Introduction - EMS COMBINATION 850 Manual

Ultrasound
Sound is mechanical vibration. The human ear responds to these vibrations in
the range 20 Hz to 20 kHz. Sound above 20 kHz is called ultrasound.
Therapeutic ultrasound is sound in the range 500 kHz to 5 MHz.
Sound waves are produced by some disturbance in a material medium causing
the particles or molecules of the medium to vibrate. For this reason sound will
not pass through a vacuum. If the vibration is continuous and regular a constant
tone or frequency is produced. The vibration or sound wave propagates through
the medium as particles in the medium pass on their vibration to neighbouring
particles and series of compressions and rarefactions are produced in the
direction of travel of the wave. Therefore, sound waves are longitudinal waves.
Direction of wave motion
Mean
The diagram shows a sound wave travelling from left to right. The vertical bars
represent thin slices of the medium which are displaced to form areas of
compression and rarefaction. The sinewave represents their displacement
relative to their mean position. The distance over which the vibration repeats
itself is called the wavelength. The number of complete vibrations in one
second is called the frequency of the sound wave. The velocity of sound in the
medium is given by:
Sound will travel faster through media where the molecules are closer together
and so the velocity is higher in solids than in liquids, and higher in liquids than
in gasses.
COMBINATION 850

Introduction

compression rarefaction
Velocity = frequency x wavelength
compression rarefaction
One wavelength
6
compression
ISSUE 2