PASCO WA-9612 RESONANCE TUBE User Manual

Page 14

Resonance Tube

012-03541E

➁ Slowly increase the frequency and listen carefully. In general, the sound will become louder as

you increase the frequency because the function generator and speaker are more efficient at
higher frequencies. However, listen for a relative maximum in the sound level—a frequency
where there is a slight decrease in the sound level as you increase the frequency slightly. This
relative maximum indicates a resonance mode in the tube. Adjust the frequency carefully to find
the lowest frequency at which a relative maximum occurs. (You can also find the relative
maximum by watching the trace on the oscilloscope. When the signal height is a relative maxi-
mum, you have found a resonant frequency.)

➤ NOTE: It can be difficult to find resonant frequencies at low frequencies (0-300 Hz). If you

have trouble with this, try finding the higher frequency resonant modes first, then use your
knowledge of resonance modes in a tube to determine the lower resonant frequencies. Be sure
to check to make sure that resonance really occurs at those frequencies.

➂ Mount the microphone on the end of the probe arm and insert it into the tube through the hole in

the speaker/microphone stand. As you move the microphone down the length of the tube, note
the positions where the oscilloscope signal is a maximum and where it is a minimum. Record
these positions in table 2.1. You will not be able to move the probe completely down the tube
because the cord is too short. However, you can move the probe around to the opposite end of
the tube, to examine the other end of the tube. Pay particular attention to the wave characteristics
near the open end of the tube.

➃ Repeat the above procedure for at least five different resonant frequencies and record your

results on a separate sheet of paper.

➄ Insert the piston into the tube, as in Figure 2.2, until it reaches the maximum point that the

microphone can reach coming in from the speaker end.

➅ Find a resonant frequency for this new tube configuration. Use the microphone to locate the

maxima and minima for this closed tube configuration, recording your results in table 2.2.
Repeat this procedure for several different frequencies.

WA-9612

RESONANCE TUBE

Plunger

Piston

Figure 2.2 Using the Plunger