It’s hard not to stop what you’re doing and watch when a fire truck comes barreling down the road with its sirens blaring. But once the truck passes, something strange occurs: the sound of the siren changes pitch. Why is that?

In this lesson, students come up with equations in several variables to explore the Doppler Effect, which explains how sound from a moving object gets distorted. Along the way, they’ll explore the relationship between the wavelength, frequency, and pitch of a sound, and will also explore what happens when an object breaks the sound barrier.

### Students will

• Explore the relationship between frequency, wavelength, and pitch of a sound
• Write an equation for wavelength in terms of frequency for the sound emitted by a stationary object
• Describe how the frequency of sound emitted by a moving object will appear different to stationary observers in front of and behind the object two observers
• Write a formula for the observed wavelength of a sound wave in terms of the wave’s actual frequency and the speed of the object emitting the sound
• Write a formula for the observed frequency of a sound wave in terms of the wave’s actual frequency and the speed of the object emitting the sound
• Describe how observers will experience sound emitted from an object as it moves past the speed of sound

### Before you begin

Students should be relatively comfortable working with variables in expressions and equations, and should be able to solve for one variable in terms of another.