How strong is the sound?

The idea

Intensity of sound is the amount of sound energy passing through a unit area perpendicular to the direction of propagation in unit time.

Energy through a window

Imagine a window held facing the source of a sound, so the sound waves hit it straight on. Energy from the sound passes through the window each second.

What do you think makes the sound at the window more intense?

• More energy passing through the same area each second
• The same energy passing through a larger area
• Less energy passing through each second

Energy and area

Change how much energy passes each second and the area of the window. The numbers are only examples.

Energy per area per second

Intensity tells us how much sound energy goes through each unit of area, facing the sound, in one second. More energy through the same area means a higher intensity. The same energy spread over more area means a lower intensity.

The intensity of sound is the amount of sound energy passing through a unit area perpendicular to the direction of propagation in unit time.

Notes

Intensity is the sound energy passing through a unit area perpendicular to the direction of propagation in unit time.

Check yourself

Which of these describes the intensity of sound?

In one example, 6 units of sound energy pass through an area of 2 units in 1 second. How many units of energy pass through each unit of area in that second?

Answer: 3

6 ÷ 2 = 3 units of energy per unit of area in 1 second.

The same sound energy passes through a window each second. The area of the window is doubled. What happens to the energy passing through each unit of area?

In the definition of intensity, how must the area be placed?

• The number of oscillations in one second. That is the frequency. Intensity is the sound energy passing through a unit area in unit time.
• The distance between two consecutive crests. That is the wavelength. Intensity is the sound energy passing through a unit area in unit time.
• Sound energy passing through a unit area, perpendicular to the direction of propagation, in unit time — correct. Yes!
• It becomes less — correct. Yes!
• It becomes more. The same energy is shared over a larger area, so each unit of area gets less.
• It stays the same. The energy per unit area depends on the area. With double the area, each unit of area gets less.
• Parallel to the direction of propagation of the sound. The definition uses a unit area perpendicular to the direction of propagation.
• Perpendicular to the direction of propagation of the sound — correct. Yes!
• It does not matter how it is placed. Intensity is defined for a unit area perpendicular to the direction of propagation.