The sound level of fans is expressed in a unit called the sound pressure level and denoted by 'dB(A)'. The '(A)' indicates that sound pressure levels are 'A-weighted.' This A-weighting corrects frequency measurements to replicate the frequency response of human hearing, and the calculation is defined by a JIS standard.
The sound pressure level of fans is measured by a sound level meter located 1 m from the fan's intake side in an anechoic chamber. Since the measurement is done with the fan suspended in the air with almost no obstructions around it, the measured value is the sound pressure level when the airflow is equal to the maximum airflow.
By using two formulas that can calculate "SPL (sound pressure level) when fan speed is changed" and "SPL from multiple sound sources," you can make the following comparison.
If you want to double the maximum airflow, there are two methods: increase the fan speed, and add a fan. However, the above calculations show that the two methods resulted in significantly different sound pressure levels. So, if you want to keep the sound pressure level low, it can be said that using two fans is the right thing to do. As shown above, changing the fan speed and number of fans in the device design stage is a way to solve sound level problems.