Fundamentals of Sound and Subtractive Synthesis
An audio Assignment
“Sound is a physical phenomenon produced by the vibration of
matter. ... As the matter vibrates, pressure variations are created in the air
surrounding it. This alternation of high and low pressure is propagated through
the air in a wave-like motion. When the wave reaches our ears, we hear a sound.”
Concept
of Sound
When something
makes a sound, it is sending vibrations (the movement of particles) through the
air (or other medium) that is perceived either by the ears or felt otherwise.
This energy is represented as sound waves, the key characteristics being:
·
Amplitude
– the amount of air pressure
·
One
period – cycle of “wave” - Wavelength
·
Time
– the frequency of the cycle
The S.I unit of frequency is hertz or Hz. A vibrating body
emitting 1 wave per second is said to have a frequency of 1 hertz. That is 1
Hz is equal to 1 vibration per second.
Sometimes a bigger unit of frequency is known as kilohertz (kHz) that is 1 kHz = 1000 Hz. The frequency of a wave is denoted by the letter f.
Sometimes a bigger unit of frequency is known as kilohertz (kHz) that is 1 kHz = 1000 Hz. The frequency of a wave is denoted by the letter f.
Figure 2-1 graphs
the oscillation of a pressure wave over time.
Various Type of Sound Waves
There are two types of waves:
Longitudinal waves and Transverse waves.
Longitudinal Waves: A wave in which the
particles of the medium vibrate back and forth in the ‘same direction’ in which
the wave is moving. Medium can be solid, liquid or gases. Therefore,
sound waves are longitudinal waves.
Transverse Waves: A wave in which the particles of the medium
vibrate up and down ‘at right angles’ to the direction in which the wave is
moving. These waves are produced only in a solids and liquids but not in gases.
Crazy fact: Most musical sounds are not made up of just one frequency. Instead, sounds are a combination of many different frequencies, called partial tones. The lowest frequency is its fundamental frequency—this is what our ears perceive as the sound's "pitch," while all of the other partial tones combine to create a sense of timbre, or tone. So when you hear a piano playing a “middle C,” that tone is actually made up of a combination of different frequencies, or partials, that come together to create the sound of “middle C on a piano” to our ears.
Each waveform has a spectrum of partials associated with
it—basically, lots of different sounds go into defining the quality of any
given sound. As you alter a waveform’s shape, the partials themselves begin to
rearrange, creating audible changes in the sound.
Sine Wave
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DEFINITION: A sine wave sounds as
though it were: smooth and clean. At its most simple it is tone. The vibration
of a sine wave consists only of one thing, known as the fundamental. Not to see
any partials! Start whistling one note or imagine a tuning fork's tone. Those
are both approximations of what sounds like sine waves, while real-life sine
waves are uncommon. Every other waveform can be made from multiples of sine
waves.
Square Wave
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DEFINITION: Remember how only one
thing, the fundamental, forms a sine wave? Not with the square wave.
Square waves sound rougher and noisier. It looks different too. This is
because the square wave also contains harmonics, in addition to the
fundamental. A harmonic is a kind of partial tone that is a multiple of a
fundamental pitch. Such harmonics occur in a square wave as odd-number multiples
of the fundamental frequency. The harmonics, combined with the fundamental,
give this wave a square shape. A square wave is balanced on both sides of its
zero centre point however if there is an imbalance it is referred to as a rectangular
wave.
Triangle Wave
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DEFINITION: A triangle
wave contains odd harmonics similar to a square wave. Unlike a square
wave, they reduce quickly in amplitude as they distance themselves from
the fundamental and give it its shape. Similar to a sine wave in
appearance however it sounds like it is between a square wave and a
sine wave. It's not as noisy as a square wave but not as smooth as a
sine wave. It sounds plainer, perhaps shinier than a sine wave. Think about a
recorder, or a breathily-played flute—that sounds similar to a triangle wave.
Sawtooth Wave
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DEFINITION: Also known as a saw
wave, a wave of the sawtooth is much more jagged and looks like a saw. Its
amplitude reduces less than the triangle in the higher frequencies. It's the buzziest
sound wave of all of them, sounding even harsher than a square wave, and
that's because it's the richest in harmonics (odd and even). This ensures
that when you deal with subtractive synthesis, which is when you create a sound
by filtering out frequencies, it can be ideal.
