We could simplify things by saying that a fat sound is simply a well-filled sound spectrum. 

In reality, a sound spectrum filled with frequencies that all have the same PSD (Power Spectral Density) is white noise. Not exactly the idea of a ‘fat synth sound’. 

So let's imagine that the spectral content of our ‘big sound’ isn't a bunch of frequencies with identical PSD but, on the contrary, full of hollows and bumps in the ‘right places’. All right, so let's take a simple sound, a basic waveform, and add some bumps and hollows... Yes, but how? 

If you use an equaliser, you'll soon realise that this little game only works for one note and that you'll have to change your settings for all the other notes.

What we call the ‘right places’ will be different from note to note. 

Rather than simply amplifying portions of the signal, it is essential to understand a fundamental concept: harmonics.

Harmonics are like echoes of a given frequency at higher frequencies. They are known as ‘overtones’ or ‘partials’. 

In simple terms, a sound without harmonics is a sinusoid. This does not mean that a sinusoid cannot vary in frequency and therefore produce a note. No, it just means that the harmonic content of the note played will consist solely of the exact frequency of that note. The exact frequency of the note is called the ‘fundamental frequency’, so a note may or may not be enriched by several harmonics. 

Is the harmonic content of a sound sufficient to obtain a fat sound? Unfortunately not. But it is a first requirement.

So what other elements could define a fat sound?
Let's look at a very specific point: psychoacoustics.

It's a subject that will displease some people, since it has an unfortunate tendency to pit our perception of sound against its physical reality. Our senses deceive us!

One bias that particularly concerns us here is the perception of loudness. We tend to think that louder is better. Except when it comes to neighbourhood noise.

This is an important point in the explanation of ‘fat sound’, since we are inclined to think that between two identical sounds, the louder one is ‘better’. It's a point that makes it difficult to compare two synthesizers, since the first thing to do is to make sure that the gain is the same for both.

One way of objectively enlarging the sound is to multiply the sources. A single oscillator will give less harmonic richness than two or three together. The result is all the more striking when the oscillators are not perfectly tuned to each other (with a very small deviation).

But another point comes into play in this attempt to explain in a very simplistic way what ‘fat sound’ is: harmonic distortion.

Rather than embarking on a complex explanation that would involve notions of even and odd harmonics (among others), remember that distorting a sound can enrich it. If this applies to a single oscillator, it also applies to multiple oscillators.

More harmonics, more oscillators, more output level... 

Music is all about the delicate relationship between frequencies, their level and their number, and the idea of ‘fat sound’ is no exception. 

Imagine you had all the synthesizers known for their ‘fat sound’ (in the collective imagination). How would you go about composing and mixing with these ‘monsters’? 

There are two possibilities: either you go ‘all out’ and have little problems when mixing, or you leave enough room for each sound by using it in the right place at the right time.

Fat sound is above all an effective relationship between sound design and composition. Not all the sounds can be ‘’full on‘’.  Fat sound is a way of composing with frequencies.

It's entirely possible to make a sound fat by gradually adding harmonics, by giving it the support of a sub-oscillator one or two octaves lower and by using filters in a way other than a DJ effect. And it works with all forms of synthesis, analogue, VA, FM, samplers... including software.

The quest for the ‘fat sound’ arrived shortly after the first cheap second-hand analogue synthesizers, at the very end of the 1990s.

With the digital synthesiser craze in full swing, many artists and studios decided to get rid of their analogue synthesizers, which were deemed too heavy, too unstable and not modern enough (with little or no memory and lacking MIDI). Available en masse on the second-hand market, these analogue synthesizers have since experienced a new wave of popularity. So much so, in fact, that they've gone from irrelevant old-timers to pop culture icons. 

This wave of affordable second-hand analogue synthesizers has accompanied music production ever since. This has prompted many manufacturers to re-release them or offer emulated versions.

All this is not unrelated to ‘fat sound’, since it was at this point that the term began to be used, in opposition to digital synthesizers (particularly FM and Romplers).

What sounds fat to you may not be fat to everyone. Perhaps your vision of ‘fat sound’ is guided more by harmonic richness. Or, on the contrary, you identify as ‘fat sounding’ a synthesiser that delivers a high output level with slight compression.

All this being highly subjective, the notion of ‘fat sound’ is in fact very relative. First of all, it depends on your perception and understanding of sound. But it is above all relative to the use you make or will make of a sound. A very broad harmonic content is not necessarily of the greatest use for a bass line... in theory. But in practice, in your practice, it's you who redefine whether it's a good thing or not. 

And that's the conclusion of this essay on the subject: it's up to you and you alone to define and make your own fat sound. You have the tools to do this, and we've put together a small selection of tools to help you chisel out your sound.