In a nutshell, all the aforementioned. I forgot to mention that the soundproofing is rather good. Amazing to see from above. Common, simple plugs with silicone liners offer the best protection from engine noise. The explanation is likely due to the Bose silicone earbud's design; being larger and further away from the speaker than typical earplugs allows more ambient noise in. They are, thus, ineffective in extremely noisy environments. Honestly, I can live without it.
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