State the approximate range of frequencies audible to humans as 20 Hz to 20000 Hz

Topic 3.4 – Sound

Learning Objective

State the approximate range of frequencies audible to humans as 20 Hz to 20 000 Hz.

What is Sound?

Sound is a longitudinal wave that travels through a material medium (solid, liquid or gas) by the successive compression and rarefaction of particles.

Key Concepts

• Frequency (\$f\$) – the number of complete vibrations (cycles) per second, measured in hertz (Hz). \$f = \dfrac{1}{T}\$ where \$T\$ is the period.
• Pitch – the perceived highness or lowness of a sound, directly related to its frequency.
• Amplitude – the maximum displacement of particles from their rest position; related to the loudness of the sound.
• Speed of sound (\$v\$) – depends on the medium and its temperature; for dry air at 20 °C, \$v \approx 343\ \text{m s}^{-1}\$.

Human Audible Frequency Range

The average healthy human ear can detect frequencies roughly between 20 Hz and 20 000 Hz. Frequencies below 20 Hz are called infrasound, and those above 20 kHz are called ultrasound.

Why the Range is Limited

1. The ear’s eardrum and cochlea are tuned to respond most efficiently to vibrations within 20 Hz–20 kHz.
2. At very low frequencies, the displacement of air particles is too slow to stimulate the hair cells effectively.
3. At very high frequencies, the wavelength becomes comparable to the dimensions of the inner ear structures, reducing the ability to detect the wave.

Practical Example – Determining Pitch

Consider a tuning fork that vibrates at \$f = 440\ \text{Hz}\$. This frequency corresponds to the musical note A₄, which is used as a standard pitch reference for musical instruments.

Summary

• Human audible range: 20 Hz – 20 000 Hz.
• Frequency determines pitch; amplitude determines loudness.
• Sound requires a material medium to travel.