The folks at Anandtech who did the original analysis of the iPhone signal attenuation problem have tested the latest iPhone software updates, both iOS 4.1 beta and the official version iOS 4.0.1 that Apple (AAPL) released on Thursday.

Their verdict: Apple, for what it’s worth, did what it promised it would do — it made the bars more accurate.

“The results are conclusive,” wrote Brian Klug and Anand Lal Shimpi. “Apple has dramatically changed the signal strength to signal bar mapping in iOS 4.0.1 and the iOS 4.1 beta, making the dynamic range not only much broader, but the range values for each bar much wider. The range of signals that correspond to bars three and four are the same width, and bar two is only slightly less…

“While the software update obviously does not and cannot address the design of the antenna itself — or make the drop from holding the phone any less — it does change the way the issue is perceived among users.” (link)

The chart at right shows the changes in height and dBM ranges of the iPhone’s signal bars. But the total effect can be seen more clearly in the two charts posted below the fold. One compares the dBM mapping of iOS 4.0 with beta iOS 4.1, the second compares Apple’s two versions with Android’s bar mapping scheme.

Note: Klug and Shrimpi wrote a separate review of the software update for CNNMoney. You can read it here.

See also:

• Diagram: The iPhone’s lost bars
• Death Grip: Apple blames the bars
• How bad is the iPhone 4’s signal loss?
• The iPhone 4 Death Grip saga

[Follow Philip Elmer-DeWitt on Twitter @philiped]