One speaker louder


What could cause one speaker of a consecutive pair to sound brighter than the other? Both tweeters are working but one speaker sounds a little louder and sharper. A slight but noticeable difference.
neilmc
You can tell they are the same speaker, but one has an airy quality, the other is more mellow

A dead or half dead tweeter perhaps?
Is there tone generating software that I can use to listen to high frequencies of both speakers?