The higher the impedance, the more voltage is needed to make the headphones sound loud enough. A smartphone or notebook usually does not have as much "power" as a modern hi-res player or stereo system. Headphones below 100 Ω are therefore more suitable for mobile use. Headphones over 100 Ω, on the other hand, are well suited for stationary systems with headphone amplifiers or are used in the professional sector.