RPM ANALYSIS: Simple & Easy

When evaluating order based noises, it is imperative to consider both orders, resonances, and cross overs or stack frequencies.? Note: Stack frequencies are titled as such when devices under test have resonance the interact with orders based sounds, essentially amplifying their influence and making them more audible and apparent in the data.

?However to do this accurately and provide a full picture an engineer cannot simply pick on analysis. A multitude of RPM and time based analyses can be considered based on the type of metric being evaluated and source material being used.? For example, when simply using an RPM based analysis while a part is operating at a steady speed, the data could become smeared or not very useful due to the lack of definition in the signal.? True RPM based analyses need a range of speed and variability to provide meaningful data points to consider and review.? In these cases you would want to use a combination approach of both RPM and Time based analyses.

With this in mind we will use an example file that has initiates RPM from a 0 RPM, ramp up to initial PWM %, steadily increase in speed to top PWM %, hold at a steady speed for a period of time, then ramp down (aka coast down) back to 0.? In one recording using microphone data, accelerometer data, and pulse channel.? NOTE: the pulse channel could be derived from acceleration, or audible sounds provided clear orders can be selected via RPM Generator with a known order or prominent order signal.? Here is an example of the what the time signal may look like.

Next you will want to define your ramp up, steady state, and coast down segments of the data, by creating new marks within ArtemiS SUITE and saving each file separately.? This can be done my changing the start stop time in the Mark Analyzer and then saving the segmented data in a new HDF file.

Then once completed you will have 3 new HDF files stored in your folder (defined in your settings) to work with in your pool project.? From here one may wish to consider what analyses to use.? In this example, we will use FFT vs. RPM / vs. Time, Order Spectrum, and Order Spectrum vs. RPM.

As explained earlier the steady state is better served using FFT vs. Time for clarity and impact while using the RPM based calculation for the ramp up and ramp down can be used effectively.

At this point you will also want to consider the properties of your selected analyses. For example in the FFT you may want A-Weighting and you may want to change the abscissa to a meaningful range to better focus the analytical lense of your analysis.? You may also want to change the slope detection from the default 50 RPM to 5 for even more clarity.

Lastly, you may want to align your analyses with in a group to better understand the influence from both Order (RPM) and Time analyses.? To do within a mark analyzer you right click on the tab(s) and “Move to” “Group 3”. Then Right click once more on the tab and “Arrange here Horizontally”

With this information you can now begin extracting vital details, for example what RPM’s to specific order begin to appear, what frequencies are resonances, and lastly what frequencies actually contain order sourced sounds.

Within Order Spectrum you can define the orders you are most interested in, using the data from above.? Isolate the influence of these specific orders and determine their impact.

As an example of this we can see the 5th, 8th 10th, and 13th Orders.? You can also see what the the impact on a dB scale as well. Then the last step in this example is to consider the impact to the overall sound of your product and the perceptible / source of your sounds as they are operating.

Using the Order Spectrum vs. RPM calculation and considering the overall sound over RPM. (Order 0) you can compare the other orders (via Analysis -> Properties -> Cuts) and determine how many and what orders are influencing or impinging into the overall noise.? In the case above we can see the overall noise is driven by the 2nd? Order at low speeds (375 RPM) then by 1st order (460, 865, 1220 RPM), and 13th order at 1830 RPM

Hopefully you find this work flow usable and supporting your own products.

#innovation #nvh #orderanalysis #soundquality