Physics Grade 11

Unit 4: Waves and Sound

Note 3: Sound Intensity and Doppler Effect


Reference: Chapter 8.5, 9.5


Sound Intensity: 

Sound intensity decreases with distance away from the source.

Intensity is the power of sound energy per unit are (units W/m2

               where A is often

Ask students to prove why the ratio of intensities emitted from one source at different distances is


Example 1: Person A is 707m from a hovering helicopter. Person B is 1000m  from the same helicopter. How much has the intensity decreased when it reaches person B?

Decibel System:

Named after Alexander Graham Bell

More commonly used to measure intensities than W/m2

It is a logarithmic scale. Explain how this is in powers of 10.

Example 2: Someone is trying to whisper so the teacher cannot hear them :) . The whisper has a sound intensity of about 1.0 x 10-10  W/m2 . They now increase their voice by 30. dB. What is the intensity in W/m2 now?


Doppler Effect (ALL):

Ask students:

1. What does it sound like when an ambulance is moving towards you and after it passes you and is moving away?

2. What does it sound like when you are watching a car race and the cars are moving towards you and after they pass you and are moving away? (frequency gets higher as it approaches, frequency gets lower as it goes away).


Doppler Summary:

- A stationary person with a sound source moving toward them will hear a higher frequency (pitch) of the sound than another stationary person with the same sound source moving away from them.

- Doppler effect only applies to objects moving less than Mach 1.
- It's the apparent changing frequency of sound in relation to an object's motion.

- Equations:

           1)                      + moving away   - moving toward          


            2)        is called the Doppler Shift


http://www.youtube.com/watch?v=O5rqMPdQMQ8  (Doppler Effect Train Horn)
http://www.youtube.com/watch?v=Y5KaeCZ_AaY (Big Bang theory Doppler Effect comedy)

video: http://videos.howstuffworks.com/discovery/27963-assignment-discovery-doppler-effect-video.htm

Example 3: A car travelling at 100 km/h sounds its horn as it approaches someone. If the horn's frequency is 440 Hz and the air temperature is 0 degrees celsius, what frequency will be heard as the car               

 a) approaches (ans: 480 Hz, shift is 40 Hz)       b) passes by (ans: 406 Hz, shift is 34 Hz)



Human hearing range is 20Hz - 20 000Hz. The range diminishes with age and with exposure to loud sounds.

In industries where workers are exposed to 85 dB or more, ear protection is required by law.

A walkman can be harmful because of its proximity to the ear. It may not seem loud but it can deliver 100dB of sound to the eardrum. It doesn't have a chance to use the 1/r rule to decrease the intensity before it enters the ear canal.   

Intensity and Decibel Questions:

Pg 397 #8, 10

Doppler Questions:

Pg 435 #3-6    

Old text question on Doppler percent decrease/increase in apparent frequency