MIDI synthesizers are generally capable of producing some maximum number of simultaneous notes, also referred to as voices. A voice is a stream of successive single notes, and the process of assigning incoming MIDI notes to specific voices is known as voice allocation. However, the voice-allocation algorithm and the contents of each voice are normally internal to a MIDI synthesizer and hidden from outside view. One can, of course, learn from MIDI messages which notes the synthesizer is playing, and one might be able deduce something about the assignment of notes to voices. But MIDI itself does not provide a means to report which notes a synthesizer has assigned to which voice, nor even to report how many voices the synthesizer is capable of synthesizing.

In Java Sound, however, a Synthesizer class can expose the contents of its voices through its getVoiceStatus() method. This behavior is recommended but optional; synthesizers that don't expose their voice allocation simply return a zero-length array. A Synthesizer that does report its voice status should maintain this information at all times for all of its voices, whether they are currently sounding or not. In other words, a given type of Synthesizer always has a fixed number of voices, equal to the maximum number of simultaneous notes it is capable of sounding.

If the voice is not currently processing a MIDI note, it is considered inactive. A voice is inactive when it has been given no note-on commands, or when every note-on command received has been terminated by a corresponding note-off (or by an "all notes off" message). For example, this happens when a synthesizer capable of playing 16 simultaneous notes is told to play a four-note chord; only four voices are active in this case (assuming no earlier notes are still playing). Usually, a voice whose status is reported as active is producing audible sound, but this is not always true; it depends on the details of the instrument (that is, the synthesis algorithm) and how long the note has been going on. For example, a voice may be synthesizing the sound of a single hand-clap. Because this sound dies away so quickly, it may become inaudible before a note-off message is received. In such a situation, the voice is still considered active even though no sound is currently being produced.

Besides its active or inactive status, the VoiceStatus class provides fields that reveal the voice's current MIDI channel, bank and program number, MIDI note number, and MIDI volume. All of these can change during the course of a voice. While the voice is inactive, each of these fields has an unspecified value, so you should check the active field first.