3B Scientific Acoustics Kit - Manual (Page 4)

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7. Infrasound
• Without the plotter pen attached, make the
tuning fork (21 Hz) vibrate by pressing its
prongs together and suddenly releasing them.
The tuning fork produces slow vibrations that can
be perceived by the naked eye. When held close to
the ear, a very deep (barely audible) tone can be
heard.
Reasons: the prongs of the tuning fork vibrate in
opposite directions and give rise to compressions
and rarefactions in the surrounding air. When this
reaches the ear, it makes the eardrum vibrate. A
tone is thus perceived.
The tuning fork vibrates at approximately 20 vibra-
tions per second. The lowest note that can be per-
ceived by human hearing has a frequency of ap-
proximately 16 vibrations per second. Vibrations
below 16 Hz are not audible to the human ear. The
sound produced by these vibrations is called infra-
sound. (Latin: infra = below).
8. Ultrasound
• Blow the Galton whistle.
No sound can be heard, simply a hiss.
Reasons: owing to its short length, the Galton whis-
tle produces very high tones which are not audible
to the human ear. This phenomenon is called ul-
trasound. (Latin: ultra = above).
9. Tuning fork with plotter pen
• Attach the pen (8) to the prongs of the tuning
fork (21 Hz).
• Make the tuning fork vibrate by pressing the
prongs together and move a sheet of paper as
uniformly as possible under the pen so that the
motion is plotted onto it. Make sure that the
surface on which the paper rests is not too soft.
The pen traces a wavy line of a constant wave-
length but decreasing amplitude on the paper.
Reasons: sound is produced by harmonic oscilla-
tions of solids, liquids or gases. The locus of the
oscillating particles of the body in relation to the
time traces a sine curve. When struck once, vibrat-
ing bodies exhibit a “damped” oscillation (continu-
ous decrease in amplitude). If the supply of energy
is uninterrupted (constant sound of a car horn,
constant blowing of an organ pipe), the result is an
undamped oscillation of constant amplitude (loud-
ness or volume).
10. Progressive waves
• Make a simple knot in the resonance rope and
attach it by the loop to the handle of a door.
• Make the wire moderately taut and jerk it
suddenly to the side.
From the centre of motion (the hand), a wave is
produced which runs along the wire with an in-
creasing velocity, gets reflected at the fixed end
and returns to the point of origin.
Reasons: every solid, liquid and gas produces vibra-
tions when disturbed suddenly. These vibrations
spread through a medium with a definite propaga-
tion velocity.
11. Doppler effect
• Strike the light-metal tuning fork (1700 Hz)
hard with the metallophone beater. Hold it
still for a short while and then rapidly move it
to and fro through the air.
In a state of rest, the tuning fork produces a clear
tone of uniform pitch. In a state of motion, the
pitch constantly changes. If the tuning fork is
moved towards the ear, the pitch rises, and if it is
moved away from the ear, the pitch decreases.
Reasons: when the distance between the source of
sound and the ear is decreasing, the time interval
between two compressions also decreases as a
second compression has to travel a shorter distance
to reach the ear compared to the first. The ear
registers a higher frequency. The tone thus gets
higher. When the source of sound is moved away
from the ear, the intervals between compressions
and rarefactions get longer. The tone thus becomes
deeper.
12. Chladni figures
• Use the table clamp and plastic block to attach
the Chladni plate to the workbench. Scatter
some bird sand or a similar material onto the
plate. Allow it to spread in a thin layer so as to
cover a third of the plate.
• With one hand, bow the plate exactly half way
between two corners with a good violin bow,
simultaneously touching one other corner
lightly with the finger of your other hand.
• Bow several strokes across the plate, preferably
quite forcefully so that the vibrations of the
plate are vigorous and well audible.
When the plate is being bowed, a very distinct
acoustic tone can be heard. At certain points, the
grains of sand experience lively resonance and
begin to bounce up and down on the surface of the
plate, accumulating in unusual figures on the sur-
face.
Reasons: “standing waves” are formed on the plate.
When bowed, the plate does not vibrate uniformly
across its surface. At certain points (antinodes), the
plate begins to vibrate, whereas it is in a state of
complete rest at other points (nodes). By touching
the plate at one corner, the point is forced into
being a node.