Questions: FM and Additive Synthesis

Question 1

In FM synthesis, what happens to the timbre when the modulation index is increased?

• A. The sound becomes quieter
• B. New sideband frequencies are generated, dramatically changing the harmonic content
• C. The carrier frequency shifts higher
• D. The sound becomes more sinusoidal

Answer: (B)

Increasing modulation index generates more and stronger sidebands at carrier ± (n × modulator). This is why small modulation index changes in FM synthesis produce dramatic, nonlinear timbral shifts.

Question 2

True or false: Additive synthesis generates sound by starting with harmonically rich waveforms and filtering them.

• T. True
• F. False

Answer: False

That describes subtractive synthesis. Additive synthesis constructs sound by summing many individual sine wave partials, each with its own frequency, amplitude, and phase — building up complexity from the simplest components.

Question 3

What is the relationship between the carrier and modulator oscillators in FM synthesis?

Model answer: The modulator's output is applied to the carrier's frequency input, varying the carrier's instantaneous pitch at audio rates. This frequency modulation generates sidebands at carrier ± (n × modulator frequency).

When modulation rate is in the audible range, it produces sidebands rather than simple pitch vibrato. The ratio of modulator to carrier frequency determines whether the sidebands are harmonic (integer ratio) or inharmonic (non-integer ratio).

Question 4

Why do non-integer ratios between carrier and modulator frequencies produce metallic or bell-like tones in FM synthesis?

• A. Non-integer ratios produce lower sideband frequencies
• B. Non-integer ratios generate inharmonic sidebands that don't fit the harmonic series, creating the inharmonic spectra characteristic of metal and bells
• C. Non-integer ratios increase the amplitude of all harmonics equally
• D. Non-integer ratios cause FM synthesis to revert to subtractive behavior

Answer: (B)

When carrier:modulator ratios are non-integer (e.g., 1:1.4), the sidebands fall at non-harmonic frequencies. Physical objects like bells and metal bars have inharmonic partial series, so FM's inharmonic sidebands naturally mimic those timbres.