Smartphones have become an extremely popular playback device because there is no more convenient speaker than the one in your pocket. But for a mixer, this small speaker can prove to be extremely useful to see how your mix translates. The cellphone is great for checking if your bass is clear, and is very helpful for checking balances including vocal levels. If your mix doesn’t pass the cellphone test, it’s a good indication to go back to the studio speakers and make some changes.