When you review Speakers/Headphoes/Audio, you see how accurate the sound reproduction of mids, lows, and highs are.

But how is this measured, with your ears or with testing equipment?

I like to use my ears, hehe, and I think that's the best way... but true sound techies have a swag of devices they can use. One really cool gadget I saw had seperate Db meters for each range (ie, low/mid/hi). ;D

heheh... i know far too much on this..
for example, a proffessional test on say.. a set of speakers firstly involves selecting the right environment. to do an accurate measurement of the frequency response manufacturers use an anechoic room. this is a room which is completly full of foamalong walls, ceiling, floor, typically made of reainforced concrete. this means that the measured sounds from the speakers will 1) be only the sounds of the speakers (not next doors dog barking too) and 2) the foam stops measurement of reflections of sounds from the walls, ceiling.
to measure what comes out of the speakers, a special sound has to be put through them. this sound has to contain all frequencies that us humans can hear. a precision microphone (very expensive) is used to pick up what comes out of he speakers. the output of the mic is sent to a device which can interpret the info into a graph for analysis.
alternativly, to measure the loudness capability, a sound level meter can be used which measures the levels of particular frequencies emmited from the speaker.
hope this helps!!!