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Sound power testing at SATRA
How different factors must be considered when a product’s noise level is being tested.
When your ears detect a sound, what exactly are you hearing? Imagine a scenario in which you are placed in an empty room with a noisy machine. You might think that the sound you are hearing is the inherent noise output level from the machine, but this may not necessarily be the case.
The sound level you are hearing is the sound pressure level produced by that particular machine within that particular room, and there are several factors which can affect this. The boundary conditions of the room will play a large part in the noise level which you experience. You may be hearing only the sound pressure caused by the machine, but you could also be experiencing a contribution from the acoustic reflections coming back from the boundary of the room.
Your proximity to the machine will also affect the perceived noise level. You will almost certainly notice an increase in sound pressure level as you approach the machine, and a decrease as you move further from it. How noisy is the room? There is a possibility that you are also hearing a contribution from ambient noise, making the machine seem louder than it actually is. Where is the machine positioned within the room? Is it suspended, so that it can radiate its acoustic energy equally in all directions, or is it positioned in the corner of a room, forcing its sound radiation to be concentrated into a smaller space?
Each of these factors will alter the sound pressure level which may be experienced within our imaginary room. However, the machine will also have a specific noise level which is inherent to its design – known as the 'sound power level'. The sound power level is a very useful figure defined as 'the inherent sound level of a sound source independent of the acoustic environment in which it is placed'.
Measuring sound power
Considering the factors above, it is clear that the measurement of sound power levels is a fairly complicated task, requiring some very sophisticated equipment. There are four common measurement methods used when assessing the sound power level of a device: 'free-field', 'sound intensity', 'reverberant room' and 'substitution'.
The free-field method
In an ideal scenario, sound power measurements should be made in free-field conditions. This is a sound field in which there are no acoustic reflections. One way to visualise this is to imagine being suspended high in the air – any noise that you make is constantly travelling away from you; there is no sound reflected at all. Obviously, this is very impractical in real life, so we use an anechoic chamber. This is lined with acoustically absorbent foam which virtually eliminates acoustic reflections, giving a relatively accurate approximation of free-field conditions.
We also ensure that background noise levels are minimised by having an extremely low ambient noise level within the chamber, thanks to its 'room within a room' design. Because of the way the decibel scale works, if the ambient noise level is more than 10 decibels (dB) below the level of the noise source we are measuring, there will be negligible contribution from the background noise.
The actual noise measurement involves a minimum of ten measurements taken over a hemispherical, cylindrical or parallelepiped surface (a three-dimensional figure formed by six parallelograms) surrounding the noise source, with each measurement point on the surface at a precise position determined by the dimensions of the source under test. The overall level is calculated by logarithmically summing the measurements from the ten microphone positions and averaging over the entire measurement surface.
The sound intensity method
The intensity of an acoustic wave is defined as the rate at which energy travels through a unit area, perpendicular to the waves propagation direction. It is measured in watts per metre squared (W/m2), and is a vector quantity (a combination of quantity and direction).
Most commonly, an intensity probe will be used to conduct measurements of this type, simultaneously measuring sound pressure and particle velocity using two closely spaced microphones. This method is similar to the free-field method, as the measurements are made over a hemisphere and averaged over the total measurement surface.
The reverberation method
A true reverberant room is one in which acoustic energy becomes so diffuse, due to the large number of reflections, that the sound pressure level throughout the entire room is virtually constant. Measurements made in this type of environment are corrected for the reverberation time of the room. One drawback of this method is that the directivity of the source under test (the acoustic emissions from the source at different angles) cannot be determined, due to the diffuse nature of the room. The directivity can only be measured in free-field conditions.
The substitution method
In some situations, it may not be possible to move a source into a test chamber. In these situations, the substitution method is used. A reference sound source (calibrated to output a known sound power level) is positioned in the same room as the device under test. The difference between the sound pressure levels measured from the two devices is taken to be the difference in the sound power level output.
All the above methods will determine the sound power level of a device. Sound power is denoted as ‘LwA’ and expressed in dB. The LwA acronym shows us that the value is a sound power Level (denoted by the L), measured in watts (reference 1 pico watt) and finally, A-weighted. A-weighting mimics the human ear’s response to sound, as we do not hear all frequencies with equal sensitivity. Our hearing is much more sensitive at around 4kHz, and considerably less so at lower frequencies.
Why do we need the sound power level?
As previously mentioned, the sound power level is a very useful measurement. One of its main uses is to allow acoustic modelling, as it permits predictions of noise levels in given situations. For example, the sound power level could be used to estimate the sound pressure level a source would produce in a given room, or to estimate how a home near to a prospective wind turbine site would be affected acoustically. It also gives the ability to directly compare the noise output of devices, as the measurement is made in a controlled environment and is very much repeatable.
The EU Noise Emission Directive, 2000/14/EC, makes it compulsory for 57 types of equipment to be tested for sound power level output, before they can be marketed in the EU. This ranges from construction machinery to lawnmowers. Further, since December 2011, the European Energy Labelling Directive has made it the responsibility of consumer goods suppliers to ensure that their products are properly labelled, including the sound power output of the device. This covers a wide range of white goods (such as washing machines and dish washers).
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