the exact meaning of impedance is quite complex and won't really tell you much about the headphones...
(google it if you want to know more)
basically the impedance of a headphone is a measure for how much resistance is on the spool, so in other words, higher impedances require a more powerful output signal (ie. a mixer or amplifier instead of a simple mp3 player)
Now you might wonder why don't all headphones have low impedances ?
If you know how headphones/speakers work, you know theres an electric spool in there somewhere, basically an electric wire wrapped around several times...
if you're listening to high pitch sounds the spool needs to be able to vibrate very fast and be very responsive, if its wrapped with a heavy/thick copper wire to reduce the resistance, it's gonna be heavy and won't be able to vibrate very fast, thus high tones will generally sound a bit less sparkling...
however if it's wrapped with a thin wire, it's gonna be lighter and much more responsive, however since it's a thin diameter wire, it's gonna cause a lot of electrical resistance, so you're gonna need a stronger output signal...
so in short and general, higher impedance = a bit more sparkling sound but needs higher output signal
(i personally found that 60ohm still works ok on an mp3 player)
while lower impedance will work with anything but will have to give in a bit on the high notes...
(however, high impedance headphones will most likely work on your mp3 or iphone, they just won't be very loud, ie. my dad's dt770 with 250ohm still works on my phone but is very quiet)
So if you're planning to listen through an mp3 player or iphone, check that your impedance is max 80ohm i'd say...
since you're planning to spend around 150$ i assume you're a bit of an audiophile and want to hear what a real set of headphones sounds like
me and my dad both love music, so i'll just let you know what we have..
i have a semi-open AKG 121, i love the sound, it's crystal clear, low impedance, but still ok on the high notes and not very expensive + it still looks kinda normal and stylish, so you can still use it on the bus or something....
my dad has 2 Beyer dynamics DT770 headphones, 1 pro with 80ohm impedance and 1 regular with 250ohm impedance, these are both closed headphones and have similar sound quality as my akg121..
however, since they're fully closed they sound about 10x more awesome!
if you're listening through an amp i'd suggest the 250ohm, it's just orgasmic for the ears, if not, the 80ohm will sound almost exactly the same but will still work on your mp3
dT770 is around 180$ but can be found for 150 if you look around carefully....
and the akg 121 is around 80$ but is semi-open so good for everyday use on the bus or at school, but is a bit less impressive since it doesnt completely block out surrounding sounds...
also, if you're buying in a store, be sure to try them on first, ie. my dad gets a headache from the headband on my akg 121s ..... (however for me they fit perfectly)
Beyer dynamics dt770 (mind if you're buying this one, the impedance varies 80 or 250ohm but the headphones look exactly the same on the outside !)
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