Ch.3 Describing Sound Waves

Hertz is another way to say

"per second"

Power (Typical Values) In clinical imaging, typical powers range from

-0.004 to 0.090 watts (4 to 90 milliwatts) -depending on the diagnostic ultrasound technique.

Intensity (typical values) In clinical imaging, intensity ranges from

-0.01 to 200 W/cm^2

Period (Typical Values) The typical value of period in diagnostic ultrasound is

-0.06 to 0.5 microseconds(𝝻s). This may also be written as : -6 X 10^-8 to 5 X 10^-7 seconds -0.00000006 to 0.0000005

Wavelength (Values) In clinical imaging, wavelength in soft tissues ranges from

-0.1 to 0.8 mm.

Amplitude (Typical Values) In clinical imaging, pressure amplitude ranges from

-1 million pascals (1 MPa) to 3 million pascals (3 MPa).

What other terms describe stiffness?

-Bulk modulus

Propagation of Speed (Typical Values)

-In the body, the speed of sound ranges from 500 m/s to 4000 m/s, depending on the tissue through which it is traveling.

How is intensity related to power?

-Mathematically, intensity is proportional to power. -Intensity ∝ power -See p. 32 for more examples

How is intensity related to amplitude?

-Mathematically, intensity is proportional to the wave's amplitude squared (the term "squared" means that a number is multiplied by itself. -Intensity ∝ amplitude^2 -See p. 32 for more examples

Propagation of Speed (Units) Speed is measured in units of

-Meters per second -mm/𝝻s, or any distance divided by time.

Period (Adjustable) It it adjustable?

-No. The sonographer cannot change the period while using a basic ultrasound system with a particular transducer.

Amplitude (Units) 1.) Amplitude can have units of any of the acoustic variables:

-Pressure - Pascals -Density - g/cm^3 -Particle motion - cm, inches, any distance

Power (Determined by) Just like amplitude, initial power is determined only by the

-Sound Source, the ultrasound system.

How does stiffness affect speed?

-Stiffness and speed are directly related. -As materials become stiffer, the speed of sound in the material increases.

Sound is classified based on

-The ability of humans to hear it.

What is the formula to calculate intensity?

-To calculate the intensity, divide the beam's power by the beam's cross-sectional area. intensity (W/cm^2) = Power (w)/area (cm^2)

Wavelength (Determined By)

-Wavelength is determined by both the source and the medium.

What is the difference between amplitude and peak-to-peak amplitude? Amplitude

-is measured from the middle, or undisturbed, value to the maximum value.

What is the difference between amplitude and peak-to-peak amplitude? Peak-to-peak amplitude

-is the difference between maximum and minimum values of an acoustic variable. There, peak-to-peak amplitude is twice the value of the amplitude.

Propagation Speed

-is the rate at which a sound wave travels through a medium.

Power

-is the rate of energy transfer or the rate at which work is performed. -Power, like amplitude, describe the "bigness" of a wave.

Wavelength (Units) Wavelength is measured in units of

-mm, meteres, or any other unit of length.

Frequency (Units) 1.) Frequency is reported in units of

-per second -1/second, hertz, or HZ.

Frequency (Determined By) Frequency of a sound wave is determined by the

-sound source only, not by the medium through which the sound is traveling.

Period (Determined By) Period is determined by the

-sound source only, not the medium.

Propagation of Speed (Determine By)

-speed is determined only by the medium through with the sound is traveling.

Mathematically, power is proportional to the

-wave amplitude squared -power ∝ amplitude^2 -The symbol ∝ means proportional to.

1,000,000 cycles/second =

1 MHz

1 cycle/second =

1 hertz

1,000 cycles/second =

1 kHz

1.) As frequency increases, 2.) As frequency decreases, 3.)If one of these parameter remains constant,

1.) As frequency increases, period decreases. 2.) As frequency decreases, period increases. 3.) If one of these parameters remains constant, then the other remains unchanged.

Both wavelength and period describe a single cycle in a sound wave. 1.) Wavelength refers to? 2.) Period refers to?

1.) Period refers to the length or distance of a single cycle. 2.) Period refers to the time that it takes to complete a single cycle.

Three "Bigness" Parameters Three parameters describe the size, magnitude, or strength of a sound wave:

1.) amplitude 2.) Power 3.) Intensity

It is more precise to think of hertz as a two-part process:

1.) hetz means events per second 2.) to which event are we referring? Read "More about Hertz" p. 24

Fill in the blank If you know whether the parameter is determined by the __________ __________ or the __________, you will be able to understand how these parameters relate to the physical principles of ultrasound physics.

1.) sound source 2.) medium

The source of a sound wave is the

1.)Ultrasound System 2.) Transducer

Frequency (Typical Values) In clinical imaging, frequency ranges from approximately

2 MHz to 15 MHz. -Another way to say tis is 2 million to 15 million per second.

The values of how many parameters are required to completely characterize a sound wave

7

Is the "bigness" of a wave. It is the difference between the maximum value and the average or undisturbed value of an acoustic variable. It is also the difference between the minimum value and the average value of the acoustic variable.

Amplitude

What is the relationship between wavelength and frequency?

As long as wave remains in one medium, wavelength and frequency are inversely related. -

Describes the relative weight of a material

Density

How does density affect speed?

