The three things you hear
Pitch, loudness, and timbre — every knob on every synth ultimately moves one of these three.
Turn a speaker up and watch the cone: it moves forward and back, pushing air. When the pushes repeat fast enough — about twenty per second or more — your ear stops hearing separate movements and hears one continuous tone. Every sound a synthesizer makes is this single physical event, repeating. Three measurable properties of the repetition decide everything you hear: how often it repeats, how far the cone travels, and the shape of each repeat.
A waveform drawn as a graph: time runs left to right, air pressure swings up and down. One full repeat is a cycle; cycles per second is the frequency; the height of the swing is the amplitude.
How often: frequency is pitch
Repetition rate is called frequency and is measured in Hertz (Hz), cycles per second. Human hearing spans roughly 20 Hz to 20,000 Hz. Musical fundamentals sit in the lower part of that range: bass lines mostly between 40 and 200 Hz, melodies between about 200 Hz and 2 kHz. Content above 5 kHz is mostly brightness and air rather than notes you could whistle — worth remembering when the filter lessons start talking about "highs".
Pitch works by multiplication. Doubling any frequency raises the note by exactly one octave: 110, 220, 440, and 880 Hz are all the note A. Equal musical steps are equal ratios, so a 100 Hz change is an enormous jump at the bottom of the range and barely noticeable at the top. This is why the frequency slider in the toy below moves in octaves rather than in equal Hz steps, and why analog tuning knobs feel "faster" at the top of their travel.
The note A at five octaves. Each arc is the same musical step (one octave) and the same multiplication (×2); on a plain Hz axis the rungs crowd left and spread right.
How far: amplitude is loudness
Amplitude is the size of each pressure swing — how far the cone travels. Bigger swing, louder sound, though the ear compresses the scale: doubling the amplitude does not sound twice as loud, which is why audio equipment measures level in logarithmic dB rather than linear units. Amplitude is the property that volume faders, velocity, and (from lesson 3 on) envelopes operate on, and it is fully independent of frequency: any pitch can occur at any loudness.
What shape: waveform is timbre
Play the same note at the same loudness on a flute and on a violin. Nobody confuses them, so a third property must exist. It lives in the shape of each cycle: the flute's is smooth and close to a pure sine; the violin's is jagged with fine detail. This property is called timbre, and shaping it is most of what synthesis is. Lesson 2 explains where shape actually comes from; for now the goal is only to hear the three properties move independently of each other.
One oscillator, one control per property. A suggested experiment, in order: 1) keep frequency and level still and switch waveforms — character changes, pitch does not; 2) keep the waveform and sweep frequency — the spectrum comb slides as one piece; 3) lower the level — the scope trace keeps its exact shape and only shrinks.
On the hardware this lesson lives on the TRIG page. NOTE chooses the oscillator frequency, quantized to the musical ladder; VEL (velocity) scales the amplitude per step. Open either and the panel highlights the exact knob: