A fan Blade Pass Frequency noise can be very intense - and vary with the number of blades and the rotation velocity
The fan Blade Pass Frequency noise level intensity varies with the number of blades and the rotation speed of the wheel and can be expressed as
BPF = n t / 60 (1)
BPF = Blade Pass Frequency (Hz)
n = rotation velocity (rpm)
t = number of blades
Example - Blade Pass Frequency of Fan
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 for fans with few blades.
Be aware that the Blade Pass Frequency may not visible in charts common for fans and that fan noise in general is documented in octaves.