Woman posts about ‘Happy’ song on Facebook seconds before fatal Business 85 crash

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16. To be honest, earphones may block out less background noise than a standard stereo system.

Earphones are notoriously bad at filtering out background noise. You can experience this by putting on a pair and walking around noisy locations. This is especially true with high pitched noises like sirens but is also true for lower frequency sounds.

Car stereos, especially powerful ones, can very effectively mask noise outside of the car because it can better mimic, and thus better interfere with, louder, fuller sounds.