Felicia Cherry
Product Manager for Physical Science, Physics, and Earth Science

Mechanical waves require a medium through which to travel. The medium may be any material. The medium examined in this demonstration is the air around us. Sound waves are mechanical waves. Remove the air and sound cannot travel. However, we do need air to breathe. So let’s use a vacuum pump with a bell jar and battery-powered buzzer or with a buzzer and jar device to evacuate the air from a smaller defined space to demonstrate what happens to sound waves when air is removed.

Materials

• Buzzer and Jar Device or Small Battery-Operated Buzzer
• Vacuum Pump
• Bell Jar
• Base Plate
• Connecting Hose
• Vacuum Grease (optional)
• Battery, 6 V

Safety

Follow closely the instructions for this activity and abide by established laboratory safety practices, including the use of appropriate personal protective equipment (PPE).

Ensure that students understand and adhere to safe laboratory practices when performing any activity in the classroom or lab. Demonstrate the protocol for correctly using the instruments and materials necessary to complete the activities, and emphasize the importance of proper usage. Model proper laboratory safety practices for your students and require them to adhere to all laboratory safety rules.

Make sure that neither the pump nor the base nor the jar falls off the table from the motor’s vibration. Read through the instructions carefully and be familiar with the various gauges and valves on each piece of equipment.

Before the demonstration

Access students’ prior knowledge of sound or sound waves by asking, “What is sound?” Answers will vary.

Procedure

• It is a good idea to practice a demonstration before you perform it for your class, even if you have done the demo before. This demo requires multiple pieces of equipment with lots of connections, valves, and parts. Practicing to become familiar with the equipment is the best way to ensure a safe, successful performance.
• Prepare the vacuum pump for use. Check the batteries in the buzzer.
• Turn on the buzzer. If you are using a battery-powered buzzer, place it on the base plate and cover with the bell jar. If you are using the bell jar assembly, place the buzzer in the jar. For a more airtight seal on the jar, apply a small amount of vacuum grease around the neck before screwing on the cap.
• Attach the pump and turn it on.
• Run the pump until the buzzer is silent or the sound is greatly diminished. A stronger vacuum will completely silence the buzzer, while less powerful pumps will greatly diminish the sound.
• Turn off the pump. Wait a few moments to allow students to notice the change in volume.
• Release the valve to allow the air back in. The buzzer sound will return to its starting level.

After the demonstration

Ask students to revisit their answers to the question “What is sound?”

Explanation

Mechanical waves rely on particle interaction to transport their energy between locations. The medium through which they travel provides said particles. Mechanical waves cannot travel in an area void of particles.

In this demo, removing the air (medium) creates a vacuum (void) and the buzzer sound becomes inaudible. After releasing the valve to equalize the pressure, the buzzer is heard once again. Thus the silence was due to the absence of air. Sound waves are indeed mechanical waves.

Extension

Repeat the demonstration with a small battery-powered light beside the buzzer to kick off a discussion on properties of light waves vs. sound waves. Challenge students to find both accurate and inaccurate depictions of light and/or sound in movies about space.

Resources

Below is a selection of vacuum pumps and equipment you can use to perform this demo.

• Two-Stage Vacuum Pump with Gauge (item #752836)
• Oilless Vacuum Pump (item #752830)
• Vacuum Chamber System, PC (item #752951)
• Vacuum Tubing (item #752840)
• Bell Jar, Glass (item #752970)
• Vacuum Plate, Metal (item #752960)
• Bell Jar and Vacuum Set (item #752910)

About vacuum pumps

Different setups remove varying amounts of air, thereby providing differing levels of vacuum. Stronger pumps will be able to pull a near complete vacuum. Some base plates have a gasket that provides an adequate seal. Some devices will require the application of grease to provide a complete seal between the base plate and bell jar. Some pumps are strong enough when they run to move across the table. Become familiar with the characteristics of your vacuum equipment before performing a demonstration. Vacuum pumps that require oil can be stored with the oil in the pump, but the oil should be changed each year.

We recommend the Two-Stage Vacuum Pump with Gauge (item #752836) for those who need to pull a near complete vacuum or who use a pump quite often. Though it requires oil to run, the annual oil change is a simple, straightforward task that will keep this heavy-duty workhorse running for many years to come.

We recommend the Oilless Vacuum Pump (item #752830) for those who use a pump less frequently. Because it requires no oil, this unit is easy to store and easy to retrieve to run engaging demos such as the one outlined here.