I saw youtube videos that claimed to do this, although I'm quite certain the videos just excluded sound and lied.
However, I am wondering if the physics of this is actually possible - to create a completely negative sound wave that cancels the sound out to a flat line i.e. complete silence.
p.s. I am not referring to white noise, which prevents the sound from being heard by overriding it with a different sound (white noise). Rather, I am referring to making a sound be completely silent by generating the inverse, opposite, or "negative" sound wave.
Answer
Yes it can be done, and indeed it's a well established technology called active noise control.
The idea is based on destructive interference. If at some point two sound waves have the same amplitude and frequency and they're 180º out of phase then they will sum to zero and the sound intensity at that point will be zero. Your phrase negative sound just means sound that is 180º out of phase with the sound you're trying to cancel.
However it's rarely possible to cancel the sound over more than a very small region. The cancellation requires the amplitude of the cancelling sound to be precisely matched to amplitude of the noise. The trouble is that the amplitude of sound typically decreases as the inverse square of distance from its source. As a result it's hard to get the sound amplitudes to match over more than a restricted area. However noise cancellation is used in special cases like noise cancelling headphones.
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