# Round Window Reinforcement-Induced Changes in Intracochlear Sound Pressure
## 4 Discussion
### Effect of RWM Reinforcement on the Pressure Difference across the Partition  
The pressure difference \(P_{DIFF}\) across the basal part of the cochlear partition triggers the propagation of the traveling wave on the basilar membrane and, hence, is an important measure to assess the influence of RWM reinforcement on the hearing sensation. The \(P_{DIFF}\) of each HTB sample was calculated in the complex domain as the difference in pressure in the SV and ST normalized with the ear canal pressure \(P_{EC}\). To illustrate the RWM reinforcement-induced change in \(P_{DIFF}\), the ratio between \(P_{DIFF}\) after and before RWM reinforcement was calculated and depicted in dB. Figure 11A shows the corresponding ratio for cartilage and Figure 11B shows it for soft tissue reinforcement. Only data at frequencies below 2.2 kHz are considered where changes in ISCP can be attributed to RWM manipulations. For certain HTB samples, reinforcement materials, and frequencies, the pressure in SV and ST showed almost the same magnitude or an even higher pressure in ST than in SV (cf. Figure 5). Such pressure conditions are not physical and are attributed to the measurement uncertainty of the sensor system and other interfering effects. However, these invalid data were removed and not considered for further analysis (certain curves not continuous). Except for HTB-19, where the \(P_{DIFF}\) was reduced by up to 5 dB using cartilage and up to 10 dB using soft tissue, the change in \(P_{DIFF}\) was small or even negligible. The reason for this is that the \(P_{DIFF}\) is mainly pretended by the vestibuli pressure, which is significantly larger than the pressure in ST. Despite the clear increase in ST pressure after reinforcement that is obvious throughout the samples, it has a mostly negligible effect on \(P_{DIFF}\) or is compensated by an increased vestibuli pressure. Figure 11: Ratio between the pressure difference across the cochlea partition normalized to the ear canal pressure (\(\frac{P_{SV}-P_{ST}}{P_{EC}}\)) after and prior RWM reinforcement with cartilage (**A**,**C**) and soft tissue (**B**,**D**). In (**A**,**B**), \(P_{DIFF}\) was calculated in the complex space considering the phase shift between \(P_{SV}\) and \(P_{ST}\). In (**C**,**D**), zero phase shift was assumed for the \(P_{DIFF}\) calculation. Invalid data (discontinuous curves) arise due to the uncertainties in the sensor measurements, and other interfering effects were neglected. Another reason for this that might explain the small effect of RWM reinforcement on the \(P_{\text{DIFF}}\) is the phase shift between the pressure in SV and ST, which deviates significantly from zero for certain HTB samples (cf. Figure 4C,D) and, hence, reduces the effective \(P_{\text{DIFF}}\). An existing phase shift between the two pressure readings at low frequencies contradicts the theory of a fast wave propagating down the perilymph-filled cochlea duct [28]. Considering an approximately 70 mm-long channel between the oval and round window [29] and a propagation speed similar to the speed of sound in water, a phase shift of less than 18\({}^{\circ}\) at 1 kHz should arise between the pressure in the basal part of the SV and ST. Moreover, the phase shift of 180\({}^{\circ}\) that was measured in the present study between the motion of the oval and round window at low frequencies (cf. Figure 3B) stipulates a vanishing phase shift in the simultaneous ICSP readings at low frequencies. Thus far, it is not clear why for certain HTBs the phase shift deviates from the expected range. We do not assume a measurement (system)-related problem, mainly because a similar phase shift in the pressure readings can also be observed in the study of [PERSON] et al. (2009), where another pressure measurement system was used. We believe, rather, that the hydrophone signals might be distorted by mechanical vibrations of the sample, which are triggered by the acoustic stimulation and inducing pressure fluctuations in the cochlear fluid.