Bug 97290 – Loud sounds turning up when using Web Audio API where no sound expected

• Status: UNCONFIRMED
• Component: Web Audio

[Srikumar K. Subramanian]

OS: iOS6
Device Type: iPhone4

Steps to reproduce:

The following code is not expected to produce any sound since the volume set is 0, but once in a while you get a loud "ping" if you keep reloading the page in Mobile Safari on iOS6. Less than 10 attempts are needed to produce this effect.

{{{
<script>
    var ac = new webkitAudioContext();

    var attack = 0.01;
    var decay = 0.5;

    function note(t, f, v) {
        var osc = ac.createOscillator();
        osc.frequency.value = f;
        var gain = ac.createGainNode();
        gain.gain.setValueAtTime(0, t);
        gain.gain.linearRampToValueAtTime(v, t + attack);
        gain.gain.setTargetValueAtTime(0, t + attack, decay);
        osc.connect(gain);
        gain.connect(ac.destination);
        osc.noteOn(t);
        osc.noteOff(t + 12 * decay);
    }

    note(ac.currentTime, 880, 0);
</script>
}}}

I've occasionally heard such loud sounds in Chrome 21.0.1180.89 and Safari 6 on MacOS 10.8.1 and so I believe it might be present in the WebKit implementation across the board.

Some other observations -

1. Changing the line "gain.gain.setValueAtTime(0, t);" to "gain.gain.value = 0;" seems to result in the code doing the right thing - i.e. I never got the loud ping on several attempts.

2. Changing the line "note(ac.currentTime, 880, 0);" to "note(ac.currentTime + 0.01, 880, 0);" also seems to resolve this issue.

[Srikumar K. Subramanian]

Btw with either of the changes (1) and (2), if I give a non-zero gain value, the note is played at that level correctly upon page load.