Speed of Sound Pre-Lab Purposes

Speed of Sound Pre-Lab
Purposes:
The three purposes of this lab are (1) to use resonance to determine the speed of sound in air
trapped within a closed tube from the wavelength and frequency of a set of tuning forks, (2) to
determine the speed of sound in air from room temperature, and (3) to determine the relationship
between wavelength and frequency when the speed of sound is held constant.
Introduction:
We know that the speed of a mechanical wave is set by the medium in which it travels. In this
case, the sound waves are traveling through a column of air inside the tube. They hit the water
boundary and reflect back through the air to the open end of the tube. Since they are traveling
through air the entire time, the speed is set by the air itself. The speed of sound in air can be
calculated with the equation Vsound = 331.5 m/s + (0.6 m/s/°C • Tempair).
The speed of sound waves (v) is related to the frequency (f) and wavelength (λ) by the wave
formula V = f λ. We also know that for a pipe filled with air, with one open end and one closed end,
a standing wave is possible if there is a node at the closed end and an antinode at (or near) the
open end. In this experiment, the ‘closed end’ corresponds to the surface of the water, and the
‘open end’ is always at the top of the tube. The apparatus allows you to change the length of the air
column by changing the water level. A tuning fork of known frequency (f) held near the open end
will be used to send sound waves down the pipe. When the water level is at a standing wave node,
the standing wave pattern is reinforced, and the sound intensity increases noticeably due to
resonance. We also know that sound is a mechanical wave; therefore, the speed of sound
depends on the properties of the air itself. The primary factor that determines the speed of sound
in air is the air temperature.
We also know the sound waves will resonate inside a closed tube when the length of the tube is
equal to ¼ λ. We can use the length of a column of air within a resonating tube to determine the
wavelength of the sound wave; and then, combine that with the frequency of the sound wave to
determine the speed of sound within the resonating tube.
Procedure
1. Obtain a tuning fork. Record its known frequency.
2. Record the air temperature of the room.
3. Strike the tuning fork with the tuning fork hammer, a shoe with a rubber sole, or on a book. Do
not allow the ends of the fork to hit each other. Hold the tuning fork about 2 cm above the tube.
Slowly lower the water level until the sound is amplified. Move the water and down to be sure
to find the loudest point.
4. When the sound is at its loudest point, measure length (L), the distance from the top of the
water to the top of the tube. Convert this distance into meters and record your data.
Centimeters ÷ 100 = meters i.e. 15.5 cm = 0.155 m
5. Repeat steps 2-4 until you have tested 8 different tuning forks with different frequencies.
Always start with the water at its highest point. (You should do 512 Hz first and then do the
other tuning forks in order)
6. Calculate wavelength and velocity of sound by using the equations given in the data table.
Name Speed of Sound Pre-Lab
Hypothesis Statements: (Make a prediction statement and use a “because” statement to
justify your prediction)
1. Write a hypothesis statement about the relationship between the wavelength and frequency.
2. Write a hypothesis statement describing a wavelength vs. frequency graph. Describe the graph
in terms of “direct or indirect” and “linear or exponential”.
Understanding the Lab
In this lab we will be using two methods to determine the speed of sound in air. We will measure
the air temperature and we will use resonance within the tube.
1. Describe the factor(s) that determine the speed of a mechanical wave (in general – not just
sound waves). Give at least two examples to support your answer.
2. What single factor is most responsible for the speed of sound in air? Explain why.
3. In the space below draw a quick “sketch” of what you think the wavelength vs. frequency graph
will look like. Include a line/curve for each set of data. Briefly explain why you think it will take this
shape.