One of the less-discussed features in iOS 7 is the new male voice for Siri, but why is Apple adding it now? According to Stanford professor Clifford Nass, one cringeworthy reason could be because female voices are sometimes seen as less intelligent than male voices. These cultural perceptions are the reason why Siri originally received male voices in some countries, like the United Kingdom, but not in others. For example, BMW had to recall a car in Germany, because male drivers didn’t trust the female voice chosen for the GPS. A better reason? Apple wants to let users customize the iOS experience more fully, though it seems unlikely that Apple would go as far as including the celebrity voices used by GPS makers TomTom and Garmin. follow link
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Why Apple Added a Male Voice to Siri
Although I don't know why we need a reason for the added male voice choice, to those familiar with hearing loss, the obvious reason is for accessibility. Hearing loss usually begins in the high ranges making a female voice harder to understand than a male voice.
I have some hearing loss. I have noticed that I trouble understanding some women's voices, air conditioning, wind noise, or other conversations can mask the sound.
It's well know SAC in the 50's/60's used female voices in high stress situations to give male pilots warnings. This example is for the B-58 but there were others. No idea about current practices.