(\text{FBH}_{\text{E}}\) & 4.51 & \(-\)8.1 & 13.1 & 18.6 & 18.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Contrast-to-noise ratio (CNR) for all FBHs of the \(\text{CFRP}_{\text{FBH}}\) coupon sample. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multicolumn{6}{c}{**Defect-to-Background Ratio (–)**} \\ \hline  & \multicolumn{3}{c}{**Time-Domain**} & \multicolumn{3}{c}{**Frequency-Domain**} \\ \hline  & **Depth** & **DTT** & **TSA** & **FDT** & \\  & **(mm)** & \(\text{maxigram}^{\text{T}}\) & **(0–50 s)** & \(\text{maxigram}^{\text{F}}\)** & **ASBI** \\ \hline \(\text{FBH}_{\text{A}}\) & 0.85 & 10.3 & 3.9 & 174.7 & 6341.0 \\ \(\text{FBH}_{\text{B}}\) & 1.64 & 6.5 & 3.5 & 126.5 & 1159.4 \\ \(\text{FBH}_{\text{C}}\) & 2.47 & 3.5 & 2.4 & 73.4 & 307.3 \\ \(\text{FBH}_{\text{D}}\) & 3.68 & 1.4 & 1.4 & 26.2 & 45.5 \\ \(\text{FBH}_{\text{E}}\) & 4.51 & 1.2 & 1.2 & 8.8 & 11.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Defect-to-background ratio (DBR) for all FBHs of the \(\text{CFRP}_{\text{FBH}}\) coupon sample. cross-section before dividing by the then obtained maximum value. The timegram and frequencygram were not represented here since their values were not directly related to a measure of how well a defect is detected (i.e., not magnitude based). Cross-section A-A' crosses FBH\({}_{\text{B}}\) (at 1.64 mm depth), and cross-section B-B' went through the deepest FBH\({}_{\text{E}}\) (at 4.51 mm depth). In order to evaluate the defects' lateral size and sharpness, the defect locations are indicated on the cross-sectional views. Providing a quantitative comparison of the estimated defect sizes would require establishing appropriate decibel drop levels (e.g., -3 dB drop cuts a defect off at 50% of its maximum amplitude [19; 39]) for each post-processing technique. This, however, was out of the scope of the present article. In cross-section A-A' (Figure 6a), all post-processing techniques detected FBH\({}_{\text{B}}\) (see also Figure 5, Tables 1 and 2). DTT's maxigramT, FDT's maxigramF, and ASBI all provided a comparable defect contour with a similar sharpness. In contrast, TSA had a significantly wider defect indication due to lateral heat diffusion, and therefore did not obtain a sharp defect contour.