Sound Waves and the Acoustic Spectrum | Ultrasound Physics | Radiology Physics Course #1

Introduction to Ultrasound Physics

Understanding Sound Waves

• Dr. Michael Nell introduces the ultrasound physics module, emphasizing the importance of understanding sound waves.

• Sound is defined as mechanical energy that propagates through a continuous elastic medium via compression and rarefaction.

• Unlike electromagnetic radiation, sound requires a medium to propagate; it cannot travel through a vacuum.

• The medium must be elastic, allowing molecules to move, transfer energy, and return to their original position for effective wave propagation.

• An example illustrates that water can transmit waves due to its elasticity, while sand cannot because it lacks this property.

Wave Properties

• Sound waves consist of regions of compression (high pressure) and rarefaction (low pressure), which can be graphed as sine waves.

• The amplitude of compression is typically higher than that of rarefaction; however, both are represented in textbooks for simplicity.

• Energy transfers from one point to another without the actual movement of molecules across the entire distance; they oscillate back and forth instead.

Key Characteristics of Waves

• Wavelength is defined as the distance between successive compressions or rarefactions in a wave.

• Frequency measures how many cycles pass a point in one second, expressed in hertz (Hz); 1 Hz equals one cycle per second.

Speed and Medium Dependency

• The speed of sound differs from electromagnetic radiation; it varies based on the medium rather than being constant like light speed.

• While frequency can be controlled (e.g., by vocal cords or ultrasound transducers), sound speed depends solely on the material it's traveling through.

Relationship Between Frequency and Wavelength

• The wavelength adjusts according to changes in frequency and speed; they are interdependent but not directly linked like in electromagnetic radiation.

• Understanding these differences is crucial for interpreting questions related to ultrasound physics effectively.

Differences Between Sound Waves and Electromagnetic Radiation

• Distinctions between acoustic properties and electromagnetic spectrum characteristics are highlighted as critical knowledge areas for exams.

Understanding Acoustic Waves and Their Properties

The Relationship Between Wavelength, Frequency, and Medium

• The link between wavelength and frequency is not tight when the medium changes; thus, frequency is used to categorize the acoustic spectrum instead of wavelength.

• Audible sound ranges from 20 Hz to 20 kHz (20,000 Hz), where frequencies below this range are classified as infrasound, which humans cannot hear.

• Frequencies above 20 kHz are termed ultrasound; these high-frequency sounds (2 to 20 MHz) are also inaudible to humans.

Characteristics of Electromagnetic vs. Sound Waves

• Electromagnetic waves are transverse waves that can propagate through a vacuum without needing a medium; they maintain constant velocity regardless of frequency or wavelength.

• In contrast, sound waves are longitudinal waves where particle movement occurs parallel to energy transfer within a medium.

Medium Requirements for Sound Propagation

• Sound requires a continuous and elastic medium for propagation; variations in the medium's properties affect sound speed and wavelength but not frequency.

• While the speed of sound changes with different tissues, the set frequency from an ultrasound probe remains constant throughout.

Energy Transfer Mechanisms