Clear results with NVH source path contribution

17 May 2013

'Before-and-after' results show the difference SPC makes by quickly finding the contributions from specific components to the NVH experienced by the driver, and the significance of the NVH path.

Automotive | Application

Enlarge image
A spectrogram showing measurements at the driver’s ear during 6th gear run-up. A high amplitude around 215 Hz is apparent throughout the run-up, with several orders exciting this area as they pass through it.

Source Path Contribution (SPC) determines the sources of the sound or vibration perceived by occupants of a vehicle, and determines the routes they take. The results include a quantification of the source level, the sensitivity of the path, and the contribution of each individual path to the receiver. But it is only with the recent innovation of time-domain SPC that any type of event, including transient events, can be analysed.

“The best way to analyse the data is in an interactive manner, listening to the contributions, viewing the live spectrum, and turning individual contributions or groups of contributions on and off,” says Senior Application Engineer Dave Bogema. Consequently, pronounced differences between specific measurement locations on this diesel engine were able to show the origin and frequency components of high-frequency airborne sounds emanating from the turbocharger.

A diesel truck is a very good example of a case where time-domain SPC is required, as the characteristic transient sounds of a diesel truck engine make it very difficult to accurately assess using only traditional frequency-domain analysis. In the example used here, several airborne and structure-borne paths from the engine are measured and analysed using time-domain SPC techniques.

By contrasting spectrograms that show the contributions from different directions on different locations, with others that show the sensitivity functions of the transmission paths taken by each contribution, it is easy to identify problems. The benefits are clear to see in the difference between the original and the modified mount contributions in the graph below. 

A graph showing the sum of all contributions to the driver’s ear – with the original (green) and with the modified X and Y front cab mount contributions (red). A significant reduction of 12 dB at the driver’s ear can be observed at 215 Hz.

On the front-right cab mount of this truck, it was found that path sensitivity was normal for the X-direction, yet measuring the force here showed a clear resonance at 215 Hz. On the same mount, the Y-direction sensitivity function showed a peak that doesn’t correlate to the force for the same frequency. The outcome is that by modifying the force level for the X direction and the sensitivity for the Y-direction, a significant reduction in the noise level experienced at the driver’s ear can be observed at 215 Hz, of approximately 12 dB. 

Interactive troubleshooting

"This is highly specific information for troubleshooters to use to make a precisely targeted fix, and which can be easily communicated to others by letting them hear the difference"

This is highly specific information for troubleshooting that allows a precisely targeted fix to problems, and which can be easily communicated to others by letting them hear the difference. All the content and nuances of the actual sound bring a far better understanding than merely looking at graph plots. Listening to simulated changes takes it even further, into the realm of virtual prototyping, giving the ability to make changes to a virtual model and predict and perceive these changes.

Overall, working with time-domain data for source path contribution makes a great deal more data available than a traditional frequency-domain approach, meaning there are many more ways to analyse the data and interrogate the model at leisure, and it is even possible to merge subjective sound quality assessment. Making changes and performing "what-if" studies are invaluable to fast development, and for almost any given application there is the potential to develop and streamline the workflow using time-domain source-path-contribution.

You can get more information about source path contribution here

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