To evaluate the safety of diagnostic ultrasound, a precise and practical technique for measuring instantaneous acoustic pressure using the frequency response of hydrophone sensitivity has been investigated. We previously confirmed that the extrapolation of the frequency response using constants that are equal to extremes of the frequency range of certificated hydrophone sensitivities is generally effective when this frequency range is narrower than that from 0.5 to 8 times the center frequency of the measured ultrasound. However, this method is not always effective for hydrophones with large frequency response fluctuations. Here, we study whether the effectiveness of the extrapolation could be improved by using numerical simulation that includes assumptions about the materials and construction of the hydrophone and present the prospect that diagnostic ultrasound can be precisely measured using certificated sensitivity even if the upper frequency of certificated sensitivity is only up to twice the center frequency of the diagnostic ultrasound.