In many respects, the role of sound in a film is as crucial as the visual imagery. Not only does it convey information in the form of dialogue and sound effects, but the proper use of sound can set a mood and help establish the surroundings before an actor enters or a word is spoken. There are many important elements to sound in film, and in order to truly understand it, you must familiarize yourself with the science on which it's based. You also need to recognize the part that sound plays in the production and postproduction process and the techniques your specialists employ.

Sound is created by vibrations. Vocal cords, guitar strings, breaking glass, and crashing cars all create a disruption that vibrates through the air, creating compressions and rarefaction until they reach our ears. Sound itself is defined by four parts: wavelength, harmonics, resonance, and amplitude.

Wavelength

Put simply, the faster the vibrations, the shorter the distance between compressions and the higher the frequency and pitch. Frequency is the number of compressions per second. This means that low-frequency sounds have a longer wavelength; high frequencies correlate to shorter wavelengths and create a higher pitch.

The cycles from the beginning of one compression to the beginning of the next are measured in hertz (Hz). The average human ear is capable of detecting sounds from 15 Hz at the low frequency up to 20,000 Hz.

Harmonics

The source of any sound vibrates at its fundamental frequency and at other frequencies called harmonics. Harmonics are always a higher frequency, rising in multiples of the fundamental. If the fundamental is 200 Hz, the next harmonic would be 400 Hz, and the next would be 600 Hz, and so on.

Resonance

Most string instruments have large, hollow areas to help intensify and prolong the sound produced by the strings. The space in our nasal cavities, chest, and lungs produces a similar effect when we speak. This intensifying effect is called resonance. The natural resonant frequency is determined by the physical parameters of the vibrating object. Any object, from your skeleton to a suspension bridge, can be made to vibrate if acted upon at its resonant frequency.

Amplitude

The actual measurement of sound is defined by two parameters: the scientific and the psychological. The scientific measurement of amplitude is often confused with its psychological equivalent of loudness. Amplitude is measured in decibels (dB). A rise of 10 dB results in a perceived doubling in loudness.