Experiment VIII

Waves and Resonance:

The Velocity of Transverse Waves On a Vibrating String

Introduction and Objective

The velocity of propagation of vibrational waves through a medium can depend not only on the properties of the medium, but also on the external conditions imposed on it. It is reasonable to suppose, for example, that the velocity with which transverse vibrational waves propagate down a string or flexible wire would depend both on the tension, T, in the string and the mass per unit length (linear mass density), µ, of the string. The tension supplies the restoring force when the string is given a transverse displacement and the linear density determines the inertia of the string and, therefore, its speed of response to the tension. On this basis it would be expected that the velocity of propagation would increase with increasing tension T and decrease with increasing mass density, µ. A first guess might be V = KT/µ where K is a dimensionless constant, but as you can and should show, this cannot be correct from dimensional considerations. This being the case, a next logical step is to assume a relationship of the form V = KTa/mb and to take as your objective the determination of the coefficients K, a and b experimentally.

Equipment

60 Hz electric vibrator, several strings of different linear density, pulley, swivel clamp, two table clamps, two 12" rods, a set of slotted weights, and a meter stick.

The string is held horizontally by the vibrator at one end and by weights hung over a pulley at the other end so that the tension may be varied. A sketch of the set-up is shown below.

Theory of the measurements

The velocity of propagation of waves on a string can be readily measured by setting up a standing wave pattern using a simple harmonic vibrator of known frequency. The vibrator sends down the string a wave whose mathematical representation is y1 = A sin (2ft - 2p/lx) where x is any location on the string between 0 and L (L being the length) t is the time, f is the frequency and l is the wavelength. The number of waves generated per second (f) multiplied by the length of each (l) is just the distance traveled per second by the wave, that is, the velocity V = fl. The wave sent down the string by the vibrator will reflect at the end (x = L) and return to the vibrator where it will again reflect. Leaving the vibrator again it will now have the form Y2 = A sin [2ft - 2p/l(x + 2L)] as it has already traveled 2L along the string. Since the vibrator also continues to create waves of the form of Y1, the net effect is obtained by adding Y2 to Y1.

How the two waves combine depends on the relationship l between and L; suppose 2L = ln where n is an integer (1,2,3, etc). Then the length L is an integer multiple of one-half wavelength or L = nl/2. In this case, 2p/l(x + 2L) = 2p/l(x + n) = 2p/l x + 2pn. Now when an integer multiple of 2p is added to the argument of the sine function nothing is changed, thus when 2L = nl, Y1 = Y2 and as the 2 waves add peak to peak or what is called constructive interference. As waves Y2 continue one after another to reflect off the vibrator they combine constructively with waves Y1 generated by the vibrator. The net result is the build up of a large wave (much larger than the amplitude of the vibrator) and the result is called resonance. The result is also called a standing wave as it does not appear that waves are moving along the string. To see why this is so, consider now the reflection off the far end of the string. This wave can be represented by Y3 = -A sin [2ft - 2p/l(2L - x)]. (The minus sign in front corresponds to the fact that when the wave reflects off the fixed end at x = L it flips over. Note that when the wave is at x = L the expression shows that the wave has traveled a distance L and when it returns to x = 0 the distance traveled is 2L as required). Now if the condition for resonance, 2L = nl, is fulfilled, then 2p/l(2L - x) = 2pn - 2xp/l and Y3 = -A sin (2ft + 2p/l x). If this wave is added to the wave Y1 = A sin (2ft - 2p/l x) coming down from the vibrator, the sum Y1 + Y3 is

Y1 + Y3 = 2A cos 2ft sin 2p/l x. (8.1)

(This follows from the formulas for sin (a + b) and sin (a - b).)

Note that this sum is always zero at certain values of x, namely when x = nl/2 for when sin 2p/l x = sin 2pnl/l2 = sin pn = 0. These stationary points which are separated by one-half wavelength are called nodes as the string does not move there. Hence at resonance one does not see any wave traveling back and forth. The vibration pattern shows motion only in the Y direction. The maximum Y or transverse displacement (antinodes) occur halfway between the nodes as sin 2p/l x has its largest values (±1) at these points. The net results is called a standing wave pattern. The figure below shows some of the possible wave patterns in order of increasing numbers of nodes.

