Properties of sound

You are at home and a sound comes in from the street. Before you even know what it was, your ear has already answered four questions. Was it over in an instant, or did it go on? Was it strong or weak? Was it low or high? And what made it: a car horn, a dog, someone calling your name?

Those four questions have names in music. They are the properties of sound, and every sound you hear answers all four at once.

Four questions, four names

• Duration answers how long? A knock on the door is short; a kettle's whistle can last a minute.
• Intensity answers how strong? It is what changes when you turn the volume knob.
• Pitch answers low or high? A radio announcer's voice tends to be low; a bird's song, high.
• Timbre answers what made it? It is what lets you recognise a friend's voice on the phone before they say who they are.

A word of caution. In everyday speech, "turn it up" and "the volume is too high" are about how strong a sound is. In music, high and low are about pitch: a high sound is shrill, a low sound is deep. Strength has its own name, intensity. A whistle can be soft and high; thunder, low and very loud.

One changes, the others stay

The four properties are independent: you can move one without touching the others. Play C4 and then C5. It is the same instrument, with the same strength and the same length, and still you hear that the second is higher. Only the pitch changed.

Now imagine the other three, one at a time. Hold the same key for longer, and the duration changes. Press it harder, and the intensity changes. Play the same note on a guitar, and the timbre changes, even though the pitch stays exactly the same.

What happens behind each one

Every sound is vibration. Something moves back and forth very quickly, that movement travels through the air in waves, and it reaches your ear. Each property corresponds to one aspect of that vibration:

• pitch depends on how many times per second the vibration repeats. That count is called frequency and is measured in hertz. The A that orchestras tune to vibrates 440 times per second; the more times, the higher;
• intensity depends on the size of the vibration: the wider the movement, the stronger the sound;
• duration is simply how long the vibration goes on;
• timbre comes from the fact that a real sound is never a pure vibration. Alongside the main vibration several others sound, weaker, and each instrument mixes them in its own recipe.

That recipe has a name: the harmonic series, and it deserves a topic of its own. If you want to read ahead, Med compares different instruments playing the same note (Med 1996, ch. XVI), and Blatter describes sound as a sum of simple waves (Blatter 2007, art. 1).

Not every sound has a definite pitch

Try a test: if you can sing a sound back, it has a definite pitch. A piano note passes the test. A handclap, a shaker or a slamming door does not. They have duration, intensity and timbre, but no note you could sing.

That does not leave them out of music: just imagine a samba circle without its shakers. The books differ in emphasis. Lacerda describes these instruments as closer to noise than to musical sound (Lacerda n.d., ch. I), while Med prefers to stress that music uses both kinds of sound (Med 1996, ch. I).

A score answers the same questions

A score is, among other things, a way of writing down the four answers for each sound:

• pitch: where the note sits on the staff. The higher up, the higher it sounds;
• duration: the shape of the note. A hollow note lasts longer than a filled one;
• intensity: dynamic letters and signs, such as p for soft and f for loud;
• timbre: the name of the instrument or voice written at the start of the staff.

In this passage, the first note sits lower on the staff and has a different shape: it sounds lower and lasts twice as long as each of the two that follow.

These signs were not invented together. Pitch was written down long before the other properties, and marking timbre and intensity is much more recent. Blatter (Blatter 2007, art. 1) and Med (Med 1996, ch. II) tell that story with dates that do not match.

Going further

The four questions work as a map. Pitch leads to notes, the staff and intervals; duration to note values, metre and rhythm; intensity to dynamics; timbre to voices and instruments. Lacerda's Compêndio organises the whole of elementary theory around these four properties, and it works as a route for anyone who wants to study them in that order (Lacerda n.d., preface).