An example would be the sound of a bow rubbing
across violin strings, the friction is pulled in one direction (the saw’s ramp
up) until it snaps back (the saw’s return) which gives the violin it’s buzzy
sound
While other waveforms certainly exist, these four are the
primary foundation for the sound of most analogue synthesizers. Whether working
with a keyboard, modular, or even software, these shapes are likely to be there
at the core of your synthesizer's voice.
White Noise
White noise is a type of noise that
is produced by combining sounds of all different frequencies together. If you
took all of the imaginable tones that a human can hear and combined them
together, you would have white noise. It has the same amplitude
across its frequencies. The previous waves have a fundamental frequency and
harmonics, white noise has all harmonics and tones
White Noise contains all audible frequencies with random amplitudes.
Pink Noise
Pink noise is white noise, but
with reduced higher frequencies. It resembles the sounds of steady rainfall or
wind and is often considered to be more soothing than white noise, which some
people find unpleasant. The human ear perceives the sound as being even across
the frequencies. An ''equal-loudness contour'' is a measure of sound
pressure dB SPL, over the frequency spectrum, for which a listener
perceives a constant loudness. http://www.lindos.co.uk/cgi-bin/FlexiData.cgi?SOURCE=Articles&VIEW=full&id=17
Pink
Noise also
contains all audible frequencies, but they are reduced by 3 dB per octave as
you move up the spectrum.
Synthesizer Architecture
In terms of signal flow, the starting point
is the Oscillator.
The oscillator generates a waveform with
a wide frequency spectrum (square wave, noise) at a frequency, 
and an amplitude 
, although there can be two or perhaps even three oscillators in
the design; any more begin to distract from the purpose of a subtractive
synthesis technique although there are various designs which exist.
and an amplitude , although there can be two or perhaps even three oscillators in
the design; any more begin to distract from the purpose of a subtractive
synthesis technique although there are various designs which exist.
The filters, which can be
composed of many various filter forms (lowpass, bandpass), shifts the character
of the oscillator by manipulating the frequency spectrum distribution,
before being further changed by additional impact modules integrated into the
system. Use other oscillators or value envelopes such components can be
defined or automated by the user. These elements can be user specified or
automated using other oscillators or value envelopes. The
subtractive approach is favoured by many musicians due to the fact that it is
comparatively imprecise and therefore requires tweaking and experimentation of
control parameters to get a desired sound.
VCO - stands for Voltage Controlled Oscillator.
Oscillators are the building blocks of synths.
VCF - stands for Voltage Controlled Filter. A filter is the tool you
use to shape the tonal balance of a sound.
Filters
come in several different styles. The most common type is a low pass filter
or LPF. A low pass filter is named for the frequencies it leaves
unaffected, rather than those it cuts.
That’s
the same for other types of filter like high pass and band pass.
If you start with a rich tone from an oscillator like a square or sawtooth wave
your synth’s VCF can do a lot of tone shaping by filtering out
frequencies. This technique is called subtractive synthesis. It’s the
basis of the method used for most analogue synthesis.
VCA - VCA stands for Voltage Controlled Amplifier. A VCA deals with
the amplitude level of your synth sounds. A VCA normally accepts
voltage input from an envelope generator to create variation in the
synth’s level over time. These variations have a surprisingly large influence
on the character of the sound.
LFO - LFO stands for Low Frequency Oscillator. It creates a signal the
same way the VCO does except much lower in frequency.
An LFO’s frequency
range is so low that it doesn’t sound like a distinct tone like a regular
oscillator. Instead the LFO is used as a modulator to change the
characteristics of a sound.
ENV - Envelope generator – An envelope
generator controls how a signal evolves over time. Envelope typically contain
controls for attack, decay, sustain and release.
When
connected to a VCA, an envelope generator creates the variation in
amplitude that gives a sound its identity.
But
envelope generators aren’t just for synth loudness. They can be a
modulation source for almost any destination you can think of. Modulators such
as LFO’s and ENV’s can be used to sound like almost any instrument by matching
the attack, vibrato,etc.
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