Sound effects editing and sound design

Sound effects give a film its physical world. They tell us what objects weigh, how fast things move, how far away the storm is and how dangerous the creature might be. Effects work divides into several overlapping crafts: cutting hard effects in sync with picture, building backgrounds that place each scene in a time and space, and sound design, the creation of sounds that do not exist or that must be heightened far beyond reality. This lesson covers how effects editors find, record, edit and layer sounds, how designers create new ones, and how effects are organized for the mix. Foley, the performed movement sounds, has its own lesson next.

Categories of effects

Effects editors use a shared vocabulary for different kinds of sound. Hard effects, also called spot effects, are sounds tied to specific on-screen events: a door slam, a gunshot, a car pass, a phone ring. Backgrounds, or ambiences, are continuous sounds that establish place: a city street, a forest at dawn, an office hum. Designed effects are created or heavily processed sounds for things that do not exist or need to feel extraordinary: spaceships, magic, creatures, transitions into memories. Foley covers human movement and is usually handled separately. Production effects are useful sounds kept from the production recording.

Sources: libraries and custom recording

Most effects begin as recordings. Commercial libraries contain hundreds of thousands of categorized sounds, licensed for use in productions under the terms of each library's license. Effects editors search them by keyword and metadata, often using a standard naming system such as the Universal Category System, which gives each sound a category, subcategory and descriptive name so libraries from different vendors can be searched together. Libraries are fast and affordable, but heavily used sounds can be recognizable, and a film that relies only on libraries can sound generic.

Custom recording gives a film its own identity. Effects recordists capture specific vehicles, weapons, locations and props, often at high sample rates such as 96 or 192 kHz so the sounds can later be slowed down or pitched down without losing high-frequency detail. Recording sessions require planning, permissions and safety: recording firearms needs licensed armorers and range permission; recording vehicles needs closed roads or tracks; recording in public places may need permits.

Licensing matters. A sound effect from a library is usually licensed for synchronization in your production but not for resale as a sound effect. Sounds recorded from television, films or other people's recordings are not free to use. Even sounds that seem generic, such as a famous scream, may be owned. Keep records of library sources and licenses with the project files.

Cutting hard effects

Cutting hard effects means placing sounds precisely in sync with their visual events and choosing sounds that match the size, material, distance and story weight of each event. A car door on a luxury sedan should sound solid and expensive; on a rusty pickup, rattly and tired. A door slammed in anger should feel louder and heavier than the same door closed gently. Effects editors usually cut each event from several elements and place them on separate tracks so the mixer can balance them.

Sync is usually frame-accurate, but perception guides small adjustments. Impacts often sound better a frame early, landing on the frame where the hand or object makes contact, because the eye anticipates contact. Distant events in wide shots may be slipped late to suggest the time sound takes to travel, though film convention rarely uses the full physical delay: a lightning strike 1.7 km away would produce thunder 5 seconds later, which audiences accept, but a cannon 300 m away would be about 1 second late, which can feel like a sync error unless the film has established the convention.

Layering: building a sound from elements

Few powerful film sounds come from a single recording. Designers build them in layers, each contributing part of the frequency range and envelope: a sharp transient for the attack, a body for weight, a low-frequency element for size and impact, a tail for space and decay, and sometimes a sweetener, a subtle element such as an animal growl inside an engine, that gives the sound character.

Processing tools of the sound designer

Sound designers transform recordings with a toolkit of processes. Pitch shifting changes pitch, often with time-stretching to keep duration constant or by simple speed change, which changes both. Slowing a recording down by half lowers its pitch by an octave; this is why recording at high sample rates helps, because the high-frequency content that drops into the audible range after slowing must have been captured in the first place. Equalization shapes tone; reverberation and convolution place sounds in spaces; distortion and saturation add aggression; granular synthesis breaks sounds into tiny grains and reassembles them into textures; and synthesis creates tones from scratch.

The equation gives the frequency ratio \( r \) for a pitch shift of \( n \) semitones: 12 semitones is an octave and doubles or halves the frequency. A sound shifted down 7 semitones plays at about 0.67 times its original frequency.

