Subtractive Synthesis Explained – What is Subtractive Synthesis?

If you’ve ever wanted to know what subtractive synthesis was, without having to perform the intellectual acrobatics it sometimes requires to understand the principles behind, fret not…

This post will aim to break down the concept of subtractive synthesis to its fundamental, basic, easy to understand principles.

A thorough reading of this guide will help you in your next sound designing project or electronic music production.

Let’s take a look then!

Read the other articles on audio synthesis:

• Additive synthesis
• FM synthesis
• Wavetable synthesis
• Music Production Courses Online

Introduction – Subtractive Synthesis 101

Synthesis is at the foundation of music production knowledge. Particularly in electronic music. A producer like Skrillex is a genius because he has an excellent grasp of audio synthesis. Knowing synthesis allows a producer and musician like him, or you, to identify and choose the sorts of unique sounds you need for your music. And it all starts with a proper understanding of the various types of syntheses that are available to create these sounds.

With that, if you have a sound in mind that you need to design, you’d know right away what sort of waveforms, envelopes, filters, as well as your audio source to use, and which instrument you will be utilizing to produce these sounds. Sounds cool, doesn’t it?

Well, with that said, what is subtractive synthesis?

Basics

Subtractive synthesis refers to a type of synthesis where the partials of a harmonically rich sound are attenuated using a filter. The result of filtering out your harmonic overtones (also called sidebands) allows the synthesist (the person using the synthesizer) to form and sculpt any desired sonic texture or timbre of their choice from their sound source. This can be achieved using either a hardware device (called a synthesizer) or a virtual instrument (called a softsynth).

I’m aware that this definition is highly technical and full of jargons. But don’t worry, I’ll make it easy for you to understanding. So much so, that by the end of this post, you, my dear reader, shall become the next great synthesis jargonaut.

Corny jokes aside, let us proceed, shall we?

How does a subtractive synthesizer work?

To know how a subtractive synthesizer works, we need to know how a synthesizer itself works.

Synthesis Basics

To quickly recap, a synthesizer is a device that produces audio signals using an oscillator or a series of oscillators. This oscillator produces vibrations of hundreds to thousands of times per seconds. If something vibrates at 20 cycles per second, we can perceive it as a low rumbling bass sound. We can also hear vibrations all the way up to 20,000 cycles per second. We call “cycles per second” Hertz, or Hz.

So when I say that the middle C on the piano is producing a frequency of 262 Hz, I mean that the piano string (or rather, wire) is vibrating at a rate of 262 times per second. Anything that vibrates at this frequency will give you the sound you hear at the piano when you play middle C.

Harmonic Overtones

However, no sound produced in nature or on real instruments is going to give you such a clean harmonic tone. If that were the case, all C notes, whether sung by a man, woman, child, or played on guitar, piano, or ukelele, would sound exactly the same. Yet, we can usually tell when a man is mumbling a C note, or if that note is being played on the piano.

That’s because each instrument or voice produces a combination of various overlapping waveforms (harmonic overtones) that are unique to the source of the sound. The dominating frequency is what we perceive as a note, sung or played. All other frequency vibrations are what “colors” the note with the particular characteristics, or “timbre”, of the instrument or mouth that the sounds are coming from. On the other hand, if there is no dominating frequency, it is simply a noise.

The only way to produce a pure tone with no harmonics is by using a synthesizer, and that sound produced would be called a “sine wave.”

A Traffic of Frequencies

I want you to think of traffic for a second. How does it make you feel? I know, we don’t like traffic. Boo. But keep with me for a second…

How does traffic usually sound? Sometimes loud and chaotic, sometimes just lots of wush-wushing sounds of tire, asphalt, horns, and engine revs.

Now, I want you to think of the sounds of traffic as simply a bunch of frequencies producing the sound we call traffic.

That sound, traffic, is a harmonically rich sound (or noise).

Any sound in the world, then, that is produced naturally via an instrument will produce a traffic of frequencies. When there is a dominant frequency in the bunch leading the rest of the frequencies along the way, we no longer have so much noise, we have a discernible note.

Back to our traffic example. Imagine that there was a way for you the “comb out” a desirable, discernible sound from all that traffic? What if, you just wanted to get the sound of a certain horn you hear? You’d need to cut out the frequencies that are consistent with the sounds of engine revs, tires on asphalt, etc.

Are you getting the point yet? You are “subtracting” sounds until you get a sound you like. That is the basis of subtractive synthesis.

