Sound power emitted by an object: The amount of energy emitted by that object which propagates through the environment producing instantaneous pressure changes making it vibrate at audible frequencies. It is measured in Watts [W]
Sound power is a parameter that is inherent to mechanical elements or objects. It serves to characterise and compare the sound level emitted by an object. Sound power does not depend on the distance to or from the object emitting the sound being measured as is the case of acoustic pressure. Neither does it depend on the area where the emitting object is located. It exclusively depends on the emitting object itself.
Essentially, sound power can be determined by 4 mechanisms:
- Free field methods: The sound power is determined by the acoustic pressure measurements taken around the emitting object in an area where sound is propagated without any reflection. This is typically conducted outdoors or in partially sound-proofed test rooms where the surfaces are treated to absorb sound waves from the sound pressure they receive.
- Reverberating field methods: This is a variation on the former method which requires a reverberating area in which the surfaces in the test room reflect the sound pressure waves in multiple directions. The acoustic pressure measurement is not taken around the emitting object, but rather within the acoustic field induced by the emitting object in the room.
- Acoustic intensity (the average value over time resulting from acoustic pressure at a single point, and then multiplying the instantaneous speed of the particles at that point.[W/m^2]) assessment around the emitting object.
- Based on the speed of vibration of the surfaces of the emitting object.
At CEIS we determine the power by employing the reverberating field method and intensimetry.
Reverberating field methods:
Special test rooms are required to apply this method. CEIS implements the methods defined in EN ISO 3741:2010
There are two reverberating field methods. The so-called direct method and the comparison method.
When using the direct method, we determine the sound power on the basis of acoustic pressure level measurements in the room when the test object is operating, and from the reverberation time measurements (T10 or T15 extrapolated to obtain T60), which allow us to then determine how the sample itself, the air surrounding it and the surfaces dampen the acoustic energy.
When we use the comparison method, a known acoustic source is used for which there is an sound power calibration chart with emissions under certain conditions. The test consists of comparing average acoustic pressure levels recorded in the test room when the sample is tested while operating, and when only the reference sound emitting source is operating. The difference between the recorded pressure levels in both cases is added to the power emitted by the reference source in test conditions to obtain the power of the test sample.
For larger equipment or those with multiple internal units, we generally use intensimetry methods to determine the sound power levels. This test method does not require a specific test environment and also has some interesting advantages owing to its immunity to background noise, although not all acoustic environments allow working with high-precision levels.
In intensimetry methods we use the set of EN ISO 9614 standards. We draw an imaginary surface around the test sample at a set distance from it. We determine the acoustic intensity levels on the defined surfaces by using an intensity sensor (acoustic instrument fitted with 2 facing condenser microphones separated by a few millimetres, and their electronics work in a strictly coordinated manner or in phases.), sweeping or sampling at given positions to measure the intensity on the defined surfaces with the intensity sensor. Finally, the sound power is calculated as the result of the acoustic intensity and the area of the measurement surfaces.
CEIS has two twin 215 m^3 reverberating chambers, with climate capabilities which mean we are able to determine the sound power levels in air-conditioning equipment, heat pumps, liquid coolers, etc., in controlled temperature and humidity conditions.
Larger equipment is tested in large climate chambers where cooling, heating and performance tests are also carried out, capable of volumes of up to 550 m^3.