Chapter 6: Interaction of Sound and Media

A reduction in the intensity of a sound beam to one-half of its original value is _____ dB.

-3dB

A reduction of the intensity of a sound beam to one-quarter of its original value is ______dB.

-6dB

A sound waves strikes a boundary with normal incidence. The impedances of the two media are identical. What percentage of the sound wave is refracted?

0% Remember, refraction cannot occur with normal incidence.

Sound traveling from bone to soft tissue. The impedances of the media differ significantly, and 90% of the beam's intensity is reflected. What % of the intensity is transmitted?

100 % of the energy must be accounted for. If 90% is reflected, 10% must be transmitted

A wave's intensity is 2mW/cm^2. There is a change of +9dB. What is the final intensity?

16mW/cm^2

The impedance of medium 1 is 8 rayls. The propagation speed is 1,450 m/s. The impedance of medium 2 is 6 rayls and the speed is 1.855 km/s. A sound beam strikes the boundary b/w the media and both partially transmitted and reflected. The angle of the incident beam is 30 degrees. What is the reflection angle?

30 degrees. This question is quite simple. The angle of reflection equals the angle of incidence.

Sound is traveling in a medium and strikes a boundary with normal incidence. If 63% of the wave's intensity is reflected back toward the transducer, what % is transmitted?

37% of the intensity will be transmitted. Conservation of energy occurs at a boundary, and as a result, the sum of the relected and transmitted intensities must equal 100%. 63% + 37% = 100%

A sound wave with an intensity of 50 W/cm², strikes a boundary and is totally reflected. What is the reflected intensity?

50 W/cm²

What is better to use while examining a carotid artery, a 7.5 or 3.0 MHz transducer?

7.5MHz because the carotid artery is a superficial structure.

A pulse of ultrasound propagates in soft tissue, such as liver. The pulse strikes a boundary with a different soft tissue at normal incidence. What portion of the intensity is reflected back toward the transducer? Why?

A very small % of sound, typically less than 1%, is reflected at a boundary b/w two soft tissues. The impedances of two soft tissues are similar, and the difference in impedance directly determines the intensity reflection coefficient. Very little reflection occurs when the impedances have similar, but not identical, values.

What processes occur as the ultrasound passes through all media? What are the units of this process?

Attenuation (scattering, absorption, and reflection) Units: dB

Which of the following terms does not belong with the others? A. orthogonal B. oblique C. normal D. perpendicular

B. oblique

Sound that is traveling in Jell-O passes through an interface at 90 degrees and continues to travel in whipped cream. The impedance of Jell-O and whipped cream are nearly identical. What percentage of the intensity is transmitted? A. 2% B. 25% C. 78% D. 99%

D. The best choice is 99%. If the impedances of Jell-o and whipped cream are nearly identical, only a very small % of the sound beam's intensity will reflect. The remainder, of course will transmit.

True or False. We need one intensity to calculate decibels?

False

A sound pulse travels in Medium 1 and strikes and interface with another tissue, Medium 2, at 30 degrees. The angle of transmission is 10 degrees. From these facts alone what can be said about * the speed of sound in medium 1 * the speed of sound in medium 2 * the difference b/w the speeds of Media 1 and 2?

Given only this info, we can say nothing about the speed of sound in medium 1 or medium 2. However, b/c the beam refracted significantly ( there was a 20 degrees change in refraction), the speeds of these two media are very different. Refraction depends on the difference in the speeds of sound in the two media, not the actual speeds.

A sound wave with an intensity of 50 W/cm², strikes a boundary and is totally reflected. What is the intensity reflection coefficient?

IRC is 100%

What is the relationship between ultrasound frequency and the attenuation coefficient in soft tissue?

In soft tissue, the attenuation coefficient in dB per centimeter is approximately one half of the ultrasonic frequency in MHz

Two media A and B have the same densities. The speed of sound in medium A is 10% higher than in medium B. Which medium has the higher acoustic impedance?

Medium A's acoustic impedance is higher than medium B's. Recall that impedance equals speed x density. Since both have identical densities and medium A's speed is 10% higher, then medium A's impedance is 10 % higher.

A sound pulse travels in medium 1 and strikes an interface with another tissue, medium 2 at 30 degrees. The angle of transmission is 10 degrees. In which medium is the impedance higher?

