Many factors affect the perception of timbre. Three such factors are spectrum, transposition, and temporal envelope as demonstrated below.
DEMONSTRATION 1: Effect of Spectrum on Timbre
The sound of two musical instruments are synthesized in six steps by adding successive partials with their original frequency, phase and temporal envelope. How many partials does it take until you can identify the musical instrument?
Guess: Instrument 1
Guess: Instrument 2
DEMONSTRATION 2: Change in Timbre with Transposition
High and low tones from a musical instrument normally do not have the same relative spectrum. A low tone on the piano typically contains little energy at the fundamental frequency and has most of its energy at highter harmonics. A high piano tone, however, typically has a strong fundamental and weaker harmonics. If a single tone from a musical instrument is spectrally analyzed and the resulting spectrum is used as a model for the other tones, one almost always obtains a series of tones that do not seem to come from that instrument. This is demonstrated with a recorded 3-octave diatonic scale played on a bassoon.
The purpose of this demonstration is to show that the temporal envelope of a tone, i.e. the time course of the tone's amplitude, has a significant influence on the perceived timbre of the tone. A typical tone envelope may include an attack, a steady state, and a decay portion (e.g., wind instrument tones), or may merely have an attack immediately followed by a decay portion (e.g., plucked or struck string tones). By removing the attack segment of an instrument's sound, or by substituting the attack segment of another musical instrument, the perceived timbre of the tone may change so drastically that the instrument is no longer recognizable.
You will hear a recording of a Bach chorale played on a piano:
Now the same chorale will be played backwards:
Now the tape of the last recording is played backwards so that the chorale is heard forward again, but with an interesting difference:
The tape recording of the backward chorale played in reverse yields the original (forward) chorale, except that each note is reversed in time. The instrument does not sound like a piano any more, but rather resembles a kind of reed organ. The power spectrum of each note, measured over the note's duration, is not changed by the temporal reversal of the tone.
