The short answer. A screw compressor in its acoustic canopy typically produces about 63 to 80 dB(A) at 1 m; an open piston compressor 80 to 95 dB(A); an unenclosed roots blower can exceed 95 dB(A). Inside a room, levels rise because walls reflect sound, and two machines add 3 dB, not double. Outside, what matters is the level at the boundary against the zone limit, which at night is 45 dB(A) in residential areas and 40 dB(A) in silence zones. Noise can usually be cut by 10 to 20 dB(A) with enclosures, silencers, isolation and treated openings, as long as the cooling air the compressors need still gets through.
Who this page is for: plant engineers planning a compressor house, factory managers dealing with a neighbour’s complaint or an inspection, safety officers responsible for workers’ hearing, and owners of workshops and garages with compressors close to people.
Key facts
- dB(A) is sound pressure level weighted to match the human ear. Every 10 dB(A) increase sounds roughly twice as loud.
- Two equal sources together are 3 dB louder than one. Ten equal sources are 10 dB louder.
- In open air, from a compact source, the level falls by about 6 dB each time the distance doubles.
- Ambient noise limits in India under the Noise Pollution (Regulation and Control) Rules, 2000, in dB(A) Leq, day / night: industrial 75 / 70, commercial 65 / 55, residential 55 / 45, silence zone 50 / 40. Day is 6 am to 10 pm and night 10 pm to 6 am. Silence zones are areas up to 100 m around hospitals, educational institutions and courts, declared by the competent authority.
- For workers, 85 dB(A) over an 8-hour day is the widely used good-practice level at which hearing protection and noise control should start. Factory rules set legal limits; the good-practice level is lower.
- Most compressor noise reduction fails not on acoustics but on heat: a room sealed for silence overheats. Every treatment must keep the cooling airflow the machines need.
How loud is each type of compressor?
Figures below are typical ranges in free field at 1 m from the package, for machines in good condition and under load. Makers quote measured values on data sheets, often to a stated test standard and sometimes at 7 m for portable units. Use the data sheet for your machine where you have it.
| Machine | Typical sound level | Main noise sources |
|---|---|---|
| Single-stage piston, open, belt-driven, 2 to 7.5 kW | 80 to 90 dB(A) at 1 m | Intake pulsation, valve and crank mechanical noise, belt and fan |
| Two-stage piston, open, 11 to 30 kW | 85 to 95 dB(A) at 1 m | Intake, cylinder valves, cooling fan, base vibration |
| Piston in a factory acoustic enclosure | 65 to 75 dB(A) at 1 m | Cooling air openings, structure-borne vibration |
| Oil-injected screw in canopy, 5.5 to 37 kW | 63 to 75 dB(A) at 1 m | Cooling fan, airend, intake |
| Oil-injected screw in canopy, 45 to 250 kW | 68 to 80 dB(A) at 1 m | Cooling fan, motor fan, airend |
| Oil-free screw in canopy | 70 to 80 dB(A) at 1 m | Two-stage airend, gearbox, coolers |
| Oil-free scroll | 55 to 65 dB(A) at 1 m | Low; mainly fan |
| Portable diesel, silenced canopy, 100 to 1,000 cfm (2.8 to 28.3 m³/min) | Typically quoted about 70 to 80 dB(A) at 7 m | Engine exhaust, cooling fan, engine mechanical noise |
| Roots blower, unenclosed | 90 to 105 dB(A) at 1 m | Pulsation at the inlet and outlet, casing |
| Roots blower in acoustic hood with silencers | 70 to 85 dB(A) at 1 m | Openings, pipework radiating downstream |
| Blowdown or unloading exhaust without a silencer | Short bursts, often 90 to 100 dB(A) or more | Release of compressed air |
Piston machines are the loudest per kW and the hardest to quieten because their noise is impulsive and their base vibration travels into floors and walls. Screw machines in canopies are far quieter, which is one reason they replaced piston machines in many plants. Blowers deserve special attention: their low-frequency pulsation passes through light walls and travels a long way.
The arithmetic: adding sources and allowing for distance
Adding levels
Decibels do not add arithmetically. The combined level of several sources is L = 10 × log10(10^(L1/10) + 10^(L2/10) + …). Useful shortcuts:
- Two equal levels: add 3 dB. 80 dB(A) and 80 dB(A) combine to 83 dB(A).
- Levels 6 dB apart: add about 1 dB to the louder. 80 + 74 ≈ 81 dB(A).
- Levels 10 dB apart: the quieter source adds almost nothing. 80 + 70 ≈ 80.4 dB(A).
So treating the loudest source always comes first. Silencing a quiet machine next to a loud one changes nothing anyone can hear.
Distance
Outdoors, from a source that is small compared with the distance, L2 = L1 − 20 × log10(r2 ÷ r1). Going from 1 m to 2 m loses 6 dB; to 10 m, 20 dB; to 30 m, about 29.5 dB. This works for a canopy or a louvre once you are a few times its largest dimension away. It does not work indoors, where reflected sound from walls and ceiling keeps the level high across the room. In a bare concrete compressor house, the reverberant level can be 5 to 10 dB above what the free-field rule predicts.
