EMS COMBINATION 850 Manual page 7

For example, the velocity of sound in stainless steel is approximately 5800 m/s,
in water 1500 m/s and in air only 330 m/s.
As the sound wave passes through the medium, causing molecules to vibrate,
some of the energy in the wave is converted from kinetic energy to heat. For a
collimated sonic beam the intensity, power per unit area, therefore, decreases
exponentially with the distance travelled. The attenuation of the beam is also
dependent upon the frequency of the sound. In solids the attenuation is
proportional to frequency whereas in liquids the attenuation is proportional to
the square of the frequency.
The usual method of specifying the degree of attenuation of ultrasound in
different media is by the half depth. The half depth is the distance the
ultrasound must travel through the medium for its intensity to be reduced to one
half of its original value. Many attempts have been made to measure the
attenuation in various types of tissue with varying results. It is perhaps more
important to remember which types of tissue have the highest absorption and
which the lowest. With the lowest absorption first the order is, fat, muscle, skin,
tendon, cartilage and bone. For soft tissue the half depth is around 50 mm at 1
MHz and 15 mm at 3 MHz.
It is also important to remember that where there is a change in medium or
tissue type there will be both reflection and refraction of the ultrasound beam.
In particular, there is almost 100% reflection at the interface of a solid or liquid
to air at therapeutic ultrasound frequencies. Any air bubbles in coupling
medium will therefore reduce the effective intensity of the ultrasound. Also
bone reflects a high percentage of incident ultrasound. It is important, therefore,
when applying ultrasound to keep the transducer orthogonal to the surface of
the treatment area, to keep the ultrasound transducer moving and to use a good
coupling medium to avoid unwanted reflections and locally high intensities.
Interferential Therapy
The Combination 850 Unit generates medium frequency currents used for 2 and
4-pole interferential therapy.
Prior to the introduction of interferential therapy in the mid 1950s, low
frequency stimulation was used for pain relief, muscle re-education etc. These
currents, however, have the disadvantage that normal human skin has a
relatively high impedance at such frequencies. In order to overcome the skin
impedance a larger voltage has to be used to achieve the desired current,
resulting in a more uncomfortable treatment for the patient.
COMBINATION 850 7 ISSUE 2