:::::::::Yeah, the space is a typographical convention stipulated by SI. As for the 30 vs. 50 - train your ear a bit and you'll find that fairly obvious - 30 Hz practically subsonic, so you get that stuttering judder on the voice (which Barbara and the Doctor hilariously simulate in "The Dalek Invasion of Earth" by waving their hands quickly in front of their mouths). Crank up to 50 Hz and you'd hear a more tonal modification. 

  
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