How much quieter is the belt-drive compared to the direct-drive?

Here is some decibel information comparing belt-drive and direct-drive whole house fans.

At low speed the 30" direct drive fan = 75.1 dB; at low speed the 30"belt drive fan = 72.9 dB.  At high speed the 30" direct drive fan = 82.7 dB; at high speed the 30 belt drive fan = 77.1 dB.  At high speed the 36" belt drive fan = 80.6 dB; at low speed the 36" belt drive fan = 78.6 dB.  The 24" direct drive should be slightly lower than the 30"