• Sound waves are caused by vibrations such as that from a tuning fork, a loudspeaker cone, or the human voice.
• These vibrations need air to travel through. They cannot travel through a vacuum.
• The air itself doesn't travel.
• The sound causes compression and decompression of the air as it moves through it.
• There is a regular spacing between one pressure peak and the next.
• This distance is called the WAVELENGTH.

• Sound travels at about 330 metres a second.
• A pure sound tone consists of a single frequency of vibration.
• The range of human hearing is about 20 Hertz to 20 KiloHertz.
• Most sounds are a mixture of frequencies. See the page on HARMONICS.
• Microphones convert sound pressure waves into electrical signals.
• Loudspeakers convert electrical signals into sound waves.
• Loudspeakers and microphones are TRANSDUCERS.
• Frequency, wavelength and the speed of sound are interrelated.

Wavelength x frequency = the speed of sound in metres per second.