Density and speed are inversely related

What are the opposite of stiff?

Elasticity and compressibility

Amplitude (Determined By) True or False Amplitude is determined only by the sound source, the ultrasound system. However, amplitude increases as sound propagates through the body.

False - Amplitude is determined only by the sound source, the ultrasound system. However, amplitude decreases as sound propagates through the body.

True or False It is not important to know the typical values for the parameters.

False - In ultrasound physics we should know the common values.

True or False The quantity, or level, of some of some of the parameters are established by the ultrasound system. The sonographer can adjust the level of all parameters.

False-The quantity, or level, of some of some of the parameters are established by the ultrasound system. The sonographer can adjust the level of some of these parameters, whereas others cannot be changed.

Why is frequency important in diagnostic ultrasound?

Frequency is important in sonography because it affects penetration and image quality.

What is the wavelength of 1 MHz sound in soft tissue?

In soft tissue, sound with a frequency of 1 MHz has a wavelength of 1.54 mm. -See p.35 for more examples

If the frequency of a sound wave is less than 20 Hz, it is below the threshold of human hearing and cannot be heard. That is, the sound is inaudible. Sound waves with frequencies this low are defined as

Infrasonic or infrasound

The concentration of energy in a sound beam is known as?

Intensity

Intensity (Determined By) How is intensity determined

Just like amplitude and power, initial intensity is determined only by the sound source, the ultrasound system.

When you think of power, think of a

Light Bulb

Propagation Speed (Adjustable)

No, speed of sound cannot be changed by the sonographer. Speed Changes only when the wave travels from one medium to a different medium.

Wavelength (Adjustable)

No, wavelength cannot be changed by the sonographer when using a basic ultrasound transducer.

Frequency (Adjustable) Is frequency adjustable?

No. The sonographer cannot change the frequency while using a basic ultrasound system and transducer.

Describe features of a sound wave.

Parameters

What is the relationship between frequency and period?

Period and frequency are inversely related to each other.

Amplitude, power, and intensity are three parameters that describe the magnitude or strength of a wave. These parameters tend to be directly related; therefore, when intensity increases....

Power and amplitude also increase

Describes the ability of an object to resist compression.

Stiffness

The lower the frequency

The longer the wavelength

What is the speed of sound in soft tissue?

The speed of sound in soft tissue is 1,540 m/s. -1.54 mm/𝝻s or 1.54 km/s. -This value is approximately 1 mile per second.

Period (Units) Period is reported in units of

Time -Such as microseconds (𝝻s), seconds, hours, or days.

Amplitude (Determined By) True or False The rate at which amplitude decreases as sound propagates depends on the characteristics of both the sound wave and the medium.

True

True or False All sound, regardless of the frequency, travels at the same speed through any specific medium. This means that sound with a frequency of 5 MHz and sound with a frequency of 3 MHz travel at the same propagation speed in the same medium.

True

True or False As materials become more dense (heavier), the speed of sound in the material decreases. Sound travels faster in media with low density.

True

True or False Both amplitude and power are parameters that describe the size, or magnitude, of a wave. When power increases, so does amplitude. Similarly, when power decreases, amplitude decreases.

True

True or False Intensity changes as sound propagates through the body. The rate at which intensity changes as sound propagates depends on the characteristics of both the sound wave and the medium.

True

True or False It is important to know whether the sonographer has the ability to change the value of a parameter while using a particular transducer. In other words, is there a knob or control on the system that will allow you to alter the parameter's magnitude?

True

True or False Power decreases as sound propagates through the body.

True

True or False Speed is not affected by the nature of the sound wave.

True

What characteristic of a medium determine the speed of sound in that medium?

Two characteristics of a medium affect the speed of sound: -stiffness -density

Sound with frequencies of 20,000 Hz (20 kHz) or greater cannot be heard by humans, they are called

Ultrasonic or ultrasound.

Power (Units) Power has units of

Watts.

Intensity (units) The units for intensity are

Watts/cm^2 (watts from power and cm^2 from beam area)

As frequency increases

Wavelength decreases

Amplitude (Adjustable) Is amplitude adjustable?

Yes, a control on basic ultrasound system allows the sonographer to alter initial amplitude of a wave.

Power (Adjustable) Is Power adjustable?

Yes, initial power, like amplitude, can be changed. A control on ultrasound systems allows the sonographer to alter the initial power of a wave.

Intensity (Adjustable) Is intensity adjustable?

Yes. Initial intensity, like power and amplitude, can be changed. A control on a basic ultrasound system allow the sonographer to alter the initial intensity of a wave.

Humans can hear sound with frequencies between 20 Hz and 20, 000 Hz. This sound is called

audible

Wavelength

is the distance or length of one complete cycle.

Frequency

is the number of particular events that occur in a specific duration of time. -In diagnostic ultrasound, the frequency of a wave is described as the number of cycles that occurs in one second.

Period

is the time it takes a wave to vibrate a single cycle, or the time from the start of one cycle to the start of the next cycle.

Other parameters are determined by the tissue through which the sound is traveling. The tissue is also called the

medium.

Period and frequency have an even more special relationship that is called

reciprocal. -When two reciprocal parameters are multiplied together, the result is 1 -Period X Frequency = 1

The average or typical biologic medium is called

soft tissue.