How to Reduce Noise From Roots Blower

Roots blowers generate 85–100 dBA – loud enough to require hearing protection and noise control measures. The primary noise source is pressure pulsation from the discharge, with mechanical noise from bearings and gears adding to the total. Silencers are the most effective noise reduction method, reducing noise by 15–25 dBA. Acoustic enclosures add another 10–25 dBA reduction.

Based on field data from hundreds of installations, proper silencer selection and installation can reduce noise to safe levels (75–85 dBA) – meeting OSHA and EU noise standards. This guide covers all noise control methods: silencers, acoustic enclosures, vibration isolation, piping, and maintenance.

Table of Contents

• Why Are Roots Blowers Noisy?

• Noise Sources

• Silencer Types

• Silencer Selection and Installation

• Acoustic Enclosures

• Vibration Isolation

• Piping Noise Control

• Maintenance for Noise Reduction

• Regulatory Compliance

• Frequently Asked Questions

• Final Thoughts

Why Are Roots Blowers Noisy?

Roots blowers are loud because of pressure pulsation. Roots blowers have no internal compression – at discharge, higher-pressure air backflows into the lobe cavity, creating pressure pulses. This is the dominant noise source (70–80% of total noise).

Typical noise levels:

• Bare blower: 90–100 dBA at 1 meter

• With inlet and discharge silencers: 80–88 dBA

• With acoustic enclosure: 70–80 dBA

• Helical rotors: 5–8 dBA lower than straight rotors

Based on field data, a roots blower at 8 psig produces 90–95 dBA – well above OSHA's 85 dBA 8-hour exposure limit. Silencers are required for operator safety and regulatory compliance.

Noise Sources

1. Pulsation (dominant source – 70–80% of noise).
Roots blowers have no internal compression. At discharge, higher-pressure air backflows into the lobe cavity – creating pressure pulsations.

• 2-lobe: 4 pulses/revolution – higher pulsation

• 3-lobe: 6 pulses/revolution – 30–50% lower pulsation

• Helical: smoother discharge – lowest pulsation

2. Mechanical noise (10–15%).

• Bearings: rolling element noise

• Gears: tooth meshing noise

• Motor: fan noise, electrical noise

3. Airflow noise (5–10%).

• Inlet turbulence

• Discharge turbulence

• Piping noise

4. Radiated noise (5–10%).

• Casing vibration

• Piping vibration

• Foundation vibration

Silencer Types

1. Reactive silencers (pulsation dampers).

• Use expansion chambers to attenuate pulsation

• Most effective at low frequencies (pulsation frequencies)

• Typically used on discharge side

• No internal media to degrade

• Attenuation: 15–20 dBA

2. Absorptive silencers (sound-absorbing).

• Use foam, fiberglass, or other absorbent material

• Most effective at high frequencies

• Typically used on inlet side

• Media degrades over time – requires replacement

• Attenuation: 10–15 dBA

3. Combination silencers.

• Both reactive and absorptive elements

• Best overall performance

• Higher cost

• Attenuation: 20–25 dBA

Silencer comparison:

Silencer Selection and Installation

Step 1 – Determine required attenuation.
Required attenuation = (blower noise level) – (target noise level)
Example: Blower 95 dBA, target 85 dBA → 10 dBA attenuation.

Step 2 – Select silencer type.

• Inlet: absorptive (filtration + noise reduction)

• Discharge: reactive or combination (pulsation damping)

Step 3 – Size for flow and pressure.
Silencer must handle:

• Flow rate (ACFM)

• Pressure (psig)

• Temperature (°F)

• Pressure drop (typically 0.5–1.0 psig per silencer)

Step 4 – Verify pressure drop.

• Inlet silencer pressure drop adds to vacuum or reduces inlet pressure

• Discharge silencer pressure drop adds to discharge pressure

• Typical: 0.5–1.0 psig per silencer

Step 5 – Check materials.

