Distance Amplitude Correction addresses how echo height changes with depth by applying a depth-based gain to the received signal. In ultrasonic testing, deeper reflectors naturally produce smaller echoes because the sound loses energy as it travels through the material (attenuation) and because the beam spreads over distance. If you relied on raw echo amplitudes, deeper flaws would look weaker simply due to their location, making it hard to compare flaw sizes across depths. DAC uses a compensation curve that increases the displayed amplitude of echoes as depth increases, so a reflector of a given size produces a more uniform, comparable height on the display regardless of how deep it is. This normalization helps you detect and size flaws consistently from near to far, rather than being biased by depth. The other terms describe related ideas but not the specific corrective technique. Attenuation is the general loss of signal strength with distance, which causes the depth-related variation DAC corrects. Transfer and interpretation refer to different parts of data handling or evaluative steps and do not denote the depth-based amplitude correction itself.

When a finite transmitting aperture emits ultrasound, diffraction from the aperture determines how wide the main beam is. The angular width of that beam is roughly proportional to the wavelength divided by the aperture diameter. For a given crystal diameter, increasing frequency shortens the wavelength (since wavelength is inversely related to frequency in the same medium). A shorter wavelength diffracts less, so the main beam becomes narrower. Hence the angle of beam divergence decreases as frequency increases.

When a ultrasonic wave encounters an interface between two materials with different acoustic impedances, part of the energy is reflected back into the first medium and part is transmitted into the second. The impedance mismatch causes this split because the boundary conditions require the pressure and particle velocity to satisfy continuity across the boundary, so the wave must partly reflect and partly continue forward. Energy conservation (ignoring losses) means the incident energy equals the sum of the reflected and transmitted energies. At normal incidence, the fraction reflected is R = ((Z2 − Z1)/(Z2 + Z1))^2 and the transmitted fraction is T = 1 − R (or T = 4Z1Z2/(Z1 + Z2)^2). If the impedances match, all energy transmits; with mismatch, some energy is reflected in addition to what is transmitted. In real materials, a small portion can also be absorbed at the boundary, but the fundamental outcome is that energy divides into transmitted and reflected waves.

The speed of ultrasonic waves through a material is called velocity. It represents how fast the wavefront travels, typically measured in meters per second, and is essential for time-of-flight calculations used in flaw sizing and material characterization. The other terms refer to how often pulses are emitted (pulse repetition rate) or are not standard descriptors of wave speed, so they don’t describe the actual travel speed.