There are too many factors to consider for a micropohone on an iPod to accurately measure tension. As UCSF2012 eluded to in an ealier post. Head size, string type, gauge of string, density of string pattern, racquet material, head shape....the list could go on. All of these will have a different effect on the 'sound' of the strings resonating in the racquet....which is how this app is measuring tension.
A good example would be a Head Microstring racquet (circa the 90s). If you strung that racquet at 30lbs it felt like 60lbs. Do you think your iPad app would say 30 or 60??