Sound Waves and Acoustic Parameters

Flashcards covering the characteristics of sound waves and acoustic variables and parameters discussed in Chapter 2, Part I of the lecture.

Sound

A mechanical, longitudinal wave that travels in a straight line.

Longitudinal Wave

Particles of the medium move (vibrate) in the same direction as the wave travels.

Particle (Sound Wave)

A small portion of the medium through which the sound wave travels, oscillating back and forth.

Transverse Wave (Shear Wave)

Particles of the medium move in a direction perpendicular to that of the wave front.

Compressions

Areas of high pressure and high density in the molecules of a medium caused by sound waves.

Rarefactions

Areas of low pressure and low density in the molecules of a medium caused by sound waves.

Sound (Pressure Variation)

A traveling pressure variation that starts at a normal value, increases to maximum, decreases to minimum, and returns to normal.

Vacuum

A space where sound cannot travel because a medium is required for wave propagation.

Energy (in waves)

The ability to do work; waves carry energy, not matter, from one place to another.

Mechanical Wave

Sound is a mechanical wave, meaning it requires a medium to travel.

Acoustic Variables

Rhythmical cycling properties of sound: Pressure, Density, Temperature, and Distance.

Pressure (Acoustic Variable)

Concentration of force, measured in lb/sq in or Pascals (Pa).

Density (Acoustic Variable)

Concentration of mass or weight, mass per unit of volume, measured in kg/cubic cm.

Temperature (Acoustic Variable)

Concentration of heat energy, measured in degrees.

Distance (Acoustic Variable)

Measure of particle motion, measured in cm, feet, or miles.

Acoustic Parameters

Properties used to describe sound waves: Frequency, Period, Amplitude, Intensity, Propagation Speed, and Wavelength.

Sound Source (XDCR)

Determines the Frequency, Period, Amplitude, Intensity, and Wavelength of a sound wave.

Medium (Body)

Determines the Propagation Speed (wave speed) and Wavelength of a sound wave.

Frequency

The number of cycles of an acoustic variable that occur in 1 second, measured in Hertz (Hz); determined only by the sound source.

Hertz (Hz)

A unit of frequency, equal to one cycle per second.

Megahertz (MHz)

A unit of frequency, equal to 1,000,000 Hertz.

Audible Sound

A frequency between 20 Hz and 20,000 Hz (20 kHz).

Infrasound

A frequency less than 20 Hz, too low to be heard by humans.

Ultrasound

A frequency exceeding the upper limit of human hearing (20,000 Hz or 20 kHz).

Diagnostic Imaging Frequency Range

Typically 2 to 20 MHz, influencing penetration (depth) and resolution (image quality).

Period (T or P)

The length of time it takes to complete a single cycle, from the start of one cycle to the start of the next; measured in units of time like microseconds (µsec); determined only by the sound source.

Microsecond (µsec)

A unit of time, equal to one millionth (0.000001) of a second.

Relationship between Period and Frequency

They are reciprocals of each other, making them inversely related (Period decreases as Frequency increases), and their product equals 1.

Wavelength (λ)

The distance or length that one complete cycle occupies; measured in millimeters (mm); determined by both the sound source (XDCR) and the medium.

Typical US Wavelength in Soft Tissue

Ranges from 0.1-0.8 mm, influencing longitudinal resolution and displayed detail.

Relationship between Wavelength and Frequency

Inversely related; higher frequency waves have shorter wavelengths, which generally produce higher quality images.

Frequency in Different Mediums

The frequency of a sound wave remains constant when it changes mediums because it is determined by the vibrating source.

Wavelength Formula

Wavelength (λ) = Propagation Speed (C) / Frequency (F).