What Does Sound Look Like? Oscilloscope Wave Forms. (page 2)
- 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.
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