Digital Sound & Music: Concepts, Applications, & Science, Chapter 3, last updated 6/25/2013
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Figure 3.6 Approximate frequencies of various instruments
Instruments also are distinguished by the amplitude envelope for individual sounds
created by the instrument. The amplitude envelope gives a sense of how the loudness of a single
note changes over the short period of time when it is played. When you play a certain
instrument, do you burst into the note or slide into it gently? Does the note linger or end
abruptly? Imagine a single note played by a flute compared to the same note played by a piano.
Although you don’t always play a piano note the same way for example, you can strike the key
gently or briskly it’s still possible to get a picture of a typical amplitude envelope for each
instrument and see how they differ. The amplitude envelope consists of four components:
attack, decay, sustain, and release, abbreviated ADSR, as illustrated in Figure 3.7. The attack
is the time when the sound is first audible and reaches its maximum loudness. The decay is the
period of time when the amplitude decreases. Then the amplitude can level to a plateau in the
sustain period. The release is when the sound dies away. The attack of a trumpet is relatively
sudden, rising steeply to its maximum, because you have to blow pretty hard into a trumpet
before the sound starts to come out. With a violin, on the other hand, you can stroke the bow
across a string gently, creating a longer, less steep attack. The sustain of the violin note might be
longer than that of the trumpet, also, as the bow continues to stroke across the string. Of course,
these envelopes vary in individual performances depending on the nature of the music being
played. Being aware of the amplitude envelope that is natural to an instrument helps in the
synthesis of music. Tools exist for manipulating the envelopes of MIDI samples so that they
sound more realistic or convey the spirit of the music better, as we’ll see in Chapter 6.
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