Speed of Sound

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  • Topic: Wave, Standing wave, Waves
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Faculty of Engineering
Physics Laboratory I

1. EXPERIMENT : Speed of sound

2. OBJECTIVE: : (1) To determine the wavelength of a sound in resonance air column. (2) To determine the speed of sound in air at room temperature.

3. APPARATUS : Resonance tube (air column) attached with water container and meter stick, thermometer, function generator, speaker.

4. THEORY: : Sound is a longitudinal wave in a medium. If n is the frequency and is the wavelength of the standing wave, than the speed of the sound at the temperature t c is given by:

vt = (1)

Wavelength (ג)

The speed of sound in air at 0 °C is 331.5 m/s, and as the temperature rises it increases at the rate of about 60 cm/s per degree centigrade. Hence the speed of sound vt at temperature t is obtained from the speed v0 at 0 °C by the relation

vt = v0 + 0.6 t (m/s)(2)
vt = [ 331.5 + 0.6 t ] (m/s)


1.Record the frequency of speaker (source of vibrating object) from the function generator.

2.Record the temperatures of the room in the region near your apparatus.

3.Fill the water in to the glass tube (column) nearly full.

4.Switch on the function generator and while holding the speaker at the top of the air column.

5.Adjust the water level for the shortest length of the air column, which gives a maximum resonant sound (very loud sound). [Raising and lowering the attached reservoir R can adjust the water level or length of air column.] Mark the point and move the water level past the mark a few times to check the correctness of the position selected.

6.Measure the distance of top of the tube to the first resonance position or the length of air column.

7.Now lower the water level until the second lower resonant position is found.

8.Continue in this until the third lower resonant position is found.

9.Measure the positions of all the resonance levels located and record the length in the date table.

10.Use different frequency to repeat all the preceding operations and record as before.

11.Subtract the reading of the first resonance position from second, third, etc. and record as L  L1 and L  L2. [Where L1 is the distance to the first resonance position and L is the resonance position from second, third, etc.]

12.The first difference of L  L1 will be , the next difference L  L2, will be . From each of these differences determine , and then compute the average .

13.Use the average value of the wavelengths and calculate the speed of sound in air at room temperature for each frequency used.

14.Calculate the theoretical value of speed of sound using the room temperature.

The simplest sound waves are sinusoidal waves, which have definite frequency, amplitude and wavelength (. ). Human ear is sensitive to waves in the frequency range from about 20 to 20,000 Hz, called audible range. The wave, which has below the audible frequency range, is called infrasonic (infrasound) and above the audible range is ultrasonic (ultrasound). If a tuning fork or a speaker is set in vibration and held over an air column the loudness of its sound will be greatly increased if the air column is of such a length as to vibrate in harmony with the tuning fork or speaker. Such an air column is said to be in resonance with the tuning fork or speaker.

Wavelength (ג) Figure 1

When we hear resonance sound, the waves set up in the air column are called standing waves. [See figure 1] The interference of two sinusoidal waves having the same wavelength, frequency and amplitude but moving...
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