html5 - getUserMedia returning stream with audio when there is no audio device -

i have case when turned off media (camera , microphone) navigator.getusermedia with constraints of {audio: yes, video: no} return stream , furthermore stream.getaudiotracks().length return 1 highly strange because there no device connected produce audio. if put stream audio tag hear weak white noise (in opposite if plug in microphone when hear myself).

what can reason such strange behavior?