[FFmpeg-user] Why do these blank spots appear in spectrograms of converted youtube videos?

[Ethan Lewis]

Hello

I was converting a bunch of youtube videos into 16kHz wav files using ffmpeg

I used the following commands

youtube-dl -f best /path/to/youtube/video/ -f 'bestvideo,bestaudio' -o
'%(title)s.f%(format_id)s.%(ext)s'

ffmpeg -i /path/to/download/m4a/audio -acodec pcm_s16le -ar 16000 -ac
test1.wav

When I use these youtube videos

https://www.youtube.com/watch?v=E1mGPeLqg2g
https://www.youtube.com/watch?v=XBFOP_pF7PY

I see many blank spots in their spectrograms  (see the ones uploaded here
https://drive.google.com/file/d/0BzNdoGke8tyRSGgwUUtSTkgxdEE/view?usp=sharing
)

Natural speech should not have such artefacts.

I am confused why this is happening. Is it because of ffmpeg conversion of
m4a audio OR was it somehow fault of the original recording?

Thanks.