Controlling sound volume and panning

An individual SoundChannel object controls both the left and the right stereo channels for a sound. If an mp3 sound is a monaural sound, the left and right stereo channels of the SoundChannel object will contain identical waveforms.

You can find out the amplitude of each stereo channel of the sound being played using the leftPeak and rightPeak properties of the SoundChannel object. These properties show the peak amplitude of the sound waveform itself. They do not represent the actual playback volume. The actual playback volume is a function of the amplitude of the sound wave and the volume values set in the SoundChannel object and the SoundMixer class.

The pan property of a SoundChannel object can be used to specify a different volume level for each of the left and right channels during playback. The pan property can have a value ranging from -1 to 1, where -1 means the left channel plays at top volume while the right channel is silent, and 1 means the right channel plays at top volume while the left channel is silent. Numeric values in between -1 and 1 set proportional values for the left and right channel values, and a value of 0 means that both channels play at a balanced, mid-volume level.

The following code example creates a SoundTransform object with a volume value of 0.6 and a pan value of -1 (top left channel volume and no right channel volume). It passes the SoundTransform object as a parameter to the play() method, which applies that SoundTransform object to the new SoundChannel object that is created to control the playback.

var snd:Sound = new Sound(new URLRequest("bigSound.mp3"));
var trans:SoundTransform = new SoundTransform(0.6, -1);
var channel:SoundChannel = snd.play(0, 1, trans);

You can alter the volume and panning while a sound is playing by setting the pan or volume properties of a SoundTransform object and then applying that object as the soundTransform property of a SoundChannel object.

You can also set global volume and pan values for all sounds at once using the soundTransform property of the SoundMixer class, as the following example shows:

SoundMixer.soundTransform = new SoundTransform(1, -1);

You can also use a SoundTransform object to set volume and pan values for a Microphone object (see Capturing sound input) and for Sprite objects and SimpleButton objects.

The following example alternates the panning of the sound from the left channel to the right channel and back while the sound plays.

import flash.events.Event;
import flash.media.Sound;
import flash.media.SoundChannel;
import flash.media.SoundMixer;
import flash.net.URLRequest;

var snd:Sound = new Sound();
var req:URLRequest = new URLRequest("bigSound.mp3");
snd.load(req);

var panCounter:Number = 0;

var trans:SoundTransform;
trans = new SoundTransform(1, 0);
var channel:SoundChannel = snd.play(0, 1, trans);
channel.addEventListener(Event.SOUND_COMPLETE, onPlaybackComplete);

addEventListener(Event.ENTER_FRAME, onEnterFrame);

function onEnterFrame(event:Event):void
{
    trans.pan = Math.sin(panCounter);
    channel.soundTransform = trans; // or SoundMixer.soundTransform = trans;
    panCounter += 0.05;
}

function onPlaybackComplete(event:Event):void
{
    removeEventListener(Event.ENTER_FRAME, onEnterFrame);
}

This code starts by loading a sound file and then creating a new SoundTransform object with volume set to 1 (full volume) and pan set to 0 (evenly balanced between left and right). Then it calls the snd.play() method, passing the SoundTransform object as a parameter.

While the sound plays, the onEnterFrame() method executes repeatedly. The onEnterFrame() method uses the Math.sin() function to generate a value between -1 and 1, a range that corresponds to the acceptable values of the SoundTransform.pan property. The SoundTransform object's pan property is set to the new value, and then the channel's soundTransform property is set to use the altered SoundTransform object.

To run this example, replace the filename bigSound.mp3 with the name of a local mp3 file. Then run the example. You should hear the left channel volume getting louder while the right channel volume gets softer, and vice versa.

In this example, the same effect could be achieved by setting the soundTransform property of the SoundMixer class. However, that would affect the panning of all sounds currently playing, not just the single sound being played by this SoundChannel object.