Basics of Sound and Sound Waves

Study Notes

Basics of Sound and Sound Waves

Sound is energy that travels as a mechanical, longitudinal wave through a medium.
Sound is measured in Hertz (Hz).
- One hertz equals one cycle per second.
- 1 kHz equals 1000 Hz.
- 1 MHz equals 1000 kHz, or 1,000,000 Hz.

Classification of Sound

Audible sound is between 20 to 20,000 hertz.
Infrasound is below the range of human hearing – less than 20 hertz.
Ultrasound is above the range of human hearing – greater than 20,000 hertz.

Categories of Sound

Sound requires a medium through which to travel.
Sound waves travel by compressing and rarefying the matter it goes through.
The velocity and direction of a sound wave depend on the matter it’s moving through.

Sound Waves

Sound waves are mechanical vibrations called condensations (compressions) and rarefactions (decompressions).
One cycle is one compression and one rarefaction.

Terminology

Sonogram: The pictorial record of an ultrasound examination (the ultrasound image itself).
Scattering: Occurs when a sound wave strikes an interface between two small objects equal to or smaller than the wavelength.
Scatterer: An object that causes scattering.
Acoustic: Pertaining to sound waves.
Amplitude: The height of a wave. Amplitude and intensity of sound represent the energy of sound.
Specular reflectors: Interfaces responsible for the major organ outlines seen in ultrasound imaging.
- Smooth surface: The interface is large compared with the wavelength size of the sound wave.
- Rough surface: When the undulations are approximately one wavelength or less, sound is reflected in all directions, which is known as diffuse reflection.
Exposure: The condition of being scanned with an ultrasound transducer.
- Increasing the power to the transducer produces a high-intensity ultrasound pulse.
Exposure time: The total amount of time the transducer assembly is delivering ultrasonic energy to the subject.
Pulse Repetition Frequency (PRF): The PRF is set to allow time for the most distant echoes to return before sending the next pulse.
Time Gain Compensation (TGC): TGC compensates for the loss of signal strength so that equal amplitudes can be displayed from all depths as the same brightness on the image.
Gain: The degree of amplification of the returning wave displayed on the ultrasound image, usually expressed in decibels (dB). Gain is an input function.
Artifact: An echo feature, present or absent in a sonogram, that does not correspond to the presence or absence of a real target (e.g., reverberations and shadowing).

Wave Terminology

Frequency (f): The number of cycles per second. Expressed in Hertz (Hz).
- 1 Hz equals 1 cycle per second.
Wavelength (λ): The distance between two consecutive identical points on a wave, typically between 0.1-1 mm for medical ultrasound imaging.
- Wavelength depends on both the frequency (f) and the propagation speed (c).
- λ = c/f
Velocity (c): The speed of sound with direction specified.
The wavelength and frequency of ultrasound are inversely related. High-frequency ultrasound has a short wavelength, and vice versa.
Period (T): The time taken for a particle in the medium through which the wave is traveling to make one complete cycle. As frequency increases, period decreases
- If frequency is in Hz, period is in seconds/cycle
- If frequency is in kHz, period is in msec/cycle
- If frequency is in MHz, period is in microsec/cycle