The fan Blade Pass Frequency noise level intensity vary with the number of blades and the rotation speed and can be expressed as
BPF = n t / 60 (1)
BPF = Blade Pass Frequency (Hz)
n = rotation velocity (rpm)
t = number of blades
If a fan with 10 blades rotates with 2400 rpm - the Blade Pass Frequency - BPF - can be calculated like
BPF = (2400 rotations/minute) (10 no/rotation) / (60 s/minute)
= 400 Hz
Note that the energy in the sound may be very high and annoying for the surroundings, especially with fans with few blades.
Be aware that the Blade Pass Frequency is not visible in charts common for fans where sound in general is documented in octaves.