Sound waves in air are a combination of dense areas of air particles, called compressions, and 'thinned out' areas called rarefactions. In compressions, air pressure is high because all the particles are in a small space and not moving. In rarefactions, air pressure is low, however the velocity of the particles is high as they are moving very quickly, rushing to the next compression. We can see that sound waves are made of high pressure, low velocity (not moving) parts, and low pressure, high velocity parts. The difference between high pressure or high velocity is very important when we consider ways to control sound such as absorbing it.
In an electrical sound signal, the alternating high and low pressures are represented by positive and negative voltage, usually relative to mains ground.
Every time a wave alternates from high pressure to low, and back to high again, this is called one cycle. The physical distance the sound travels while it makes 1 cycle is known as the wavelength. The number of times it cycles in 1 second is known as the frequency. 1 cycle in 1 second is called 1Hz. We know the speed of sound in air, so can calculate wavelength from frequency and vice versa. Wavelength (meters) = 342 (speed of sound in meters per second) / Frequency (Hz). Alternatively we can say Frequency (Hz) = 342 ( Speed of sound) / Wavelength (meters).
The frequency of sound, as we know, is written in Hz. However, if we simply chart Hz in a linear fashion, it doesn't correspond very well to how high or low a sound seems to us humans. We hear by octaves, hence their use in music. An octave is simply a doubling or halving of frequency. One octave above 1000Hz is 2000Hz, then 4000Hz and so on. A tone of 8000Hz would subjectively seem about three times higher than a 1000Hz tone.