Understanding Sound

Editor's Note: The Windows Sound System v2.0 software user's guide is mostly concerned with the setup and control of audio into and out of a computer running Microsoft Windows. Within this rather dry manual we found two especially useful chapters that describe, in simple terms, audio compression, sampling frequencies and related subjects.
The material is so straightforward and helpful that we wanted to include them as part of the EUonline Continuing Education page.
Please keep in mind that this material is from 1993, and that specific technology has advanced such that 32kHz sampling is really a minimum for broadcast quality recording, storage and transmission---but the underlying concepts are still valid.

Did You Hear That?

Sound is created when something (for example, a spoon) comes into contact with something else (for example, a pot), creating a physical disturbance in the second object because it "feels" the contact. For example, when you strike a pot with a spoon, the surface of the pot feels the energy conveyed in the blow. This energy creates a disturbance (a movement or vibration) on the surface of the pot and the spoon that affects the air molecules on both sides of the surface. When air molecules are disturbed successively over a short period of time, a sound is transmitted through the air by the rapid vibration (disturbance) of these molecules.

This is similar to the way that vibrations travel through water. When you drop a rock into a pool of water, ripples spread from the point where the rock touched the water, appearing as vertical displacements on the surface of the water. Sound, likewise, travels away from the sound source, with one significant difference.

While water waves travel vertically along the flat surface of water, sound waves are not vertical displacements of air molecules. Instead, they are consecutive compressions, or a series of regions of higher and lower density, in the air.

Can You See What I Hear?

Since sound waves are caused by a change in the density of air, they are not visually received. You hear sound through your ears, but you can use an instrument called an oscilloscope to look at sound waveforms.

To see how this works, you could connect a microphone ro an oscilloscope and speak into the microphone. The oscilloscope displays the waveform of your speech (the sound signal) as a series of repetitive curves resembling a cross-sectional view of ocean waves. The curves have crests and valleys which repeat regularly across the oscilloscope's display in a pattern. Because of this visual similarity to ocean waves, these repetitive curves are called sound waveforms. A sound waveform is simply the graphical representation of the sound that you hear.

How to Hear from There

As stated earlier, sound waves travel through air as consecutive compressions, or a series of regions of higher and lower density, in the air.

The microphone detects these changes in pressure, converts the changes into a voltage, then sends the voltage to the display unit. The greater the sound pressure level that the microphone detects, the higher the voltage, therefore the higher the line on the oscilloscope's display. A waveform crest on the oscilloscope represents the greatest pressure in the sound wave, and a valley represents the least pressure.

When you hear a sound, the dimensions in the representational sound waveforms correspond to the pitch, timbre, and amplitude (loudness) of the sound.

Pitch: What Makes A Dog Howl

Pitch is the high or low quality of the sound. It is not determined by, and does not affect, the volume of the sound, but the frequency of the waves producing it. For example, since a violin string is much thinner and shorter than the string of a bass fiddle, the violin string vibrates at a much higher rate when the bow is drawn across it. The result is that the violin creates sounds with a much higher pitch than the bass fiddle.

People and animals have different sensitivity levels to pitch—while the siren of an emergency vehicle may be irritating but tolerable to most people, it can be painful to animals, especially dogs.

In an oscilloscope image, pitch corresponds to the rate of repetition of the waveform.

On the oscilloscope, the distance between repetitions is known as a wavelength. Wavelength is used to calculate the frequency,or how many times the waveform is repeated in one second. The frequency determines the pitch of the sound. The longer the time between waveform repetitions, the lower the frequency, and the lower the pitch of the sound. The shorter the time between waveform repetitions, the higher the frequency, and the higher the pitch.

Timbre: The "Color" of Sound

Timbre is the "tone color" of a sound, or the quality given to sound by subtle differences in its tones. Timbre enables you to tell the difference between sounds, for example, voices and musical instruments. On an oscilloscope screen, timbre is represented by the shape of the repeated curve. If the shapes of two sound waveforms are different, the sounds differ.

A very smooth waveform shape is called a sine waveform. It represents a sound like a note played on a tuning fork.

A violin waveform is quite complex and looks nothing like the sine waveform. This waveform complexity may be perceived as a more "nasal" or "whiny" sound quality.

If a waveform does not repeat, the sound is usually perceived by the human ear as a hiss. Noise is perceived as "hiss"; it is similar to the sound of the wind or a fan.

Amplitude: Turn It Down!

Amplitude is the strength or loudness of a sound, without regard to its frequency. On an oscilloscope screen, amplitude is represented by the distance between the highest point and the lowest point of the waveform (from the crest to the valley). Amplitude has a direct impact on how loud the sound is; the higher the crests and the deeper the valleys, the greater the amplitude and the louder the sound.

There is, however, one catch to the relationship between amplitude and loudness. You may expect a waveform with twice the amplitude of another to sound twice as loud, but it doesn't. Instead, it sounds only about fifty percent louder. A sound that is twice as loud as another actually has about four times the amplitude.

To determine the amplitude of a sound from its waveform, decibels (dB) provide a more accurate measurement than the height and depth of the waveform. A decibel more accurately reflects human hearing because it measures what is actually heard. A sound that has twice the amplitude of another is 6 dB, or is perceived as fifty percent louder. A sound that is twice as loud as another is about 12 dB louder and has about four times the amplitude.

Can You Hear That?

Because of the unique way each person perceives (and appreciates) pitch, timbre, and amplitude, what sounds pleasant and comfortable to you may be discordant, disturbing, or painful to someone else. When you are recording, whether through conventional methods or with your computer, experiment on a sample in advance. It's a good idea to give special consideration to the quality of sound in playback, for this is where it affects you, if you plan to store your recording for posterity, and your audience, if you plan to play it now or in the future.