3B Scientific Acoustics Kit - Manual (Page 5)

5
13. Chimes
• Secure the bell dome to the bench with its
open end facing upwards using the table
clamp and plastic block.
• Strike the edge of the bell at different points
with a hammer. (Alternatively, the edges can
also be bowed with a violin bow.)
The pitch depends on the point at which the bell
has been struck. It is easily possible to obtain dif-
ferences of a whole tone. If the bell is struck at
definite points, both tones are excited and the
result is a familiar “beating” (periodic increase and
decrease in volume at varying speeds).
Reasons: bells are curved vibrating plates. The
overtones are mostly not in harmony with the
fundamental tone. Bells too exhibit specific vibrat-
ing regions while they are chiming
14. Standing waves
• Make a simple knot in the resonance wire and
attach it by the loop to the handle of a door.
• Make the wire moderately taut and gently
move it round in circles.
• Now make the wire tighter and spin it faster.
When moved gently, nodes arise at both ends of
the wire and an antinode is created in the middle
of the wire. When moved faster, three nodes and
two antinodes are formed, and when moved even
faster, four nodes and three antinodes are formed.
Reasons: owing to the reflection at the door han-
dle, standing waves are formed. Due to persistence
of vision, the original and reflected waves appear
to be simultaneous. In its fundamental mode, the
whole of the wire vibrates in one length, thus de-
scribing one half-wave. One antinode is observed in
the middle of the wire with nodes at both ends. In
the case of a first harmonic (octave), the wire vi-
brates describes the form of a complete wave (two
antinodes and three nodes); for the second har-
monic, there are three antinodes and 4 nodes; and
so on.
15. Overtones
• First blow the whistle gently, then blow it very
hard.
Initially, a fundamental tone is heard. When the
whistle is blown hard, a much higher tone can be
heard.
Reasons: since the whistle is closed at one end
standing waves are always formed with a node at
the base and an antinode at the blade opening.
This is the case when the length of the whistle is
exactly 1/4 of the wavelength. It is also the case if
the distance of the opening from the base is 3/4,
5/4, 7/4, etc. of the wavelength.
Apart from the fundamental tone, all the possible
odd overtones or harmonics from the harmonic
series are produced at varying degrees of intensity.
The fact that every musical instrument has a very
characteristic timbre can be attributed solely to the
presence of individual harmonics of this kind ap-
pearing to a greater or lesser degree.
16. Measurement of wavelength
• Seal off the end of the 45-cm glass tube (21)
with the rubber cap and, holding the tube at
an angle, put a small quantity of lycopodium
powder into the tube using a teaspoon. Care-
fully spread a moderate quantity of the powder
uniformly to form a fine yellow strip in the
tube.
• Attach the glass tube by means of the retaining
clip, table clamp and plastic block.
• Strike the tuning fork (1700 Hz) hard on the
handle of the hammer and hold one prong di-
rectly alongside the opening of the tube. If
necessary, repeat this acoustic excitation sev-
eral times.
At the antinodes, the lycopodium powder begins to
resonate strongly, whereas it is absolutely static at
the nodes. The powder particles fall to the base of
the tube and form periodic clusters that repeat 4½
times along the axis of the tube.
Reasons: the light-metal tuning fork has a fre-
quency of 1700 vibrations per second. According to
the following equation:
Frequenncy
Speed
Wavelength
=
m
Hz
s m
⋅ =
⋅
⋅
2 . 0
1700
/ 340
The corresponding wavelength is 20 cm. Thus, 4½
half-waves or 2 full waves and one quarter wave can
“fit” in a 45-cm-long tube, as demonstrated in the
experiment. At the opening of the tube, there is
always an antinode and there is always a node at
the base of the tube.
17. Soundboard
• Hit the tuning fork that produces the note
a’ = 440 Hertz hard using the metallophone
beater and push the stem down onto the table
top.
Simply by holding it on the table, the barely audi-
ble tone produced by the tuning fork is amplified
to such an extent that it is now clearly heard
throughout the room.
Reasons: owing to the rising and falling vibrations
in the shaft of the tuning fork, the surface of the
table begins to resonate. Since the effective table
surface is much larger than the tuning fork, the
loudness of the tone is considerably intensified.