Wobble Bass

 The classic wobble bass heard extensively in drum & bass and dubstep can be synthesized using three basic techniques. These include:

• Beat Frequency

• Amplitude Modulation

• Filter Cutoff

 Beat Frequency

By far the easiest way to synthesize wobble bass is to exploit the beat frequency between two tones of similar frequency. Suppose that two sine tones are played simultaneously, SineA with a frequency of X Hz and SineB with Y Hz. If the difference of the two frequencies |X-Y| is less than roughly 20 Hz, the human ear will not be able to distinguish the two tones, and instead perceive a periodic change in amplitude; a wobble. 

For example, suppose that SineA has a frequency of 55 Hz. This corresponds to a low A note. SineB is played one semitone above; A# with a corresponding frequency of roughly 58 Hz. You will not hear two separate tones, rather a single tone changing volume three times per second (58 Hz - 55 Hz = 3 Hz). As shown in the figure, the envelope of SineA + SineB repeats itself three times.

Notice that if you play these same two notes several octaves higher, you will not be able to hear the wobble any longer. Suppose that SineA has a frequency of 880 Hz, which is a high A. A# in this case has a frequency of approximately 932 Hz. The beat frequency is 52 Hz, and your ear will clearly hear two separate tones; the wobble effect decreases with increasing frequency. But this is not a problem as we are only interested in /bass/.

You won't find the killer wobble bass that you are looking for just by playing two tones a few semitones apart. This technique is a start. You will need to add effects, such as saturation/distortion, in order to emphasize the natural wobble.

Presets

Operator [Ableton Live]

Amplitude Modulation

Instead of relying on psychoacoustics (the beat frequency) to produce the wobble for us, the next easiest method is to modulate the amplitude directly. This can be done simply by controlling the amplitude of the oscillator with an appropriately tuned LFO. The advantage of this technique is that the speed of the wobble can be directly controlled by adjusting the speed of the LFO. This is especially handy for making the bass wobble according to the tempo (such as, exactly four times per measures, etc.).

Filter Cutoff 

Another method for producing a wobble is to modulate the filter cutoff of the oscillator using a LFO. While also effective, remember to use any other oscillator waveform besides a sine. The filter acts on the harmonics of the oscillator, and a sine does not have any.

Tips & Tricks 

Probably no one technique will give you the sound that you are looking for, so it is worth trying out different combinations. Furthermore, the effects that are used to treat the ur-bass are really what make the sound. The synthesis stage is rather trivial.

Saturation

I find saturation/distortion critical to creating a good wobble bass. It emphasizes the existing wobble, and adds harmonics which make the sound more present 

Side Chaining

This technique allows your imagination to run wild. One approach might be to allow the bass frequencies to pass, while adding additional effects to the higher frequencies. These effects might include delay, chorus/flanger/phaser, and reverb. That way, the bass comes through clearly and strongly, while the harmonics do all sorts of crazy things.

Retriggering

Instead of using LFOs to modulate the amplitude or filter cutoff while the note is playing, try using the ADSR envelopes instead, and then repeatedly retrigger the note. For instance, if you want the bassline to wobble eight times per measure, set the filter cutoff attack to 1/8th of a measure and retrigger the note 8 times. This approach may also give you more flexibility if you decide to do other things with the bass line than just wobble.