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Many synthesizers (like the Elektron Analog Four) have two oscillators. The Novation Peak has even three oscillators. This seems to be common in subtractive synthesis:

enter image description hereSource: https://de.wikipedia.org/wiki/Subtraktive_Synthese

For what practical purpose do synthesizers have multiple oscillators?

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    For the same reason some orchestras have more than one first violin. It sounds better that way. – AJFaraday Jan 28 at 13:20
  • More is better -- just like Gillette razors :-) – Carl Witthoft Jan 28 at 14:04
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    If you've ever spent a good chunk of time working with a synthesizer and crafting sounds, you likely found out that there's not much you can do with only one oscillator. – Todd Wilcox Jan 28 at 14:05
  • The answer to 90% of questions about synthesizers: Because it sounds cool. – chrylis Jan 28 at 23:49
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Firstly, a single oscillator will tend to produce rather a subjectively 'thin' and static sound. This isn't always the case (as later stages, such as the filter, or a separate chorus stage, can add warmth and movement), and it isn't always a bad thing - but of course if you want to have a one-oscillator sound on a synth that has more oscillators, you can always decline to use more than one!

We're often told that musical sounds are periodic, and that they have overtones that are integer multiples of the fundamental frequency. This often is true of synthesizer oscillators. However, in nature, this is usually only approximately true - a vibrating string will often have higher overtones whose frequencies are a little higher than those multiples, for example. Many acoustic musical sounds also have multiple oscillating elements at slight pitch offsets - the string section is a classic example. Being able to play multiple oscillators, slightly detuned, is therefore useful for producing these subjectively more natural sounds.

You can also take this detuning further and have oscillators tuned in musical intervals (e.g. a fifth apart.) You could also use mix different shapes of oscillator to produce spectra that couldn't be produced by a single simple oscillator.

Possibly the most interesting uses of multiple audio-rate oscillators is to use one to modulate another at an audio rate - in techniques such as PWM, FM, and oscillator sync. These techniques also do a good job of producing more interesting and animated (though not always very natural!) spectra.

I've assumed here from the picture that you're talking about multiple audio-rate oscillators, that would normally be described as 'VCOs' - you'll probably be aware that synths usually have one or more low frequency oscillators (LFOs) that can also be used for modulation purposes.

  • “Not always very natural” – what's that even supposed to mean? A violin signal is also not natural. Even the human voice had to be crafted through evolutionary pressure... – leftaroundabout Jan 28 at 11:24
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    @leftaroundabout I agree that 'natural' a slightly nebulous concept, and I think it is also something of an axial, rather than binary, concept. I personally think that one part of that axis could be "sounds produced by man-made machines", which are still subject to a lot of natural tendencies and limitations that purely electronic sounds aren't. That's what I meant in that sentence. – topo morto Jan 28 at 11:47
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    I expect one other reason is that the synth that revolutionized the industry (the Moog Minimoog) had three oscillators, and then the synth that did it again (the Prophet 5) had two oscillators per voice. So releasing a synth after those that only had one oscillator would have made musicians (rightly or wrongly) think that it was not as capable. That said, I believe the Moog Taurus line (popular for bass sounds) are all single oscillator analog subtractive synths. In any case, I think an additional valid answer here is: "Marketing". – Todd Wilcox Jan 28 at 16:44
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I think that the shortest and most precise answer to your question is: to produce timbres that are not possible with a single oscillator, including an entire form of synthesis (FM) that requires at least a second oscillator to modulate the first one.

In a slightly longer answer I would describe the West Coast and East Coast approximations to synthesis, as each one takes advantage of the presence of multiple oscillators in its own way.

  • East Coast synthesis, mainly substractive, was often adopted by musicians with a more classical training to replicate or at least resemble the sound of real instruments, such as violins, brass sounds or organs. These real instruments sounds are not easy to emulate with a single oscillator, as in nature sounds are not perfect and often there are sympathetic vibrations and other small artifacts. By means of additional oscillators slightly detuned from each other, a sound can be fattened and modified. This is what is usually called a more natural sound, as in similar to what's in nature. Of course, other obvious musical usages of a second oscillator are possible and frequent in this approximation, such as harmonization: detuning the second oscillator (and the following ones) to a musical interval relative to the root note being played by the main oscillator. The 5th interval is very common, as it doesn't change in most grades of major and minor scales and sounds very rich and pleasant.
  • West Coast synthesis, on the contrary, tried to estabilish a whole new musical language not based on the classical heritage. The most popular brand of West Coast synthesizers, Buchla, didn't even put a "piano" keyboard in their first synthesizers in that regard. In this type of synthesis, additional oscillators are used to enrich and modulate the main oscillator to create a more complex tone with techniques like FM, wavefolding or additive synthesis.

Anyway, and with all due respect, I want to disagree with this affirmation in a comment by @Todd Wilcox:

If you've ever spent a good chunk of time working with a synthesizer and crafting sounds, you likely found out that there's not much you can do with only one oscillator.

It definitely depends on which oscillator are we speaking of. In the modular world, there are examples such as Mutable Instruments' Braids, that are chosen to be the only oscillator present in many small Eurorack systems, and the variety of sounds achievable is huge. However, I agree that a hard-wired synthesizer with a single oscillator is pretty limited.

Some links:

https://reverb.com/news/the-basics-of-east-coast-and-west-coast-synthesis https://en.wikipedia.org/wiki/Buchla_Electronic_Musical_Instruments https://en.wikipedia.org/wiki/Moog_Music

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    I think it's misleading to call Braids a single oscillator. So does Mutable Instruments. The second line in their description of Braids is, "Sound source... like an oscillator? Not really." Braids is a not a single analog oscillator. It's a complicated digital sound source. Two of the settings on Braids are "Dual square or sawtooth oscillator with hard sync" and "Triple saw, square, triangle, or sine". So Braids is actually re-creating the sounds of multiple oscillators in one sound source. – Todd Wilcox Jan 28 at 16:33
  • You are totally right on that, Braids is not the best example of a single oscillator. However, I stand on the premise that in the modular world you can create a lot of variety with a single oscillator module, even with an analog one. For example, Pittsburgh Modular Waveforms is like a swiss-knife, and with the right modules on its side it is possible to create very different tones. It has to be said, though, that many of the techniques used in "single oscillator patches" are actually substituting that extra oscillators with other sources that can act like one, such as filters or LFO's. – Eme Jan 30 at 9:23

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