Loudness, pitch and quality

Loud, soft, high, low

A shout is loud; a whisper is soft. A whistle is high; a big drum is low. Loud and high are different things.

Loudness and amplitude

A bigger amplitude means bigger swings of pressure and more energy: a louder sound. Hitting a drum harder makes the skin swing further. Loudness is measured in decibels ($\text{dB}$); a very loud sound damages the ear.

Pitch and frequency

A higher frequency gives a higher pitch. A short, thin, tight string vibrates faster and gives a higher note. Women's and children's voices usually have a higher pitch than men's.

Quality

A violin and a flute can play the same note, equally loud, and still sound different. Each instrument adds a different mixture of higher frequencies to the main one. This gives the wave a different shape, called the quality or timbre. It is also how you know a friend's voice on the phone.

You should find that the mixed wave repeats exactly as often as the pure tone, so the pitch is unchanged, but its shape is different. That different shape is what our ears hear as a different instrument.

Watch out. A louder sound is not a higher sound. Turning up the volume changes the amplitude, not the frequency.

A worked example

A guitar string plays $392\ \text{Hz}$ softly, then the same string is plucked harder, then pressed to play $784\ \text{Hz}$. What changes each time?

1. Plucked harder. The amplitude grows: louder, same pitch, same $392\ \text{Hz}$.
2. Pressed shorter. The frequency doubles to $784\ \text{Hz}$: a higher pitch.
3. Time periods. $\dfrac{1}{392\ \text{Hz}} = 2.55\ \text{ms}$ and $\dfrac{1}{784\ \text{Hz}} = 1.28\ \text{ms}$.

Practise with new numbers

Try it in Python

The program describes sounds by their amplitude and frequency.

Remember these

• Loudness depends on the amplitude.
• Pitch depends on the frequency.
• Quality depends on the mixture of frequencies, the shape of the wave.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A drummer hits the drum harder. Which changes: the amplitude, the frequency or the speed?

2. Which has the higher pitch: $250\ \text{Hz}$ or $900\ \text{Hz}$?

3. Two instruments play the same note at the same loudness. What lets us tell them apart?

4. Turning up a radio's volume knob changes which property of the sound?