Oil mist purifier noise testing measures the A-weighted sound pressure level and sound power level emitted by oil mist collectors under standardized operating conditions, providing objective evidence for product acceptance, procurement specifications, and occupational health compliance. This article explains the applicable standards (ISO 3744, ISO 3746, ISO 11201 and their GB/T equivalents), typical test samples, measured parameters, instrumentation, measurement procedures, and common acceptance criteria for oil mist purification equipment.
Technical Overview
Oil mist purifiers, also known as oil mist collectors or oil mist separators, are air-cleaning devices widely installed on CNC machine tools, grinding machines, gear-cutting equipment, and other metalworking systems to capture airborne coolant mist, smoke, and sub-micron oil aerosols. Because these units run continuously in production halls, often only a few meters from machine operators, their noise emission directly affects workshop acoustics, occupational health compliance, and customer acceptance of the equipment. Noise measurement, or noise determination, quantifies the acoustic output of a purifier under defined operating conditions and supplies objective data for type testing, quality control, tender documentation, and workplace noise-risk assessment.
Two acoustic quantities are typically reported. The A-weighted sound pressure level (LpA, expressed in dB(A)) describes the noise perceived at a stated distance, usually 1 m, from the machine surface, while the A-weighted sound power level (LwA) is an intrinsic property of the noise source that is independent of the measurement environment and allows comparison between different products. Determination is normally carried out by accredited third-party laboratories using Class 1 instrumentation conforming to IEC 61672, in a semi-anechoic room, a reverberation room, or a sufficiently quiet open test area that satisfies the environmental requirements of the selected standard.
Testing Samples
Noise characterization can be performed on different product configurations, and the sample preparation must reflect the intended installation condition as closely as possible.
Standalone Cabinet-Type Oil Mist Purifiers
Complete free-standing units with built-in centrifugal fans and multi-stage mechanical or electrostatic filtration stages, tested on a reflective floor with inlet and outlet conditions representative of normal field installation.
Machine-Mounted (On-Board) Purifier Units
Compact units designed to be fitted directly onto CNC machining centers or grinding machines. These may be tested individually on a rigid bench to determine unit noise emission, or together with the host machine tool to evaluate their contribution to the overall machine noise level.
Ducted Centralized Filtration Systems
Larger units serving several machines through ductwork, for which casing-radiated noise, fan outlet noise, and structure-borne noise transmitted through the mounting frame may each need to be measured at designated positions.
Electrostatic and Combined ESP/HEPA Units
Units equipped with high-voltage electrostatic precipitator cells and HEPA final stages, in which fan noise usually dominates, although high-frequency components associated with the power supply may justify supplementary frequency analysis.
Testing Items
The scope of measurement typically includes the following items, which may be selected individually or combined depending on the purpose of the test:
- A-weighted equivalent continuous sound pressure level (LpAeq) at 1 m distance from the unit surface
- A-weighted sound power level (LwA) of the complete unit
- Octave-band spectra from 63 Hz to 8 kHz, or one-third-octave-band spectra from 50 Hz to 10 kHz
- Sound pressure level at the operator position for occupational exposure evaluation
- Prominent discrete tone (tonality) assessment where a clearly audible pure tone is present
- Background noise measurement and K1 correction at every microphone position
- Environmental correction K2 verification for the test environment
- Noise levels at different fan speeds or airflow settings, including the maximum-speed worst case
Testing Methods
Reference Standards
The most commonly applied measurement standards are ISO 3744 and GB/T 3767 (engineering-grade method over an essentially free field above a reflecting plane), ISO 3746 and GB/T 3768 (survey-grade method suitable for on-site testing), ISO 3745 and GB/T 6882 (precision method in anechoic or semi-anechoic rooms), and ISO 3741 (precision method in a reverberation room). Sound pressure levels at the workstation are determined according to ISO 11201 or GB/T 17248.2, and declared noise emission values are formatted per ISO 4871 and GB/T 14574. Evaluation against occupational limits generally references ISO 1999 and GBZ 2.2, which establish 85 dB(A) as the typical 8-hour time-weighted exposure limit.
Instrumentation
Measurement equipment includes a Class 1 integrating-averaging sound level meter conforming to IEC 61672-1, optional real-time octave and one-third-octave filters conforming to IEC 61260, and a Class 1 acoustic calibrator conforming to IEC 60942 that is applied before and after each measurement series. A calibrated reference sound source conforming to ISO 6926 and GB/T 4129 is used when the environmental correction must be determined. Ambient temperature, humidity, and atmospheric pressure are recorded with calibrated instruments.
