Inside Macintosh: Sound

Square-Wave Data

Square-wave data is the simplest kind of audio data supported by the Sound Manager. You can use square-wave data to generate a sound based on a square wave. Your application can use square-wave data to play a simple sequence of sounds in which each sound is described completely by three factors: its frequency or pitch, its amplitude (or volume), and its duration.

The frequency of a sound is the number of cycles per second (or hertz) of the sound wave. Usually, you specify a sound's frequency by a MIDI value. MIDI note values correspond to frequencies for musical notes, such as middle C, which is defined to have a MIDI value of 60, which on Macintosh computers is equivalent to 261.625 hertz.

Pitch is a lister's subjective interpretation of the sound's frequency. The terms frequency and pitch are used interchangeably in this chapter.

A sound's duration is the length of time a sound takes to play. In the Sound Manager, durations are usually specified in half-milliseconds.

The amplitude of a sound is the loudness at which it is being played. Two sounds played at the same amplitude might not necessarily sound equally loud. For example, one sound could be played at a lower volume (which the user may set with the Sound control panel). Or, a sampled sound of a fleeting whisper might sound softer than a sampled sound of continuous gunfire, even if your application plays them at the same amplitude.

Amplitude is traditionally considered to be the height of a sound wave, so that two sounds with the same amplitude would always sound equally loud. However, the Sound Manager considers amplitude to be the adjustment to be made to an existing sound wave. A sound played at maximum amplitude still might sound soft if the wave amplitude is small.

A sound's timbre is its clarity. A sound with a low timbre is very clear; a sound with a high timbre is buzzing. Only sounds defined using square-wave data have timbres.