AMETEK Rotron
Search


Home
AMETEK Aerospace and Defense
Technical Corner

AMETEK Rotron Product Search

ACOUSTIC NOISE
  • Fundamentals
  • Fan Selection Hints
  • Acoustic Noise Control
  • How to Make Quiet Fans

Acoustic Noise Fundamentals
  • Sound is a change in pressure with respect to atmosphere
  • Noise is unwanted sound
  • Speed of sound in air is 1,130 ft/sec (340 m/sec)
  • Wavelength y=c/f
    • where: c = speed of sound
    • f = frequency of sound (cycles/sec)
  • Normal range of hearing is 20 Hz to 10,000 Hz
  • Most sensitive range is 1,000 Hz to 4,000 Hz
Acoustic Noise Measurement

Decibels - Logarithmic scale - 0 to 150
0 dB = threshold of hearing
20 dB = Rustling leaves
50 dB = Average Two-Person Conv
70 dB = Average Street Noise
80 dB = Vacuum Cleaner
110 dB = Subway Train
130 dB = Threshold of Feeling (pain)
140 dB = Jet Aircraft

Adding decibels:
dB1+dB2+…+dBn=10log(((10)dB1/10)+((10)dB2/10)+…+((10)dBn/10))

Examples:
1 - Two equal decibel values (I.e. two identical fans) add to produce a 3 dB increase.
86 + 86 = 89 dB
100 + 100 = 103 dB
2- Two decibel values differing by 6 dB add to produce a 1 dB increase
86 + 80 = 87 dB
100 + 94 = 101 dB


Acoustic Noise Fundamentals
  • Noise Analysis
    • Microphone - Measures pressure fluctuations
    • Analyzer - Takes amplified pressure fluctuations and processes the information into useful information.
    • Narrowband analysis
    • Octave Band Analysis
    • Weighted Scales
      dBa, dBb, dBc.
  • Octave Bands
    • 1/3 and Full Octave bands
    • Typical curves are full octave bands with center frequencies from 63 to 8,000 Hz
    • 1/3 Octave band measurement simply divide each octave band into thirds

Typical Full Octave Band   Typical 1/3 Octave Band 
 Tech Corner - 11  Tech Corner - 12

How to Make Quiet Fans

  • Vary # of blades to shift blade passing frequency
  • Modify trailing edge of blade to control vortex shedding noise
  • Vary # of stationary vanes in vaneaxial fans
  • Proper material selection
  • Proper balancing techniques
  • High grade bearing
  • Control speed of air through the fan by changes in geometry

Air Mover Selection Hints

  • Run as slow as possible
  • Pick a fan that will operate at peak efficiency
  • Be aware of blade passing freq and harmonics
  • Blade Passing Freq = NZ/60 (Hz)
    where: N = # of blades
    Z = rotational frequency in RPM
  • Fundamental noise freq must not equal mounting structure natural freq.
  • limit line of sight to prop / rotor
  • Acoustic damping foam
  • Filters can damp noise (EMI + Air Filters)
  • Mechanical isolation
  • Control air velocity in system. The lower the better.
  • Keep sharp objects away from prop / rotor. This will create another fundamental frequency
  • Put fan as far away from noise critical area as possible

Acoustic Noise Control

  • Limit line of sight to rotor
  • Acoustic dampening foam
  • Filters can dampen noise (EMI + Air Filters)
  • Mechanical isolation
  • Control air velocity in system. The lower the better.
  • Keep sharp objects away from rotor. This will create another fundamental frequency
  • Put fan as far away from noise critical area as possible