Sound power level determination quantifies the total acoustic energy radiated by a noise source in decibels, providing an objective and largely environment-independent indicator of product noise emission. Accredited third-party laboratories perform these measurements in accordance with the ISO 3740 series, the IEC 60704 series, and the corresponding GB/T standards to support CE marking, noise labeling, product development, and regulatory compliance. This article explains the technical principles, applicable sample types, test items, measurement methods, and typical applications of sound power level testing.
Technical Overview
Sound power is the total amount of acoustic energy radiated by a noise source per unit of time, expressed in watts. The sound power level, denoted Lw, is defined as ten times the logarithm to the base 10 of the ratio of the radiated sound power W to the reference sound power W0 of 1 pW (10 to the power of minus 12 watts), expressed in decibels (dB). Because it describes the source itself rather than a single listening point, the sound power level is the internationally preferred quantity for characterizing and comparing the noise emission of machinery, appliances, and equipment.
Unlike sound pressure level, which varies with measurement distance, direction, and the acoustic properties of the surrounding environment, the sound power level is an intrinsic characteristic of a noise source operating under a specified mounting and operating condition. This makes it the standard metric for regulatory noise declarations, limit-value verification, procurement specifications, and benchmarking between competing products.
Test results are typically reported as the A-weighted sound power level LWA in dB(A) and, where required, as octave-band or one-third-octave-band sound power levels over the frequency range of interest, typically 50 Hz to 10 kHz. Depending on the test environment and the measurement method applied, results are classified into three accuracy grades defined in the ISO 3740 framework: Grade 1 (precision method), Grade 2 (engineering method), and Grade 3 (survey method).
The governing framework consists of the ISO 3740 series for sound pressure-based methods, the ISO 9614 series for sound intensity methods, the ISO 11200 series for emission sound pressure level at workstations, and ISO 4871 for the declaration and verification of noise emission values. These are mirrored by Chinese national standards such as GB/T 6881, GB/T 6882, GB/T 3767, GB/T 3768, and GB/T 16404, and supplemented by product-specific test codes such as the IEC 60704 series for household appliances and GB/T 4214.1.
Testing Samples
Household and Similar Electrical Appliances
Washing machines, refrigerators, range hoods, vacuum cleaners, dishwashers, electric fans, air conditioners, air purifiers, hair dryers, and kitchen machines. These products are normally tested in a semi-anechoic or reverberation environment according to IEC 60704-1 and the applicable IEC 60704-2 product part, with mounting, installation, and operating conditions defined by the specific test code.
Power Tools, Lawn and Garden Machinery
Electric drills, angle grinders, circular saws, sanders, screwdrivers, lawn mowers, string trimmers, and chain saws. For equipment covered by the EU Outdoor Noise Directive 2000/14/EC, determination of the A-weighted sound power level is a mandatory conformity requirement and must be marked on the product.
Industrial Machinery and Components
Electric motors, fans, pumps, air compressors, gearboxes, generators, transformers, hydraulic power units, and machine tools. Rotating electrical machines are commonly evaluated using ISO 1680 in combination with an ISO 3744 measurement environment, while large equipment is often measured on the user's site.
Information Technology and Office Equipment
Desktop and notebook computers, servers, printers, copiers, and data storage equipment, tested in accordance with ISO 7779 with declared values reported under ISO 9296.
Large or Fixed Installations Measured In Situ
HVAC systems, cooling towers, air-handling units, refrigeration plants, production lines, and construction machinery that cannot be moved into a laboratory. These are addressed by the in-situ survey and comparison methods of ISO 3746 and ISO 3747, or by the sound intensity technique of ISO 9614.
Testing Items
Depending on the applicable product standard and the target market, the following quantities are determined:
- A-weighted sound power level LWA, expressed in dB(A), as the principal single-number result.
- Octave-band and one-third-octave-band sound power levels, providing the spectral signature of the noise source for engineering analysis.
- Emission sound pressure level at the operator position or other specified positions, determined directly or derived from the sound power level in accordance with ISO 11201 and ISO 11203.
- Declared noise emission values, calculated from measured data together with production spread and measurement uncertainty according to ISO 4871 and the ISO 7574 series.
- Audible tone and impulsive noise characteristics, where the product standard or the customer specification requires tonality assessment.
- Measurement uncertainty of the sound power level, stated for each accuracy grade and frequency band.
Testing Methods
Precision Methods (Grade 1)
ISO 3741 determines sound power in a qualified reverberation room using the direct method or by comparison with a reference sound source calibrated to ISO 6926. ISO 3745 applies to qualified anechoic and semi-anechoic rooms, where the sound field is measured over an enveloping surface around the source. The corresponding Chinese standards are GB/T 6881.1, GB/T 6881.2, GB/T 6881.3, and GB/T 6882.1. Precision methods deliver the lowest measurement uncertainty and are used for type testing, reference measurements, and legal disputes.