Test Environment and Corrections
Background noise at each microphone position must be at least 6 dB below the level measured with the purifier operating; a difference of 15 dB or more eliminates the need for correction. The background correction K1 and the environmental correction K2 are calculated according to the selected standard, with K2 limited to 2 dB for engineering-grade determinations and 7 dB for survey-grade determinations.
Measurement Procedure
- Mount the sample on a hard reflecting plane using its normal supports, and install accessories, filters, and duct connections as specified by the manufacturer.
- Operate the unit at rated voltage and frequency with the fan at maximum speed, and allow at least 10 minutes of stabilization before recording data.
- Select an enveloping measurement surface, typically a hemisphere of radius equal to at least twice the largest source dimension, or a rectangular parallelepiped at a reference distance of 1 m.
- Distribute microphones over the measurement surface at standard key positions defined by the standard, with microphone height of at least 1 m above the reflecting plane.
- Use an averaging time of at least 30 seconds per position, and repeat measurements at each point, including background readings taken before and after the test series.
- Calculate the surface-averaged sound pressure level and derive the sound power level using LwA = LpA + 10 lg(S/S0), where S is the measurement surface area and S0 equals 1 m².
- Report the measurement uncertainty, with a standard deviation of approximately 1.5 dB(A) for ISO 3744 determinations and approximately 3 dB(A) for ISO 3746 survey determinations.
Acceptance Criteria
Typical procurement and factory specifications require an A-weighted sound pressure level of no more than 75 dB(A) to 80 dB(A) at 1 m for general industrial units, while low-noise models are frequently specified at 70 dB(A) or below. Regardless of the unit rating, the combined noise at the operator position must remain compatible with the 85 dB(A) daily exposure limit applied in most jurisdictions. Acceptance values are normally declared as a double-number pair of LwA and LpA in accordance with ISO 4871.
Applications
Noise determination results for oil mist purifiers are used across the complete product life cycle, from research and development to installation acceptance:
- Type testing and performance certification of new purifier models before market release
- Noise benchmarking of competing fan, filtration, and casing designs during product development
- Batch quality control checks in serial production to detect fan imbalance, bearing defects, or assembly faults
- Tender submissions and procurement verification for CNC machining centers, gear hobbing and shaping machines, grinding and honing machines, cold-heading and thread-rolling lines, and heat-treatment quench installations
- Workplace noise mapping and occupational risk assessment in machining workshops
- Verification of retrofit silencers, acoustic enclosures, or low-noise fan upgrades
- Export documentation and regulatory submissions supported by accredited third-party laboratories
FAQ
What noise level is considered acceptable for an oil mist purifier?
Most industrial specifications accept 75 dB(A) to 80 dB(A) measured at 1 m, while premium low-noise units achieve 70 dB(A) or less. The governing requirement in practice is that the total noise at the operator position, including the host machine tool, remains below the occupational exposure limit of 85 dB(A) as an 8-hour average.
Which standard should be chosen for the measurement?
Use ISO 3744 or GB/T 3767 when a sound power level with engineering-grade accuracy is required, for example for product labeling or export. Use ISO 3746 or GB/T 3768 for quicker on-site survey measurements, and ISO 11201 when the objective is to assess noise at the operator workstation rather than the machine emission itself.
Can the test be performed in the customer workshop?
Yes, provided that the background noise is sufficiently low and the environmental correction K2 stays within the limit allowed by the survey method. Where these conditions cannot be met, the unit should be tested in a laboratory semi-anechoic or reverberation room.
How many samples are required and how long does testing take?
One representative unit per model is generally sufficient for type testing. A complete test sequence, including setup, stabilization, multiple microphone positions, background measurements, and reporting, typically requires one to two working days.
What information is contained in the test report?
The report identifies the unit and its operating conditions, the applied standards, the instrumentation with calibration traceability, the measurement surface and microphone arrangement, K1 and K2 corrections, the measured LpA and LwA values, frequency spectra where applicable, the stated uncertainty, and the ambient conditions during testing.
Must the test be performed by accredited third-party laboratories?
Internal laboratory measurements are adequate for routine quality control and development work. However, regulatory submissions, public tenders, customer acceptance disputes, and export documentation usually require test reports issued by accredited third-party laboratories holding ISO/IEC 17025 accreditation in the acoustic field.
