PASCO PS-2006 GLX Power Amplifier - Manual (Page 21)

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P S - 2 0 0 6 G L X P o w e r A m p l i f i e r S a m p l e E x p e r i m e n t s : St a n d i n g W a v e o n a S t r i n g
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4. Setup the GLX for manual sampling:
a. Press + to open the Sensors screen.
b. Press to open the Mode menu.
c. Press the down arrow key to highlight Manual and press .
The Data Properties box opens.
d. With Measurement Name highlighted, press , type n, and
press .
The Data Properties box should now look like Figure 15.
e. Press (OK) to close the Data Properties box.
5. Hold and press to return to the Output screen.
Procedure
1. Press . The flashing icon appears to indicate that the GLX is
ready to record manually triggered points.
2. Turn the output on ( ).
3. Use the up or down arrow key to highlight the Frequency setting.
4. Press (or press and hold) to increase driving frequency until it
gets close to a resonant frequency. (See sidebar, “How to find a
Resonant Frequency.”)
5. Change the frequency step size to 0.1 Hz (press ).
6. Press and to adjust the frequency up and down until you
have found the resonant frequency (to the nearest 0.1 Hz).
7. Press . The GLX records the frequency and prompts you to
enter the number of segments.
8. Enter the number of segments and press .
9. Set the step size back to 1 Hz.
10. Repeat steps 4 through 9 for all harmonics up to 5 or 6 segments.
Adjust the output amplitude as needed.
11. When you have finished collecting all data, press to end the
data run.
12. Turn the output off ( ).
Analysis
1. Open the Graph screen and configure it as with Output Frequency
on the vertical axis and n on the horizontal axis (Figure 17).
2. From the Tools menu ( ), select Linear Fit.
Figure 15: Data Properties Box for
Manual Sampling
F4
F1
F1
Figure 16: (a) Node at tongue
(b) & (c) Node not at tongue
a
b
c
How to find a Resonant Frequency
At the string's resonant frequency, there
should be a node (a point of minimum
movement) where the string is attached
to the string vibrator (Figure 16a). Maxi-
mum amplitude does not necessarily
indicate the resonant frequency, espe-
cially at lower frequencies.
If the tongue of the string vibrator is mov-
ing excessively (Figure 16b), increase
the frequency until tongue movement is
almost imperceptible.
If a node occurs on the string close to
the tongue, but not at the attachment
point (Figure 16c), decrease the fre-
quency to move the node toward the
tongue.
F1
+
F2
+
Figure 17: f versus n
F1
F3