Inside the compressor house: room acoustics and workers
A compressor room with hard walls, a concrete floor and a sheet roof is a reverberant box. Sound builds up, and anyone working inside is exposed to the combined, reflected level of every machine. Two open piston compressors each giving 88 dB(A) at the working position combine to about 91 dB(A). On the widely used 3 dB rule, each 3 dB increase halves the allowable daily exposure: 85 dB(A) for 8 hours is equivalent to 88 dB(A) for 4 hours or 91 dB(A) for 2 hours. A maintenance technician spending a full shift in that room needs hearing protection, and the room needs treatment.
Inside treatments that work:
- Sound-absorbing panels on walls and under the roof cut the reverberant level by about 3 to 8 dB. They do nothing for the direct sound near a machine.
- Acoustic enclosures around piston machines and blowers cut direct sound by 10 to 20 dB, provided the cooling airflow is designed in.
- A separate control or operator room with a window, so people are not inside the compressor house for long periods.
- Isolation: anti-vibration mounts under piston machines and blowers, and flexible connections on pipes, stop the floor and pipework becoming loudspeakers.
Outside: boundary levels and neighbours
For neighbours and inspections, the level that counts is at the site boundary, compared with the limit for the zone, and the night limit is the hard one. The main ways compressor noise leaves a building are, in order of usual importance: open louvres and intake and exhaust openings; the roof, if it is light sheeting; doors left open; and pipework or ducts that radiate outside.
Worked example (illustrative): a compressor house near housing
A factory compressor house holds three screw compressors in canopies, giving 75, 75 and 72 dB(A) at 1 m. The house has an intake louvre about 1 m square facing a residential boundary 15 m away. The site runs two shifts, until 11 pm.
- Combined level of the three machines: 10 × log10(10^7.5 + 10^7.5 + 10^7.2) = 10 × log10(79.1 × 10^6) = 79.0 dB(A) near the group.
- Measured outside, 3 m in front of the louvre on its centre line: 68.5 dB(A). The reading is taken a few louvre widths away because the distance rule does not hold close to an opening. (The room barely reduces the level reaching an open louvre.)
- At the boundary, 15 m from the louvre: 68.5 − 20 × log10(15 ÷ 3) = 68.5 − 14.0 = 54.5 dB(A).
- Against the residential limits: below the 55 dB(A) day limit, but 9.5 dB(A) over the 45 dB(A) night limit, and the second shift runs until 11 pm.
- Fix: replace the plain louvre with an acoustic louvre and add an attenuator in the intake path, together about 12 dB of reduction, sized so the added pressure drop stays within what the compressor fans can overcome. Boundary level becomes about 54.5 − 12 = 42.5 dB(A), within the night limit with a small margin.
Note what was not done: the louvre was not closed. Closing it would have met the noise limit for a week and then shut the compressors down on high temperature.
How to quieten a compressor installation without killing the cooling
| Treatment | Typical reduction | Watch out for |
|---|---|---|
| Factory acoustic canopy or enclosure | 10 to 25 dB(A) | Enclosure must have engineered cooling air paths and fan capacity |
| Intake silencer on piston compressors and blowers | 5 to 15 dB(A) at the intake | Pressure drop; clean and replace filter elements |
| Blowdown and unload exhaust silencer | 10 to 20 dB(A) on bursts | Blocked or oil-soaked silencers raise back-pressure |
| Acoustic louvres and duct attenuators | 8 to 20 dB(A) through the opening | Pressure drop must stay within fan capability; size for full cooling airflow |
| Absorptive wall and roof lining | 3 to 8 dB(A) reverberant level inside | Use oil-resistant, cleanable facings; no help to direct sound |
| Anti-vibration mounts and flexible pipe connections | Large cut in structure-borne noise | Correct mount selection for weight and running speed |
| Barrier wall outside | 5 to 15 dB(A) behind the wall | Must break line of sight; reflections from nearby buildings |
| Moving loud machines away from the boundary or behind the building | About 6 dB per doubling of distance | Pipe runs and pressure loss |
| Replacing old open piston machines with canopy screw machines | Often 15 to 20 dB(A) at the machine | A capital decision; energy and reliability usually improve too |
Ventilation and silencing must be designed together. A canopy screw compressor’s cooling fan can typically overcome only a limited extra pressure drop in ducts, louvres and attenuators, given by the maker. Exceed it and the machine runs hot. Use our compressor room ventilation worksheet and the compressor room installation guide alongside any noise plan, and see thermal performance and overheating for what happens when a room runs too hot.
Measuring compressor noise properly
- Use a sound level meter set to A-weighting and slow response, checked with a calibrator before and after.
- Measure with the machines at normal load, and separately during unload and blowdown, which may be the loudest short events.
- For a machine figure, measure at 1 m from each side at about 1.5 m height and average; compare with the data sheet.