Take a look at the video below. It starts with white noise sound with lots of frequencies, so there is no discernible note, just a “noise,” like traffic.

The synthesist applies a band-pass filter to let through only a narrow range of frequencies. The result is a sound consistent with a “whistling note.”

He does the same thing again, but with a “buzzy sound.” As he slides his filter up and down, it almost sounds like human’s voice. That’s the power of subtractive synthesis.

Watch it.

That’s great. Now, enough about traffic and noise and stuff. You want to make music. Keep reading…

How to Add and Subtract in Synthesis

In order for you to first subtract, you need to add something in the first place.

That something is a sound source.

What a subtractive synthesizer does, initially, is to create a raw sound using one or more oscillators (devices that create vibrations to produce sounds, like the vibration of a piano wire). Once you get a sound, it’s then passed through a filter which cuts away all the stuff we don’t want.

Sound Chisels

In other words, think of it like sculpting. A sculptor takes a raw boulder and chips away bits of it until you get the sonic equivalent of the Statue of David.

A subtractive synthesizer, then, is a device that creates sonic sculptures out of audio-rubble.

Of course, there are other steps involved, such as using an LFO tool and ADSR envelope. But we will get to these shortly.

For now, let’s take our sound sculpting example and attempt to sculpt a plucking electric guitar sound out of a basic audio-rubble – a buzzy square wave (audio illustration courtesy of Wikipedia).

Subtractive Synthesis in Action

#1 – First, we’d need raw complex sounds to work with. So we’ll use two fairly harmonically rich sounds created using two oscillators.

• Waveform 1 (Click to listen)
• Waveform 2 (Click to listen)

The reason why we are using a square wave instead of a sine wave is that square waves, due to their shape, produces a fairly harmonically rich tone. Sine waves, on the other hand, produce pure sounding tones. There’d be nothing, in other words, from a sine tone to “chisel away” at.

We can also use triangle waves and sawtooth waves as our audio source. See and listen to my sound examples of various waveforms in the FM synthesis post.

#2 – Now, just for fun, let’s add some dynamic variations to that pitch. So we’ll add a pulse-wave modulator (just to get it nice and fizzy).

• PWM Waveform 1 (Click to listen)
• PWM Waveform 2 (Click to listen)

Cool.

#3 – When we mix these two sounds together, this is the sonic concoction that we get.

• Combined waveform (Click to listen)

#4 – If you notice, the waveform (sound) remains sustained all the way throughout. Most naturally occurring sounds in nature decay in loudness over time, like the sound of a guitar. To alter this, we need to be able to control, or shape, the amplification of the sound so that it decays over time.

We do that using an ADSR envelope (Amplitude, Decay, Sustain, and Release. I’ll explain this in a bit, hang on).

• Enveloped waveform (Click to listen)

Now, for some sonic chiselling: Let’s get subtractive.

#5 – We want to get rid of some of the upper harmonics (the buzziness) of the sound for a more natural and clean sounding wave.

To achieve that, we need what is called a “low-pass filter“. A low pass filter simply allows the lower frequencies in the sounds harmonics to “pass-through”. This cuts out the higher frequencies (sometimes it’s called a hi-cut filter).

• Filtered waveform (Click to listen, also you might need to turn up your volume)

#6 – And to better emulate the plucking sound of a guitar pedal, the cut-off frequency of that low pass filter is set to start cutting out the higher frequencies at the beginning of the sound, then shifting over time to cutting out the lower frequencies as the pitch decays. This is controlled by a low-frequency oscillator, or LFO (will explain this a little later).

• Final sound (Click to listen, also you might need to turn up your volume)

#7 – Now we can play these sounds like one would play an instrument.

• Arpeggiated synth (Click to listen)

Sounds good.

Properties of Subtractive synthesis – Terminologies

The Amplifier

Sometimes called the voltage controlled amplifier, or just volume, this is what controls the loudness of the sound produced by the oscillator. This can also be controlled using another oscillator (in particular, the LFO) or envelope controls.

The Envelope Control

Just above, I mentioned ADSR and I promised to you that I was going to explain. If you’ve reached this far, it’s because you like knowledge. Good for you. You’re my favourite reader. You get a star ⭐

ADSR – Attack, Decay, Sustain & Release

An envelope is simply a tool that controls the amplification of the sound generated by the synthesizer’s oscillators. Do you want your sound to begin suddenly, or start with a swell? That’s what attack controls.

