Order Analysis - Sound Power
Introduction
Order Analysis Sound Power (OASP), or Rotation-based Sound Power, inspects the sound power emission of individual rotational components by looking at the rotational order content of the sound power emission.
Based on the design of individual rotational components they all have their own pattern regarding which orders radiate noise and how much is radiated relative to other orders. In that way is it possible to distinguish between components and inspect how much sound power comes from a particular component.
Example 1:
A 4-stroke engine with four cylinders will have noise at the second order component due to the fact that each cylinder ignites every second revolution (½ order) and you have four of them (½ x 4 = 2nd order).
Example 2:
A rotor fan with 64 blades will generate noise at the 64th order component since you have 64 blades, each rotating once per revolution.
Example 3:
A gear with 31 teeth that all are worn down, while one of the 31 teeth is cracked off - this will produce noise at both the 1st order and the 31st order. The 1st order noise appears due to the missing tooth making a transient noise event once for each rotation, when the gear jumps over the missing tooth. The 31th order noise happens due to the worn down connection surface between each gear tooth with another gear. This happens with a rate of 31 teeth per revolution.
Such analysis can assist in determining which rotational components might have to be modified if the tested system has to adhere to a certain sound power level criteria.
NOTE:
The OASP application module is licensed under Sound Power, DEWESOFT-PLUGIN-SOUND-POWER.
Also note that this application uses data coming from the Order Analysis application module, so license to this module is also needed, DEWESOFT-OPT-ORDTR.
Test Configuration
Below is an example of a possible order tracked sound power test configuration, that can be used for certain test objects having no major noise directivity:
The test object has to be placed surrounded by microphones. The microphones will cover a surface area which all sound power will be radiated through.
NOTE: First The test object has to be configured for Order analysis, which includes using a rotational angle or speed sensor. Please see the Order Tracking Module Help for how to configure this part.
OASP Module Setup
This OASP module is found in the application module list under the Acoustics section, as shown below:
After adding the module to a setup the following parameter settings has to be adjusted:
Order tracking setup section
Order Tracking
Select here the Order tracking module instance that you want to use as data provider for the OASP module.
The selected order tracking instance will provide the following data results for processing Sound pressure to Sound power:
- Order waterfall vs. reference
- Order domain harmonics
NOTE: You have to select sound pressure input channels in the Order tracking module instance, to get sound pressure data in the OASP module that can be processed to sound power.
If you later on have made a change to the setup of the selected order tracking instance, then click the ‘Add’ button to update. E.g. if an additional order component has been added to the Harmonic list for data extraction.
Setup settings section
Surface area
Type in the surface area covered by the used microphones, in the units of square meter, $[m^2 ]$.
Reflecting surface areas should not be included since the reflected sound power will radiate out through the area covered by the microphones.
Pressure and Temperature
The environmental pressure and temperature must be set to determine the particle density, which is used for the sound power calculation.
Weighting
Select between ‘Linear’ and ‘A’-weighting. Linear will apply no acoustic weighting, A will apply acoustic A-weighting.
Positions setup section
The table is mainly used to see how the surface area is shared between microphones, and to see which order components are extracted to individual results. The table contains a tab for each extracted order component, which has been specified in the Harmonic List for the used Order Analysis (OA) instance.
In the table example above the OA instance used has a Harmonic list defined like: $1;2;3;22$
Results
Sound power order waterfall vs. reference
Based on the sound pressure order waterfall vs. reference the OASP module converts this to a sound power order waterfall vs. reference result.
The waterfall plot shows how the sound power varies over a reference interval like e.g. a speed interval, and at the same time it sorts the sound power into specific orders of rotation.
The reference is typically rotation speed in units of $[RPM]$, but the tag-axis (reference) can also be set to use another measured quantity. This is configured in the used Order Analysis instance.
Sound power extracted orders vs. reference
Based on the extracted sound pressure orders defined in the used OA instance, the sound power for such orders are determined and outputted as individual channels vs. reference (like speed).
Markers can be added to the graph widgets to extract certain scalar values for sound power as well.