Propagation Speed

Propagation speed can be correctly recorded with which of the following units? (More than one may be correct.) A. miles per hour B. mm/msec C. km/sec D. inches per year

A, B, C, and D. Speed is recorded as a distance per unit of time, such as miles per hour, or feet/sec. Any relationship of distance per time is an acceptable answer.

The characteristics of four media are described below. Which of the media has the slowest propagation speed? A. high density and high elasticity B. low density and high stiffness C. low stiffness and low density D. low compressibility and low stiffness

A. Speed is determined by density and stiffness of a medium (the opposite of stiffness is elasticity). When a medium has high elasticity, its speed is slow. A medium with a high density also has a slow speed. The medium described in choice A has the lowest propagation speed.

When the elasticity of a medium is high, the ____ is high. A. stiffness B. propagation speed C. compressibility D. reflectivity

C. Both elasticity and compressibility describe the ability of a medium to reduce its volume when affected by a force. High elasticity means the same as high compressibility.

What is the best estimate of the distance that sound can travel in soft tissue in one second? A. one yard B. one hundred yards C. one mile D. ten miles

C. Sound travels at a speed of 1,540 meters per second in soft tissue. This is approximately one mile per second.

What is the best estimate for the speed of sound in tendon? A. 1,000 m/s B. 1,540 m/s C. 1,754 m/s D. 5,400 m/s

C. Sound travels slightly faster in tendon than in soft tissue.

T/F: An ultrasound pulse propagates from soft tissue through a mass. Sound's propagation speed in the mass is 1,575 m/s. The frequency of the wave increases as it travels through the mass.

FALSE. A wave's frequency is determined by the sound source only, and is unaffected by the tissues through which it travels.

T/F: Three sound waves with identical frequencies of 3 MHz have powers of 2 mW, 5 mW, and 15 mW. They all travel through three media--wood, brick, and fat--with identical thicknesses of 5 cm. The waves travel through all three media at the same speed since they have identical frequencies.

FALSE. The determinant of the propagation speed in a medium is the medium only. The fact that these three waves have the same frequency is irrelevant. The waves are traveling in media with vastly different densities and stiffnesses and as a result will travel at different speeds through the wood, brick, and fat.

T/F: Three sound waves with identical frequencies of 3 MHz have powers of 2 mW, 5 mW, and 15 mW. They all travel through three media--wood, brick, and fat--with identical thicknesses of 5 cm. The sound waves travel through all three media at different speeds because the waves have different powers.

FALSE. The fact that the waves have dissimilar powers is not the reason they travel at different speeds. The waves travel at different speeds because of the dissimilarities in the densities and stiffnesses of the media through which they travel.

T/F: An ultrasound pulse propagates from soft tissue through a mass. Sound's propagation speed in the mass is 1,575 m/s. The period of the sound wave decreases as it travels through the mass.

FALSE. The period of a wave is determined by the sound source only, and is unaffected by the tissues through which it travels. Recall that period and frequency are reciprocals. Since the frequency remained unchanged, so too must the period.

T/F: Soft tissue is an imaginary construct that actually does not exist.

TRUE. Soft tissue is an imaginary structure with characteristics that represent an "average" of body tissues, including muscle, blood, kidney, and spleen. It is used as an approximation.

T/F: Three sound waves with identical frequencies of 3 MHz have powers of 2 mW, 5 mW, and 15 mW. They all travel through three media--wood, brick, and fat--with identical thicknesses of 5 cm. The waves travel through all three media at different speeds because the media are different.

TRUE. This is correct! Finally, we have a statement that attributes variations in propagation speeds to the differences in the media. Again, the characteristics of the media alone determine the speed that sound can travel through it.

T/F: An ultrasound pulse propagates from soft tissue through a mass. Sound's propagation speed in the mass is 1,575 m/s. The wavelength increases while the wave travels through the mass.

TRUE. Wavelength is dependent on both the sound source and the medium. The speed of sound in soft tissue is 1,540 m/sec, and through the mass is 1,575 m/sec. As a wave travels in a faster medium, its wavelength increases.

T/F: Shorter wavelength sound creates higher quality images with greater detail.

TRUE. Wavelength plays a very important role in image quality. Shorter wavelength (higher frequency sound) produces higher quality images with greater detail.

The speed of sound traveling through bone is ____ soft tissue. A. higher than B. lower than C. equal to D. cannot be determined

A. The propagation speed of sound in bone is higher than in soft tissue. Sound travels at a speed of 3 to 5 km/sec in bone. This is two to three times faster than soft tissue.