After the attack comes decay, which controls how long it takes for the sound’s volume to decay to a sustained level.

Sustain is the level of the main body of the sound. If you press a key with a fast attack and delay, you get a sound with a strong “pluck” in the beginning, but the rest of the sound will continue to “sustain” until you release the key.

Finally, you can decide how long the note continues to sound after you release the key. The release function then controls how long the note’s volume takes to decay to zero.

The Filter

Also called the cut-off filter, these are your sonic-chisels. There are different kinds: Hi-pass, low-pass, band-pass, and band-reject pass.

The filter’s job is to control the ‘tone’ of the sound. Do you want a high tone, a low tone a muffled tone, or a sharp tone? Use the filter control to manipulate the sound by controlling what type of frequencies reach your ears.

The filter that’s most popularly used in subtractive synthesis is the low-pass filter, since it allows you to ‘cut-off’ the higher harmonics of a sound, the part of the sound that produces its buzzy characteristics.

But you can also use the high-pass filter to “cut off” the bottom ends of noises like “white noise“, which you can shape into snare or hi-hat sounds using the ADSR tool.

The Low-Frequency Oscillators

The low-frequency oscillator is what controls the other parameters of the synthesizes. It can be used to control the amplifier from loud to soft, or the cut-off filter’s setting.

For example, as the filter is usually controlled by a knob which you can turn up and down (to let through high or low frequencies), it makes sense that you’d like a sound that can variate over time from a high tone to low tone (or vice versa).

The guitar example we used above is an example of the use of an LFO. When the note is triggered, the LFO tells the cut-off filter knob to begin sweeping the cut off frequency filter from high to low, so that only the very initially attack gets a bright plucking sound. The LFO, like the envelope tool, is what allows you to shape your sounds.

It’s a “low-frequency oscillator” for the very fact that this control “oscillates” at a much slower rate, unlike the modulator oscillator in an FM synthesizer which oscillators at super fast speeds.

What is Subtractive Synthesis good for?

In the post about FM synthesis, you were told that FM synthesis is good for creating sonically complex textures. It does this by going in opposite direction to how we’ve just sculpted our guitar sound.

It takes a basic clean sound wave as the basis for our sound, and modulate it’s pitch so fast we end up with another frequency. These frequencies give us sideband frequencies which we can alter by controlling the ratio of the carrier frequency to the modulation frequency, giving us limitless amounts of textures to play with, which include all sorts of delightful noises and effects.

Even though with FM synthesis you could get rich harmonic sounds from a simple source, back in the day of analogue hardware, this method was expensive, difficult to use, and required lots of wires to connect oscillators to modulators and filters to get certain simple yet natural sounding notes.

To remedy this, companies came out with a type of synth that would create basic harmonically rich tones that you could alter by simply filtering out (“chiselling away”) the frequencies that you didn’t want. The result is that you have an electronic instrument that could emulate easily and cheaply the sounds of pianos, guitars, organs, and so on.

This same principle can be used to create drum sounds such as snares, hats, and symbols. Since you’d be able to create these sounds by cutting off the low frequencies of a noise, and shaping the sound using the LFO tool and ADSR envelopes.

Great Examples of Virtual Subtractive Synthesizers (Softsynths)

Included in a Digital Audio Workstation

• Retrologue by Steinberg
• Analog by Ableton
• Subtractor Polyphonic Synthesizer by Propellerhead Reason
• Mojito by PreSonus Studio One

As a Third-party Plugin

• Sylenth1 by LennarDigital
• Circle2 by Future Audio Workshop
• Diva by u-He
• One by Fabfilter

Final Thoughts

From this post, you’ve hopefully learned by now the essential fundamentals of subtractive synthesis, and how it works.

This knowledge should equip you with an understanding of what sort of sounds you’d like to create, how they are created, and what device you’d use and techniques to apply to create them.

You’ve learned that a subtractive synthesizer uses oscillators to create sound waves that are harmonically rich. And that these harmonically rich waves are your source sounds which you will shape into a sonically designed masterpiece.

You learned that the shaping and process takes place by applying an ADSR envelope tool and a cut-off filter to the sound source. And that you can further form and shape these sounds using an LFO tool. These tools shape and chisel your audio into beautiful sounds.

You also learned about harmonic frequencies, which are frequencies that accompany the dominant note’s frequency, giving that note the characteristics of the instrument or sound in nature.

You’ve also learned a bunch of jargons and terminologies to make you sound educated. Good job!