The same resonating object sounds completely different depending on how energy is put into it. A hammer strike, a soft mallet, a bow, and a scrape each create a distinct signature that the resonator then amplifies and colours.
Same object, different interaction
The resonator is fixed: a marimba-like wooden bar at 260 Hz. Only the excitation changes. Listen to how the interaction — not the object — determines the initial character of the sound.
Excitation signal
Output (resonator response)
The resonator
The resonator is unchanged across all excitations. Adjust its tuning and see how the same excitation signal produces different pitches and textures from the same bar.
260 Hz
Medium
Excitation × Resonator
This separation is the foundation of physical modeling. The total sound is the product of two independent systems, which can be designed and modified separately.
Impulse excitation (hard hammer) contains energy at all frequencies simultaneously. This reveals the resonator's complete mode structure in a single strike — the resonator acts like an amplifier for whatever mode frequencies it has, filtering out everything else.
Shaped excitation (soft mallet) is like a short burst of noise with a rise-time. The slower attack reduces high-frequency content, so fewer high modes are excited. This is why hitting a drum with a soft beater sounds rounder than a hard stick hit.
Sustained excitation (bow) continuously feeds energy into the resonator. Instead of dying away, the sound sustains or even grows. The excitation must match the resonator's frequency to build — random noise would cancel out. Only the modes near the excitation frequency are reinforced.
Stochastic excitation (scrape) is a series of tiny random impacts. Each mini-impulse excites all modes, but the random timing smears the attacks into a continuous texture. This models rough surface contact — dragging a finger across a table or scraping objects together.