Compressor operating noise measurement is a specialized acoustic test that quantifies the sound pressure level and sound power level emitted by a compressor under defined, reproducible operating conditions. Performed in accordance with internationally recognized standards such as ISO 2151, ISO 3744 and GB/T 4980, this testing enables manufacturers to verify noise specifications, optimize low-noise designs and demonstrate regulatory compliance. This guide explains the applicable standards, sample types, test items, measurement methods and typical applications of compressor noise testing.
Technical Overview
Compressors generate noise through several interacting mechanisms, including intake air pulsation, gas flow through valves and ports, mechanical impacts such as valve closing and gear meshing, electromagnetic motor noise, bearing noise, and structure-borne vibration radiated by the casing, piping and mounting frame. Because these mechanisms combine in complex ways, the acoustic performance of a compressor can only be assessed reliably through a standardized measurement of its operating noise.
Compressor operating noise measurement determines the A-weighted sound pressure level (LpA) at specified microphone positions and derives the A-weighted sound power level (LWA), which characterizes the machine as an intrinsic noise source independent of the measurement environment. Frequency spectra in octave or one-third-octave bands are typically recorded to identify dominant sources such as blade-pass tones, valve impacts or gear mesh frequencies.
The principal governing documents include ISO 2151 (the noise test code for compressors and vacuum pumps), the ISO 3740 series (particularly ISO 3744 for the engineering-grade sound pressure method), ISO 3741 and ISO 3743 for reverberation room methods, ISO 9614 for sound intensity methods, and the Chinese national standard GB/T 4980 for the determination of sound power levels of noise emitted by positive displacement compressors. Instrumentation is verified against IEC 61672-1 for sound level meters, IEC 60942 for acoustic calibrators and IEC 61260 for octave-band filters. Depending on the environment and method applied, results are classified as precision grade (Grade 1), engineering grade (Grade 2) or survey grade (Grade 3).
Testing Samples
Reciprocating (Piston) Compressors
Single-acting and double-acting piston compressors, including air-cooled and water-cooled types, oil-lubricated and oil-free designs, and portable single-stage or two-stage units. Samples are mounted on a rigid base over an acoustically reflecting plane and tested at rated speed, rated discharge pressure and rated supply voltage.
Screw Compressors
Oil-injected and oil-free rotary screw compressor packages, both bare-shaft and complete boxed units with enclosures, coolers and separation equipment. Packaged units are tested in their delivered configuration, since the enclosure and internal silencing strongly influence the radiated noise.
Scroll and Rotary Vane Compressors
Compact rotary machines widely used in air conditioning, refrigeration, medical air systems and laboratory applications. These units are typically bench-mounted on resilient supports, and their low noise output requires Class 1 instrumentation and low background noise in the test environment.
Centrifugal (Dynamic) Compressors
High-speed turbo and integrally geared centrifugal compressors, tested at rated speed, pressure ratio and flow point. Aerodynamic broadband noise and high-frequency blade tones are characterized using spectral analysis in addition to overall A-weighted levels.
Refrigeration and Hermetic Compressors
Hermetic, semi-hermetic and open-type refrigerant compressors evaluated under representative suction and discharge conditions, in line with the noise clauses of the relevant product standards and purchaser specifications.
Testing Items
A comprehensive compressor noise test program typically covers the following quantities:
- A-weighted sound pressure level (LpA) at specified microphone positions, commonly 1 m from the reference surface of the machine
- A-weighted sound power level (LWA) of the complete compressor package
- Octave-band and one-third-octave-band frequency spectra, typically covering 63 Hz to 8 kHz or wider
- Identification and quantification of prominent discrete tones (tonality assessment)
- Intake (inlet) noise and discharge or exhaust noise where relevant to the machine configuration
- Background noise level and environmental correction values (K1 and K2)
- Simultaneously recorded operating parameters, including rotational speed, discharge pressure, flow rate, power input, and gas temperatures
- Measurement uncertainty for the reported sound power level
Testing Methods
Sample Preparation and Operating Conditions
The compressor is installed on an acoustically reflecting plane in accordance with the manufacturer's instructions, connected to a suitable load system, and operated at rated conditions until thermal and mechanical steady state is reached. Auxiliary equipment that is not part of the delivered package is either acoustically shielded or excluded from the measurement surface, and its contribution is treated as background noise.
Engineering-Grade Sound Pressure Method (ISO 3744 / ISO 2151 / GB/T 4980)
A hemispherical or rectangular (parallelepiped) measurement surface is established around the reference box enclosing the source, normally at a distance of 1 m. Sound pressure levels are sampled at the standard microphone array defined by the standard, from which the surface-averaged sound pressure level is calculated. The background noise correction K1 and the environmental correction K2 are applied, and the sound power level is obtained by adding the logarithmic term for the measurement surface area. For valid engineering-grade results, the background noise should be at least 6 dB below the combined level, and the environmental correction K2 should not exceed 2 dB.
