5- pitch and timbre
One unifying characteristic of human life is music. In fact, no known human culture lacks music. But what is there about human perception that allows us to hear sound as musical notes? Why do instruments playing an Identical note sound different?
The answer to these questions requires some insight into how humans perceive pitch. When a musical instrument is played properly, it vibrates in a predictable way and pushes on the air in and around the instrument. This action creates waves or pulses that travel through the air. You might think of these waves as brief fluctuations in air pressure. Pitch relates to is how close together these waves or pulses are. If the musical instrument vibrates 120 times a second, about the same as a typical adult male speaking voice, we say the sound has a frequency of 120 cycles per second, or in current terminology 120 Hz (pronounced Hertz. the name of a 19th century German physicist).
The typical female speaking voice has a vibration frequency of around 220 Hz. Notes with a low frequency of vibration are referred to as low notes and those with a high frequency as high notes.
Pitch is tied to the vibration of air, but it is ultimately a product of how our ear and brain interpret these vibrations. Vibrating air molecules push against our eardrums, causing them to vibrate at the same frequency. The vibration is then amplified by mechanisms in the middle ear. The amplified vibration stimulates nerve sensors that convert the vibrations into electrical signals that the brain can analyze. What we perceive as pitch is a mental image of those vibrations. Although the vibrating air molecules are quite real, pitch occurs only in the brain. So we may need to reconsider the philosophical question "If a tree falls in a forest and nobody is present to hear it, does it make a sound?" The air vibrates, of course, but can there be a sound without eardrums present to vibrate and a brain to interpret the vibrations?
The human ear and brain have limits and cannot assign a pitch to all frequencies of vibration. We cannot hear sounds below 20 Hz or so, and if a sound is below 30-35 Hz, we do not perceive it as a distinct musical note. It sounds toneless, like a rumble. The same is true at the high end. Human hearing tops out at about 20,000 Hz even though air can vibrate at frequencies many times higher. As with the very tow frequencies, frequencies above about 4,000 Hz do not sound like musical notes. They begin to sound like snaps, hisses, clicks, and squeaks. You can test this aspect of human perception by playing the very lowest and highest notes on the 88-key piano. To most people, they seem a little musically "off or lifeless.
The brain does interesting things with the arithmetic of pitch. If an instrument plays a note with a frequency of 220 Hz and another one at twice as many cycles per second at 440 Hz. we hear the same musical note (both an A in the C major scale). We say they are an octave1 apart. Likewise, we hear an A if the frequency doubles again and vibrates at 880, 1760, and 3.520 Hz. At the lower end. we hear an A note at 110 Hz and 55 Hz. All told, we can hear between seven and eight octaves. Outside these ranges the notes become indistinct.
The fascinating arithmetic of musical notes allows the brain to play a trick on us that helps us distinguish sounds. Due to the physics of sound and the materials that make sound, there is no such thing as a pure tone. We may think a note is pure, but we are hearing much more. If a piano plays an A note with a frequency of 110 Hz, it actually plays a note at that frequency plus all the whole number multiples above it—220 (2 x 110), 330 (3 x 110), 440 (4 x 110), 550, 660, and so forth. The loudest frequency, the one with the most energy, is usually the lowest frequency (in this case 110 Hz). It is called the fundamental frequency, the frequency we identify as the pitch of the note. The higher frequencies are called overtones, or harmonies. You hear only one note, rather than dozens of evenly spaced notes, but that is because your brain works behind the scenes and uses the harmonies for other purposes.
You can experiment with harmonies using a guitar (or any string instrument). Pluck the thickest string on the guitar. If your guitar uses standard tuning, you will hear an E (about 82.4 Hz). Now very lightly rest your finger against the string at its exact midpoint (the i2th fret). Pluck the string again and you will hear a softer, rather pretty-sounding E note one octave higher. By lightly touching the string, your finger has absorbed the vibration produced by the fundamental frequency before it could reach the guitar body and be amplified. What's left are the higher harmonies.
Overtones and harmonies are also involved in shaping a musical instrument's tone or sound quality—its timbre (pronounced TAM-ber or TIM-ber). Timbre is the principal feature of sound we use to recognize each other's voices or distinguish a dog's bark from a baby's cry. With musical instruments, timbre is partly determined by the way an instrument amplifies or dampens harmonies. A trumpet, for example, which is made of brass, amplifies the harmonies rather evenly. A clarinet, because it is shaped differently and made of different materials, amplifies some harmonies more than others.
You can see the effect that an instrument's shape has on tone by considering what your mouth does when you make vowel sounds. If you sing the words "tea" and "too" and use the same musical note, the fundamental frequency is the same for both words. But "tea" sounds different because you changed the shape of your mouth in such a way as to dampen the overtones between about 500 Hz and 2,000 Hz. To make the vowel in the word "too," your mouth amplifies the overtones between 500 and 1,000 Hz and dampens the higher ones. Confusing, for sure, but your brain is hard-wired to handle such calculations automatically and adjust for the different fundamental frequencies of high and low voices.
Timbre is determined by more than just the loudness of overtones. All musical instruments make sounds when a musician commits some sort of controlled violence against the device. We strike keys, blow horns, pluck strings, scrape violins with a bow, beat drums. These actions all make noises that occur before a note can sound. This initial burst of sound, called the attack, is not particularly musical. It is the sound of clicks as keys are hit, hollow thumps as fingers cover holes, the friction of fingers rubbing against strings, hissing before a horn has enough energy to vibrate, tongues tapping and pulling away from mouthpieces. After the attack phase, there is a more stable phase when the note's fundamental frequency and overtone patterns emerge. Experiments show that if the attack phase is removed from a recording of an instrument, people have trouble identifying the instrument. Clearly, the way an instrument moves into a note is part of its distinctive sound.
The final dimension of timbre is the instrument's flux. Flux refers to the fluctuations that occur throughout the duration of a note, such as slight variations in pitch and volume. A trumpet, like many wind instruments, has very little flux. Its tone is fairly stable as the note proceeds. Percussive instruments tend to have a lot of flux. Think of the sound of a large gong struck with a lot of force. Its sound alters significantly as the gong returns to its nonvibrating state.
Other dimensions of sound contribute to a musical instrument's character—its volume, the duration of its notes, the speed at which it can be played, its register (the lowest and highest notes it can play), and the number of notes it can play concurrently. The workings of these elements may seem more intuitively obvious than the puzzling physics of pitch and timbre. But they too contribute to what we perceive as music by taking advantage of the way our brains process the subtle vibration patterns of air molecules. In the end, the only place these instruments make music is in our heads.
