Roots Blower Noise Level

Roots blower noise level typically ranges from 85–100 dBA at 1 meter – loud enough to require hearing protection and noise control measures. The primary noise source is pressure pulsation from the discharge backflow, with mechanical noise from bearings and gears adding to the total. Proper silencers can reduce noise to 75–85 dBA, and acoustic enclosures can achieve 70–80 dBA.

Based on field data from hundreds of installations, roots blowers are among the loudest industrial equipment. OSHA requires hearing protection above 85 dBA for 8-hour exposure. Without silencers, roots blowers exceed this limit. Understanding noise sources and reduction methods is essential for operator safety and regulatory compliance.

This guide covers noise levels, sources, measurement, silencer selection, and noise reduction strategies.

Table of Contents

• What Is Roots Blower Noise Level?

• Typical Noise Levels

• Noise Sources

• Noise Measurement

• Silencer Types

• Silencer Selection

• Acoustic Enclosures

• Noise Reduction Strategies

• Regulatory Compliance

• Frequently Asked Questions

• Final Thoughts

What Is Roots Blower Noise Level?

Roots blower noise level is the sound pressure generated by the blower during operation, measured in decibels (dBA). The noise is primarily from pressure pulsation at the discharge, with additional contributions from mechanical components and airflow turbulence.

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.

Why roots blowers are loud:

• Pulsation from discharge backflow (dominant)

• Mechanical noise from bearings and gears

• Airflow turbulence

• Radiated noise from casing

Typical Noise Levels

Noise level reference table:

Noise vs pressure:

Noise vs size:

• Small blowers (20 HP): 80–85 dBA

• Medium blowers (50 HP): 85–90 dBA

• Large blowers (100+ HP): 90–100 dBA

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

Frequency characteristics:

• Pulsation: low frequency (pulse frequency)

• Mechanical: mid-frequency (bearing, gear frequencies)

• Airflow: broad spectrum (turbulence)

• Radiated: structure-borne (vibration)

Noise Measurement

Measurement standards:

• ISO 2151: Compressors and vacuum pumps (noise measurement)

• ISO 3744: Sound pressure level measurement

• ISO 9612: Workplace noise measurement

Measurement locations:

• 1 meter from blower

• At operator position

• At property line (if applicable)

Measurement conditions:

• Operating at rated pressure and speed

• Steady-state operation

• Background noise correction

What to measure:

• Overall sound level (dBA)

• Octave band analysis (for silencer selection)

• Peak levels (for hearing protection)

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

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

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

Noise Reduction Strategies

1. Use three-lobe blowers.
Three-lobe is 5–8 dBA quieter than twin-lobe. Helical rotors reduce noise another 5–8 dBA. Select the quietest design for your application.

2. Install silencers on inlet and discharge.
Inlet silencer: 10–15 dBA reduction. Discharge silencer: 15–20 dBA reduction. Total: 15–25 dBA reduction. Required for noise compliance.

3. Use acoustic enclosure.
Additional 10–25 dBA reduction. For noise-sensitive locations. Consider for indoor installations.

4. Locate blower away from workers.
Distance reduces noise. Moving blower outdoors or to a separate room reduces worker exposure. Plan layout for noise reduction.

5. Isolate vibration.
Vibration transmits through foundation. Use neoprene pads or spring isolators. Isolate piping with flexible connectors.

6. Use larger piping.
Lower velocity reduces noise. Larger diameter pipes reduce turbulence noise. Keep velocity below 3,000 ft/min.

7. Maintain equipment.
Worn bearings increase noise. Loose bolts rattle. Worn clearances increase pulsation. Regular maintenance keeps noise down.

8. Consider helical rotors.
Helical rotors reduce pulsation and noise. 5–8 dBA quieter than straight rotors. Worth the premium for noise-sensitive sites.

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

• 140 dBA: Peak limit – instantaneous

EU noise standards:

• 80 dBA: Action level – hearing protection provided

• 85 dBA: Limit level – hearing protection mandatory

• 87 dBA: Exposure limit – must be reduced

Other standards:

• NIOSH: 85 dBA recommended limit

• ACGIH: 85 dBA TLV

• Local regulations may vary

Compliance checklist:

• Measure noise levels at operator positions

• Install silencers (inlet and discharge)

• Provide hearing protection

• Post warning signs

• Train operators on noise hazards

• Monitor hearing (audiometric testing)

Frequently Asked Questions

1. What is the typical noise level of a roots blower?
Bare blower: 90–100 dBA at 1 meter. With inlet and discharge silencers: 80–88 dBA. With acoustic enclosure: 70–80 dBA. Pressure affects noise – higher pressure = higher noise. Size affects noise – larger blowers are louder.

2. Why are roots blowers so loud?
The primary noise source is pressure pulsation from the discharge backflow. Roots blowers have no internal compression – at discharge, higher-pressure air backflows into the lobe cavity, creating pressure pulses. Mechanical noise from bearings and gears adds to the total.

3. What is the OSHA noise limit for roots blowers?
OSHA requires hearing protection above 85 dBA for 8-hour exposure. A bare roots blower (90–100 dBA) exceeds this limit. Silencers are required to reduce noise to safe levels. Without silencers, operators must wear hearing protection.

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. A bare blower at 95 dBA with both silencers: 70–80 dBA.

5. 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. Both are required for effective noise reduction.

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. Common for indoor installations and noise-sensitive locations.

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. For noise-sensitive applications, choose three-lobe helical.

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. Energy savings plus noise reduction.

9. 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. Correct for background noise.

10. What is the noise difference between 2-lobe and 3-lobe?
3-lobe is 5–8 dBA quieter than 2-lobe at the same pressure and flow. At 8 psig, 2-lobe is 95–100 dBA. 3-lobe is 88–92 dBA. The difference is noticeable – equivalent to halving the perceived noise.

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. 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. Silencers are a small investment compared to hearing loss claims and OSHA fines.

13. 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. Discharge silencers have no foam – inspect baffles.

14. 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.

15. 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. For remote outdoor locations, standard rotors may be acceptable.

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. The cost of silencers is small compared to hearing loss claims.

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.

Measure and monitor. Noise levels vary with pressure and operating conditions. Measure noise at operator positions. Monitor for changes – increasing noise indicates wear or problems. Use data for maintenance decisions.

The bottom line. Roots blower noise level is a critical safety and compliance issue. Zhanggu and other manufacturers provide noise data and silencer options. Specify silencers, use three-lobe designs, and consider acoustic enclosures for sensitive locations. The investment in noise control protects workers and ensures compliance.