Survey-Grade Method (ISO 3746)
Where a qualified acoustic environment is unavailable, a simplified measurement with fewer microphone positions may be performed on-site. The method tolerates larger environmental corrections (up to 7 dB) and delivers results with survey-grade accuracy, suitable for comparative and acceptance purposes.
Reverberation Room Method (ISO 3741 / ISO 3743)
Smaller compressors may be tested in a qualified reverberation room using the comparison method with a reference sound source calibrated per ISO 6926. This precision-grade approach provides accurate sound power levels and spectral data without requiring free-field conditions, although it does not yield directivity information.
Sound Intensity Method (ISO 9614-1 / ISO 9614-2)
For large packaged compressors that cannot be moved into a laboratory, sound intensity probes scanned over a closed enveloping surface determine the sound power in situ. This method is inherently less sensitive to steady background noise and is validated through pressure-intensity index and field indicator checks prescribed by the standard.
Instrumentation and Quality Assurance
All measurements employ Class 1 integrating sound level meters and real-time frequency analyzers meeting IEC 61672-1 and IEC 61260, verified with an acoustic calibrator conforming to IEC 60942 immediately before and after each measurement series. Ambient temperature, relative humidity, barometric pressure and wind conditions are recorded, and measurements are corrected or repeated when environmental limits are exceeded.
Data Processing and Reporting
The test report documents the installation and operating conditions, instrumentation with calibration status, measurement surface geometry and microphone positions, all correction values, the A-weighted sound power level with its measurement uncertainty, and the frequency spectra. Typical standard deviations of reproducibility are approximately 0.5 to 1.5 dB for precision and engineering grades and up to 3 dB for survey grade.
Applications
Compressor operating noise measurement data support a wide range of engineering and commercial objectives:
- Research and development benchmarking, low-noise design validation, and ranking of silencers, enclosures and vibration isolation measures
- Production quality control and batch-to-batch acoustic consistency checks
- Demonstration of compliance with occupational health noise exposure limits and plant boundary (environmental) noise regulations
- Product data sheets, tenders, and acceptance testing against purchaser or contractual noise specifications
- Support for certifications, labels and market-access documentation prepared with accredited third-party laboratories
- Acoustic input data for room and plant acoustic calculations, including predicted sound pressure levels at workstations and neighboring properties
- Application-specific verification in HVAC and refrigeration, medical and dental air systems, laboratory air supply, and household appliances where quiet operation is a key selling point
FAQ
What noise level is considered acceptable for a compressor?
Acceptance limits depend on the machine type, size and application. As an indicative range, small hermetic and scroll compressors typically emit 55 to 70 dB(A) at 1 m, packaged oil-injected screw compressors around 65 to 85 dB(A), and large industrial reciprocating machines 85 dB(A) or more. Many specifications also reference workplace exposure limits of 85 dB(A) over an eight-hour shift.
What is the difference between sound pressure level and sound power level?
Sound power level, expressed in decibels relative to 1 picowatt, is an intrinsic property of the noise source and does not depend on distance or surroundings, making it the preferred figure for comparing products and performing acoustic calculations. Sound pressure level, expressed in decibels relative to 20 micropascals, varies with measurement distance, direction and the room, so it must always be quoted together with the measurement position.
Which standard should be selected for my compressor?
ISO 2151 and GB/T 4980 are the dedicated noise test codes for compressors and positive displacement compressors respectively. ISO 3744 is applied where a qualified free-field environment over a reflecting plane is available, ISO 3746 for survey-grade on-site checks, and ISO 9614 for large packages measured in situ. An accredited third-party laboratory will recommend the most appropriate standard based on the machine size, accuracy requirement and intended use of the data.
How is background noise dealt with during testing?
Background noise is measured with the compressor shut down at each microphone position. If the difference between the combined level and the background level is at least 6 dB, a standard correction K1 is applied with full validity; for differences between 3 dB and 6 dB, corrections are applied but the accuracy is downgraded, and differences below 3 dB generally invalidate the measurement at that position.
How long does a compressor noise test take?
A typical single-condition test, including installation, warm-up, measurement and processing, can be completed within half a day to one full day in a laboratory. Multi-condition test matrices covering several speeds, pressures or load points require additional time, and the formal report is normally issued within a few working days after the measurements.
Can the measurement be performed at the customer's site?
Yes. Survey-grade sound pressure measurements per ISO 3746 and sound intensity measurements per ISO 9614 are specifically designed for in-situ use, allowing sound power determination in factory halls or installed plant environments where a laboratory test is impractical.