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

New Trends in Physics Teaching IV
Figure 14.
So far, we have provided examples of single reeds, but a double reed can be made from a plastic
soda straw by cutting slits on either side at the end, as shown in figure 14b. Some practice is
required in blowing this device. It is said that chewing the cut end between one’s molars so that it
is flat greatly improves the response. Press the reed between the lips while blowing - the range of
pressure under which you can make the reed sound is restricted, and you need to blow rather
hard. This is similar to the double reed in an oboe. As before, you can insert it into a paper cone
or Styrofoam cup, to demonstrate how the cone matches the impedance of the instrument to the
air, and hence greatly increases the volume of sound. Now try cutting the tube of the reed pipes
shorter and shorter. Note how the pitch of the reed goes up and at some point the reed stops
functioning. Clearly, the air in the tube is necessary to the functioning of the reed, even though
the length does not determine the pitch in the same way as it does for an open pipe. After
cutting the pipe shorter, roll a tube of paper around the outside and tape it as shown in figure
1 Sa. Slide the outer tube up and down. The pitch goes up and down as you slide the tube in and
out and you can measure the frequency as a function of the length. Listen to the timbre of the
note produced. It is almost a quacking or bleating kind of tone. This is because a puff of air is
allowed into the pipe each time the flap opens and closes, and such sharp puffs have a lot of high
frequencies in them, in addition to the lowest frequency, or fundamental. Now, cut the corners
off the paper reed, as shown in figure 15b. (Take care that the reed covers the tube - otherwise
it wil not work.) Observe that the pitch goes up. The natural frequency of vibration of the reed
is higher, the smaller the mass oscillating.
Figure 15.
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