In June, when news about the ATX12V 1.3 Power Supply Design Guide was posted in SPCR's Recommended PSUs, I failed to notice a most important change in section 5 of the Guide on environmental factors: Section 5.7 on page 54 covers Acoustics!
The guideline calls for a maximum sound power level no greater than 4 Bels at 50% load under well defined conditions. This is "for power supplies designed for low noise". Four bels is not exactly whisper quiet, but it is a good starting point, and Intel's inclusion of this definition is very good news for silent PC enthusiasts. The full text of section 5.7 is now available in the Recommended PSUs page.