A wave is a means of transferring energy through a substance with no movement of the substance itself.

Title: To show that sound needs a medium to travel

Apparatus: Bell jar, alarm clock, vacuum pump

Method: Set up the apparatus as shown in the diagram.
Set the alarm on the clock and listen to it ringing.
Start vacuum pump and remove air from the bell jar.
Note how the bell seems quieter.

Result: When all the air is removed the bell can no longer be heard.

Conclusion: Sound needs a medium to travel.
The pitch of the sound is determined by the frequency.