Doppler Effect Experiment

Grade Level: 8th-12th grade
Time Required: 1 hour
NDT Focus Area: Ultrasonic Testing (UT) 

Students will observe how sound changes when a moving object emits it and define the Doppler Effect. The activity introduces the physics behind ultrasonic waves and their use in real-world nondestructive testing.

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Learning Objectives

By the end of this activity, students will be able to: 

  • Explain the Doppler Effect in terms of motion and frequency.

  • Describe how ultrasonic sound waves are used in NDT.

  • Observe and interpret changes in pitch related to a moving sound source.

  • Relate the demonstration to real-world technology (e.g., ultrasound, radar, flaw detection).

Key Concepts 

  • Sound travels in waves.

  • Frequency increases when a sound source approaches (higher pitch).

  • Frequency decreases when it moves away (lower pitch).

  • Ultrasonic waves are sound waves above the range of human hearing (>20 kHz).

  • In NDT, these waves reflect off internal features, helping technicians "see" inside solid materials.

Vocabulary

Here are some key terms you will need to know to support your understanding during the experiment.

Doppler Effect

A change in the frequency or pitch of a sound due to the motion of the source.

Frequency

The number of wave cycles per second, measured in Hertz (Hz).

Ultrasonic

Sound waves with frequencies above human hearing (>20,000 Hz).

Discontinuity

A flaw or abnormality in a material.

Materials Needed

Before you start, collect the following items for the experiment.

Nerf-style foam ball

Small buzzer

(continuous sound)

Battery connector

9-volt battery

Electrical tape

Wire strippers or scissors

(for teacher prep or advanced students)

Experiment Procedure

Follow these steps one at a time to carry out the experiment. Read carefully and take your time to make sure you complete each part safely and correctly.

Prep the wiring

  • Strip about ½ inch of insulation from the ends of the wires on the buzzer and battery connector.

Connect the wires

  • Twist one buzzer wire together with one battery connector wire. Wrap with electrical tape.

  • Attach the second wire to the buzzer terminal (screw or post), then secure it.

  • Tape the wires neatly around the buzzer.

Hands connecting wires to a small electronic component on a white surface.

Assemble the Doppler Ball

  • Cut a small slit into the Nerf ball.

  • Hollow out a space big enough to fit both the buzzer and the battery.

  • Turn the buzzer on by connecting the battery.

  • Insert the buzzer and battery into the ball.

  • Close the slit with tape to keep the buzzer in place.

Hands holding a green and blue textured toy ball with a small hole. The hands are pressing in a battery and wires into the ball.

Test and Demonstrate

  • With the buzzer sounding, gently toss the ball across the classroom.

  • Ask students to listen carefully as the ball approaches and moves away.

Two people play catch with a small ball. One person is throwing, while the other prepares to catch.

Expected Observations 

Students will hear the pitch of the buzzer rise as the ball approaches and lower as it moves away. This change is the Doppler Effect, where the motion of the sound source causes the sound waves to compress (higher pitch) or stretch (lower pitch).

Real-World Connection:
NDT and Ultrasonic Testing

In nondestructive testing, ultrasonic waves are sent into a material to check for hidden flaws, just like bats use sound to "see" their prey. The waves reflect back from internal surfaces or discontinuities.

  • Shorter wavelength (higher frequency): detects small flaws but doesn’t travel deep.

  • Longer wavelength (lower frequency): travels deeper but may miss tiny flaws.

Technicians use this science to inspect aircraft parts, pipelines, bridges, and more—without taking them apart.

Learn More About Ultrasonic Testing
A technician performing an ultrasonic testing inspection on a large welded pipe. The technician is using a handheld transducer connected to a portable ultrasonic testing device with a screen displaying data. The setting appears to be outdoors, with industrial structures visible in the blurred background.

Apply What You've Learned

  • Define the Doppler Effect in your own words.

  • Draw a diagram showing how the sound changes when the object is moving.

  • Briefly explain how ultrasonic testing uses sound to find problems inside materials.

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