What is blade-pass frequency and why does my fan whistle?

Blade-pass frequency (BPF) is the tone produced each time a blade passes a fixed disturbance: BPF = number of blades × revolutions per second. A 7-blade fan at 3,000 rpm produces a tone at 7 × 50 = 350 Hz, plus harmonics. Whether it annoys depends on level, on what the tone excites in your chassis, and on the obstruction pattern in front of the fan.

Calculating and predicting the tone

BPF (Hz) = blades × (rpm / 60). Examples: 7 blades at 3,000 rpm → 350 Hz; 9 blades at 4,500 rpm → 675 Hz; 5 blades at 1,800 rpm → 150 Hz. Harmonics appear at integer multiples (2×, 3× BPF). Check the tone against your structure: if BPF or a harmonic lands on a panel's natural frequency, the chassis amplifies the whistle far beyond the fan's airborne spec — a fan swap that changes blade count or speed moves the tone off the resonance.

What amplifies BPF

Anything fixed immediately in front of or behind the impeller turns each blade passage into a pulse: close-clearance grilles, straighteners, brackets, and closely spaced finger guards all strengthen the tone. Distance and streamlining weaken it. Tonal level also tracks speed strongly — a fan run 20% faster can produce a noticeably louder tone from both the level increase and the shift toward more sensitive hearing bands.

• Increase grille-to-impeller clearance where the design allows.
• Prefer aerodynamic grilles over flat perforated plates close to the blade tips.
• If a resonance is hit, changing speed (control table) or blade count escapes it.

Verification

Listen (or measure with a phone-class spectrum app as a screen, a proper mic for the record) at the customer's ear position, not beside the fan. Sweep duty to find duties where the tone aligns with resonances, and avoid parking there in the control table. When submitting an RFQ for a quieter variant, specify the forbidden frequency bands and the acoustic target at the ear position — that gives the fan maker room to change blade count, speed and inlet geometry deliberately.