The following note describes measuring exhaust noise using a Prosig P8000/DATS system for the refinement of an automotive muffler design for a major after-market exhaust manufacturer in Europe. The particular vehicle under test was required by local legislation to have an overall radiated noise level of less than 70 dB. When tested, the vehicle was found to be producing 71.8 dB of radiated noise. The design of the exhaust system clearly needed to be reviewed and modified.
Using the Prosig P8000 hardware and DATS software, the following test procedure was developed. The objective was to compare and contrast different modifications to the vehicle exhaust, in order that a solution that achieved less than 70 dB could be found. Since the vehicle was only available to the design team for a short period of time, it was also important that the testing and analysis could be carried out quickly and accurately.
The vehicle was tested using on a rolling road dynamometer and could therefore accelerate to the desired engine speed in the confines of a semi-anechoic chamber while the noise levels were being recorded.
The measurement system was a Prosig P8012 Data Acquisition unit with the Prosig DATS Signal Processing software. The Prosig P8012 used one analogue input channel connected to the vehicle ignition coil and a second analogue channel connected to an IEPE microphone. The signal produced by the ignition coil exhibited two electrical pulses per revolution of the engine. The signal from the microphone measured the radiated noise from the exit of the exhaust pipe. The microphone was mounted at a distance of 100 cm from the end of the exhaust pipe at an angle of 40 degrees to the direction of flow.
The data was captured and processed in the DATS signal processing package.
In Figure 1 the radiated noise from the exhaust can be seen as captured by the microphone.
Figure 2 shows the tachometer train and Figure 3 shows a small segment of the pulse train.
Using the DATS Signal Processing software the speed versus time curve (Figure 4) was created from the tachometer pulse train.
The DATS waterfall analysis function was then used to create a waterfall plot of the noise signal. Figure 5 shows the parameter entry screen for the waterfall analysis in which most of the parameters are given suitable default values – it is only necessary to enter essential information such as the number of tachometer pulses per rev.
In Figure 6 the initial waterfall plot is displayed. This shows the speeds and frequencies that are present when there is significant noise generation. To view some of the phenomena more clearly it is possible to switch to a color intensity plot. As can be seen in Figure 7, there is only significant structural noise above 500 Hz. This type of noise has the characteristic that it does not follow any of the diagonal engine order lines. However, looking below 500 Hz it can be seen that nearly all the energy is related to the speed of the engine and therefore is not structural.
From a close inspection of the data below 500 Hz (Figure 8 ) it is clear that the 2nd order is dominant as would be expected with a 4 cylinder engine. However, the largest amplitude can be see in the 4th order at approximately 350 Hz.
So, by capturing the data with a P8012 and analysing it with DATS it has been possible to see where the largest contributing factor is with respect to the radiated exhaust noise, and the frequency at which it occurs.
From the results of the test the development engineers were able to isolate which part of the exhaust muffler design was causing the noise to be generated at the identified critical frequency range and were able to dampen it by modifying the first stage of the exhaust muffler.
Various modifications to the exhaust were tested and the test the engineers were able to produce the plot shown in Figure 9.
The 350Hz to 380Hz range showed a marked reduction of the amplitude of the noise. This had the effect of reducing the overall radiated noise to less than 70 dB, in fact the new level was measured at 69.5 dB.
This study showed that it was possible to analyse the problem with reproducible results, then to re-run the test using different exhaust muffler configurations and determine the optimum design modification in a short timeframe.
James Wren was Sales & Marketing Manager for Prosig Ltd until 2019. James graduated from Portsmouth University in 2001, with a Masters degree in Electronic Engineering. He is a Chartered Engineer and a registered Eur Ing. He has been involved with motorsport from a very early age with a special interest in data acquisition. James is a founder member of the Dalmeny Racing team.