Lesson 16.2: Frequency determines pitch

Lesson Focus

This lesson explores how a sound wave's frequency creates its pitch. You'll learn what makes sounds high or low, from musical notes to the everyday sounds around you.

Learning Objectives

• Understand how a sound wave's frequency determines its pitch, distinguishing between high and low sounds.
• Learn how sound quality differs from pitch, making different instruments and voices sound unique.
• Discover how the Doppler effect changes a sound's perceived pitch when its source is moving, like a passing siren.

A sound's highness or lowness is its pitch.

This is caused by the vibration speed of the source.

Fast vibrations create high-frequency waves, resulting in a high pitch, while slow vibrations produce low-frequency waves and a low pitch.

Frequency, measured in hertz (Hz), defines a sound's pitch.

Humans can only hear frequencies between 20 and 20,000 Hz. Sounds below this range are infrasound (used by elephants), and sounds above are ultrasound (used by bats).

This shows our perception of the world is limited by our biology.

Every object has a natural frequency at which it vibrates.

When a sound wave with that same frequency hits the object, resonance occurs. This constructive interference amplifies the vibration, making the sound stronger.

For example, striking one tuning fork can make an identical nearby fork vibrate without being touched.

Two instruments playing the same pitch sound different because of their sound quality, or timbre.

While the main pitch (fundamental tone) is the same, each sound also includes a unique mix of higher-frequency overtones.

This complex combination of waves gives each voice and instrument its distinct character.

The Doppler effect explains why a siren's pitch changes as it passes.

As the source moves toward you, sound waves are compressed, increasing their frequency and making the pitch sound higher.

As it moves away, the waves spread out, decreasing the frequency and making the pitch sound lower.