Monday’s Lesson — Investigating Sound

by Ed Hazzard

The world of sound is extraordinarily rich—rich in meaning, emotion, cultural content, and opportunities for scientific investigation. All sounds are mixtures of different frequencies, which the ear hears simultaneously and the brain interprets with exquisite subtlety. On the one hand, our perception blends together the parts of a sound and gives us a single idea, such as which friend is on the other end of the phone, what species of bird is calling, or whose baby is crying. On the other hand, it teases sounds apart so we can comprehend a single conversation at a noisy party or identify each of the instruments in an orchestra.

Figure 1: Recordings of two vowels— “ahh” and “eee”—with the same pitch. The graph is amplitude vs. time, over about 30 milliseconds.

To investigate sound, it is essential to be able to “see” frequencies and “freeze” snapshots of sound waves. Frequency and waveform displays can pull apart the components of a sound and stop it in time, allowing students to develop a scientific understanding of sound that expands their natural appreciation of the world around them.

The Sound Grapher

Displaying sounds is relatively easy on a computer. Indeed, most laptops have a built-in microphone. With this basic equipment, students have a ready-made sophisticated scientific instrument. All they need is some simple software and ideas about how to use it.

The Concord Consortium has developed a cross-platform Sound Grapher for the Technology Enhanced Elementary and Middle School Science (TEEMSS) project, which creates sensor-based investigations for students in grades 3-8.

You can find the Sound Grapher on the enclosed CD or at our website.

Technical notes: If you need to connect an external microphone, refer to the technical hint at the download site. Although there is a volume control in the Sound Grapher, you should increase the sound input level on your computer for best results.

The Sound Grapher represents a sound in two ways. Waves shows a snapshot of the waveform, like an oscilloscope. The vertical axis is air pressure, and the horizontal axis is time (about 30 milliseconds). Frequencies displays the amplitude (y-axis) of each frequency that is present in the sound (x-axis).

Compare two sounds

The Sound Grapher has two screens so sounds can be compared.

1. Click on either screen to make it active. Press Record and speak into the microphone. Press Stop after a few seconds.
   
   
2. Choose the second screen, record a sound, and take another snapshot (by pressing Record, then Stop).

Observe the shapes of the two graphs. Try speaking the vowels—a, e, i, o, u. Do they look different as well as sound different? Try to get pure sounds, and ask yourself what pure means. Try low and high sounds, loud and soft sounds. Also try toneless noises, like “shhh.” Is a beautiful sound also a beautiful wave?

Figure 2: Two vowels—“ahh” and “eee”—using the Frequencies mode.

You can adjust various parameters, such as amplitude and the time length of the graph by clicking on the settings.

A single frequency appears as a simple sine wave. When more than one frequency is present in the sound, the waveform is a combination of several sine waves and looks more complex. It is difficult to tell what is happening from the waveform alone, so an additional analysis tool is provided. Click on the Waves button, and change the drop-down menu to Frequencies. The screen now displays the distribution of frequencies that are present in the sound.

The “ahh” sound (figure 1, top screen) has a primary frequency and lesser amounts of several higher frequencies, called overtones. The “eee” (figure 1, bottom screen) is a single frequency plus one overtone.

The Frequencies mode (figure 2) also includes a settings button, so you can change the amplitude and the range of frequencies displayed. The default range is 0 – 2000 Hz.

Try whistling, humming, clapping, and making a “shhh” noise. The whistle is a nearly pure note—it will have just one frequency. Humming includes a mixture of several frequencies, with a main pitch and a number of overtones, which are two and three times the frequency of the main pitch. The “shhh” noise, by contrast, has many frequencies mixed together, with little relationship to each other.

Music to my ears

The traditional musical definition of a beautiful sound is one in which the relationships among the frequencies are simple ratios, such as 2:1, 3:2, and so forth. On a stringed instrument, these are the same ratios as the lengths of the string. For instance, if you make a violin string half as long by pressing it in the middle, the frequency is twice as high.

1. Strike a bell very sharply and record it with the Sound Grapher’s Frequencies mode. The first sound is jarring and discordant, and the mix of frequencies is chaotic. As the bell sound fades, it becomes more pleasant. Notice that the waveform becomes smoother and overtones appear that are simple multiples of the basic pitch.
   
   
2. Play pairs of notes on a piano and record them with the Sound Grapher. The ratio of the two frequencies for an octave is exactly 2:1; the ratio for a fifth is 3:2; the ratio for a fourth is 4:3, and so on. As this ratio becomes a pair of larger whole numbers, such as 17:16, we perceive the interval to be more dissonant.
   
   Another factor in “beautiful” sounds is the quantity of overtones compared to the basic pitch. Many singers and instrumentalists work hard to add overtones, giving their sound greater richness, depth, and carrying power.
   
   
3. Explore the characteristic overtones of several instruments. A flute is quite pure, an oboe is quite complex with very prominent overtones, and a clarinet has only odd-numbered overtones!

Figure 3: Two superimposed sounds—one person singing “eee” and another whistling— viewed as Frequencies (top) and Waves (bottom).

Superimposed sounds

Try recording two simultaneous sounds (figure 3).

The Frequencies mode approximates the way the ear works. Thousands of hairs in the inner ear each respond to a different frequency. The ear sends signals to the brain indicating the intensity of the sound at each frequency. But that is only the simplest description of what happens. What the brain does with this information is truly astonishing.

The Sound Grapher does not begin to reveal the subtleties the ear is capable of recognizing, such as the flow of language, inflections, dialects, and rhythms. Still, with the Sound Grapher, students can begin to delve into the science of sound. Perhaps then they will ponder the question: how can a bird pick out its offspring from a thousand others in a nesting ground only by its tiny peeping voice?

Ed Hazzard (ehazzard@concord.org) is Curriculum Developer of the TEEMSS Project.