Differences in audio frameworks on iPhone 3G, 3GS and 4

I'm building an app that measures sound volume. I understand that audio hardware in the iPhone is not as accurate as profes开发者_JAVA技巧sional hardware, which is OK, but I need to know if there are any differences between the different iPhone models. For example, is it possible that the volume measured on an iPhone 3G will be different on an iPhone 4? Unfortunately I do not possess any models earlier than the 4 so I'm unable to test this myself.

The audio frameworks seem to be identical for identical iOS versions (except for the 2G). However the physical microphones (and acoustical environments) are different. People have published various test results, such as:

http://blog.faberacoustical.com/2009/iphone/iphone-microphone-frequency-response-comparison/

and

http://blog.faberacoustical.com/2010/iphone/iphone-4-audio-and-frequency-response-limitations/

But it's possible that the mic response may vary with manufacturing batches as well. YMMV.

I'd just like to add that no matter what I try, there seems to be no way of exporting audio through AVAssetExportSession using an iPhone 3G. It's working with 3GS, iPod Touches, 4, iPad and so on.