A wide variety of sounds can be generated by tweaking knobs on a synthesiser. Understanding the basics of synthesis can take you a long way towards creating a unique and interesting pallete of sounds.
All sounds that we hear can be broken down into three fundamental categories:
Pitch refers to the percieved frequency of a sound. The higher the pitch, the higher its frequency will be. Every sound has a fundamental frequency and almost always has overtones above it. These overtones can drastically change the nature of the sound. They can be classified into harmonic frequencies (whole number multiples of the fundamental) and inharmonic frequencies (non-integer multiples of the fundamental). Harmonic sounds tend to sound more pleasant and musical, whereas inharmonic sounds can be harsh and noisy.
Timbre is the tone or colour of the sound. It is the quality which distinguishes one instrument from another even when both instruments play the same note. This quality is independent of pitch and loudness. For example, it is clearly possible to distinguish a piano from a violin or a saxophone because we can identify different timbres.
Loudness is the perceived sound pressure level of a sound wave. In other words, it is how loud a sound is.
To recreate sound artificially, these three parameters are mapped to a synthesizer and given specific terms:
Pitch - Oscillator
Timbre - Filter
Loudness - Amplitude
Oscillators are the sound generators of synthesizers. They operate at frequencies within the human hearing range (20 Hz – 20,000 Hz). This oscillation can produce different types of waveforms and they can sound very unique based on the waveform. Some basic types of waveforms are:
A sine waveform is a pure fundamental frequency with no overtones above it. Many hip-hop and trap songs contain bass lines primarily derived from a simple sine wave in the lower octaves. Further processing with audio effects such as distortion are applied to taste.
Square waveform contains all the odd harmonics. For example, say 110 Hz is the fundamental frequency, then a square wave sound would also have 330 Hz, 550 Hz, and so on. It has a glassy, hollow sound. A modified version of the square wave which is also commonly used is through pulse width modulation (PWM). It changes the dimensions of the squares into rectangles, and this results in a slightly saturated sound which is often desirable.
Sawtooth, or saw waveform contains all the harmonics, even and odd. So for example if 110 Hz is the fundamental frequency, then a saw waveform sound would contain 220 Hz, 330 Hz, 440 Hz, 550 Hz, and so on. Sawtooth sounds have been popularized by their use in famous records by various artists over the years. A filtered gritty saw bass line is now a common sound used in various styles of music. Hip hop in the 90s and 2000s made use of high pitched smooth saw synth lines.
Triangle waveform sounds almost the same as sine, the only difference being that it has some harmonics above the fundamental, whereas sine has no harmonics. Triangle sounds are commonly used to make pads and other harmonic elements.
Oscillators are primarily known to control two main factors of the sound; the waveform and the pitch. After selecting a waveform, the pitch can be controlled through the semitone knob, as well as a fine tuning knob to adjust the cents. Every oscillator will also have its own gain knob to control the output level of the signal. In some modern synthesizers, there are additional parameters to customize the waveform, modulation, and more.
Once the signal is generated, it is then routed to the filter. This filter can modify the frequency spectrum of the raw sound by either cutting or boosting frequencies. This is done with the help of different types of filters, the main ones being:
Low Pass (LP) / High Cut
High Pass (HP) / Low Cut
Band Pass (BP)
Band Reject (BR)
A filter primarily contains two parameters, cutoff and resonance. The cutoff is the point where the filter begins to act. The resonance or Q is the slope of the filter. Based on the type of the filter, the cutoff and resonance do different things.
In a low pass filter, the all the frequencies higher than the cutoff point are rolled off. The curve can be gentle or steep, based on the filter Q. Similarly, in a high pass filter, the low frequencies are cut off.
A band pass filter is a combination of both low pass and high pass filters. The band of frequencies passing through is determined by the filter Q. A band reject filter is the opposite of a band pass, where a band of frequencies is not accepted.
Shelving filters are similar to HP/LP filters, but they do not cut off the frequencies entirely at the cut off point. Instead, they are brought down in level. The filter Q determines the slope characteristics of the filter. Low shelf acts on the frequencies below the cutoff, and high shelf acts on the frequencies above the cutoff.
Several filters also have other parameters other than the cutoff and resonance, like overdrive. Overdrive saturates and distorts the sound in a controlled manner to get a brighter tone.
A key parameter that helps shape sounds artificially is called an ADSR (attack – decay – sustain – release) envelope. It defines the shape of the sound from the beginning to the ending.
