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Technical Corner

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

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

How to Make Quiet Fans

• Modify trailing edge of blade to control vortex shedding noise
• Vary # of stationary vanes in vaneaxial fans
• Proper material selection
• Proper balancing techniques
• 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