General Operating Principles; Ultrasonic Principles; Nature Of Ultrasound - NDT Systems NovaScope 5000 User Manual

4. GENERAL OPERATING PRINCIPLES

This section presents an elementary overview of the ultrasonic principles and general
instrumentation concepts for the NovaScope. Detailed discussions on the specific controls/features
and operational/calibration procedures for the NovaScope are presented in subsequent sections.
The small amount of time spent in review of this section will lead to a more thorough understanding
if thickness gaging in general.

4.1 ULTRASONIC PRINCIPLES

4.1.1 Nature of Ultrasound - The basic physical principle behind the NovaScope is ultrasound.
Ultrasound refers to sound waves whose frequency (pitch) lies beyond the upper limit of hearing
for humans, which is about 20 kHz (kilohertz). The NovaScope employs high frequency ultrasound
(and electronics) in the range of about 0.1 - 50 MHz (megahertz).
Ultrasound, like any frequency of sound, is basically mechanical vibrations that propagate or travel
through a medium (gas, liquid or solid) in a wave-like fashion. The velocity at which ultrasonic
waves travel depends upon the physical and chemical properties, as well as the temperature of the
medium. If these properties are virtually constant throughout the medium supporting the
ultrasound, then velocity is also constant. Table I lists the nominal velocity for a variety of common
materials. Note that sound waves travel relatively slowly through gases (like air), with medium
velocities through liquids, and fastest through solids (metals).
As a sound wave travels through a material, it loses a portion of its energy due to a process known
as attenuation (a combination of wave scattering from inhomogeneities and absorption). Sound
waves typically attenuate much more in gases than in many common liquids and solids. Also,
attenuation normally increases rapidly with frequency (for example, high frequency [MHz-range]
ultrasonic energy travels only exceedingly short distances through air before it is virtually
attenuated).
Ultrasonic waves behave quite similarly to light waves and microwaves (radar) in that they also
reflect, refract, interfere, and travel as beams (radiation patterns). Higher frequencies permit
ultrasound to be shaped into fairly well-collimated and even sharply-focused beams.
Ultrasound is highly reflective at boundaries (surfaces) between most dissimilar materials
(technically, those with substantially different acoustic impedances). The greater the impedance
mismatch between two materials, the greater the reflection at their interface.
Ultrasound is almost totally reflected at a solid-gas (i.e., solid-air) interface. The ultrasonic
reflectively is so high at a metal-air interface that even the interface between two pieces of flat
polished metal tightly pressed together still contains enough air molecules to produce a strong
reflection. Typically, the great majority of energy in an ultrasonic beam is reflected from a solid-
liquid interface, while considerably less is typically reflected from a molecularly-bonded solid-solid
interface (between dissimilar solids).
Because of its beam-shaping and high reflectivity characteristics, plus the ability to travel through
optically opaque materials (like metals), ultrasound is very well suited for measuring the dimensions
of and inspecting the interior of solid materials, while requiring access to only one surface of the
material.
Manual No. OM5000
Jan X/04 Ver 1.40
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