Engineering Methods (Grade 2)
ISO 3744, identical in principle to GB/T 3767, is the most widely applied method. The source is installed on a reflecting plane in an essentially free-field environment, and time-averaged sound pressure levels are measured at specified microphone positions on a hemispherical or rectangular-parallelepiped measurement surface. ISO 3743-1 and ISO 3743-2 describe similar engineering-grade procedures in hard-walled and special reverberation test rooms. The environmental correction K2 is limited to 2 dB, and the background noise correction K1 is applied where necessary.
Survey Methods (Grade 3)
ISO 3744's companion survey standard ISO 3746, reflected by GB/T 3768, allows sound power determination in situ without a special test environment, with an environmental correction of up to 7 dB. It is suitable for acceptance checks, on-site diagnostics, and machinery where engineering-grade conditions cannot be achieved.
In-Situ Comparison Method
ISO 3747 uses a calibrated reference sound source operated alongside the machine under test. The sound power level is obtained from the difference between the sound pressure levels produced by the machine and by the reference source, which largely eliminates the influence of the room acoustics.
Sound Intensity Methods
ISO 9614-1, using discrete point measurements, and ISO 9614-2, using scanning over a measurement surface, corresponding to the GB/T 16404 series, determine sound power from the sound intensity vector normal to the surface. Because steady background noise outside the enveloping surface contributes little to the net intensity, these methods are particularly effective for testing large machinery in noisy workshops.
Standard Measurement Procedure
A typical determination proceeds as follows: the source is installed and operated under the conditions prescribed by the applicable test code; a measurement surface of known area S is defined around it; sound pressure levels are sampled at the prescribed microphone positions using a class 1 sound level meter complying with IEC 61672, with octave filtering to IEC 61260 and field calibration to IEC 60942; the background correction K1 and the environmental correction K2 are applied; the surface-averaged sound pressure level Lp is computed on an energy basis; and the sound power level is obtained as Lw = Lp + 10 lg(S/S0), with the reference area S0 equal to 1 square meter. For household appliances, the dedicated procedures of IEC 60704-1 and the relevant IEC 60704-2 parts take precedence, and for IT equipment the procedures of ISO 7779 apply.
Applications
Determined sound power levels serve a wide range of regulatory, commercial, and engineering purposes:
- CE marking under the EU Machinery Directive 2006/42/EC, which requires the A-weighted emission sound pressure level and, above specified thresholds, the A-weighted sound power level to be declared in the instructions for use.
- Conformity assessment under the EU Outdoor Noise Directive 2000/14/EC, which sets noise limit values and marking requirements for equipment such as compressors, concrete breakers, and lawn mowers.
- Compliance with EU Ecodesign implementing measures, for example noise limits for household vacuum cleaners and noise declarations for fans.
- Compliance with national limit standards such as GB 19606 for the noise of household and similar electrical appliances.
- Support for eco-label and energy-label programs, green building assessment, and corporate environmental reporting.
- Product development activities such as noise source ranking, benchmarking against competitor products, verifying the effectiveness of acoustic design measures, and quality control in serial production.
- Procurement specifications, tender documents, and customer acceptance criteria in industrial and infrastructure projects.
FAQ
What is the difference between sound power level and sound pressure level?
Sound pressure level describes the noise at one specific point and changes with distance, direction, and surroundings. Sound power level describes the total acoustic energy radiated by the source and is essentially independent of where it is measured, which is why regulations and product standards use sound power level for declaring and comparing noise emission.
Which method should be selected for my product?
The choice depends on the product type, its size, the required accuracy grade, and the available test environment. Household appliances generally follow IEC 60704 or GB/T 4214.1; general machinery is commonly tested to ISO 3744 or GB/T 3767 at engineering grade; precision requirements call for ISO 3745 or ISO 3741; and equipment that cannot be brought to a laboratory can be assessed in situ to ISO 3746, ISO 3747, or ISO 9614. Accredited third-party laboratories can confirm the applicable standard during a pre-test consultation.
Can the measurement be performed at the installation site?
Yes. The survey method of ISO 3746, the comparison method of ISO 3747, and the sound intensity methods of ISO 9614 are all designed for in-situ use, provided that background noise is sufficiently stable and the environmental correction limits of the respective standard are respected.
What accuracy grade is required for EU market declarations?
Engineering grade measurements to ISO 3744 are generally expected for Machinery Directive declarations, while the Outdoor Noise Directive 2000/14/EC specifies the applicable measurement standards and conformity procedures for each equipment category. Declared values must account for uncertainty and production spread in accordance with ISO 4871.
What information should be provided before commissioning a test?
Typical requirements include the product description, dimensions and mass, intended mounting and installation arrangement, the operating modes and load conditions to be tested, the target market and applicable standard, the number of samples available, and any declaration or verification scheme to which the results will be submitted.
How is the declared noise emission value obtained from the measurement?
The measured sound power levels of one or more units are statistically evaluated together with the measurement uncertainty, and the declared value is derived according to ISO 4871 and the ISO 7574 series so that it reliably represents the noise emission of the production batch rather than a single individual machine.