It is the time taken for a sound to go from silence to the peak level of the sound. A sharp hit such as a hi-hat or a snare have very fast attack times. A swelling pad or a strings crescendo would have comparatively slower attack times.
Decay time is the time taken for a sound to drop to the sustain level after the initial peak.
Sustain time is the constant volume that the sound takes after decay until the note is released. For example, legato strings have higher sustain times as compared to pizzicato strings.
Release time is the time taken for a sound to die out after the sustain period. In case of a piano, it is the time taken for it to quiet down after the keys are released.
Envelopes can be mapped to several parameters, such as the volume, filters, and so on. Based on what it is mapped to, the envelope changes the sound in different ways.
To make a classic pad sound, a volume envelope with slow attack and slow release does the job. A pluck would have very fast attack fast decay and release times. Envelopes, when mapped to filters, would modulate the sound even more. A bright pad can also be made to sound swelling when the filter cutoff is mapped to the filter envelope with a slow attack, so that the filter opens gradually as the pad reaches its peak level.
LFOs, or low frequency oscillators, are essentially oscillators that usually operate below the human hearing range (20 Hz). It is not possible to hear them on their own but by mapping them to other parameters such as filter cutoff or oscillator level, they can produce interesting effects. Many of the sounds created in heavy electronic music such as dubstep make use of LFOs mapped to several different things such as filter cutoff, distortion level, and so on.
An LFO usually has two main parameters, namely the amount and rate. The amount controls the ratio of the signal being split and sent into the LFO and the rate controls the speed.
A common use of LFOs is to push the rate to the maximum level to generate a sound which contains some artifacts. This happens because since the rate of the LFO is do high, it starts getting into the frequencies within human hearing range. So, what we end up hearing is a somewhat FM-based synthesis happening in the synthesizer, which sounds interesting in some applications.
Some synthesizers also have a shape option for the LFO. The shapes are similar to an oscillator’s waveforms, such as sine, square, triangular, sawtooth. A typical LFO’s shape will set to sine by default. By having more and more customization options, it becomes easier to generate unique sounds.
A simple dubstep wobble bass can be generated by using a sawtooth wave on a lower octave, routing it to a filter and applying an LFO on the filter. The rate of the LFO can be automated to change rhythmically to suit the song and create interest.
Some common sounds made using these techniques:
Bass slides or drops:
For a clean subby sound, sine wave is ideal. Saw and square result in a grittier slide or drop. Start off with in a low octave and set low-medium attack on the volume envelope to avoid pops and clicks. Map another envelope onto the pitch of the oscillator and set the sustain time to zero. On the oscillator, let the envelope peak be at a higher pitch compared to the oscillator base pitch.
If the attack time is very fast, the resulting sound would be a bass drop, starting from the envelope peak pitch and dropping down to the oscillator base pitch. If the attack time is increased, the pitch rises from the base pitch, reaches the envelope peak and then drops back down to the bass pitch. This results in the sound of a bass slide, depending on the attack time.
High-energy lead sounds:
Start with a sawtooth or a PWM waveform. Duplicate it onto at least one or two other oscillators. Detune the oscillators slightly from each other (a few cents). If the option is available, increase the unison to around 6-8 voices.
Map an envelope to the pitch of all these oscillators. Modulate it downwards such that the pitch starts higher and drops to the note being played. Have very little attack time on the envelope to make the effect hidden in the back. Add some glide time to make the notes flow into each other.
House-style pluck sounds:
Set the oscillator to square or sawtooth, or any other buzzy waveform available in the synthesizer. Set the volume envelope with almost zero decay time and sustain time. Change the attack time based on the amount of pluckiness required.
Route the oscillator to a low pass filter and map a second envelope on the filter. Have the envelope move the filter cutoff from high to low very quickly using the attack envelope parameters. It can also be mapped onto the resonance parameter to make it sound sharper.
DnB sub bass:
It is a combination of a sine wave sub and filtered, detuned saw waves in the lower octave. Stereo spread between the saw waves gives it more room in the mix. Modulation in the filter resonance adds movement to the bass.
Dubstep wobble bass:
This can be done through several ways. One of the most common ways to achieve this sound is to start with a metallic waveform, or a modification of a saw wave, route it through a low pass filter, and map an LFO's rate to the filter cutoff. A more interesting method to make wobbles are through FM synthesis. After setting up bright operators with a lot of harmonics, they can be routed to an envelope which makes the operators open up along with the note duration, or an LFO can be set to increase the amount of FM at a specific rate. This creates a metallic timbre which is common in heavy electronic music.