Understanding Pitch, Loudness, and Timbre in Sound Waves
To begin, let's delve into the characteristics of waves, focusing on those that define sound. A fundamental aspect of any wave, including sound waves, is its repetitive nature. Whether observing a vibrating string on a violin or waves crashing on the shore, a pattern of repetition is always present.
Pitch: The Frequency of Sound
Musical notes, or tones, possess a quality known as pitch. The pitch of a specific note is often represented numerically. For instance, the "A" note in the middle of a piano is typically designated as A=440. But what does "440" actually refer to?
Essentially, the number indicates the frequency of oscillation. A string vibrating at 440 Hz will oscillate back and forth 440 times per second, resulting in a specific pitch. To simplify this, several shorthand notations have been developed.
Frequency always denotes a number of oscillations, so we can omit the word "oscillations." Furthermore, "per second" can be shortened to "/second," and "second" can be abbreviated as "sec." Even more concisely, "/sec" is often represented as "Hz," which stands for Hertz, named after the physicist Heinrich Hertz.
Once we grasp the meaning of a pitch or frequency of 440 Hz, we can explore a related concept: the duration of a single oscillation. If a string oscillates 440 times in one second, then each individual oscillation takes 1/440 of a second.
Another perspective is that if each oscillation lasts 1/440 of a second, then 440 oscillations will occur in one second. Shorthand notation is used here as well. For very small periods, scientific notation is employed to avoid writing numerous zeros after the decimal point.
For example, 10-3 seconds corresponds to 1 millisecond, which is abbreviated as 1 msec.
Consider the Earth's orbit around the Sun. Is this motion repetitive? What would an oscillation correspond to?
Repetitive sounds can originate in various ways. One common source is, of course, a musical instrument. Another example is a drummer playing near a staircase; the individual echoes of the drum off each step are delayed from one another. To the drummer, these echoes create a repetitive sound, which is then perceived as a pitch.
Loudness: The Amplitude of Sound
Besides pitch, loudness is perhaps the most noticeable characteristic of a musical note. The loudness of a sound wave is determined by its amplitude. While loudness is specifically associated with sound waves, all types of waves possess an amplitude.
Waves on a calm ocean may have an amplitude of less than 1 foot, while good surfing waves might reach 10 feet or more.
Factors affecting loudness include:
- Distance from the sound source
- Intervening material
- The detector of the sound wave (e.g., ear vs. microphone)
Timbre: The Tone Quality of Sound
Although a wave repeats in time, its motion during one oscillation can be simple or highly complex. Consider two sound waves with the same frequency and amplitude. In fact, their amplitudes are also the same. Thus, if these two waves represented sound waves, the pitch and loudness would be the same in both cases. But would they sound exactly the same?
The answer is no, because there is one more attribute to sound waves that you are familiar with, and that is tone quality. This is what makes different instruments sound different. A violin and a trumpet can play the same pitch with the same loudness, but we can easily tell them apart, because they have a different tone quality. In fact, the same instrument can create different tone qualities. If you pluck a guitar in different ways, you can get quite different tones. Try it!
The technical musical term for tone quality is timbre.
Harmonic partials on a guitar string demonstrate the complexity of timbre.