Resonance Tubes - Florida State University



Resonance Tubes

Background:

A pendulum swings back and forth at a natural frequency in the same way a child moves on a swing a natural frequency. If you push the child too quickly (or too slowly) they won’t continue to swing higher and higher. If you add energy to a system (like pushing the child on the swing) using the same resonant frequency, you will receive the maximum energy output. In short, resonance happens every time an object is allowed to vibrate at its natural frequency.

We know the speed of sound in air is 340 meters/second. We also know the frequency of the sound from the tuning forks used in the lab. If we take our wave speed formula and rearrange it, we can find out the wavelength of the sound wave produced in the tube.

Materials:

Tuning fork #1 – 256 Hz

Tuning fork #2 – 384 Hz

Tuning fork #3 – 512 Hz

Rubber hammer

Resonance tube

Tall glass or plastic water-tight container

Metric Ruler

Water

Procedure:

1. Obtain the three tuning forks. Record the frequency of the forks in the data table.

2. Fill the container to the top with water.

3. Gently strike the first tuning fork with the hammer and hold the vibrating prongs about 2 cm above the end of the tube.

4. As you hold the vibrating fork over the tube, move the tube and fork up and down until you hear the loudest sound.

5. Hold the resonance tube still at the level that produced the loudest sound measure the length from the top of the tube to the surface of the water. Write this in your data table under “Length of air column”. Note: If you measure it in centimeters, you will need to convert it to meters.

If you want to increase your accuracy, repeat the experiment two more times and take the average distance.

6. Repeat steps 2-5 using the 2nd and 3rd tuning forks. Record your results in the data table.

7. Complete the data table by calculating the wavelength of each tuning fork.

Data and Results:

|Tuning Fork |Frequency (Hz) |Length of air column (meters) |Wavelength (m) |

| | | | |

|1 | | | |

| | | | |

|2 | | | |

| | | | |

|3 | | | |

Questions:

1) Which tuning fork produced the shortest wavelength sound wave?

2) Which tuning fork produced the longest wavelength sound wave?

3) Resonance occurs when an object vibrates at its ____________ frequency.

a) Lowest

b) Highest

c) Natural

d) Longest

4) True or False: If you continued this experiment with tuning forks that had higher frequencies, you would find they produce sound waves with longer wavelengths.

5) The length of a resonant air column that is closed at one end is _______________ times the length of the fundamental wavelength of the sound wave it produces. For a hint, look at the picture of instrument and the wave inside of it.

a) 1

b) 4

c) 3

d) ¼

-----------------------

[pic]

[pic]

Closed end

Open end

¼ wavelength

¾ wavelength

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