These aren't the same kind of earplugs you get in a 10-pack at Home Depot. They are designed to have a more-or-less flat frequency response (at least above 500 Hz), so that the SPL hitting your eardrum is reduced but everything still sounds the same. This particular model yields a 12 dB reduction. To extend that flatness below 500 Hz you have to go with much more expensive types that are custom-molded to your ear by an audiologist. But those are frickin'
expensive.
By comparison, the cheap foam plugs they hand out at NAMM drop a good 25 dB in the mids, 30 dB or more in the highs. That nonlinearity is why you can't hold a conversation when they're in your ears. Great for protecting construction workers' ears from chop saws, but terrible for musicians.