Whenever I look on the box of headphones, or even here on head-fi I see people compare headphone specs, and I don't know a thing about any of them. For example, freq. response, sound pressure level, THD, normal impedence, etc
What do all of those mean and what exactly says what is bad or good in these specs? Is there a bad freq. response, pressure level, thd, or low impedence? Is there a threshold when it becomes hi-fi?
Also what confuses me the most is about freq. response. I've seen headphones that are like 20-39500 in the response, but I thought the human ear can only indentify between the spectrum of 20-20k?