If sound travels at exactly 1,540 m/sec in a particular medium, then the medium ____. A. must be soft tissue B. may be soft tissue C. cannot be soft tissue

B. Sound waves travel exactly 1,540 m/sec in soft tissue. However, other media have the same propagation speed as that of soft tissue. The medium in question could be soft tissue, or it could be something else.

The speed at which a wave travels through a medium is determined by: A. the sound wave's properties only B. the medium's properties only C. properties of both wave and medium D. none of the above

B. Speed is determined by the characteristics of the medium only. The characteristics of the wave do not affect its speed. All sound waves of any frequency, period, intensity, and power travel at the same speed in a particular medium.

What is the speed of sound in air? A. 1,540 m/s B. 330 m/s C. 100 m/s D. 3,010 m/s

B. The speed of sound in air is 330 m/s, substantially lower than the speed of sound in soft tissue.

Compared to soft tissue, the speed of an acoustic wave through lung tissue is: A. faster B. slower C. equal D. cannot be determined

B. The speed of sound in lung tissue is slower than in soft tissue. Sound travels at speeds in the range of 0.5 to 1.0 km/sec in lung tissue.

The propagation speeds of ultrasound waves in muscle, liver, kidney, and blood are ____. A. exactly the same B. very similar to each other C. vastly different

B. The speed of sound in these media are less than 5% different from each other. The characteristics of muscle, kidney, liver, and blood that determine the wave's speed in the media are quite similar.

If sound doesn't travel at 1,540 m/sec in a medium, then the medium ____. A. must be soft tissue B. may be soft tissue C. cannot be soft tissue

C. Sound waves travel exactly 1,540 m/sec in soft tissue. Since the propagation speed of sound in this medium is not 1,540 m/sec, the medium cannot be soft tissue.

Compressibility describes a characteristic of a medium. Which two terms describe the same property as compressibility? A. density and stiffness B. density and conductance C. stiffness and elasticity D. elastance and impedance

C. The compressibility of a medium describes its ability to reduce its volume when a force is applied to it. For example, a marshmallow is compressible because when it is squeezed, it gets smaller. Elasticity has the same meaning as compressibility. Stiffness describes this same characteristic; however, stiffness is the opposite of compressibility.

The propagation speed of continuous wave ultrasound is 1.8 kilometers per second. The wave is then pulsed with a duty factor of 50%. What is the new propagation speed? A. 0.5 km/sec B. 0.9 km/sec C. 1.8 km/sec D. 3.6 km/sec E. cannot be determined

C. The speed of sound in a medium is determined only by the medium. There is no difference in sound's speed whether the wave is continuous or pulsed. Thus, the new and old speeds will be identical, 1.8 km/sec.

Two sound waves with frequencies of 5 and 3 MHz travel to a depth of 8 cm in a medium and then reflect back to the surface of the body. Which acoustic wave arrives first at the surface of the body? A. the 5 MHz wave B. the 3 MHz wave C. neither D. cannot be determined

C. They both travel at the same speed and reach the surface of the body at exactly the same time. All sound waves, regardless of their features, travel at the same speed in a specific medium. The fact that these waves have different frequencies is irrelevant.

As a general rule, which of the following lists media in increasing order of propagation speeds? A. gas, solid, liquid B. liquid, solid, gas C. solid, liquid, gas D. gas, liquid, solid

D. Generally, sound travels slowest in gases, faster in liquids, and fastest in solids.

What is the propagation speed of a 5 megahertz sound wave in soft tissue? A. 1,450 meters per sec B. 1,540 km/sec C. 1.54 m/s D. 1.54 mm/us

D. The speed of any sound wave moving through soft tissue, regardless of frequency, is 1.54 mm/usec, 1,540 m/sec, or 1.54 km/sec.

What two properties establish sound's propagation speed in a given medium? A. elasticity and stiffness B. stiffness and impedance C. conductance and density D. density and stiffness

D. The two properties of the medium that affect sound's propagation speed are density and stiffness.

T/F: An ultrasound pulse propagates from soft tissue through a mass. Sound's propagation speed in the mass is 1,575 m/s. The power in the wave increases as it travels through the mass.

FALSE. Typically, the power of a beam diminishes as it travels. This is a result of attenuation. If anything, the wave's power diminishes as it propagates through the mass.

T/F: The wavelength of an acoustic wave is shorter when it is pulsed rather than continuous.

FALSE. Wavelength reports the length of one cycle in a wave. The wavelength does not change based on whether the cycle is part of a puulsed or continuous wave.