I'd guess you could find the DB figures for the Noctua at Noctua's site.
But.....published ratings are not particularly helpful to determine your satisfaction.
Differences of 3 or 4 DB may not even be perceptible in actual use.
2 fans rated at 40 db and spinning at the same RPM may be virtually indistinguishable from 1 such fan.
You may find that the characteristics of a specific fan model may be more annoying to you, even though "quieter".
Let alone variations between 2 fans of the same model.
Let alone fan speed....they are all a bit noisy as they creep above 1000 rpm.
Let alone the frequency response curves of your 2 ears.
Let alone that you may find that 7 fans is no "better" than 5.
I wouldn't get hung up on the spec sheets as a deciding factor. You'll have to hope and experiment a bit.