What Is a Sound Power Level?
Sound power levels are the ratio of the total sound power of the energy that a sound produces in a unit of time to a reference sound power of 10 to the -12th power, converted to ordinary logarithm and multiplied by 10.
The unit is dB, and the same unit is used for sound pressure level, but the sound pressure level is defined differently. The sound pressure level is the ratio of the measured sound pressure to the reference 2 x 10 to the -5th power, converted to the ordinary logarithm and multiplied by 20. Sound power levels refer to the measured value at the source, while sound pressure levels represent the sound intensity at the point of measurement.
Usage of Sound Power Levels
Since sound power levels are independent of the measurement environment and location, they can be used to describe product specifications and indicate noise limits without specifying conditions. Once the sound power levels are known, the sound pressure power level due to distance attenuation can be calculated from them.
Since 1970, ISO standards have been created mainly in the United States, Germany, and France for general-purpose sound power levels measurement methods. First, sound power levels measurements were widely used in air conditioning equipment, construction machinery, and copying machines.
Principle of Sound Power Levels
Sound power levels can be measured by the following methods, depending on the sound field.
1. Practical Semi-Free Field Method (Method A)
This method is applied to a large room or an outdoor space with little resonance, which can be regarded as a semi-free sound field, and is measured with accuracy according to the precision method and corresponds to ISO3744.
2. Simple Semi-free-Field Method (Method B)
This method is used to measure the approximate sound power levels in a narrower room than Method A, where the measurement points can be placed according to the principle of the semi-free-field method, and where there is the influence of reflected sound.
3. Simple Diffuse Sound Field Method (C Method)
This is a simple method to measure sound power levels according to the principle of the diffuse sound field method in a room where boundary surfaces, such as walls, floors, and ceilings can be identified, assuming a situation where there is some degree of reverberation. There is no ISO standard that directly corresponds to this method; the C method was added so that it can be widely applied to measurements in machine rooms, general rooms, and factory test rooms in buildings and factories.
Each method can measure octave band sound power levels and A-weighted sound power levels, and only Method A can also measure 1/3 octave band sound power levels, directivity index, and directivity coefficient.