Additive synthesis is a sound synthesis practice that creates timber (tone color or tone quality) by adding sine waves together. On the other hand, Subtractive synthesis is a sound synthesis practice that alters the timber of a sound by using a filter that reduces (attenuates) any sine waves (partials) in a given audio signal.
Additive synthesis basically aims to construct timbre from the ground up. To do this, pure frequencies which are sine waves of varying frequencies have to be added to together in order to precisely define the end result of the timbre we wish to have.
Subtractive synthesis relies on a filter to attenuate, reduce and alter the overall timbre of a sound. Therefore an alternative definition could be,
Subtractive synthesis is a method of subtracting overtones from a sound via sound synthesis, characterized by the application of an audio filter to an audio signal.
Which is basically the opposite of what additive synthesis does.
Additive Synthesis
The main pillar and essence of additive synthesis is the mixing of frequencies.
Each of the frequency components has its own amplitude envelope. This allows for these components to behave independently.
Basic Theory
The basis theory of additive synthesis is what is called the Fourier theory which when used in relation to additive synthesis would say that the timbre of instruments consists of multiple harmonic or inharmonic partials or over tones.
Each partial or overtones is basically a sine wave of different frequency and amplitude that swells and decays over time due
to specific modifications done from an ADSR envelope or a low frequency oscillator.
Additive synthesis most directly generates sound by adding the output of multiple sine wave generators.
Alternative implementations may use pre-computed wavetables which can be commonly found in most new digital and analog synths or the inverse Fast Fourier transform.
Applications of Additive Synthesis
Organs
Organs are considered additive synthesizers by most sound designers.
If you have heard an organ playing before then you have either consciously or unconsciously heard what additive synthesis is capable of.
In the criterion of organ tones, additive synthesis is utilized by combining various unaltered sine waves.
Bells
Most bell sounds are results of additive synthesizers doing what they do best.
The majority of Bell sounds you hear today basically consist of a fundamental along this, sine waves that shape shape original and overall pitch, tone and amplitude.
With additive synthesizers, you can basically replicate an original bell sound at an almost near perfect level.
And this is also the same for a wide variety of sounds.
Plucks and percussion
Additive synthesis is also heavily utilized in the creation of drum sounds as well as plucked strings.
The combination of sine waves of varying lengths, timbres, and amplitudes you can basically recreate the component sections of more complex
percussive sound utilizing various volume envelopes like attack, decay, sustain and release.
Miscellaneous Sounds
Additive synthesis can also create some of the more harsh and dissonant sounds some of which sound like the result of Frequency Modulation synthesis.
These sounds are created by combining various inharmonic partials and the resulting sound depends on the difference in pitch of the partials.
Frankly, additive synthesis is not given enough credit because it can create most of the warm soundscapes that would be easily delegated to Subtractive synthesis.
If you are looking for an alternative to the more familiar sounds produced by subtractive or even FM synthesis,
you should consider looking into incorporating additive synthesis in your next musical project.
The Downside to additive synthesis is that a great deal of data has to be specified to define a sound of any complexity in detail.
This can be difficult if you haven’t had much experience with it.
Subtractive Synthesis
The huge difference between additive and subtractive synthesis is that while additive synthesis adds sine waves to create sound, Subtractive synthesis subtracts overtones using synthesis characterised by the use of a filter.
A common example of subtractive synthesis is the human voice.
The vocal cords act as the oscillator while the mouth and throat act as the filter.
For example singing “ooooh” and “aaaaah” at the same pitch which produce a similar sound but the mouth and throat is what acts as the filter changing the sounds.
To site Wikipedia:
“By changing the shape of the mouth, the frequency response of the filter is changed, removing (subtracting) some of the harmonics. The “aaah” sound has most of the original harmonics still present; the “oooh” sound has most of them removed (or, to be more precise, reduced in amplitude). By gradually changing from “oooh” to “aaah” and back again, a spectral glide is created, emulating the “sweeping filter” effect that is the basis of the “wah-wah” guitar effect.”
Three useful concepts :
Source signal: Common source signals are the basic shapes we all know I.e. square waves, pulse waves, sawtooth waves and triangle waves.
Modern synthesisers (digital and software) may include more complex waveforms or allow the upload of
arbitrary waveforms. For example the VST called Serum has so many different waveshapes and also allows the user to upload their own.
Filtering: The cut-off frequency and resonance of the filter are controlled in order to simulate the natural timbre of a given instrument. These controls are quite effective when it comes to sound design.
Amplitude Envelope: Further envelope control of signal amplitude
(strictly: not subtractive synthesis but frequently
used). Also used with other synthesis techniques.
Final Thoughts
Both Additive and Subtractive synthesis have their own unique applications and work differently.
Additive synthesizers will create sounds by adding various sine waves together.
Subtractive synthesizers on the other will create sounds by the use of filters to subtract certain frequencies from a sound signal.
