iOS 4.0.1 signal strength indicator changes explained

As those who have successfully upgraded their iPhone 3G, iPhone 3GS, or iPhone 4 units to iOS 4.0.1 may have noticed, Apple made a visual change to its signal strength indicator bars in its latest update, increasing the size of bars one, two, and three, while keeping the tallest two bars the same. A more dramatic change was made behind the scenes, however, as Apple changed its formula for signal strength to signal bar mapping resulting in what is hopefully a more accurate representation of the phone’s current connection. Anandtech reports that Apple made the dynamic range broader, while also adjusting the range values for each bar to make them wider. As a result, in some areas where signal strength would have been great enough for the iPhone to report five bars under iOS 4.0 and earlier, it may now show only three. As the report notes, this means that the worst case drop of 24 dBm — seen when applying the left-hand “death grip” on the iPhone 4 — longer makes all the signal bars disappear, but just two. The new formula is also used in the iOS 4.1 beta released earlier this week to paid iPhone developers; illustrations of the difference in bar to signal strength mapping are available below.