Acoustical Noise

Acoustical noise is typically measured by the Sound Pressure Level (Lρ), expressed in decibels and is dependent upon the distance from the source as well as its surroundings.

The Sound Pressure Level is defined as:
Lρ = 10 log (ρ²/ρ_o²)
Where:
Lρ = Sound pressure level (dB)
  ρ = Measured sound pressure (Pascals)
  ρ_o = Sound reference level 20 µN/m²

Noise Generated by Air Movers
There are many different ways to reduce the noise generated by an air mover. Some of the more common are:
• Avoid obstructions to airflow
• Run larger fans at lower speeds
• Lower the system impedance
• Minimize inlet losses and obstructions

The most significant factor influencing the noise from a given air mover is the speed of rotation. This is given by the following equation:
dB₂ = dB₁ – 50 log (rpm₁ / rpm₂)
Where:
dB₁ = Sound pressure of the air mover operating at rpm₁
dB₂ = Sound pressure of the air mover operating at rpm₂
rpm₁ = Operating speed of impeller at condition 1
rpm₂ = Operating speed of impeller at condition 2

Therefore, a blower at half speed will be 15 dB quieter than at full speed. Airflow will also be half and, since static pressure is a squared function of speed, it is reduced by a factor of four.    

Decibel Loudness Comparisons:

Sound and Distance: When the distance from a Point source doubles, the sound level decreases six decibels.

Addition and Subtraction of Decibel Levels: Doubling sound energy yields an increase of three decibels. In this example, each source is 50dBA. Note the characteristics of logarithmic addition or subtraction of decibel levels.