Sound and Ultrasound: Acoustic Parameters
All our lives we are surrounded by sounds. In fact, it is our ability to create and comprehend sounds in the form of speech that is integral to our human development. As physicians we assess heart sounds, breath sounds, and bowel sounds, but few will contemplate the nature of sound. Without understanding the physical properties of sound and their interactions with the surrounding medium, it is difficult to understand the images produced in clinical ultrasound. The critical care practitioner also often acts as a sonographer, whose responsibility is to operate the equipment, obtain images, distinguish between real structures and artifacts, and manipulate the transducer. Without a solid knowledge of basic sound principles, these tasks are virtually impossible.
A sound is a wave created by a moving (vibrating) object and comprises areas of increased (compressions) and decreased (rarefactions) densities. This wave moves through a medium with a fixed speed (propagation speed), transmitting its energy, while the vibrating matter of the medium returns to its original position with each cycle (see ch002 in enclosed DVD). When the sound wave reaches an object, it is unable to penetrate, such as a wall; it may go around it (diffraction). This allows one to hear music around a corner. If the object is larger, such as a mountain, sound will bounce off (reflection) and return back to the source, creating a familiar phenomenon known as an echo. Echo was first described and named by the ancient Greeks.
Depending on the movement of the sound-generating object, the sound wave will acquire different characteristics known as acoustic parameters (Table 2-1). Some of those are related, while others are independent of each other. Though a sound wave is longitudinal with energy traveling in the same direction as the propagating wave, for the ease of representation it will be pictured as a transverse wave with energy distributed perpendicular to the direction of propagation like a wave on the surface of a pond (Figure 2-1).
Guitar string vibration is creating a longitudinal wave. It is represented, however, as a transverse wave.
Summary of Acoustic Parameters
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Table 2-1 Summary of Acoustic Parameters
|Acoustic Parameter ||Units ||Determined by ||Values in Diagnostic Ultrasound |
|Period ||μs ||Sound source ||0.1–0.5 μs |
|Frequency ||MHz ||Sound source ||2–10 MHz |
|Amplitude ||dB ||Sound source ||— |
|Power ||Watts ||Sound source ||— |
|Intensity ||Watt/cm2 ||Sound source ||0.001–100 W/cm2 |
|Wavelengths ||mm ||Source and medium ||0.1–0.6 mm |
|Propagation speed ||m/s ||Medium alone ||1500–1600 m/s |
The time necessary for the sound wave to complete one cycle is known as its period. The cycle is complete when the sound source has produced one ...