Final answer:
The frequency of a 2.3 MHz ultrasound wave remains unchanged as it passes through human tissues. Frequency is a property determined by the wave's source and is not affected by the medium through which the wave travels.
Step-by-step explanation:
The property of a 2.3 MHz ultrasound wave that remains unchanged as it passes through human tissues is the frequency. While the wavelength, amplitude, and velocity of the wave can change depending on the medium's properties, the frequency is a characteristic of the source and does not change as the wave enters different media. This is analogous to light, where the frequency remains constant as light moves from air into water, though its velocity and wavelength change. Similarly, when speaking more loudly, the amplitude of the sound waves increases, but not the frequency. The speed of sound depends on the properties of the medium, not on the frequency or wavelength of the sound wave.