- For the boundary, measure at the boundary at the time that matters, usually night, and record background noise with the compressors off if you can.
- Record positions, times, readings, load state and weather. A simple sketch with distances is worth more than a long description.
Exceptions and cautions
- The free-field distance rule does not apply indoors, close to large openings, or where buildings reflect sound. Treat its results as estimates.
- Low-frequency noise from blowers and large piston machines travels further and passes through light walls more easily than A-weighted figures suggest. Neighbours may complain about a hum even when dB(A) readings look acceptable.
- Do not remove or bypass a canopy’s internal ducting or seals to improve cooling; that often raises noise and can make cooling worse.
- Hearing protection is the last line of defence, not a substitute for noise control at the source.
- The zone limits on this page are the ambient standards in the schedule to the Rules. Your site’s consent conditions or the local authority may set specific requirements, and separate limits apply to some equipment, such as DG sets.
Compressor noise survey sheet
| Item | Value | Notes |
|---|---|---|
| Date, time, weather | ||
| Meter and calibrator check before / after (dB) | ||
| Machines running and load state | ||
| Level at 1 m from each machine, loaded (dB(A)) | ||
| Level during unload or blowdown (dB(A)) | ||
| Level at operator positions inside the room (dB(A)) | ||
| Time people spend in the room per day (hours) | ||
| Level outside each louvre or opening at 1 m (dB(A)) | ||
| Distance from each opening to the boundary (m) | ||
| Level at the boundary, compressors running (dB(A)) | ||
| Background at the boundary, compressors off (dB(A)) | ||
| Zone category and applicable day / night limit (dB(A)) | ||
| Room temperature at the compressor intakes (°C) | ||
| Proposed treatments and expected reduction |
Common mistakes
- Closing louvres and doors to cut noise, and then chasing high-temperature trips.
- Treating the quiet machine first. The loudest source decides the total.
- Using the outdoor distance rule inside a room and underestimating worker exposure.
- Ignoring blowdown bursts, which are often what neighbours actually complain about.
- Bolting piston compressors and blowers rigidly to a suspended floor, turning the building into a sounding board.
- Buying attenuators by size alone without checking the pressure drop at full cooling airflow.
How Reddy Compressors helps
Noise and cooling are two sides of the same compressor-house design, so we look at them together. Reddy Compressors has sold, rented, serviced and supplied spare parts since 2002 for screw, piston and oil-free compressors and blowers of all makes, as an independent multi-brand business. Our service engineers check noise sources and ventilation on site, replace silencers and anti-vibration mounts, and help plants choose quieter machines when replacement makes sense. While work is done, our own rental fleet, from 50 to 2,000 cfm (about 1.4 to 56.6 m³/min) and 2 to 30 bar, can keep air on. We cover Telangana, Andhra Pradesh and the districts within about 300 km of Hyderabad, with supply across India.
What to send us
- Photographs of the compressor and blower nameplates and of the compressor house, including louvres, doors and the boundary side.
- A simple sketch with distances from the machines and openings to the boundary and to the nearest houses or offices.
- Any noise readings, the time they were taken, and the nature of any complaint.
Use our equipment enquiry page or WhatsApp 99590 60718 or 92468 88812.
Frequently asked questions
How loud is a typical screw compressor?
In its acoustic canopy, a small to medium oil-injected screw compressor is typically about 63 to 75 dB(A) at 1 m, and larger machines about 68 to 80 dB(A). The data sheet for your machine gives its measured value.
Why is my piston compressor so much louder than a screw compressor?
Piston machines produce impulsive noise from intake pulsation and valves, and their base vibration travels into the floor and walls. Open piston machines typically reach 80 to 95 dB(A) at 1 m, against about 63 to 80 dB(A) for screw machines in canopies.
What is the noise limit at the factory boundary?
The Noise Pollution (Regulation and Control) Rules, 2000 set ambient limits by category of area, in dB(A) Leq: 75 / 70 day / night in industrial areas, 65 / 55 in commercial, 55 / 45 in residential and 50 / 40 in silence zones, with day from 6 am to 10 pm. Where a factory adjoins housing, the limit for the neighbouring area is usually the one that matters in a complaint. Your consent conditions may add specific requirements.
How much quieter is it if I move the compressor further away?
Outdoors, about 6 dB quieter for every doubling of distance from a compact source. Indoors the reduction is much smaller because walls reflect the sound.
Can I just close the louvres to reduce noise?
No. The compressors need that cooling air. Closing louvres raises room temperature and leads to high-temperature trips. Use acoustic louvres and attenuators sized for the full cooling airflow instead.
Do workers in the compressor room need hearing protection?
If the level they are exposed to over the day reaches about 85 dB(A), yes, as good practice, together with steps to reduce noise at the source. With two open piston compressors in a bare room, levels around 90 dB(A) are common.
Related pages
- Compressor room installation
- Compressor room ventilation worksheet
- Thermal performance and overheating
- Rotary screw compressors
- Reciprocating compressors
- Roots blowers
- Compressor monsoon care
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 5 November 2026.