3B Scientific Acoustics Kit - Manual (Page 3)

3
3. Technical data
Dimensions: 530 x 375 x 155 mm 3 approx.
Weight: 4.5 kg approx.
4. Sample experiments
1. String tones
• Pluck the monochord string hard when it is
moderately taut.
• Subsequently increase the tension on the string
by turning the peg to the right. Pluck the string
again.
At first, a low tone is heard. As the string is tight-
ened the tone gets higher.
Reasons: vibrating strings generate acoustic tones
by inducing alternating compression and rarefac-
tion of the surrounding air. The greater the tension
in the string, the faster the vibrations are and the
higher the tone.
2. Pure acoustic tones
• Hit the 440 Hz tuning fork hard with the metal-
lophone beater.
A pure acoustic tone of a very specific, unchanging
pitch can be heard. This tone dies away very slowly.
Reasons: a tuning fork consists of a U-shaped steel
piece which merges into the stem at its vertex. As
the tuning fork only vibrates in one oscillation
mode (with both prongs either both moving apart
or both moving towards one another), it produces a
pure tone of an unchanging pitch. Owing to its
property of producing a constant pitch, tuning
forks are used for tuning musical instruments.
3. Vibrating air columns
• Attach the glass tube for demonstrating a
closed air column by means of the table clamp,
plastic block and retaining clip.
• Insert the tuning plunger into the glass tube.
• Hit the 440 Hz tuning fork hard with the metal-
lophone beater. By pulling out the plunger to a
greater or lesser degree it is possible to alter
the length of the closed air column.
There is only one plunger position at which the air
column resonates strongly. At any other position
there is no sound. Resonance can be detected by
the increase in sound volume.
Reasons: a closed air column starts resonating
when its length corresponds to one quarter of the
excitation wavelength. The tuning fork vibrates
with a frequency of 440 vibrations per second.
Applying the following equation:
Frequency
pagation Speedofpro
Wavekength
=
cm
s freq Exciting
s cm
⋅ =
⋅
⋅
2 . 77
/ 440
/ 34000
the wavelength of the tone produced is 77.2 cm.
One quarter of this wavelength is therefore
19.3 cm.
The distance between the plunger and the opening
at the end of the tube is 19.3 cm when resonance
occurs.
4. Open air column
• Conduct the same experiment with an open air
column (14).
The open air column, which is exactly double the
length of the closed air column, starts resonating
when the tuning fork is brought into its vicinity, as
can be heard by means of the increased volume.
Reasons: an open air column starts resonating
when its length is half that of the wavelength or
multiples of that length. Antinodes are formed at
the ends of the open air column and a node at the
middle.
5. Whistle
• Blow the whistle and change its length by
gradually drawing out the plunger.
Depending on the length of the whistle, its note
gets higher or lower but the character or timbre of
the note remains the same.
Reasons: blowing a uniform air stream into the
opening of a whistle causes the air trapped in the
pipe to vibrate and eddies then occur at regular
intervals air the air passes over the blade. The
resulting tone depends on the length of the air
column. In the case of a closed air column, the
length of the whistle (measured from the edge of
the blade to the base of the whistle) corresponds to
a quarter wavelength of the base tone. A node is
formed at the blade of the whistle and an antinode
is formed at the end of the pipe
6. Vibrating bars
• Use the striking hammer supplied to strike
several bars of the metallophone. When the
metal bars are struck, they produce a distinct,
melodious note, each of which has a similar
timbre. The shorter the length of the bar, the
higher the tone.
Reasons: elastic rods form systems capable of oscil-
lating if they are resting upon a point where a node
is formed (about 22% of the total length between
the two ends).