• Standard: carbon steel

• Corrosive: stainless steel

• High temperature: appropriate materials

Installation guidelines:

• Inlet silencer: mount directly on blower inlet or within 18 inches

• Discharge silencer: mount within 18 inches of blower discharge

• Silencers must be supported independently

• Do not use silencer as pipe support

• Allow access for maintenance

Acoustic Enclosures

What is an acoustic enclosure?
A soundproof housing that surrounds the blower and silencers. Provides additional noise reduction beyond silencers alone.

Enclosure types:

• Full enclosure (covers entire blower package)

• Partial enclosure (covers noise sources)

• Modular panels (prefabricated, site-assembled)

Enclosure construction:

• Steel panels with sound-absorbing material

• Double-wall construction for higher attenuation

• Access doors for maintenance

• Cooling air intake and exhaust (with silencers)

• Viewing windows (optional)

Attenuation:

• Standard enclosure: 10–15 dBA

• Heavy-duty enclosure: 15–25 dBA

When to use:

• Noise limit below 80 dBA

• Blower near offices or residences

• No space for larger silencers

• Multiple blowers in one area

Vibration Isolation

Why vibration isolation matters:
Vibration transmits through the foundation and piping – radiating noise. Isolation reduces structure-borne noise.

Isolation methods:

1. Neoprene pads.

• Standard for most industrial blowers

• 60 Shore A durometer

• 20 mm thickness

• Simple and cost-effective

2. Spring isolators.

• Higher isolation efficiency

• For sensitive applications

• More expensive

3. Flexible connectors.

• Inlet and discharge piping

• Within 18 inches of blower flange

• Rubber or metal bellows

• Reduces pipe vibration

Isolation guidelines:

• Use neoprene pads under base frame

• Use flexible connectors on piping

• Isolate piping supports

• Avoid rigid connections

Piping Noise Control

Piping noise sources:

• Airflow turbulence

• Pulsation transmission

• Radiated noise from pipe walls

Piping noise control methods:

1. Larger diameter piping.

• Lower velocity = less turbulence

• Keep velocity below 3,000 ft/min

2. Flexible connectors.

• Isolate vibration

• Reduce noise transmission

3. Pipe supports with isolation.

• Rubber pads under pipe supports

• Prevent vibration transmission

4. Pipe lagging (insulation).

• Sound-absorbing wrap on pipes

• Reduces radiated noise

5. Avoid sharp bends.

• Use long-radius elbows

• Reduce turbulence

Maintenance for Noise Reduction

How maintenance affects noise:

1. Bearings.

• Worn bearings increase noise

• Replace bearings at 40,000–50,000 hours

• Listen for grinding or rumbling

2. Clearances.

• Increased tip clearance = more pulsation = more noise

• Replace rotors when clearance exceeds 0.35 mm

• Maintain proper clearances

3. Silencers.

• Foam degrades (inlet silencer)

• Baffles can fail (discharge silencer)

• Inspect silencers annually

4. Loose parts.

• Loose bolts rattle

• Loose guards vibrate

• Check and tighten hardware

Maintenance checklist for noise control:

• Check and tighten hardware

• Inspect silencer foam (replace if degraded)

• Listen for bearing noise

• Measure vibration

• Check for loose guards or panels

Regulatory Compliance

OSHA noise standards:

• 85 dBA: 8-hour TWA – hearing protection required

• 90 dBA: 8-hour TWA – hearing protection mandatory

• 115 dBA: Peak limit – no exposure without protection

EU noise standards:

• 80 dBA: Action level – hearing protection provided

• 85 dBA: Limit level – hearing protection mandatory

• 87 dBA: Exposure limit – must be reduced

Compliance checklist:

• Measure noise levels at operator positions

• Install silencers (inlet and discharge)

• Provide hearing protection

• Post warning signs

• Train operators on noise hazards

Frequently Asked Questions

1. How much noise does a roots blower make?
Bare blower: 90–100 dBA at 1 meter. With inlet and discharge silencers: 80–88 dBA. With acoustic enclosure: 70–80 dBA. OSHA requires hearing protection above 85 dBA for 8-hour exposure.

