# Round Window Reinforcement-Induced Changes in Intracochlear Sound Pressure
## 4 Discussion  
### Validation of the HTB Functionality  
The functionality of the HTB samples was assessed by using the METF against the standard ASTM and verifying the phase difference between the stapes and the RWM at low frequencies (cf. Figure 3). The METF of several samples was below the ASTM standard range across the whole considered frequency range (200 Hz-8 kHz). We assume that the lower METF is mostly the result of age-related effects, such as a less compliant tympanic membrane that reduces the sound energy transmitted to the middle ear [23]. The mean age of the sample was 74 years. In addition, the ossicles and soft tissue structures in the middle ear are subjected to common diseases that can affect the METF [24]. The freezing of the HTBs after extraction might affect the membranous structures such as the RWM and TM. Therefore, using fresh samples might lower the variability of the RWM reinforcement effect on the ICSP between individual samples. During the experiment, the samples were kept from drying with assisted moisturization. However, the drying of the sample during the time taken for preparations and measurements cannot be completely ignored [25]. After the cochleostomy in ST and SV were drilled and the sensors were inserted the surgeon visually confirmed that no damages occurred to the middle or inner ear structures. Additional confirmation of the HTB functionality after sensor insertion by measuring the stapes and RWM motion was not feasible due to the obstructed visual access to the stapes by the two hydrophones.  
The measured ICSP data were compared with the corresponding data reported by [PERSON] et al. [11]. A trend towards lower values than the reference data was also observed during the ICSP measurements. Mainly at frequencies below 2 kHz, where the effect of RWM reinforcement on the ICSP is significant, the measured ICSP data were well below the lower limit of the reference data. While in the present study we are more interested in changes in the ICSP due to RWM reinforcement than in absolute pressure values, it is important to assess the hydrodynamic condition of the cochlea under investigation and thus confirm that the comparatively low METF and ICSP data are not related to the inner ear mechanics. This was accomplished by calculating the input impedance (\(Z_{\text{C}}\)) of the cochlea, which is defined as the ratio between the vestibuli pressure \(P_{SV}\) and the volume velocity of the stapes (\(V_{\text{STAP}}\) * A), where A (3.2 mm\({}^{2}\)) indicates the area of the stapes footplate [11; 23]. The \(V_{\text{STAP}}\) was measured prior hydrophone insertion, as simultaneous measurements of ICSP and the \(V_{\text{STAP}}\) were not possible due to space restrictions in the opened middle ear cavity. The data were then compared with several reference data from the literature [11; 23; 26]. The behavior of the \(Z_{\text{C}}\) closely resembles resistive behavior, as the phase data show values close to zero across the considered frequency range and similar to the reference data (cf. Figure 7). The wider SD in the phase data above 1 kHz suggests that the complex motion of the stapes gains significance. This could have been alleviated by using a 3D LDV for stapes velocity measurements instead of a single-point LDV, as available for the present study [11]. The median magnitude of \(Z_{\text{C}}\) is in good agreement with [PERSON] et al. (2001) and slightly below the data of [PERSON] et al. (2009) until 6 kHz. Above 6 kHz, a larger deviation can be identified between our data and the reference data. Since the effect of RWM reinforcement on the ICSP is limited to frequencies below 2 kHz, the HTBs used in the present study can be classified as being valid for this study.