Physics 101: The Basics

Keywords:
Medium
Acoustic Variables
Pressure
Density
Compression
Rarefaction
Propagation Speed
Stiffness
Longitudinal
Mechanical
Frequency
Infrasound
Ultrasound
Wavelength (Lambda)
Period

Sound is described as a wave that propagates through a medium (i.e air) associated with acoustic (hearing) variables. These acoustic variables are the following: Pressure, Density, Compression, Rarefaction, Temperature, and Particle motion. Without a medium, sound cannot travel (as in a vaccum). Gas, liquids, solids, and soft tissues are the types of mediums that sound can travel with differences of propagation speeds. Essentially, the medium determines the Propagation speed, and the Propagation speed, denoted as c (mm/microsecond), depends upon the stiffness of the material. In other words, the stiffer the material (more incompressible), the higher the propagation speed. Therefore, bone has the highest propagation speed, whereas gas or air has the lowest. In soft tissue, the propagation speed is "constant" and is calculated to be at 1.54mm/microsecond (1540 m/s). With that in mind, whenever the frequency changes (thus wavelength changes), the speed remains unchanged.

As sound travels through a medium, pressure and density go through cycles (frequency) of increase and decrease, and particles of the medium oscillate back and forth. As a result, sound waves can be broken down into two types: longitidunal and mechanical (or transverse). In longitudinal, the sound oscillate back and forth in the same direction of the linear plane or wave travel. In mechanical, sound inceases and decreases becoming perpendicular to the linear plane (90 degree angle). Therefore,sound is a wave of pressure and density variations with particle vibrations and is broken down into longitidinal and mechanical that describe the frequency or wave cycle.

Frequency is defined as the number of cycles in one second. A single cycle consists of high and low pressure associated with density. Pressure is proportional to density, that is, as pressure increases, density increases as well (and vice versa). Density is defined as mass/volume, and should not be confused being proportional to "intensity". Regions of high pressure and density are called compressions, whereas regions of low pressure and density are called rarefactions. Therefore, frequency can also be defined as the number of compressions and rarefactions in one second.

The different types of frequencies are the following: Hz, kHz (1000), and MHz(1,000,000). When comparing infrasound to ultrasound, the range goes from 20Hz to 20,000Hz (20kHz). Infrasound is below 20Hz, whereas ultrasound is greater than 20,000MHz (20kHz). The frequency depends upon the transducer and is reciprocal to the wavelength and period or time. In other words, as the frequency increases, the wavelength and period decreases. Wavelength is defined as the length of space over which one cycle occurs (front to back) and is proportional to the period or time. The period is the time it takes for one cycle to occur. Therefore, as the wavelength or lambda increases, then the period or time increases as well.