The Doppler effect, the change in pitch of a sound from a moving source, is essential for vehicle passes and fly-bys. As a source approaches, waves are compressed and the pitch rises; as it recedes, the pitch falls. For a source moving at speed \( v \) directly toward or away from a listener:

where the minus sign applies when the source approaches and the plus sign when it recedes. Plug-ins can simulate Doppler, but real recordings of passes often sound more convincing because they also capture changing distance, reflections and engine load.

import math
for n in [-12, -7, -5, 5, 12]:
    print(f'{n:+3d} semitones: frequency ratio {2 ** (n / 12):.3f}')
recorded_fs, playback_fs = 192000, 48000
print(f'192 kHz file played as 48 kHz: speed {playback_fs / recorded_fs:.2f}x, pitch {12 * math.log2(playback_fs / recorded_fs):+.0f} semitones')
print(f'highest original frequency that lands at 20 kHz after that change: {20000 * recorded_fs / playback_fs / 1000:.0f} kHz')
c, f = 343.0, 120.0      # engine note at 120 Hz
for kmh in [50, 100, 200]:
    v = kmh / 3.6
    up, down = f * c / (c - v), f * c / (c + v)
    print(f'{kmh:3d} km/h: approaching {up:5.1f} Hz, receding {down:5.1f} Hz, drop {12 * math.log2(up / down):4.1f} semitones')

-12 semitones: frequency ratio 0.500
 -7 semitones: frequency ratio 0.667
 -5 semitones: frequency ratio 0.749
 +5 semitones: frequency ratio 1.335
+12 semitones: frequency ratio 2.000
192 kHz file played as 48 kHz: speed 0.25x, pitch -24 semitones
highest original frequency that lands at 20 kHz after that change: 80 kHz
 50 km/h: approaching 125.1 Hz, receding 115.3 Hz, drop  1.4 semitones
100 km/h: approaching 130.6 Hz, receding 111.0 Hz, drop  2.8 semitones
200 km/h: approaching 143.2 Hz, receding 103.3 Hz, drop  5.7 semitones

Backgrounds and ambiences

Backgrounds are often the most underrated part of a sound track. A carefully built background tells the audience where and when they are, sets the emotional temperature, and smooths the edit by providing continuous sound across cuts. Effects editors build backgrounds from several layers: a base room tone or air, a mid layer such as distant traffic or wind in trees, and specific details such as birds, distant sirens, a television through a wall. Stereo or surround recordings give width and immersion.

Backgrounds should support dialogue, not compete with it. Busy, attention-grabbing elements such as a dog barking or a car horn are placed in gaps between lines. Backgrounds also follow the story: a scene may begin with a full city soundscape and gradually thin to near silence as two characters become absorbed in conversation, then return when they are interrupted. Changing background density is a subtle way to direct attention.

Sound design: inventing new sounds

When a film needs sounds that do not exist, designers combine recordings and synthesis in imaginative ways. Classic examples include building creature voices from blends of animal recordings, making futuristic weapons from processed recordings of real objects such as cables struck under tension, and creating the sense of a vast spacecraft from low-frequency rumbles and layered machinery. The design process usually starts with the story: what should this sound make the audience feel, and what does it need to communicate about the object or creature? Designers then experiment, iterate with the director, and refine.

Designed sounds must also work in the mix. A creature roar that is thrilling on its own may mask dialogue or clash with the score's key. Designers often deliver designed effects in stems or layers so the mixer can balance them, and they work with the composer to decide where the sound design ends and the music begins. In some films, such as horror and science fiction, the line between music and sound design is deliberately blurred.

Organization and track layout

Effects sessions can contain hundreds of tracks. A consistent track layout lets the mixer find material and lets several editors work on different reels without confusion. A typical layout groups tracks by category: hard effects, designed effects, backgrounds, production effects, Foley (from the Foley team), and low-frequency effects intended for the separate subwoofer channel in surround mixes. Within each group, editors use consistent naming, color coding and clip names that describe the sound.

Effects editors deliver their tracks to the premix with notes on what each layer does, which elements are optional and where alternatives exist. Many supervisors ask editors to over-cover, providing more elements than will be used, because the mixer can always lower or mute a layer but cannot add what was never cut.

Working with the director and the composer

Effects and music share the same frequency space and the same emotional territory. When both are busy, the mix becomes cluttered, and the mixer must choose. Good communication early in post-production prevents conflict: the supervising sound editor and composer discuss which moments will be led by music, which by effects, and which by silence. Temp tracks matter too: if the director has lived with a temp mix for months, unfamiliar sounds can feel wrong at first. Presenting new design in context, with the score, helps the director judge it fairly.

In the United States, sound effects editors and sound designers on many studio and network productions are members of the Motion Picture Editors Guild, IATSE Local 700, which covers post-production sound editors. Check the production's agreements.