Ringing means the rise and decay time before and after the transducer reaches the maximum amplitude. Expressed as the mechanical Q of the transducer, which is the number of cycles it takes to get up to 90% of maximum amplitude or down to 10% above zero amplitude.

Sound and ultrasound waves consist of a mechanical disturbance of a medium such as air. The disturbance passes through the medium at a fixed speed causing vibration. The rate at which the particles vibrate is the frequency, measured in cycles per second or Hertz (Hz).
The pressure of sound is reported on a logarithmic scale called sound-pressure level, expressed in decibel (dB) referenced to the weakest audible 1 000 Hz sound pressure of 2*10^-5 Pascal (20 mP). Sound level meters contain filters that simulate the ear's frequency response. The most commonly used filter provides what is called 'A' weighting, with the letter 'A' appended to the dB units, i.e. dBA.
Sound becomes inaudible to the human ear above about 20 kHz and is then known as ultrasound. Diagnostic imaging uses much higher frequencies, in the order of MHz.
See also Spatial Peak Intensity.

Sound frequencies:

(TTE) Transthoracic echocardiography is a common type of cardiac ultrasound and is used to evaluate the size and function of the heart.

Indications:
TTE requires no sedation or special patient preparation. After the application of ECG electrodes and ultrasound couplant, the probe is maneuvered over the chest in the area adjacent to the breast bone and under the left breast, to provide the different views of the heart. Usually the images will be obtained lying relaxed on the left side. Other views can be sampled lying on the back with the knees bent, or sitting in an upright position.

See also Bicycle Stress Echocardiography and Transesophageal Echocardiography.

The traveling ultrasonic wave causes a low-level ultrasound radiation force when this energy is absorbed in tissues (absorbed dose). This force produces a pressure in the direction of the beam and away from the transducer. It should not be confused with the oscillatory pressure of the ultrasound wave itself. The pressure that results and the pressure gradient across the beam are very low, even for intensities at the higher end of the range of diagnostic ultrasound. Mechanical effects like radiation forces lead to stress at tissue interfaces. The effect of the force is manifest in volumes of fluid where streaming can occur with motion within the fluid. The fluid velocities which result are low and are unlikely to cause damage.
The effects of ultrasound radiation force (also called Bjerknes Forces) were first reported in 1906 by C. A. and V. F. K. Bjerknes, when they observed the attraction and repulsion of air bubbles in a sound field.
While incompressible objects do experience radiation forces, compressible objects driven at their resonant frequency experience far larger forces and can be observably displaced by low-amplitude ultrasound waves. A microbubble driven near its resonance frequency experiences a large net radiation force in the direction of ultrasound wave propagation. Ultrasound pulses of many cycles can deflect resonant microbubbles over distances on the order of millimeters.
In addition to primary radiation force, which acts in the direction of acoustic wave propagation, a secondary radiation force for which each individual bubble is a source and receptor causes the microspheres to attract or repel each other. The result of this secondary force is that a much larger concentration of microbubbles collects along a vessel wall than might otherwise occur.

See also Acoustically Active Lipospheres.

The waveform is the record of a signal that varies over time. A blood flow signal for example, usually varies periodically with the cardiac cycle.

See also Coded Excitation, and Pulse Volume Recording.