
I've been using the Klipsch S4
Passive means that there are no special electronics reversing the signal of the ambient sounds, and feeding the results back into the sound stream, thereby cancelling the background noise. If you've used these, you know they are amazing, but they also produce some side effects, the main one being the feeling of some kind of pressure on your ear drums. The Klipsch design simply blocks your ears as do normal earplugs, and inserts the sound from the speakers directly into your ears.
Do they work? Absolutely. In fact, they block not only the ambient background noise, but they also block pretty much everything else in the area. This makes them great for those times you want to zone out the rest of the world, and concentrate on what you want to listen to.
I attempted to use the S4 while driving, but quickly remembered that first - it's not a good idea to tune out all road noise while you're driving, and second - it's illegal in most states to do so. That gave me a good excuse to change seats with my alternate driver; my wife, so I could continue my testing.
As it turns out, my testing went well, and I was able to listen to my podcasts
and music without distratcions. In fact, I couldn't even hear my wife, as she sat next to me when she asked a question. Depending on your state of mind, and your relationship, this may not be a good thing.
I also had occasion to use the S4 on an airplane trip, and the unit did a great job filtering out engine noise. One side benefit of blocking background noise is that you don't need to turn up the volume to ear-damaging levels, in order to hear what you want to hear.
I recommend them for those times you want solitude. Just be sure you follow the rules of the road, and the rules of civility when you use them.

