Noise cancellation has a specific meaning and it's active - a microphone listens to the ambient sound and produces an out of phase waveform to actively cancel the incoming ambient noise. Things like aircraft are better as steady noise is easier for the electronics to cancel.
The ratings on earplugs and isolating earphones are in -dB - so a -33 dB rating gives you a ton of attentuation - sometimes too quiet tho I will use them for highway riding.
The isolating earphones will give you -23 to -26 tho some go higher depending on the design.
For bass response headphones have to have a good seal with the ear which also makes the attentuation work.
Good earphones preserve fidelity while preserving your hearing ( something no in helmet speakers will do )
None eliminate noise - they just move it farther away from you ( attenuation ).
Loud music will mask the attenuated ambient noise.
Aware is an app for use with earphones with a microphone.
It "listens" for you and when something like a siren, horn or other loud sound ( you set the level ) emerges from the ambient noise ( windnoise, motornoise ) then it cuts the music and kicks the microphone on so you can actually hear better than just via the helmet.......
You can set the mic to stay on and the music to stay off for a few seconds so you figure out what is going on if anything.
As I mentioned I could hear the Burgman engine almost as if I had no helmet on. This is emerging tech but for the tiny cost it does work and the big name in earphones, Etymotic has adopted it for themselves - a good recommendation.
You can easily spend more on the earphones than a helmet
http://www.amazon.com/Very-Best-High-End-Headphones-iPhone/lm/R2NVEHJCOWASUX
If you want the Aware app to work tho you need a set with a microphone which I prefer anyway for the in-line on-off button.
Like this
I've not spent more that $80 for what I consider decent sound and the correct controls. UltimateEars have been my choice and they generally fit under the helmet comfortably but some people may find it hard to get earphones correct and then put a full helmet on.
I use a 3/4 so much easier and the Bell Mag has room for the earphones and the scala speakers.
I might treat myself to a pair of Etymotics as I like the idea of multiple barriers within the ear.
Etymotic range here
http://www.amazon.com/s/ref=bl_sr_e...ld-brandtextbin=Etymotic Research&node=172282
I adore riding with music. .....and phone, and voice nav.....