2. What is the most effective way to reduce roots blower noise?
Silencers – inlet and discharge silencers are the most effective noise reduction method. Total reduction: 15–25 dBA. Acoustic enclosures add another 10–25 dBA.

3. What is the difference between inlet and discharge silencers?
Inlet silencers reduce noise from air entering the blower – typically absorptive type with filter element. Discharge silencers reduce pulsation noise from the discharge – typically reactive or combination type.

4. How much noise reduction do silencers provide?
Inlet silencer: 10–15 dBA. Discharge silencer: 15–20 dBA. Combination: 20–25 dBA. Total with inlet + discharge: 15–25 dBA reduction.

5. Do I need silencers on both inlet and discharge?
Yes – for effective noise reduction. Inlet noise and discharge noise are both significant. Inlet silencers also provide filtration. Discharge silencers also dampen pulsation.

6. What is an acoustic enclosure?
An acoustic enclosure is a soundproof housing that surrounds the blower and silencers. Provides 10–25 dBA additional noise reduction. Used when silencers alone cannot meet noise limits.

7. How does lobe count affect noise?
Three-lobe is 5–8 dBA quieter than twin-lobe. Three-lobe has 6 pulses per revolution vs 4 for twin-lobe – smoother flow, less pulsation. Helical rotors are another 5–8 dBA quieter.

8. Does VFD reduce noise?
Yes – VFD reduces noise at lower speeds. At 80% speed, noise is significantly lower. At 50% speed, noise is much lower. VFD also provides soft start – no mechanical shock.

9. How often should I replace silencer foam?
Inlet silencer foam typically lasts 12–24 months. Heat, moisture, and oil contamination degrade foam. Inspect annually. Replace if crumbling, oil-saturated, or water-damaged.

10. What is the cost of silencers?
Inlet silencer: $500–800 (4-inch). Discharge silencer: $600–1,000 (4-inch). Combination silencer: $1,000–1,800. Acoustic enclosure: $3,000–10,000.

11. Can I reduce noise without silencers?
Silencers are the most effective noise reduction method. Other measures: locate blower away from workers, use acoustic enclosure, install flexible connectors, isolate vibration, use larger piping. But silencers are the primary noise control.

12. How do I measure roots blower noise?
Use a sound level meter (Type 1 or 2). Measure at 1 meter from the blower. Measure at operator position. Follow ISO 2151 or ISO 3744. Measure at rated pressure and speed.

13. What is the quietest roots blower design?
Three-lobe helical with inlet and discharge silencers and acoustic enclosure. Helical rotors reduce pulsation. Silencers reduce noise. Enclosure provides additional reduction. Total noise: 70–75 dBA at 1 meter.

14. Are helical rotors worth the extra cost?
Yes – for noise-sensitive locations. Helical rotors add 25–35% to blower cost but reduce noise 5–8 dBA. The cost is justified for indoor installations, blowers near offices or residences, and noise compliance.

15. What should I do if my blower suddenly gets louder?
Check silencers – foam degradation or baffle failure. Check bearings – wear increases noise. Check for loose parts. Inspect clearances – increased clearance increases pulsation. Investigate immediately – sudden noise increase indicates a problem.

Final Thoughts

After decades of managing roots blower noise, here is my practical advice:

Silencers are mandatory – not optional. Roots blowers generate 90–100 dBA without silencers – exceeding OSHA noise limits. Inlet and discharge silencers are required for operator safety and regulatory compliance.

Three-lobe is quieter than twin-lobe. The 5–8 dBA difference is significant. For new installations, always specify three-lobe. For noise-sensitive locations, specify helical rotors for another 5–8 dBA reduction.

Maintenance preserves noise reduction. Worn bearings, degraded silencer foam, and increased clearances all increase noise. Regular maintenance keeps noise at design levels.

The bottom line. Reducing noise from roots blowers requires a combination of methods: silencers (primary), three-lobe design, acoustic enclosure, and regular maintenance. Zhanggu and other manufacturers provide silencer options and noise data. The investment in noise control protects workers and ensures compliance.