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Introduction |
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Sound Envelopes: Attack, Sustain, Decay The shape of a sound refers to its envelope. All sounds have a beginning, middle and end. The envelope could begin with an abrupt beginning, as heard in a bell sound, followed by a slow ending. The combined form a sound envelope. Learning to identify the difference between the beginning, middle and end of a sound event enables a better understanding of a sound structure. For example, listen to the sound of a dog's bark. How does it begin? Does it reach a peak or begin with a peak? Does the dog's bark die away quickly or slowly? These 'close-up' perspectives on a sound structure provide valuable insights into the sound event. Instead of using the words beginning, middle and end, we shall use the words attack, sustain and decay. All sound envelopes have an attack sustain and decay sound. The Attack of a sound is the beginning of the sound and refers to the time taken for the sound's loudness to establish itself. The Sustain part of the sound envelope refers to a period of constancy. Decay refers to the decrease in loudness. The sound of a car slowly approaching, passing and then departing would consist a slow attack time, a momentary period of stability at maximum volume while the car is passing, followed by a slow deacy volume as the car moves away into the distance. A drum stroke has an instantaneous attack time, vitually no sustain and a small amount of time for its decay. Compare the Attack time of an electronic buzzer to that of an electric fan motor being turned on. The Attack time of a buzzer is instantaneous whereas the fan needs time for the motor to start up. Because of the instantaneous Attack of the buzzer, and provided that the power supply is constant, the buzzer's Sustain will be continuous. The Sustain component of the fan will be achieved once the motor has reached full operational capacity. The decay time of the buzzer will be almost minimal whereas the fan will have a period of time where the sound of the motor will die away. |