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sonar
sonar m

Sonar. A system very similar to radar, but it works with ultrasound waves under the surface of the water instead of using radio waves in the air. (Radio waves do not travel far in water.)
See also echo.


sonometer
sonomètre m

Acoustic scale. A device for testing a stretched string or wire as a sound source. Strings of different densities, lengths and tensions can be used. The frequency can be determined by resonance when using tuning forks.

Force meter
Two bridges
String
screw
Voice box (empty)


sound
son m

voice. A form of radiation that includes...
Pressure waves Alternating high and low pressure on a material. The oscillating source brings the particles closer together, but they move apart on the way back.
A graph of pressure at different points in a medium at a given time gives a sine wave, as does a graph of pressure at a single point over time. Thus, sound is a wave that exhibits all the properties of waves, such as absorption, refraction, reflection, diffraction, and interference. However, sound is a longitudinal wave and therefore can be polarized.

Medial pressure
high pressure (compression)
low pressure (permeation)
Tuning fork

Pressure waves - like all waves - have a frequency spectrum. The range within which the human ear can detect appropriate sound waves is between 20 and 20,000 Hz. See also infrasound and ultrasonics. Earthquake P-waves fall into the same category.
See also speed of sound and sound sources.


sound sources
sources fpl sonores

Sound sources. Sound waves radiate from vibrating objects. The frequency of the sound generated by the source depends on the nature of this source and how it vibrates. Analysis is very difficult in the case of some sources, such as bells, for example.

The simplest form of source vibration is called its fundamental. In the case of a string or a taut wire (see sonometer), all points move in phase, as in (a) below. The two ends of the wire are fixed and the middle is the most free to move. There are nodes at the ends and antinodes in the middle. And the wavelength is . The radiated sound is twice the length of the wire “Knot = no shift.”

The second important source of sound is the closed pipe. It is a tube open at one end, like a glass bottle. The basic way the air column vibrates indoors is a node at the closed end and a belly at the open end. The wavelength of 85 here is four times the length of the tube... The distance between the two dashed lines in Figure (B) below shows the amplitude of vibration of air particles at each point.

( a )
λο= 21

(B)
λο = 4I

(c)
λο = 2I

The third main source is the 'open' pipe, which is a pipe with two ends open. There is a belly at each open end and a knot in the middle. As shown in Figure (c), the wavelength λο is twice the length of the tube.
The drawings above show the basic forms of vibration of three sound sources. The lengths of radiated waves λο are related to each natural frequency (Vo) through the wave equation c=voλο, where, is the speed of light in air.

However, more complex forms of vibration can also occur. These produce overtones. The following drawings show the first overtones of the three sources. Notice how the wavelength is related to the length of the source in each case.

(a)

λ1 = I

(B)
λ = 4I/3

(c)
λ1 = I

The drawings below show the second overtone for each source:

(a)
λ2 = 2I/3

(B)
λ2 = 4I/5

(c)
λ2 = 2I/3

There are few conventional sound sources that generate pure tones composed of only a fundamental frequency. A good signal generator that drives a good loudspeaker can do this. The quality, or timbre, of the sound produced by a source depends on the mixing of the overtones with the fundamental ones. Another musical term is pitch, which is related to the frequency of the sound wave.
See also: standing waves and noise.