The iPhone speaker is one of those elements that never stands out but that has been undergoing improvements in each generation from Apple phone.
What people are looking for in a mobile speaker it is that it sounds loud to find out when they receive a call or notification. Then there are other factors like the clarity of the sound, something very difficult to achieve seeing the tiny dimensions of this piece of hardware.
Without getting into the debate of which sounds better, Yes we can know which of them sounds louder and that is the iPhone 5. Many users who already have it have realized this detail when comparing it with their previous iPhone 4 / 4S and the sound level meter does not deceive. Below you have the audio intensity of each speaker measured in dB (decibels):
- iPhone 2G: 91.0 dBA
- iPhone 3G: 95.0 dBA
- iPhone 3GS: 97.6 dBA
- iPhone 4: 92.8 dBA
- iPhone 4S: 97.9 dBA
- iPhone 5: 100.3 dBA
More information - Passive speaker for iPhone
Source - iClarified