Chapter 2 & 3 Flashcards
Sound Waves
- All waves carry energy from one location to another.
- In ultrasonography, sound pulses must travel through biologic tissue or media.
- Sound waves are longitudinal
Definition of sound
Sound is a mechanical wave in which particles in the medium move.
- Sound cannot travel through a vacuum; it must travel through a medium where molecules are alternately compressed and rarefied.
- Sound travels in a straight line
Sound waves are also known as _____
Acoustic Waves
Acoustic variables
Pressure, density, distance, & temp (PDDT)
- Are used to identify a sound wave. If one of these variables has a rhythmic oscillation, then the wave is a sound wave.
- These make sound waves, sound waves
Pressure
Concentration of force onan area
- Unit: Pa
Density
Concentration of mass in a volume
- Unit: kg/cm3
Distance
Measure of particle motion
- Unit: cm, feet, mile…
Temperature
Warming of particles from absorption of wave energy
Unit: °C
Compression
Molecules are squeezed together creating…
- High pressure and density
- Small distance between molecules
Rarefaction
Molecules are stretched apart creating…
- Low pressure & density
- Larger distance between molecules
Acoustic Parameters
Period, frequency, amplitude, power, intensity, wavelength, and propagation speed.
- Acoustic parameters are used to describe the characteristics of a sound wave.
Transverse waves
Particles move (oscillate) perpendicular to the direction of a wave.
Longitudinal waves
Particles move (oscillate) parallel to the direction of a wave. Sound is a longitudinal wave.
In-phase
A pair of waves are considered in-phase when their peaks occur at the same time and at the same location.
- Result in the formation of a single wave of greater amplitude. This is called constructive interference.
Out-of-phase
Two waves are out-of-phase when their peaks occur at different times.
- Result in the formation of a single wave of lesser amplitude. This is called destructive interference.
Interference
When waves combine to form a single wave and lose their individual characteristics.
- Interference can be constructive or destructive.
Interference of Waves with Different Frequencies
When the frequency of the waves differ, both constructive and destructive interference occurs
Source
The source of a sound wave is the ultrasound system and transducer.
Medium
Medium is another name for the tissue the sound is traveling through.
Period
The time it takes to complete one cycle
- Units: seconds, ms
- Determined by: sound source
- Adjustable: NO!
Cycle
One positive and one negative half of a wave.
Frequency
The number of cycles per second
- Units: – 1/second, Hertz (Hz)
- Determined by: sound source
- Adjustable: NO
Infrasound
- Less than 20 Hz.
- Cannot be heard by humans
Audible Sound
- Between 20 Hz & 20 kHz.
- Can be heard by humans
Ultrasound
- Greater than 20 kHz.
- Cannot be heard by humans.
- This is what we use in sonography.
Relationship Between Frequency and Period
Period and frequency are inversely related to each other. (As frequency increases, period decreases and vice versa)
Period and frequency are reciprocals of each other. (Period x Frequency = 1)
The Three “Bigness” Parameters
The three parameters that describe the size, or magnitude, or strength of a sound wave are
- Amplitude
- Power
- Intensity
Amplitude
The difference between the maximum value and the mean value
- Units: can be any of the acoustic variables such as pressure (Pa) or density (kg/cm3)
- Determined by: sound source and
medium - Adjustable: Yes
Amplitude vs. Peak to peak amplitude
Amplitude is measured from the middle (baseline) to the maximum value
Peak to peak amplitude is the difference between the maximum and minimum values to an acoustic variable
Power
The rate at which energy is transferred
- Units: watts
- Determined by: sound source
- Adjustable: Yes
Relationship Between Amplitude and Power
Amplitude and power are directly related to each other.
- When power increases, so does amplitude
- When power decreases, amplitude decreases
Power is proportional to the wave’s amplitude squared.
- Power ∝ Amplitude2
Intensity
The concentration of energy in a sound beam
- Units: W/cm2
- Determined by: sound source
- Adjustable: Yes
Relationship Between Intensity and Power
Intensity and power are proportional to each other.
- When intensity increases, so does power
- When power increases, so does intensity
- Intensity ∝ Power
If a waves power is doubled, the intensity is also doubled.
Relationship Between Intensity and Amplitude
Intensity and amplitude are directly related to each other.
Intensity is proportional to the wave’s amplitude squared. (Intensity ∝ Amplitude^2)
Wavelength
The distance or length of one complete cycle
- Units: meters, mm, cm
- Determined by: both the source and the medium
- Adjustable: not by the sonographer
Shorter wavelength sound usually produces higher quality images with greater detail.
Relationship Between Wavelength and Frequency
Wavelength and frequency are inversely related to each other.
If a waves frequency is doubled, the wavelength is halved.
In soft tissue, sound with a frequency of 1 MHz has a wavelength of 1.54 mm
Wavelength (mm) = 1.54 mm/μs (prop. speed/frequency (MHz)
Propagation Speed
The distance that a sound wave travels through a medium in one second.
- Units: meters/sec., mm/μs
- Determined by: medium
- Adjustable: not by sonographer
Speed of Sound in Soft Tissue
The speed of sound in soft tissue is 1,540 m/s, or 1.54 mm/μs
Speed (m/s) = frequency (Hz) x wavelength (m)
Speed of Sound in Other Tissues
Tissue Type Speed (m/s)
Lung - 500
Fat - 1,450
Soft Tissue - 1,540
Liver - 1,560
Blood - 1,560
Muscle - 1,600
Tendon - 1,700
Bone - 3,500
Speed of Sound in Other Media
Material Speed (m/s)
Air 330
Water 1,480
Metals 2,000 to 7,000
As a general rule, sound travels fastest in solids, slower in liquids, and slowest in gasses.
Stiffness
The ability of an object to resist compression
Density
the mass per unit volume
The Relationship Between Stiffness and Speed
Stiffness and speed are directly related to each other.
The Relationship Between Density and Speed
Density and speed are inversely related to each other
If two media are equally stiff, the denser medium will have a lower speed.
