Measurement of the speed of sound in air

A column of air in a tube, closed at one end, has a natural frequency of vibration at a particular length. If a vibrating tuning fork is placed over the tube and the length of the column of air is altered, it is possible to find the length that resonates with the tuning fork. At this point a loud sound is heard. In it's fundamental mode of vibration the length of the air column is approximately equal to one quarter of a wavelength. By measuring the length of the air column (l) and the diameter of the tube (d) it is possible to calculate the speed of sound in air (c) using the formula: c = 4f ( l + 0.3d), where f is the frequency of the tuning fork. Note: In this simulation I have not included sound, so the position of resonance is found from the shape of the wave (1/4 wave).

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  1. Press the "Strike Fork" button.

  2. Adjust the height of the inner tube, using the slider, until resonance occurs. You will recognise resonance (at 1/4 wavelength) when the wave pattern produces a curved V shape.

  3. At resonance, press the "Get Ruler" button. Record the length (l) from the top of the water to the top of the tube and the frequency of the tuning fork.

  4. Press "New Fork". Record the frequency f.

  5. Repeat steps 1 to 3 until you have got a resonant length for each tuning fork.

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