question on open, over-ear headphones

Hi, i have a little weird question on ambient sound only, cause i've never had this type of headphones. I think only one who has can answer.

How is the ambience sound that comes through the open side of this type of headphones? Can i hear clearly my voice (or other instruments like drums, guitar etc), while singing together with the music by them?

Most important; the frequencies are decrease equally on hole spectrum? Or the ambient sound is moody etc?

If someone know how many db is the sound decrease approximatelly..

If your goal is to sing or play along with the music, it will not work.  If there is no music playing, plenty of ambient sound will come through.  But once you turn on the music, you will not hear ambient sound well enough.  The decrease in ambient sound will depend on the volume of the music you are playing