Best Earmuffs for Low-Frequency Sounds (e.g., Long Flights or Car Rides)?

I'm always reading about how low-frequency sounds, "C waves", are different and aren't blocked as well by earplugs or earmuffs. What exactly does this mean, and how does this work? I'm planning on visiting Europe this summer, which obviously means flying overseas.

What kind of earmuffs are especially good for these low frequencies? And what exactly are these C waves anyway?

Thanks.

 

Noise-cancelling headphones are great for reducing the low hum whilst in flight. You may want to pack earmuffs as well for the takeoff and landing. I've flown several times with tinnitus and only once had a slight spike for a few days after taking an indirect flight with two changes along the way. I keep filtered earplugs underneath and try not to sit too near the engines.