I am looking to buy some noise isolating (not noise cancelling) headphones, but I am very confused right now. Initially I thought that impedance was a good measure of the capacity for a pair of headphones to block outside noise, but after reading a review on Amazon to this product I am not convinced because in the description for these headphones an impedance rating of 50 ohms is given and a reviewer has made the following statement:
These headphones are not at all designed to be sound isolating. They are completely open - any other sound in your listening environment will come in, and anyone else around you will be able to hear what you're listening to.
So I ask the following:
What is a good technical spec to judge the ability of a pair of headphones to block outside noise?
What is a good pair of noise isolating over ear headphones?