Procedure

Set up standing wave patterns of 60 Hz frequency on your string. As the discussion above shows, by measuring the distance between nodes you can determine the wavelength and then, knowing the frequency, you can calculate the wave velocity. Since the length of string and frequency is fixed by the apparatus, the only quantity that may be adjusted to reach a resonance condition is the velocity which can be varied by changing the tension.

There will be nylon strings of several different linear densities available. Your instructor will assign your group one of these. Vary the tension in your string to produce at least 5 different standing wave patterns. It is a good idea to start with enough tension to get 3 or 4 nodes not counting the ends. Then you can increase or decrease T as needed for other patterns. Do not try to produce the fundamental or lst overtone as these patterns have too few nodes to be useful and the required tension is excessive.

The table below shows the approximate range of masses which must be hung on nylon fishing lines of various strengths to achieve resonance over the range of harmonics shown.

Table 8.1

Line

strength

Range of

harmonics

Range of hanging mass

(kg)

80 lb

4 - 9

2.0 - 0.40

60 lb

3 - 9

2.3 - 0.25

40 lb

3 - 9

1.1 - 0.10

20 lb

3 - 9

0.6 - 0.06

10 lb

3 - 7

0.3 - 0.05



Note that because V = fl, if you wish to decrease l so as to produce more nodes, V will have to be decreased by reducing the tension. Hence, the larger the tension, the fewer the nodes as the table above shows.

Certain combinations of vibrators, strings and tensions may result in vibration at 120 Hz rather then 60 Hz. Should you find that a particular tension produces a much shorter wavelength than the table above would indicate, it is likely that the vibration frequency is 120 Hz in that case. There is no need to discard such data. When you prepare your table of V and T for computer analysis, 120 Hz data will be recognizable.

Record the distances between nodes, the number of nodes not counting the ends and the tension. (Remember that tension is a force). Note that when you are near resonance you can lift up or pull down on the weights a slight amount to see if more or less tension is needed. Try to get as close to resonance (maximum amplitude) as possible. Note that the vibrator and pulley ends are not quite fixed and are therefore not quite true nodes. Therefore do not include the distances from either end to the first adjacent node in your measurements.

Determine the wavelength and velocity of the waves traveling on your string for each of your tensions. Since the assumption is that V = KTa/mb a log-log plot of V versus T for a given string (constant µ) should be a plot of the equation

Similarly a log-log plot of V versus µ for constant T should be a plot of the equation

The data from your group will allow you to make a log-log plot and analysis of V versus T for the fixed density of your string. Use Quattro Pro and load the file "Waves.wb3". Fill in the values in the table. Then plot V versus T. Follow the instructions on the screen to convert the linear plot to a log-log plot.

Do a "Linear Regression Fit" of log V (dependent) against log T (independent). Do your data support the assumed relationship between V and T? What is your value for a, i.e., what is the dependence of V on T? Print your data and graph.

You will need to combine your data with those of other groups to make the log-log plot of V versus µ for fixed T in order to determine b. Each group will have a fit of the data for their string to the equation,

Hence, by using each group's values for the y-intercept, c and slope, m, a value for V can be calculated for each string for a common chosen tension (say 4.0 N). These values of V can then be used to make a log-log plot of V versus µ. (Your instructor will either give you the values for the different string densities or show you how to measure them. Use units of kg/m for these values).

Enter the combined values of V and µ in the second table. Then make a log-log plot of V versus µ. Do a "Linear Regression Fit" of log V (dependent) against log µ (independent). Do the data support the assumed relationship between V and µ? What value do you obtain for ? Print your data and graph.

Discussion

From your experimental results try to infer what values for a and b are likely to be obtained on a theoretical basis. Assume that simple results are the most likely. Do your experimental values for and agree within experimental error with your inferred values? Do your inferred values for and lead to a dimensionally correct result for V?

If time permits, you can determine the value for K in the relation V = KTa/mb as follows. From the above relation,

In comparison, the equation obtained by each group from their analysis is:

Hence,

Thus, a value for K can be determined from the data for each string by first calculating log µ for each string and then combining it with the value of c for that string.

Average the values of K so obtained. What do you infer to be the theoretical value for K?



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© 1997 Dr. H. K. Ng.
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