Neither. Refraction of sound at a boundary is unrelated to the impedances of the media. Therefore, with the info provided, it cannot be determined which material has the greater impedance Refraction is affected by the speed of sound in the media

A pulse of ultrasound is propagating in bone and strikes an interface with soft tissue at 90 degrees. A giant reflection is created. * from these facts alone, what can be said about the impedance of bone? * what can be said about the impedance of soft tissue? * what can be said about the differences b/w impedance of bone and soft tissue?

Nothing can be stated about the impedance of bone or soft tissue based of the information given. However, because a large reflection was created, the impedances of these two media must be dissimilar. Reflections with normal incidence are created based on the differences in the impedances, not on the actual values of the impedances.

What happens at the boundary b/w media 3 and 4? Why?

Reflection may occur. If transmission does occur, the sound beam will refract b/c there are different propagation speeds and oblique incidence.

A sound pulse travels in medium 1 and strikes an interface with another tissue, medium 2, at 30 degrees. The angle of transmission is 10 degrees. In which medium does sound travel slowest?

Sound travels slower in medium 2. when the angle of transmission is less than the angle of incidence, sound travels slower in the second medium.

What does the 3.5 dB/cm represent?

The attenuation coefficient of the sound in the media

What does the 100 mW/cm^2 represent?

The incident intensity of the sound beam

A pulse of ultrasound propagates in soft tissue, such as liver. The pulse strikes a soft-tissue interface with oblique incidence. Some of the sound energy is transmitted. To what extent is the transmitted beam refracted?

The transmitted beam undergoes little to no refraction. A transmitted beam is refracted when the incidence is oblique and the propagation speeds are different. Because the tissues on either side of the boundary are both "soft tissues" their speeds are nearly identical and little or no refraction occurs.

Sound travels in a medium and orthogonally strikes a boundary with a different medium. Although sound waves traveling in the media have vastly different speeds, there is no refraction. How can this be?

With normal incidence, refraction cannot occur. Refraction occur only when there are different speeds and oblique incidence. Both conditions must be met. In this example, the incidence is normal-- no refraction can occur

If the media are soft tissue, what is an estimate of the ultrasound frequency?

about 7 MHz; the attenuation coefficient multiplied by 2 approximate the frequency (3.5 x 2 =7)

Name the three components of attenuation

absorption, reflection, and scattering

The Incidence b/w the sound wave and the boundary b/w media 1 and 2 is normal. What happens at the boundary b/w media 1 and 2? Why?

both reflection and transmission occur. There are normal incidence and different acoustic impedances.

What are the units of attenuation?

decibels (dB)

acoustic impedance = ____ x _____

density (kg/m cubed) x propagation speed (m/s)

What are the units of the half- value layer thickness?

distance; centimeters

Every 3 dB change means that the intensity will?

double

Attenuation in air is _______ attenuation in soft tissue.

greater than

Attenuation in bone is _________ attenuation in soft tissue.

greater than

Attenuation in lung tissue is _______ attenuation in soft tissue.

greater than

What property has units of rayls? how is determined?

impedance is calculated, not measured. impedance = density x speed

Every 10 dB changes mean that the intensity will?

increase ten times

As frequency decreases, depth of penetration

increases

As the path length increases, the attenuation of ultrasound in soft tissue _______.

increases

dB is a mathematical representation with a ________ scale.

logarithmic and relative

What type of incidence is there b/w media 3 and 4?

oblique

-10 dB means that the intensity is reduced to ______ of its original value.

one-tenth

Impedance is associated with?

only the medium

If the initial intensity of a sound beam is less than the final intensity, the gain in dB is____?

positive the beam's intensity is increasing

If the final intensity of a sound beams is more than the initial intensity, then the gain is dB is ___?

positive. the beam's intensity is increasing

Impedance is important in ____ at boundaries.

reflections

As path length increases, the half boundary layer __________.

remains the same

As the path length increases, the attenuation coefficient of ultrasound in soft tissue _______.

remains the same

The Incidence b/w the sound wave and the boundary b/w media 2 and 3 is normal. What happens at the boundary b/w media 2 and 3? Why?

transmission only. The impedances of the media are the same.

True or False. In a given medium, attenuation is unrelated to the speed of sound.

true

Sound strikes a boundary between two media orthogonally. Although the media are very different, no reflection is created. How can this be?

with normal incidence, reflection occur only when the impedances of the two media at the interface are different. Two different media can have the same impedances, and when that happens, no reflection will be created.