Sound Waves, Units and Pulse Parameters · 4 min read
The Nature of Sound and the Acoustic Variables
Many students find this material intimidating, but most SPI questions on it test a small set of relationships and a handful of short formulas. The chapters that follow define each term, state what each quantity depends on, work every calculation step by step with real numbers, and connect each idea to a control on the machine and to what appears on the screen. Master this section and the later sections on resolution, artifacts and Doppler become far easier, because they reuse the same building blocks.
This chapter explains what sound physically is, how it travels, how ultrasound differs from audible sound, and which measurable quantities change as a sound wave passes through tissue.
Sound Is a Mechanical, Longitudinal Wave
Sound is a mechanical wave. That means it is energy carried by the vibration of particles in a medium, such as tissue, water or air. Because it needs particles to push against, sound cannot travel through a vacuum. Light and radio waves are electromagnetic and can cross a vacuum; sound cannot.
Sound is also a longitudinal wave. The particles of the medium move back and forth in the same direction the wave travels. In a transverse wave, such as a wave on a rope, the particles move at right angles to the direction of travel. In soft tissue, diagnostic ultrasound behaves as a longitudinal wave.
As the transducer face pushes forward, it squeezes the particles in front of it together, creating a region of compression (higher pressure and higher density). As the face pulls back, it leaves a region where the particles are spread apart, called a rarefaction (lower pressure and lower density). A sound wave is an alternating series of compressions and rarefactions moving away from the source. The particles themselves only oscillate around their resting positions; it is the energy that travels through the body.
Memory Aid: Think of a slinky stretched across a table. Push one end and a bunched-up region (compression) travels down the slinky, followed by a stretched-out region (rarefaction). Each coil only wiggles back and forth, but the pattern moves from one end to the other.
The Three Acoustic Variables
An acoustic variable is a quantity that changes (oscillates) as a sound wave passes. If you could measure these at one point in tissue, you would see them rise and fall in a regular cycle. The three acoustic variables are:
If a question asks how you know a wave is a sound wave, the correct idea is that these acoustic variables are oscillating.
Exam Tip: A favorite recall question lists four items and asks which is NOT an acoustic variable. Frequency, period, wavelength and propagation speed are parameters (they describe the wave), not variables (the things that oscillate). Pressure, density and distance are the variables.
Infrasound, Audible Sound and Ultrasound
Sound is classified by frequency, which is the number of cycles per second, measured in hertz (Hz).
Frequency Ranges of Sound
| Category | Frequency range | Example |
|---|---|---|
| Infrasound | Below 20 Hz | Distant thunder, heavy machinery |
| Audible sound | 20 Hz to 20 kHz | Speech, music |
| Ultrasound | Above 20 kHz | Medical imaging, sonar |
| Diagnostic imaging | About 2 to 15 MHz | Abdominal, vascular, OB, small parts |
Diagnostic imaging uses roughly 2 to 15 MHz, and even higher frequencies for superficial, ophthalmic and intraoperative work. Notice that the imaging range is far above the 20 kHz boundary. A 5 MHz transducer vibrates 5 million times per second, which is 250 times the upper limit of human hearing.
Common Trap: The boundary between audible sound and ultrasound is 20 kHz, not 20 MHz. Students who mix up kilo and mega pick the wrong answer. Keep in mind that 20 kHz is 0.02 MHz.
Key Takeaways
Sound is a mechanical, longitudinal wave made of compressions and rarefactions; it needs a medium and cannot cross a vacuum.
Particles oscillate in place; energy travels through the medium.
The acoustic variables are pressure (Pa), density (kg/cm³) and distance (particle motion).
Ultrasound is any sound above 20 kHz; diagnostic imaging uses about 2 to 15 MHz.
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