Lesson: Loudness Physics • 9th Grade

In this lesson, we will learn how to relate the loudness of sounds in decibels to the intensity of sound waves.

Lesson Plan

Students will be able to

recall that loudness is a quantity proportional to intensity of sound,
recall that loudness can be measured in the unit decibel (dB),
recall that a sound of loudness dB and a frequency of 1 kHz is defined to have a loudness of phons,
recall that the intensity of sound that is at the threshold of human audibility can be defined as being 0 phons and that this is equivalent to a sound intensity of W/m²,
apply in all combinations the formula to a sound that has a loudness of phons and an intensity of W/m².

Lesson Worksheet

Q1:

The intensities of two sound waves at a point are 25 mW/m² and 150 mW/m². What is the difference between the loudness produced by the higher-intensity sound wave and the loudness produced by the other sound wave?

Q2:

The intensity of a sound wave is the average power in the wave per unit area that the wave is incident on. Which of the following is the base SI unit that corresponds to the ratio of power to area?

Q3:

The sound of a voice emitted from the speaker of a mobile phone has an intensity at the position of the ear of the person holding the phone of 1.0 µW/m². What is the loudness of the sound?