How is mid-fi defined? In a word, measurements. As in using measurements to sell mid-fi products. High end audio products are bought and sold based on the subjective experience they provide. Mid-Fi is more based on numbers, both in how it’s marketed, and how it’s designed (cheaper). Put another way, the next time somebody is screaming for measurements in an audio discussion, if you can, find out the composition of their system; mid-fi and/or old/cheap.