Fan noise is composed of wideband noise and tonal components such as rotary sound. The wideband noise is characterized by the superposition of many frequencies of the same or similar sound level. It is caused by local flow separation and vortex formation. These occur, for example, on housing walls, at the transition between the inlet nozzle and the fan inlet, and at stall points on blades.
The tonal component is characterized by a peak of any frequency that has a much higher sound level (about 5 dB) than surrounding frequencies. The tone of the rotational sound is produced by the interaction of the outflowing air at the blade end with a housing component, e.g. the housing tongue. The smaller the distance between the trailing edge of the blade and the housing component, the more pronounced the rotary sound. Further tonal components can be produced, for example, by blowing tones at holes, which are expressed in a whistling sound. Figure 7.2.1. shows how a tonal noise differs from a wideband noise by the blade repetition frequency.
Without a housing, there is also a tonal component of varying intensity, which arises from the changing, rotating pressure field of the fan impeller. Here, the transient blade forces have the greatest influence on tonality. The tonal component of the fan noise results in the frequency position from the product of the rotational speed with the number of blades, also blade frequency or blade repetition frequency (BFP) called.
Multiple components, called harmonic components, of this frequency also occur as tonal components. Typically, the amplitude decreases with the order of the harmonics. Another tonal component in the frequency spectrum of the fan can be caused by resonances. In this case, both on the inlet and outlet side, the connection geometry can be randomly designed in such a way that a resonant body is created and the actual low fan noise is massively amplified. It is therefore necessary to design the fan environment in such a way that resonance phenomena are avoided; this is referred to as acoustic mismatch.