Ultrasound uses high frequency sound waves which is inaudible to humans. Audible sound wave is between 20 - 20,000 Hz. Pulses of ultrasound is between 1 and 30 M Hz.  

Sound waves do not travel in vacuum. Instead, the propagation of sound wave is made through a particular medium. Speed of sound wave is depends on both the density and compressibility of that particular medium.

Different soft tissues in human body have different speed of sound.  

Because of the difference in speed of sound in various tissue, ultrasound system has made as assumption that all the soft tissue has a single figure of speed of sound. Therefore an average speed of sound for soft tissue is 1540 m/sec.

Pulse Echo Technique
Ultrasound pulses are introduced into the body and when these pulses meet interfaces in the body, these pulses return back to the transducer as echoes. Therefore echoes are representation of interfaces in the body. Echoes go through various processes before it is display on the screen. 

Images that are display are represented in greyscale. Structures that appear white, represents strong reflector, while structures that appear black represent poor reflector.   

Sound Waves Parameters
Sound Waves are describe in parameters such as period, intensity, wavelength, velocity, phase, power, amplitude, frequency. 

Period - Time taken for a cycle to occur. Period is inverse of frequency. As the frequency increases, the period decreases. Conversely, as the frequency decreases, the period increases.                                     

                                         T  = 1 / f                                            T - period,  f - frequency

Power - rate of energy being transfer. Power is proportional to the wave amplitude squared.                     

                                      Power = amplitude²                        Unit - Watts, W       

Intensity - define as power per unit area 

                                       I    =    Power (W) / Area (cm²)      Unit - W/cm²

Wavelength - Distance of a cycle. It is determined by the frequency of a wave and the speed of propagation in the medium.

                                              wavelength (λ ) = propagation speed (c) /  frequency (f)     

Frequency - Number of cycles per second.
                       Frequency is inverse of period
                       1 Hertz = 1 cycle per second  

                                       f    =       1 / T                                   f- frequency,  T- period 

Amplitude - Strength of a sound wave