Acoustic Principles and Sound Waves Quiz

Ultrasound Physics: Basics

• Ultrasound physics is the study of the physical principles of ultrasound.
• Graphs are used to display relationships between variables in physics and ultrasound.
• The horizontal axis (x-axis) typically represents time and the vertical axis (y-axis) represents velocity.
• In spectral Doppler, the x-axis represents time and the y-axis represents blood velocity.
• M-mode ultrasound uses the x-axis to represent time and the y-axis to represent depth of reflectors.

Relationships in Ultrasound Physics

• Relationships describe the variables' association or proportionality.
• Two items are related or proportional when one item's change affects the other.
• Unrelated items do not have a relationship or dependency.
• Example of unrelated items: Hair color and intelligence.
• Example of directly related items: Clothing size and age of a child. An example of directly proportional variables includes the relationship between the age of a tree and its size.
• Example of indirectly/inversely related items: Golf score and skills. Another example includes a car's gas mileage and its size-- larger cars typically have lower gas mileage.
• Example of reciprocal relationships: two numbers multiplied together result in one (1).
• Example of reciprocals: 2 and 1/2 are reciprocals; 1 and 1/10th are reciprocals; 1/100,000th and 100,000 are reciprocals.

Units and Presentation

• Numerical values must include units.
• Units give meaning to the numerical values.
• Examples of units: inches, feet, miles.
• A unit must be included for the numerical value to be meaningful.
• Any technically correct unit can be used.
• Example: 6 inches tall, 6 feet tall, 6 miles.
• Numerical values must have units (unless percentage or otherwise noted as a unitless number).

Scientific Notation

• Scientific notation simplifies very large or small numbers.
• Example: 0.000000124=1.24 x 10⁻⁷; 1,000,000 = 1.0 x 10⁶.
• It is a shorthand way to express very large or very small numbers.
• Counting zeros is common when expressing numbers in scientific notation.

The Metric System

• The metric system is used to express measurements in science.
• Units of measurement use prefixes to indicate multiples of ten.
• Understanding metric conversions is crucial for ultrasound physics.
• A helpful memory tool is King Henry Doesn't Usually Drink Chocolate Milk, used to remember the order of the prefixes (kilo, hecto, deca, unit, deci, centi, milli).

Converting with Metrics

• Number line conversions are common.
• Determining the number of spaces between prefixes.
• Moving the decimal point based on the calculation helps determine the equivalent value.
• Using the number line can help determine how many spaces exist between the prefix that is present in the number and the prefix to which conversion is being performed.

Ultrasound Physics: Sound

• Sound is a type of wave that carries energy.
• Sound waves must travel through a medium, they cannot travel through a vacuum.
• Sound waves travel in straight lines until they encounter something that changes their direction (like a wall).
• Sound waves are also called acoustic waves.

Properties of Sound Waves

• Sound waves move through different substances or mediums.
• Sound cannot travel through a vacuum—a space that lacks any medium.
• Sound waves travel in straight lines until they interact with objects, materials, or forces that cause them to change their pathway.

Types of Waves

• Transverse waves—particles vibrate perpendicular to wave direction.
• Longitudinal waves—particles vibrate parallel to wave direction.
• Mechanical waves are characterized by the movement of particles in a medium, vibrating back and forth.
• Sound waves are mechanical and longitudinal waves.

Acoustic Variables and Parameters

• Acoustic variables—describes changes in 3 physical characteristics: pressure, density and distance/particle motion, that occur when sound travels through a medium.
• Acoustic parameters—describe features of a sound wave: period, frequency, amplitude, power, intensity, wavelength, propagation speed.
• This is important for understanding acoustic properties of sound waves.

Phase Relationships

• Waves can be in phase (synchronized peaks and troughs) or out of phase.
• Interference—occurs when two waves overlap at the same location and time, producing either constructive or destructive interference.
• Constructive interference results in a larger amplitude than the original waves.
• Destructive interference result in a smaller amplitude than the original waves.

Waves of Different Frequencies

• Frequency—the number of cycles in one second.
• When two waves of different frequencies interfere, the interference can be constructive or destructive at different moments in time.

Acoustic Propagation Properties

• Acoustic Propagation Properties—examine the interaction between a sound wave and the medium through which the wave travels.

Describing Sound Waves and Acoustic Parameters

• Acoustic Parameters—describe features of sound waves (period, frequency, amplitude, power, intensity, wavelength, propagation speed).
• Knowing the units and how parameters are related is important to understanding ultrasound physics.

The Sound Source and Medium

• Sound source relates to the ultrasound system and transducer, which establishes the physical features of sound.
• The quantity or level of some parameters is established by the ultrasound system.
• Some of these levels can be changed by turning a knob on the machine, others cannot.
• The medium is the material through which the sound travels (soft tissue in diagnostic medical sonography).
• The speed of sound in soft tissue is 1540 m/s. This can also be represented as 1.54 km/s (kilometers per second) or 1.54 mm/µs (millimeters per microsecond).

Acoustic Variables

• Acoustic variables—pressure, density, and particle motion—describe the physical characteristics of sound waves.

Other "Bigness" Parameters (Amplitude, Power, Intensity)

• Describing the size, strength or related characteristics of a sound wave.
• amplitude—the difference between the average value and the maximum or minimum value of an acoustic variable; describing the "bigness" or height of a sound wave.
• Power—the rate of energy transfer.
• Intensity—describes how the power is distributed in a sound beam.

Wavelength

• Wavelength (L) describes the distance of one sound wave cycle.
• Wavelength (expressed in mm) is determined by the sound source and the medium through which it travels; it is not typically changed by the sonographer.
• Wavelength and frequency have an inverse relationship.

Propagation Speed

• Propagation speed—the rate at which sound travels through a given medium.

• Propagation speed is a constant when moving through soft tissue.

• Speed of sound is related to the stiffness and density of a medium. Stiffness is a medium's ability to be moved. Density is a medium's weight (directly related to stiffness and greater related to a medium's weight).

• Bulk modulus is the same as stiffness.

• Speed of sound in various mediums or tissues (different speeds).

• Example of speed of sound in different types of tissues (Example: Air—330 m/s; Lung—300-1200 m/s; Fat—1450 m/s; Soft tissue—1540 m/s; Tendon—1850 m/s; Bone—2000-4000 m/s.