Every synthesiser, sampler and effect you will ever use is a tool for manipulating a handful of physical properties of sound. Understand those properties and every control on every instrument starts to make sense. This chapter is the foundation for the whole module.
Sound is vibration
Sound is a vibration travelling through a medium - usually air. A vibrating object - a string, a drum skin, a loudspeaker cone - pushes air molecules together (compression) and pulls them apart (rarefaction). These pressure changes travel outwards as a wave, at roughly 343 metres per second in air at room temperature, until they reach your ear and move your eardrum.
A microphone does what your eardrum does: it turns pressure changes into an electrical signal. A speaker does the reverse.
Frequency and pitch
Frequency is how many times a wave repeats per second, measured in hertz (Hz). We hear frequency as pitch.
- The human hearing range is roughly 20 Hz to 20,000 Hz (20 kHz), with the upper limit falling with age and noise exposure.
- Doubling a frequency raises the pitch by an octave. A4 is 440 Hz; A5 is 880 Hz; A3 is 220 Hz.
- Below about 20 Hz is infrasound - felt more than heard. Above 20 kHz is ultrasound.
Amplitude and loudness
Amplitude is the size of the pressure change - how far the wave swings from rest. We hear amplitude as loudness.
Loudness is measured in decibels (dB), a logarithmic scale: every increase of about 6 dB doubles the amplitude, and around 10 dB is perceived as roughly twice as loud. Our ears are also more sensitive to some frequencies than others - most sensitive in the region of 2-5 kHz, where speech intelligibility lives.
Wavelength
Wavelength is the physical length of one cycle of a wave: the speed of sound divided by the frequency. Low frequencies have long wavelengths - a 50 Hz wave is nearly 7 metres long. High frequencies have short ones - 10 kHz is about 3.4 cm.
This is why bass is so hard to control in small rooms, and why low frequencies travel through walls.
Waveforms and harmonics
A pure tone - a sine wave - contains a single frequency. Almost no natural sound is a pure tone. Real sounds contain a fundamental (the lowest frequency, which sets the pitch we hear) plus harmonics (overtones) at multiples of it: 2x, 3x, 4x and so on.
The basic synthesiser waveforms are defined by their harmonics:
- Sine - fundamental only. Pure, smooth.
- Triangle - odd harmonics, falling away quickly. Soft, flute-like.
- Square - odd harmonics, falling away more slowly. Hollow, woody, clarinet-like.
- Sawtooth - all harmonics. Bright, buzzy, rich - the starting point for strings and brass.
- Noise - all frequencies at once, randomly. Wind, breath, cymbals, snares.
Timbre
Timbre is the character of a sound - why a piano and a guitar playing the same note at the same loudness sound different. It comes from the balance of harmonics, how that balance changes over time, and the noise components of a sound. Sound design is, essentially, the craft of shaping timbre.
Phase
Phase describes where a wave is in its cycle at a given moment. Two identical waves in phase add together and get louder; perfectly out of phase, they cancel completely. Partial phase differences cause some frequencies to boost and others to cancel - the basis of comb filtering, phasers and flangers, and the reason layered kicks can sound thinner than either kick alone.
The envelope of a sound
Every sound changes over time. Its loudness rises and falls in a shape called an envelope:
- Attack - how quickly it starts. A plucked string is instant; a bowed string swells.
- Decay - how quickly it falls from its initial peak.
- Sustain - the level it holds while the note continues.
- Release - how long it takes to fade once the note ends.
Synthesisers recreate this with ADSR envelopes - covered in the subtractive synthesis chapters.
Why this matters
Every control on a synth maps to one of these properties:
- Oscillator pitch - frequency.
- Waveform - harmonic content.
- Filter - which harmonics pass.
- Amplifier and envelopes - amplitude over time.
- LFOs - changes to any of these, cyclically.
Do this now
- Generate a sine wave at 440 Hz in your DAW and play it alongside one at 880 Hz. Hear the octave.
- Switch an oscillator between sine, triangle, square and sawtooth at the same pitch and look at each on a spectrum analyser. Count the harmonics.
- Play two identical sine waves, then flip the polarity of one. Hear them cancel.