Introduction

Goals

The goal of this lab is to investigate some aspects of sound and learn to use some basic data acquisition tools.

You should be familiar with the basics of sound waves and their mathematical descriptions before starting the lab.

You should keep records in a notebook of all your calculations and results from this lab. Even though you will have to enter a few results on the computer, you should also record these by hand. The data you enter on the computer may not be available later on.

Equipment check

Please make sure your station has all of the following items. If not, check again, then talk to your lab instructor.

• Tuning Fork
• Microphone
• LabQuest Mini & USB connection
• cardboard tube

Background

This lab will investigate some of the basic properties of sound.

Below is a little animation showing the basics of a sound wave. A sound wave is created by a source, like a speaker or something. That source moves back and forth and creates changes in the pressure (and density) of air molecules.

Click the button on the sim below to see a sound wave propagating outward from a source. The source in this simple animation is on the left. It moves back and forth and creates the waves in the medium, which in the case of sound, can be air. The little gray dots are all air molecules. Notice how each particle moves back and forth, and does simply move away from the source in a straight line. (Several of the air molecules are colored red to help keep track of them)

What you're seeing as sound waves is different regions of high and low pressure (or high and low density of molecules) moving across the screen. When those regions of high and low density interact with your ear, it creates the perception of sound.

If you play with the slider on the simulation, you'll notice a change in the sound wave appearance. That slider is changing the wavelength of the sound wave. You can make it shorter or longer. This also changes the frequency of the sound, which you perceive as high or low pitched sounds.

Plotting Sounds

If we wanted to begin to quantify how the pressure is changing, we could plot the pressure at a given point as a function of time.

Watch the short animation below. It shows a qualitative pressure graph for the area highlighted in the blue square. You can see it has a sinusoidal shape, consistent with a wave phenomenon.

Warm Up Exercise

Let's try to record some sounds. You'll use your voice to generate sound waves and record them on the desktop computers.

Start the LoggerPro application which can be found on the desktop. The LabQuest Mini box should be connected via USB to the computer and the microphone should be connected to CH1 of the LabQuest Mini box. After starting LoggerPro, you should see the following screen.

The areas in red show points to check out before proceeding. Make sure the upper left section says: Sound Pressure. And notice the axis of the plot. On the horizontal axis is plotted time, and on the vertical axis is Sound Pressure. The time axis should only be showing a very small region of time: approximately 30 milliseconds, or 0.03 seconds. This is because we are going to take a lot of very fast measurements.

Using your voice, try to make a sound that is very pure and smooth sounding, like a 'dooooo' or 'deeeeee'.

If you don't make a very 'pure' tone, you might end up with something like the following graph:

Play around with recording different sounds your voice can make until you can see and articulate the difference between pure and impure tones.

Frequency and Period

Just a reminder, the relationship between frequency and period is an inverse relationship. If the frequency increases, the period will decrease: $$ f = \frac{1}{T} $$ Experiment with the plot below to see how increasing the frequency decreases the length of the period (the space between maximum values of the graph.)

Experiment 1: The tuning fork

Measure the frequency of the tuning fork using by measuring the sound waves.

The tuning fork is a very special instrument. Despite its simple appearance, it has several applications in medicine, such as the Rinne Test, as well as many uses in science and engineering.

Let's begin by learning to excite the tuning fork. All you need to do is hold the tuning fork by its base, and gently tap one tine against something not too soft, but also not too hard - usually a knee or the palm of your other hand is perfect. Do not try to excite it by hitting it hard against the table. Just a light tap against something soft will set the tines oscillating. You won't be able to hear the sound it makes unless you bring it very close to your ear, in which case, you should hear a pure sine wave tone. (If you do hear a loud clang sound, then you're doing it wrong!)

Use the LoggerPro interface to measure the period of your sound wave from the tuning fork, and then use that value to calculate the measured frequency of the tuning fork. Some tips:

1. You'll have to hold the tuning fork very close to the microphone (< 1 cm).
2. Figure out which orientation works the best.
3. Since the data is only recorded for a very short time, you'll need to excite the tuning fork first, then press the start button on LoggerPro

Fill in the values below, paying attention the units indicated in brackets.

Experiment 2: Your Voice

Repeat the above experiment, however, this time, use your voice to generate a sound wave and record that. Try to make a pure tone, as you hopefully figure out how to above. You'll goal is to find the frequency of the note you sing.

Experiment 3: Measure the Speed of Sound

For this experiment, you'll use the cardboard tube to create an echo chamber in which you can measure the speed of sound.

You'll need to adjust the Data Collection settings so that instead of just recording when you press the start button, the Logger software will wait until it gets 'triggered' by a sound. This setting is found in the Experiment->Data Collection menu item. Turn on the trigger function as shown in the figure below.

Now, place the microphone just at the open end of the tube as shown in the diagram.

You can then start the acquisition software and then when you make a click with your mouth or snap your fingers near the open end of the tube, it will trigger the recording to start. The sound will travel down the length of the tube, and reflect back from the closed end. You should see two sounds measured: the first is the initial sound, the second is the echo, which the microphone will detect after the sound travels down and back the length of the tube.

Enter the following results from your measurements and calculations.

Discussion questions