On a basic level, sound can be considered as waves that travel through the air. These travel in much the same way as the waves travel through any other liquid.Frequency
Yes, I did say liquid. Technically speaking, air is a fluid.
A speaker creates sound through the physical movement of the surface of the cone in the air. When the speaker moves forward, it creates pressure in the air in front of the cone. At the same time, it also creates a negative pressure area behind the cone.
This sound 'wave' then travels through the air, creating pressure on your eardrum. This pressure then causes your eardrum to move inward. This movement is converted into an electrical signal, which is sent to your brain.
Sound waves cause localized pressure variations in the air. The rate of theses fluctuations (or more technically correct, oscillations) is referred to as the frequency, which is expressed in Hertz (Hz), which means cycles per second.
For example, a sound with a frequency of 20 Hz causes the air pressure to rise and fall 20 times every second.
Slow oscillations (low frequencies) produce bass, while high frequencies (fast oscillations) make the sounds we refer to as treble.
Not all frequencies can be heard. A person with 'normal' hearing can perceive sounds in the range from approximately 20-20,000 Hz.
Frequencies too low to be heard are referred to as being infrasonic. Although you cannot hear them, infrasonic waves can be felt.
At the other end of the frequency spectrum are ultrasonic waves, which cannot be heard by humans. Dogs can hear them however, 'dog whistles' create ultrasonic waves.
Coming when I get the time!