Fluctuations in probe intensity or frequency can lead to errors in the interpretation of the signal which are more significant than the errors induced by the detector. In general, the performance of such systems may be limited by either component. All sensors which operate by probing the external environment with signals are governed by some basic rules of wave propagation. In general, the velocity of a traveling wave is dependent on the medium through which it travels. For example, light waves move more slowly through any medium whose dielectric constant is greater than 1.
Sound waves travel more slowly through air than through any other medium.
Given these velocities, it is easy to calculate the time intervals that are needed to measure distances to target. For practical applications, the upper limit generally comes about due to the finite generating capabilities of sound wave generators. Once generated, sound waves tend to spread out as they propagate, generally filling the space in front of the transmitter. Because of this spread, the intensity of the sound waves decreases as the distance from the target increases. To be specific, the total energy in any wavefront cannot increase with time, so as the surface area of the wavefront increases, the intensity must decrease.