🌿 Sound power level of machines — survey method (accuracy grade 3)

Environment PN-EN ISO 3746

Overview

The standard belongs to the series ISO 3741–ISO 3747 specifying methods for determining the sound power levels and the sound energy levels of noise sources — machines, equipment and their subassemblies; the choice of the method is made in accordance with ISO 3740 depending on the available test environment and on the accuracy required. The sound power emitted by the source is calculated from the mean square sound pressure measured on a hypothetical measurement surface enveloping the source and from the area of that surface; the sound energy of a single event — from the power and the duration of the event. ISO 3746 gives results of accuracy grade 3 (survey) in accordance with ISO 12001:1996 for measurements in industrial halls or in the open air; for sources standing on the floor of halls, corrections have been defined for unwanted reflections from nearby objects, walls and the ceiling and for background noise. The method makes it possible to determine the sound power level and the sound energy level with the use of the A frequency weighting; where greater accuracy is needed, ISO 3744, ISO 3745 or ISO 9614 is applied.

The edition ISO 3746:2010 (the third) replaced ISO 3746:1995 and incorporated the corrigendum Cor.1:1995. The Polish version PN-EN ISO 3746:2011 has the corrigendum Ap1:2017-09 (KT 157), which corrects the area of the measurement surface for a source at a wall (S = πr²) and in a corner (S = πr²/2). The standard applies to all kinds of noise (steady, non-steady, fluctuating, single bursts of energy) and to sources of all types and sizes — stationary or slowly moving installations, machines, assemblies — provided that the conditions of measurement can be met; for very high or long sources (stacks, ducts, conveyors) the conditions may be impractical.

Method principle

The source is installed on a sound-reflecting plane (or on several planes) as in normal use, at a sufficient distance from walls, the ceiling and reflecting objects. Around the source a reference box and a measurement surface are defined — hemispherical or rectangular parallelepiped (at a wall half a hemisphere of an area S = πr², in a corner a quarter of an area S = πr²/2) — with microphone positions or scanning paths distributed over it. The A-weighted sound pressure levels are measured with the source in operation, as is the background noise; the mean level over the surface is corrected by the background noise correction K1 and by the environmental correction K2A determined in accordance with annex A (the measurement is valid only where K2A ≤ 7 dB; at a greater value ISO 3747 or ISO 9614 is applied). The sound power level is obtained from the mean sound pressure level so corrected and from the area of the measurement surface; for impulsive or transient noise the sound energy level is determined (reference value 1 pJ). The background noise averaged over the microphone positions must be at least 3 dB lower than the level with the source in operation.

Applications

• Determination of the noise emission of machines and equipment where survey accuracy grade is sufficient
• Measurements of the sound power of equipment installed in production halls and in the open air, where testing in an anechoic chamber is not possible
• Comparison of the noisiness of machines and assessment of the effectiveness of noise reduction under industrial conditions
• Acceptance and periodic testing of fans, compressors, units, pumps and machine tools
• A method connected with the assessment of the emission sound pressure level at the operator's workstation (ISO 11202 — the survey method)

Key parameters

Standard

Standard number

PN-EN ISO 3746:2011 + Ap1:2017-09 (ISO 3746:2010)

Title (PL)

Akustyka — Wyznaczanie poziomów mocy akustycznej i poziomów energii akustycznej źródeł hałasu na podstawie pomiarów ciśnienia akustycznego — Metoda orientacyjna z zastosowaniem otaczającej powierzchni pomiarowej nad płaszczyzną odbijającą dźwięk

Title (EN)

Acoustics — Determination of sound power levels and sound energy levels of noise sources using sound pressure — Survey method using an enveloping measurement surface over a reflecting plane

Step-by-step procedure

1. Test environment

   Choose a room or flat ground isolated from background noise; avoid strong electric and magnetic fields, wind, the stream of air from the source and extreme temperatures. In the open air limit the influence of the meteorological conditions; above 1500 m above sea level correct the results to the reference conditions in accordance with ISO 3744.

2. Installation and operation of the source

   Install the source with respect to the reflecting plane as in normal use, away from walls and reflecting objects; place the auxiliary equipment outside the test environment or include it in the source and extend the reference box. Test under repeatable conditions representative of the noisiest operation: in accordance with the noise test code, and without one — at a specified load, at full load, at idle, at maximum speed, under conditions of maximum emission, with a simulated load or in a characteristic operating cycle; stabilise the source and the temperature of the drive system.

3. Check of the measuring chain

   Before and after the series of measurements apply the class 1 calibrator to each microphone; without adjustment the difference of the readings must not exceed 0.5 dB — otherwise reject the results of the series.

4. Measurement surface and microphone positions

   Define the reference box and the measurement surface (hemispherical or rectangular parallelepiped) and distribute the microphone positions or the scanning paths in accordance with annexes B and C; for a source at a wall or in a corner use respectively half or a quarter of the hemisphere.

5. Measurements

   Measure the A-weighted sound pressure levels at each position with the source in operation and the background noise with the source not operating; check the background criterion (≥ 3 dB) and determine K2A in accordance with annex A (K2A ≤ 7 dB is required).

6. Calculations and test report

   Calculate the mean sound pressure level over the surface with the corrections K1 and K2A, and from it and from the area of the measurement surface — the A-weighted sound power level (for impulsive events — the sound energy level). In the test report state the conditions of installation and of operation of the source, the measurement surface, the corrections, the apparatus and information on the uncertainty (clause 9, annex D).

Required equipment and apparatus

Health and safety (OHS)

• Measurements at machines in operation under conditions of maximum emission — use hearing protectors and keep to the safety zones around moving parts
• Distribute the tripods and the microphone cables so that they do not create a risk of tripping and do not enter the working zone of the machine
• In the open air protect the microphones with windshields and break off the measurements during precipitation

Frequently asked questions