A hearing test measures how sensitive your hearing is in each ear at a range of frequencies - they send a beep at a certain frequency and ask you if you can hear it, and lower the volume until you can not hear it, raise it to confirm you can hear it. This is repeated in each ear over the normal range of hearing, nominally 20- Hz to 20,000 Hz, most older people have lost at the low and high end. The detailed frequency/volume curve allows the hearing aid to be programmed to bring hearing to the 'normal' curve. Some people may have lost hearing at certain frequencies, it may be an irrevocable loss? That said, an iphone with headphones can easily create and administer a tone to each ear that can be varies in frequency and intensity - with the customer pushing a button when he loses the tone, repeat to confirm, then on to the next frequency until the audio range is covered. They the customer is given test results and he sets an on the phone equalizer at the values needed for each frequency and he is good to go. It needs to be made to limit the intensity to a maximum, and some gaps might remain where he has lost hair cells and will have a permanent frequency gap that can not be cured unless we learn how to grow new hair cells in the right place of the right length. A huge market will open up, there will be a wailing and gnashing of teeth in the old FDA shielded crooks, who must adapt or fold. If they adapt, they will do well.