Introduction to Sound Synthesis Breaking it apart
An envelope as a pitch source
An envelope sent to an oscillator's pitch makes each note start high and fall to its base pitch. A small, slow fall is heard as a slide down into the note. A large, fast fall makes the note sound like a drum being hit, and it is how most synthesised kick drums and toms are made.
The chapter on envelopes as modulation sent an envelope to the oscillator's pitch with a modest amount. With a larger amount, the pitch starts two or three octaves above its base and falls within a few tens of milliseconds, so the start of the note is heard as the impact of a hit rather than as a pitch, and what follows is heard as the body of a drum.
The tool below is a single oscillator with two envelopes: one on pitch, which jumps up instantly and falls over the pitch decay time, and one on the amplifier, which sets how long the hit rings. The plot shows the pitch over the first second of each hit on a logarithmic frequency scale with a grid line at every octave, with the level drawn as the width of the shaded band.
A pitch envelope
One oscillator has an envelope on its pitch and another on its level. The presets set a kick drum, a tom, a laser zap and a plain tone with no pitch envelope for comparison.
Listen for With the kick preset each hit is a thump with a low tail. Lengthen the pitch decay and the thump becomes a downward swoop. Set the pitch amount to zero and the same settings give a plain low tone with no thump, because the thump comes from the first few tens of milliseconds of the pitch envelope.
Amount and decay
The pitch amount sets how far above the base the hit starts, and the pitch decay sets how quickly it falls. Together they set what the start of the hit sounds like. A large amount with a decay of 30 to 80 milliseconds is heard as a punch, because the fall is too fast to follow as a pitch. Around 100 to 200 milliseconds the fall becomes audible as a swoop, which is the sound of the electronic toms on the Simmons SDS-V drum kit of 1981. Beyond that it is a sweep or a laser zap, and it no longer sounds like a drum.
The base pitch and the amplifier's decay set the body of the drum. A kick drum usually sits between about 45 and 60 Hz with a decay of a few hundred milliseconds. A tom is higher and often has less pitch amount. Analogue drum machines such as Roland's TR-808 and TR-909 build their bass drums on the same principle, a tone whose pitch falls at the start of each hit, and the TR-909 adds a short click at the start of the hit from a separate circuit.
Why the fall is exponential
The pitch envelope here falls in a straight line measured in cents, which is a curve in hertz: the frequency halves in equal steps of time. That sounds like an even fall, because the ear hears pitch by ratio. A fall that is a straight line in hertz spends most of its time in the upper octaves and drops through the lower ones suddenly at the end. On a modular or analogue oscillator with a 1V/oct input, an ordinary linear envelope already gives the even version, because the oscillator doubles its frequency for each added volt. In software, a pitch envelope set in semitones or cents does the same.
Why it sounds like a drum
A drum head is under slightly more tension while it is vibrating widely, just after it is struck, so its pitch is a little higher at the start of a hit and falls as the vibration dies away. A synthesised kick exaggerates that fall, and a fast, large fall in pitch at the start of a sound is heard as an impact. A drum also has inharmonic partials and some noise at the moment of the strike, which is why many synthesised drums add a short burst of filtered noise at the front.
Common questions
How do you make a kick drum on a synthesiser?
Start with a sine or triangle oscillator at a low pitch, around 45 to 60 Hz. Send a fast envelope to its pitch so that each hit starts two or three octaves higher and falls to the base pitch within about 30 to 80 milliseconds, and send a second, longer envelope to an amplifier so the hit decays over a few hundred milliseconds.
Why does a falling pitch sound like a drum?
A struck drum head is under slightly more tension while it is vibrating widely just after the hit, so its pitch is higher at the start and falls as the vibration dies away. A synthesised pitch drop exaggerates the same effect, and the ear hears the fast fall as the impact of a hit.
What is a pitch envelope?
A pitch envelope is an envelope sent to an oscillator's pitch. It moves the pitch once for each note, by an amount set by its depth control, and returns it to the note's pitch as the envelope decays.
Why should the pitch envelope be exponential?
Pitch is heard on a logarithmic scale, so a fall that is linear in hertz spends most of its time in the upper octaves and drops through the lower ones suddenly at the end. A fall that is linear in octaves or cents, which is exponential in hertz, sounds even. On a modular or analogue oscillator with a 1V/oct pitch input, a linear envelope gives this automatically, because the oscillator doubles its frequency for each added volt.
Published Updated Glossary