Unit
- Sound waves: Sound propagates as longitudinal mechanical waves through a medium, typically air. The wave consists of alternating compressions and rarefactions that travel at approximately 343 meters per second at room temperature, varying with altitude and temperature.
- Frequency: Measured in hertz (Hz), frequency determines pitch. The human audible range spans roughly 20 Hz to 20,000 Hz, though this diminishes with age. Frequencies below 20 Hz are infrasound, above 20 kHz are ultrasound, both inaudible to humans.
- Amplitude: Amplitude corresponds to the magnitude of pressure variation and directly relates to perceived loudness. Larger amplitude means greater energy transfer and a louder sound. Amplitude is typically measured in decibels relative to a reference.
- Phase: Phase describes the position of a wave cycle relative to a reference point. When two identical waves align (0 degrees phase difference), they constructively interfere and double amplitude. Opposite phase (180 degrees) causes destructive interference, canceling the sound.
- Waveforms: Simple periodic waveforms include sine, square, triangle, and sawtooth, each producing distinct timbres. Sine waves are pure tones; complex waveforms like square waves contain odd harmonics, creating brighter sounds.
- Harmonics and overtones: Most natural sounds are complex, consisting of a fundamental frequency plus integer multiples called harmonics. The relative strength of these harmonics determines timbre, allowing us to distinguish a piano from a violin playing the same note.
- Inverse square law: Sound intensity decreases proportionally to the square of the distance from the source. Doubling distance reduces intensity by 6 dB, a critical principle for microphone placement and speaker positioning.
- Reflection and absorption: When sound waves encounter surfaces, they reflect, absorb, or transmit. Hard surfaces like concrete reflect most energy, creating echoes; soft materials like foam absorb high frequencies, affecting room acoustics.
- Velocity and medium: Sound speed depends on the medium’s density and elasticity. In air it is about 343 m/s; in water around 1500 m/s; in steel roughly 5000 m/s. This variation affects timing and pitch perception when sound travels through different materials, a key consideration in underwater acoustics and structural sound design.
- Timbre and spectral content: Beyond harmonics, timbre is shaped by the attack, sustain, and decay of a sound. The spectral envelope, which describes how harmonic amplitude changes with frequency, gives each instrument its unique character. Envelope followers and spectral analyzers help designers modify timbre electronically.
- Doppler effect: When a sound source moves relative to a listener, the observed frequency shifts. Approaching sources sound higher pitched; receding sources sound lower. This effect is used in sound design for simulating motion, such as a passing vehicle or flyby, by automating pitch and volume over time.
Design Principle
Principle: Constructive and Destructive Interference
Definition: When two or more sound waves meet, their amplitudes combine algebraically. In-phase waves reinforce each other (constructive), while out-of-phase waves cancel (destructive). This principle underpins comb filtering, phase cancellation in stereo, and noise cancellation technology.
Application: In sound design, use interference intentionally. For example, to create a phaser effect, mix an original signal with a slightly delayed copy; the varying phase cancellation produces sweeping comb filters. For noise cancellation, generate a mirror image of ambient noise at 180-degree phase shift to cancel it.
Quick Revision
- Sound travels as longitudinal waves at roughly 343 m/s in air.
- Frequency (Hz) determines pitch; amplitude determines loudness.
- Phase alignment causes interference: constructive (in phase) or destructive (opposite phase).
- Waveforms (sine, square, etc.) define timbre through harmonic content.
- Sound intensity follows the inverse square law: -6 dB per doubling of distance.