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propellers of ships and submarines.
sonic boom. A loud, explosive noise heard on the ground when an airplane passes overhead at a speed
faster than the speed of sound. Sound-pressure waves build up as the airplane passes through the air, but
because the airplane is flying faster than these waves can move out ahead of it, the sound pressure builds up
and forms a shock wave.
The shock wave radiates out from the airplane in the form of a cone. When this cone passes over
the ground, the pressure difference between the pressure inside the wave and the pressure of the
surrounding air causes the loud explosion-like sound.
sonic cleaning. A method of cleaning delicate mechanical parts. The parts are placed in a sonic cleaning
machine and covered with a cleaning fluid. High-intensity sound waves are produced in the cleaning fluid,
and the vibrations caused by these waves shake loose any contaminants on the parts being cleaned.
sonic frequencies. Frequencies of mechanical vibrations that can be heard by the human ear. Sonic
frequencies are normally considered to be those between 20 hertz and 20,000 hertz (20 kHz).
sonic soldering. A method of soldering metals, such as aluminum, that are normally difficult to solder
because of the speed with which surface oxides build up. The surface to be soldered is covered with a flux
to prevent oxygen reaching it, and the tip of the soldering iron used to heat the surface is vibrated at a sonic
rate. The vibrations loosen the surface oxides, and they float to the top of the flux. Since no new oxides can
form under the molten flux, the solder can reach the hot, clean metal and adhere to it.
sonic speed. The speed of sound.
sonic venturi. A venturi in a line between a turbine engine compressor or turbocharger and the
pressurization system of an aircraft. When the air flowing through the venturi reaches the speed of sound, a
shock wave forms across the throat of the venturi and restricts the flow. A sonic venturi is also called a
flow limiter.
soot. A black residue left when a hydrocarbon material is burned in an atmosphere in which there is not
enough oxygen for complete combustion. Soot is composed mostly of carbon.
sound suppressor. The airframe component that replaces the turbine engine tail pipe. It decreases the
distance sounds made by the exhaust gases can propagate by converting low-frequency vibrations into
high-frequency vibrations.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 541
sound waves. Longitudinal pressure waves within an elastic medium, such as air, water, or a solid material,
that has been disturbed. Sound waves cannot travel through a vacuum.
source region (meteorology). An extensive area of the earth’s surface characterized by relatively uniform
surface conditions. Large masses of air remain long enough over these surfaces to take on characteristic
temperature and moisture properties from the surface.
south geographic pole. The location of the southern spin axes of the earth. The south geographic pole is
located at 90° south latitude.
south magnetic pole. The location near, but not at, the south geographic pole, at which the magnetic lines
of flux of the earth’s magnetic field enter. These lines of flux are considered to leave from the north
magnetic pole and enter at the south magnetic pole.
south pole of a magnet. The pole of a magnet at which lines of magnetic flux are considered to enter after
leaving the magnet at its north pole.
space charge (electron tube). The negative charge in an electron tube between the cathode and the plate.
This space charge is caused by the cloud of electrons that collect around the cathode of the tube when the
cathode emits more electrons than the plate can accept.
spacers. Components used to fill a space between two objects. Washers are commonly used as spacers
around a bolt to take up the space between the nut and the surface being clamped by the fastener.
space shuttle. A manned vehicle launched by a rocket, usually a two-stage rocket, and carried to an
altitude at which it can orbit the earth. At the completion of its assigned mission, the space shuttle re-enters
the earth’s atmosphere and glides to a landing.
space wave (radio transmission). The portion of an electromagnetic signal that is capable of traveling
through the ionosphere.
spade drill. A special drill used for cutting holes in graphite structural materials. The flat surfaces of the
drill end allow ample room for the drill dust to leave the hole so it will not abrade the sides of the hole and
increase its diameter.
spaghetti (electrical insulation). The commonly used name for a type of insulating tubing slipped over the
end of an electrical wire. The tubing is put on at the point the wire connects to a terminal, where it is
　
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