Acoustic Waves and Properties Flashcards

An ultrasound wave travels through two different media. If the propagation speed in the second medium is lower than in the first, what happens to the wavelength of the sound wave?

Answer: It decreases.

The relationship between propagation speed (c), frequency (f), and wavelength (λ) is given by the formula c = f x λ. The frequency of the sound wave is determined by the transducer and does not change as it passes through different media. Therefore, if the propagation speed (c) decreases, the wavelength (λ) must also decrease to maintain the equality. [2, 22]

A sonographer observes that the intensity of the ultrasound beam has decreased by 3 dB. This corresponds to what change in intensity?

Answer: A twofold decrease

The decibel (dB) scale is logarithmic. A change of -3 dB represents a halving, or a twofold decrease, in intensity. Conversely, a +3 dB change represents a doubling of intensity. A -10 dB change corresponds to a tenfold decrease. [12, 16]

Which of the following properties of a medium has the most significant influence on the propagation speed of an acoustic wave?

Answer: Stiffness

Propagation speed is primarily determined by the stiffness (or bulk modulus) and density of the medium. Of these two, stiffness has the greater influence. Sound travels faster in stiffer materials. While density does affect speed (higher density generally leads to slower speed), its effect is less pronounced than that of stiffness. For example, bone is much denser than soft tissue, but it is also significantly stiffer, resulting in a much higher propagation speed. [11, 13]

Acoustic impedance is calculated by multiplying which two properties of a medium?

Answer: Density and Propagation Speed

Acoustic impedance (Z) is a fundamental property of a medium that determines how much sound is reflected at an interface. It is calculated as the product of the medium's density (ρ) and its propagation speed (c). The formula is Z = ρc. [1, 3, 7, 9]

While scanning a patient's liver, you switch from a 3 MHz transducer to a 6 MHz transducer. Which of the following is the most likely consequence of this change?

Answer: Increased attenuation of the ultrasound beam

Attenuation, the weakening of the sound beam as it travels through tissue, is directly related to frequency. Higher frequency waves are attenuated more than lower frequency waves. Therefore, switching from a 3 MHz to a 6 MHz transducer will increase the attenuation rate. This leads to decreased penetration but improved axial resolution. Propagation speed is a property of the medium (liver tissue) and is not affected by the transducer frequency. [4, 5, 17]

The average propagation speed of sound in soft tissue, which is assumed by ultrasound systems for depth calculations, is:

Answer: 1540 m/s

Ultrasound systems are calibrated to an assumed average propagation speed of sound in soft tissue, which is 1540 m/s. This value is used to accurately place returning echoes at the correct depth. While the actual speed varies slightly among different soft tissues (e.g., fat is ~1450 m/s, blood is ~1570 m/s), 1540 m/s is the standardized average. 330 m/s is the speed in air, and 4080 m/s is the speed in bone. [2, 3, 4, 6, 13]