New trends in physics teaching, v.4; The ... - unesdoc - Unesco

String and tape experiments
In an orchestral wind instrument the reed is not free to vibrate at its natural frequency, but is
forced to oscillate at the resonant frequency of the tube of the instrument. The paper reed can
only vibrate at a low frequency, so you would need a tube about a metre long to be able to bring
this reed into resonance with the fundamental mode. If you have such a tube you could try it out.
We can examine the way the voice works using this device. The paper or styrofoam cup, which
we placed over the end of the tube, has its own resonant frequency. The vocal tract (larynx,
mouth) behaves similarly in the case of the voice. The resonance is at a high frequency and tends
to emphasize frequencies produced by the reed or vocal cords in this vicinity - these resonant
frequencies are called formants in the case of the voice, and determine whether you are saying
‘00’ or ‘ah’, even if your voice holds the same basic fundamental pitch.
While sucking on the reed, close the cup partially with one hand, then open it again. Doing
so alters the formants, and it is quite easy to get the device to say ‘ma ma’ or even more difficult
vowel sounds with a little practice. (This is similar to the ‘wa wa’ sound obtained by opening and
closing the mute on a trumpet.)
Transverse waves
It is difficult for students to get an intuitive feeling for wave motion. The next experiment is a
simple modification of a well-known wave machine. It also makes an excellent mobile to hang
in the classroom. You wil need only 1 m sticky tape, about two dozen drinking straws, and
twice as many paper clips. Unwind about 70 cm of the tape. Fold over one end and stick it to the
table. Run the tape along the table, and fold over and stick the other end down, as shown in
figure 16a. Place one paper clip in each end of every drinking straw, as shown in the figure, and
stick the centre of the straws at 2.5 cm intervals along the tape, until all twenty four are attached.
Then stick the device to the edge of the table, as shown in the lower part of the figure.
Figure 16a.
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