Based on discussion in crbug/679552:

Jimmy, while discussing this issue, we also were discussing whether we should add a pure 'silence' test. I thought there could be two variations to it:
1. Validate silence when there is no playback activity.
2. Validate silence when a silent audio file is playing. 

Do you think such a silence test would be useful? Do the two variations make sense? 

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Re#4
- Using separate test is good too. We can keep basic test simple.
- Measurement tool, that is, quality_measurement function in client/cros/audio/audio_quality_measurement.py returns noise at different places.

    @returns: A dictoinary of detection/estimation:
              {'artifacts':
                {'noise_before_playback':
                    [(time_1, duration_1), (time_2, duration_2), ...],
                 'noise_after_playback':
                    [(time_1, duration_1), (time_2, duration_2), ...],
                 'delay_during_playback':
                    [(time_1, duration_1), (time_2, duration_2), ...],
                 'burst_during_playback':
                    [time_1, time_2, ...]
                },
               'volume_changes':
                 [(time_1, flag_1), (time_2, flag_2), ...],
               'equivalent_noise_level': level
              }

  so it is easy to see where is the noise.

  This is also why I suggest in #2, where we let chameleon record full silence -> play -> silence duration.

- The basic check only check if dominant frequency is correct. Silence does not contribute to any value on the spectrum, so that half silence does not affect dominant frequency.

- About silence test, we should also exercise mute function. For example, while in mute state, play an audio file, and check if it is silence.


Currently we don't have a function to check whether recorded audio is silence. I think we can use a threshold and a for loop to check every sample value is below that threshold. We should have a separate issue tracking adding mute test, for catching issue like https://bugs.chromium.org/p/chromium/issues/detail?id=681890.