## 2 Methods
### Measuring Principle
#### 2.2.2 Measurement of Ultrasonic Reflection Coefficient  
In the above acoustic evaluation model, the measurement of the reflection coefficient spectrum \(H_{0}^{r}(\theta,\omega)\) of the fluororubber to the couplant is the key for quantifying the three metrics \(K_{s}\), \(H\), and \(\beta\). From Equation (10), it can be seen that the ultrasonic probe's impulse response function to the electrical pulse \(E(\omega)\) and the impulse response function to the acoustic pulse \(R(\omega)\) in the ultrasonic measurement system are difficult to quantify.  
Under the same experimental parameters and conditions, \(E(\omega)\), \(R(\omega)\), and \(A\Delta(\omega)\) are invariants. Therefore, a reference specimen with known material properties and acoustic parameters can be used, allowing Equation (10) to be written as:  
\[F(\omega,0)_{ref}=E(\omega)H_{0}^{a}(\omega)\Big{[}H_{0}^{r}(\omega)_{ref}+H_ {0}^{s}(\omega)\Big{]}H_{0}^{a}(\omega)H_{0}^{s}(\omega)R(\omega)A\Delta(\omega) \tag{16}\]  
Equation (16) represents the ultrasonic measurement of the reference specimen, where \(F(\omega,0)_{ref}\) is the Fourier transform of the echo signal from the reference specimen. The equation incorporates the electrical and acoustic impulse responses \(E(\omega)\) and \(H_{0}^{a}(\omega)\), the reflection and scattering coefficients of the reference specimen, and the system's impulse response function \(R(\omega)\). This provides a baseline measurement for comparison with the unknown specimen. It can be further derived that:  
\[\frac{F(\omega,0)}{F(\omega,0)_{ref}}=\frac{H_{0}^{r}(\omega)+H_{0}^{s}(\omega)} {H_{0}^{n}(\omega)_{ref}+H_{0}^{s}(\omega)} \tag{17}\]  
In the above equation, \(F(\omega,0)\) and \(F(\omega,0)_{ref}\) can be obtained by performing a Fourier transform on the first interface echo obtained from the ultrasonic measurement experiment, \(H_{0}^{r}(\omega)_{ref}\) can be calculated based on the material properties and acoustic parameters, and \(H_{0}^{s}(\omega)\) can be determined experimentally. It compares the echo signals from the unknown specimen and the reference specimen by calculating the ratio of their Fourier transforms. The right-hand side includes the known reflection and scattering coefficients for the reference specimen, allowing the material properties (e.g., bulk modulus, attenuation, and thickness) of the unknown specimen to be derived. This ratio eliminates the influence of system response, improving the measurement accuracy.  
In the nuclear power industry, fluororubber sealing gaskets and rings are widely used, and these two types differ greatly in their geometric shapes. To ensure that the method proposed in this paper can be used for the nondestructive evaluation of mechanical properties, a point-focused transducer was used to measure the reflection coefficient spectrum for the sealing gaskets, while a line-focused transducer was used for the measurement of the reflection coefficient spectrum for the rings. As shown in Figure 2b, to ensure that the measurement area of the ring is straight, its axis must be aligned parallel to the axis of the line-focused transducer. For the specific measurement process, the reader can refer to the method used by [PERSON] and [PERSON] et al. who measured the two-dimensional reflection coefficient spectrum using \(V(z,t)\) data [24; 25].