What does your voiceprint look like? You cannot see sound. But you can change the sound waves into electrical signals that can be displayed on a screen. Just as you found ways to visualize motion and to represent motion using various graphs, in this section you develop techniques to visually represent waves. This can enable you to study basic wave properties and to observe how waves combine to form new patterns.
You can go about this in two ways. One way is to use an oscilloscope, which is an instrument that takes an electrical signal and displays it in graphical form. Recently, a much lower cost alternative has become available that makes it possible to turn a computer into an oscilloscope.
This project focuses on how either type of oscilloscope can be used to study the wave properties of sound.
What You Need
- oscilloscopes, which range in cost from just under $600 to thousands of dollars
- sound card oscilloscope. You can turn your computer into a oscilloscope in several ways:
- PC sound card distributed for private and noncommercial use in educational institutions at www.zeinitz.de/Christian/Scope_en.html. (Oscilloscope images shown in this and other sections are based on this sound card oscilloscope and appear courtesy of C. Zeinitz.)
- Zelscope is available for a small charge at www.zelscope.com (this used to be called Winscope).
- tuning fork
- To connect microphone to computer. Microphones are either high- or lowimpedance connections and the computer input is typically a mini.
- Microphone output to oscilloscope input (typically BNC connector).
- Depending specifically on what connections you need to make, you can most likely find connectors at Radio Shack or build the connector you need.
- Caution: Sound card oscilloscopes can handle only low-voltage inputs, such as from microphones. Attempting to use a sound card oscilloscope for larger electrical signal may damage your sound card. A reference for how to assemble a high-impedance circuit that can enable using a sound card oscilloscope for higher voltages is given in Project 115.
- wave generator
- stand-alone device designed for this purpose
- keyboard with appropriate connectors
- waveform generator available with some computer oscilloscopes
Setting up the oscilloscope
- Connect the microphone to the oscilloscope input.
- Collect a test signal, such as your voice or a musical sound.
- Adjust the vertical scale, so the entire wave is displayed.
- Adjust the horizontal (time) scale, so the wave is displayed.
- If necessary, adjust the trigger to enable the wave to be properly displayed. (Chose continuous rather than single-event settings for the trigger.)
- Generate a pitch audibly with a tuning fork, a keyboard synthesizer, or by a waveform generator. (Depending on your setup, you can use the waveform generator to produce an audible signal through a loudspeaker or send it directly into the input of the oscilloscope.)
- Increase the pitch (frequency) and compare to the previous shape.
- Decrease the pitch and compare to the previous measurements.
- Increase the volume (amplitude) of the sound and observe how the wave changes.
- Try your voice using the microphone. How does that compare to a pure tone, such as produced by the tuning fork?
- Observe different waveform shapes, such as sinusoidal, triangular, square wave, and sawtooth. How do they sound? What musical instruments do each of the previous waveforms most closely resemble?
- Play various musical instruments and identify fundamental waveforms that appear to be present in the instruments' waveforms.
- Just for fun: Observe various samples of music. Can you distinguish various musical styles just by looking at the waveform?
- Can you recognize the "voice signature" of different people as crime labs do all the time on TV?
- Generate a tone or frequency. Let's say we start with 440 hertz (Hz), a concert A. Display this on Channel 1.
- Generate a second tone or frequency. Let's say we use 100 Hz. Display this on Channel 2.
- Many oscilloscopes let you display two signals on one display. If your oscilloscope has the capability to display two inputs on one display, show the combined signals from 1 and 2. How does the combined signal compare to the two individual signals?
- You can also accomplish this by generating two audible tones at the same time, such as playing two notes on a keyboard synthesizer at the same time. Sounding two tuning forks at the same time will also work.
Increased pitch shows up on the oscilloscope as increased frequency.
Increased volume is displayed as increased amplitude.
A tuning fork or a wave generator produces a pure sine wave. Figure 64-1 shows the relatively pure sine wave pattern produced by the flute setting of an electronic synthesizer playing a 440 Hz tone.
Sawtooth and triangular waves sound more "reedy," like a clarinet or saxophone.
Other sounds are complex mixtures of simpler forms. For instance, a synthesized rock organ consists of a wider range of overtones combined with the fundamental tone. Figure 64-2 shows several higher frequencies combined with a 440 Hz fundamental.
Adding two waveforms results in a combined sound. Figure 64-3 shows a 100 Hz tone and Figure 64-4 shows a 400 Hz tone.
Figure 64-5 shows both of these tones combined. The overall pattern shows how both of these tones add to produce a combined wave pattern.
Musical sounds are complex mixes of many individual frequencies with a large variety of overtones. Figure 64-6 is a sample from The Beatles and Figure 64-7 is an Allison Krause fiddle solo.
An oscilloscope can also show the mix of frequencies in a particular sound. For instance, a synthesizer violin sound when playing a 440 Hz tone also has some overtones at 880 Hz and 1360 Hz, as shown in Figure 64-8.
The mix of overtones contributes to establishing the musical identities of various instruments. For instance, a recorder has a very pure tone with very few overtones. Other sounds, such as a rock organ or a distorted bass, have a much more complex mix of overtones.
Why It Works
An oscilloscope processes an electrical signal and displays it in various ways. The origin of the electrical signal may be a microphone that converts a sound pattern into an electrical pattern, which the oscilloscope can work with. The most basic form of display is a single signal versus time. The scales are adjustable to permit a wide range of signals to be displayed. Oscilloscopes also display two signals both individually on the same screen or added. A plot of one signal against the other and a distribution of frequencies are also common options.
Other Things to Try
Here is a low-tech way of picturing sound: Cover a soup can (clean, empty, and with the top removed) with Latex or other rubbery material. Put it on tight, like a drum. Attach it with a wire tie, hose clamp, or good string. Glue a small (roughly 1 centimeter on a side) piece of mirror to the top of the Latex. To use it, hold the can in one hand and shine a laser on the mirror, so the beam projects onto the ceiling (or a wall). If you don't have a laser, direct sun works as well. With the light reflecting off the mirror, create sounds that will cause the Latex to vibrate. Because of the optical geometry, the movement of the reflected laser is larger (amplified) than the smaller movement of the mirror. Because the "drum" will be vibrating in two dimensions, it is not hard to generate the Lissajous patterns where the reflected light retraces a curved path.
Sound is a wave that, if converted into an electrical signal, can be displayed in a graphical form.