The process begins with driver selection. Depending on the specific acoustic goals a single, dual, or multi-way driver may be chosen. Dual drivers are those with two units of the same mechanical size joined together. The two units are either tuned identically or with one tuned higher than the other. Multi-way drivers combine units of different sizes as for example in woofer-tweeter combinations. Often crossover circuits are used in multi-way drivers. One can purchase a pre-assembled combination or create one from separate drivers.