The box on this page controls a synth running in your web browser, but synthesizers come in many shapes and sizes.

They can be physical instruments that musicians play on stage, noise-makers inside electronic devices and toys, or software running on a phone or computer.

Synthesizers make sound using electricity, usually with controls that let you change their sound. Some controls change just one specific aspect of the sound, while others, like the box above, change many aspects of the sound at once.

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One of the most common aspects of sound you can control with a synth is amplitude.

Increasing the amplitude makes a sound louder. Decreasing the amplitude makes the sound softer. The volume on your phone or computer is an amplitude control.

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Another important aspect of sound is pitch — its relative “highness” or “lowness.”

Many musical instruments allow you to control pitch. For example, keys on the left side of a piano play lower pitches, while keys on the right side play higher pitches. Synthesizers can play a much wider range of pitch than any acoustic instrument — from the very lowest to the very highest sounds that humans can hear.

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Have a play with amplitude and pitch

This box makes our synth work a bit like an early electronic instrument called a theremin, which is also played with two controls: one for pitch and another for amplitude.

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Some synthesizers are physical instruments with piano-like keyboards. These start playing a note when a key is pressed, and stop when the key is released.

On a synthesizer with a keyboard, the keyboard is also a control for the synth’s pitch. Keys to the left make the pitch lower; keys to the right make the pitch higher.

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Both physical synthesizers and software synths ave controls such as knobs and sliders.

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As you draw in the box, you’re creating an envelope — the shape of a sound changing over time.

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Synthesizers use envelopes to create shapes designed for making musical sounds. You adjust the envelope’s controls to tell it what kind of shape to make.

You don’t hear the envelope directly — the synth uses the envelope’s shape to determine how one or more other aspects of the sound change over time. This process of using some part of the synthesizer to affect other parts is called modulation. An envelope is an example of a modulator.

Most synthesizers have envelopes with four sections or stages, called attack, decay, sustain, and release. You’ll often see these abbreviated as ADSR.

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An envelope’s attack control determines how long it takes for the envelope to reach its peak at the start of a note.

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An envelope’s decay control determines how long it takes for the envelope to decrease from its peak to the sustain level. The envelope will then stay at the sustain level as long as the note is held.

Try turning the sustain all the way down and then adjust the decay.

Now turn the sustain all the way up and play with the decay again.

Notice the difference? With sustain all the way up, decay has no effect; the pitch stays at its maximum level.

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An envelope’s release control determines how long it takes for the envelope to decrease to its minimum level after the note is released.

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Envelopes can also be used to change other aspects of the sound, such as amplitude.

Now, instead of adjusting an envelope that modulates the pitch, we’re adjusting an envelope that modulates the amplitude.

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These oscillators are called low-frequency oscillators (or LFOs) because they move relatively slowly compared to the audio oscillators. You can think of LFOs as robots that make a repeating change to some aspect of the sound.

This LFO modulates the pitch with a constant speed and shape.

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The number tells you the LFO’s frequency — how many times the oscillator moves from its highest point to its lowest and back in one second.

Frequency is measured in a unit called hertz (abbreviated Hz). A frequency of one hertz means that a full oscillation (or cycle) happens once per second.

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Here, the number tells you how much the pitch will change in semitones. (A semitone is the difference in pitch between one key and the next on a piano.) The highest value (48 st) means 48 semitones above and below the central pitch (or a total of eight octaves).

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So far, our LFO has always moved smoothly back and forth between high and low. The specific shape of this motion — a bit like a rolling hill — is referred to as a sine wave.

But changing the LFO’s shape (or waveform) can completely change the character of the modulation.

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Imagine a buzzing bee. The tone we hear is caused by the bee flapping its wings quickly.

When the flapping is fast enough it will start to produce a low tone. In general, we start to hear a tone when something oscillates faster than around 20 Hz.

Most synths generate tones with audio oscillators. These are similar to the low-frequency oscillators used for modulation, but they move much faster so they can produce tones directly.

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Some oscillators produce a fixed shape, but other oscillators have controls that allow that shape to change.

You can change the square oscillator’s timbre by changing its pulse width: the amount of time the oscillator spends in the high and low part of its cycle.

The wave has its “fullest” sound at the center, where the waveform is symmetrical.

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You can also modulate the square oscillator’s pulse width with an LFO:

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Imagine there’s music playing in the next room. If the music is loud enough, you can hear it through the wall, but it usually sounds “muffled” or “dull.” This is because the wall is acting as a filter. It blocks the high frequencies but lets the low frequencies through.

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A filter behaves a bit like a wall or door. It lets some parts of the sound through and makes other parts quieter.

Dragging adjusts the cutoff frequency of the filter. In this case, the filter is removing high frequencies. This type of filter is called a low-pass filter (it passes low frequencies and cuts high frequencies).

More high frequencies make a sound “brighter.” Less high frequencies make it “darker.”

As you adjust the cutoff frequency slowly, you’ll probably recognize this effect. The sound of a filter being “swept” open or closed is a classic technique in many kinds of music.

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Filters often have another control, called resonance, that increases the amplitude around the cutoff frequency. Filter sweeps take on a very different character with different resonance settings.

Notice that high resonance values create a kind of high “whistling” sound. With the resonance at a high value, try adjusting the cutoff and hear how this whistling sound changes.

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Now that you’ve learned a bit about synthesizers, we’ll combine the various parts of the synth together and learn to make complete sounds.

We’ll also introduce a new way of showing the controls, which is closer to the way you might find them in a typical synth. Related controls are grouped together in panels like this:

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This sound is inspired by classic West Coast hip hop, and is great for long melodic lines.

The secret to this sound is that some notes glide smoothly to the next note, rather than changing pitch abruptly.

Some of the notes in the sequence aren’t released until after the next note begins. In these cases, the last note’s pitch will glide to the new note’s pitch. This type of smooth, connected playing is called legato.

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For this classic bass sound, we’ve used a slower filter envelope to make the synth sound like it’s talking.

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Synths are great for making siren sounds. We’ll start with an American police siren.

The most noticeable thing about this sound is the repeating pitch change. This is done with an LFO modulating pitch.

You might have noticed that the pitch of a siren goes up a bit as it comes towards you, then down a bit as it goes away again. This phenomenon is called Doppler shift.

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This synth is great for making classic B-movie laser sounds, and also for punchy electronic kick drums. In fact, these are almost the same sound!

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We can turn our laser into a punchy electronic kick sound by changing just a few controls.

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Here’s an aggressive bass sound inspired by trap and hip hop.

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We can make more classic movie sound effects, like this retro-futuristic sci-fi computer sound.

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This simple hihat sound uses only noise; both oscillators are turned all the way down.

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Let’s look at a classic “string”-like sound, useful for melodic lines. This sound is a bit like a violin or cello, but has a synthetic character of its own.

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Here’s a bright, noisy sound with lots of energy and “sizzle.” This type of sound was popular in some early 90’s dance music, and is sometimes called a Hoover because of its similarity to the sound of a vacuum cleaner...

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This recipe uses very fast LFO modulation of amplitude to create a noisy, distorted sound that has a different timbre depending on which pitch is being played.

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This sound behaves like a rubber ball dropped on a hard surface; it starts bouncing slowly and high in the air. With each bounce it loses energy, so it gets faster and stays closer to the ground. It also gradually gets quieter.

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This sound has an interesting timbre change when you release the note, rather than when you play it.

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This sound uses a filter modulated by a square wave LFO to switch between two different “states” of the sound.

By default, we hear only one of these states.

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