It is time to get back to basics with a recap on sound waves and waveforms. If you want to master the art of recording then it is a good idea to know the fundamentals of sound.

So what exactly is a sound wave? A sound wave is a longitudinal pressure vibration set in motion by any event that generates energy, such as a vibrating object. If a sound wave is moving from left to right through air, then the particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it.

The 'vibrating' air molecules will cause the human eardrum to vibrate, which the brain interprets as sound. Note that air molecules do not actually travel from the noise source to the ear. Each individual molecule only moves a small distance as it vibrates, which causes the adjacent molecules to vibrate in a rippling effect all the way to the ear.

It is important not to get longitudinal waves (sound waves) confused with transverse waves. Most waves ARE actually transverse, including light and the ripples we see on water. In transverse waves the vibrations are at 90 degrees to the direction of the wave. In contrast, longitudinal waves have vibrations along the same axis as the direction in which the wave is travelling. Think of the way a slinky behaves if two people are holding each end and one person quickly sends a number of vibrations down it. Compression (squeezing-high pressure) and rarefaction (spreading-low pressure) occurs in the slinky until no more energy is left.

Waveforms

A waveform is essentially a graphical representation of a soundwave (or voltage) as it moves through a medium over time. Any waveform has several fundamental characteristics:

The basic fundamentals of a soundwave

Wavelength (λ)

The wavelength of a wave is the length in metres from the start to the end of one full cycle of the waveform e.g. from crest to crest.

Amplitude

The amplitude is the maximum displacement of a periodic wave from the middle line to the peak, NOT from peak to peak. The greater the distance from the centre line of a wave form, the more intense the pressure variation will be within a medium, hence the louder it is perceived. Amplitude is measured in two ways; first is the ‘zero to peak’ value which measures the maximum positive or negative signal level; second, is the root-mean-square (RMS) value which measures a more meaningful average level, like that at which humans hear.

Frequency

Frequency is how many complete waves there are per second passing a certain point. In other words the frequency essentially indicates the rate of pressure variations or 'cycles per second' of a wave.  Frequency is measured in Hertz. The frequency of a sound determines the pitch (the sensation of how high or how deep a sound is). Higher frequency sound waves have shorter wavelengths and a higher pitch; conversely, lower frequency sound waves have longer wavelengths and a lower pitch.

Frequency comparison diagram

The frequency range in which humans can hear is 20Hz to 20,000Hz and is called the audible range. 

Velocity

The velocity is the speed and direction of a sound wave. Sound waves travel at different speeds through various mediums. Through air, sound travels at 344 metres per second. Generally speaking, the denser the medium the faster sound travels through it. The speed of sound is constant irrespective of the frequency.

To find the velocity of a wave the following equation is used:

Equation: Velocity (V) = Frequency (f) x Wavelength (λ)

Remember not to confuse a waveform, which displays a transverse wave, with the true nature of sound. A real sound-wave is longitudinal. The waveform which represents the longitudinal wave is usually shown in audio diagrams as a transverse wave to help us visualize what is going on.