The rate at which intensity changes as sound propagates depends on which two factors?

• A. Frequency and speed of sound
• B. The position of the operator and the patient
• C. Characteristics of the medium and shape of the sound beam
• D. Depth of the tissue and temperature

Answer: (C)

The rate at which ultrasound intensity changes as it propagates is governed by two main factors: how the medium absorbs and scatters energy, and how the beam’s shape causes energy to spread. The medium’s characteristics determine how much energy is lost per unit distance through attenuation (absorption and scattering), which depends on tissue type and frequency. At the same time, the beam’s geometry—whether it stays tight and focused or naturally diverges—controls how the energy is distributed over an increasing area as it travels. A focused, narrow beam maintains higher intensity over longer distances, while a broad or unfocused beam causes faster geometric spreading and a quicker drop in intensity. The other options don’t capture both the energy-loss properties of the medium and the way the beam spreads, which together set the rate of intensity change.