Because the speed of sound depends only on the state of the gas, some interesting physical phenomena occur when a sound source moves through a uniform gas. You can study some of these phenomena by using the interactive sound wave simulator. As the source moves it continues to generate sound waves which move at the speed of sound. Since the source is moving slower than the speed of sound, the waves move out away from the source. Upstream (in the direction of the motion), the waves bunch up and the wavelength decreases. Downstream, the waves spread out and the wavelength increases. The sound that our ear detects will change in pitch as the object passes. This change in pitch is called a doppler effect . There are equations that describe the doppler effect. As the moving source approaches our ear, the wavelength is shorter, the frequency is higher and we hear a higher pitch. If we call the approaching frequency fa , the speed of sound a , the velocity of the approaching souce u , and the frequency of the sound at the source f , then
Although TCD is not so accurate due to relative velocity of blood flow, but it is still useful for diagnosis of arterial occlusions in patients with acute ischemice stroke, especially for middle cerebral artery. A research has been done to compare Power Motion Doppler of TCD (PMD-TCD) with CT angiography (CTA), both are valid, but PMD-TCD accuracy is not higher than 85 percent. The advantages of PMD-TCD is portable, so can be used in bed side or in emergency room, no radiation as CTA, so can be repeated, if necessary for monitoring and less expensive than CTA or Magnetic Resonance Angiography. 
Uncalibrated pulse pressure devices utilise an algorithm that is based on the principle that pulse pressure is proportional to Stroke Volume (SV). The algorithm uses statistical analysis of the arterial pressure wave to generate an estimate of stroke volume. These systems use a multivariate polynomial equation that continuously quantifies arterial compliance and vascular resistance. By analysing the shape of the arterial pressure waveform, the effect of vascular tone is assessed allowing calculation of SV and Cardiac Output is then derived.