Why mobile conversations are annoying

From Jakob Nielsen's latest Alertbox. A study by the University of York found that public mobile phone conversations were much more annoying to bystanders than were equivalently loud face-to-face conversations -- even though the latter "produced double the audio output". Conclusion: when we can't hear half of what is being said it is somehow more intrusive. Perhaps we're biologically inquisitive. It's a bit like the phenomenon whereby, when people are trying to sleep, muffled voices tend to keep them awake more than clearly intelligible ones.
(From UseIt.com)