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Tire noise inside out

Technology from Bay Systems offers a new approach to tire noise measurement

by Alan Bennetts (Bay Systems) and Neil Crookes (Cooper Tire & Rubber Company Europe Ltd)

 

Vehicle comfort is made up of many factors, with road noise being one that vehicle and tire manufacturers seek to minimize. EU legislation introduced in 2012 reduces tire noise limits by 2dBA and has provided a great deal of work for tire test departments: all tire companies have been busy, measuring, compiling data, and producing labels showing ratings for economy, grip, and noise.

There are, however, some concerns around the procedures for testing tires to conform with the labeling directive. Most testing is done, as is required, on an ISO surface. Yet when in use, tires run on regular roads where their performance may be different.

Another concern is that the radiated noise of the tire does not always correlate with the interior cabin noise, which can be an issue for customers. So is there a way to measure tire noise that gives a better insight into the complex dynamics that produce the noise, both inside and outside the vehicle? Is it possible that the sound pressure level inside the tire cavity can be used to optimize tire design?

 

Tire cavity sound pressure measurement

The input at the contact patch is transmitted through the casing to the air inside the tire. Some of what happens at the road-tire interface is therefore transmitted to the air in the tire, having been modified by cavity and structural resonances in both the tire and wheel. One or more Tire Cavity Microphones (TCMs) may sit in the cavity and measure the changing sound field, as in Figure 1.

Figure 1: TCM on 17in wheel

It is known that tires from different manufacturers may have different characteristics: rolling resistance, handling, wet grip and noise to name but four of a possible 20+ parameters that characterize a tire. Data taken in the laboratory for a number of coast-down runs, from 100 to 30km/h, show almost perfect repeatability. The data shown in Figure 2 shows near identical results for two runs completed 10 minutes apart; this agreement has been confirmed again after three and six months.

Figure 2: TCM data from two runs of 4x4 tire, 100-30kmph

Good repeatability on the road, where the surface is irregular, would seem more unlikely. However, the time signals for two runs at 77km/h are in good agreement (see Figure 3, below).

Figure 3: Time average of four tires' TCM. North going in red, South going in blue

This time data was translated to the frequency domain and the data was separated into cavity-mode-dominated and the tread-dominated frequencies regimes (see Figures 4 and 5), for the overall Awt levels plotted against time. The cavity-dominated, lower-frequency regime, exhibits good < 1dBA differences at the center of the run, however there are differences < 4dBA at the edges of the comparison. Although further work needs to be done to confirm this increased variance, it is suspected that there was some interaction with the powertrain due to the test vehicle not permitting true free wheel. At higher frequencies, in the area dominated by tread and road surface noise, the comparison of the north and south going runs shows an agreement to within <0.5dBA at the center, and <2dBA at the edges.

Figure 4: TCM Cavity mode resonance frequencies – averaged Awt levels 

Figure 5: TCM Tread frequency – averaged Awt levels

Taking the linear third octave spectra at the center of the runs and again splitting the frequency regime at 700Hz, we can see that the lower frequencies (see Figure 6) are dominated by the first and second cavity modes at 200 & 400Hz and to a lesser extent the third cavity mode at 600Hz.

Figure 6: Cavity mode resonances

The agreement in spectral shape is good with the north run slightly higher in level than the south run. The higher frequencies (Figure 7) exhibit remarkably good agreement in all third octave bands.

Figure 7: TCM High Frequency at 77km/h

 

Reliable response
A critical test that must be passed before tire cavity noise data can be used to guide tire noise development is that it reliably reflects actual changes made to the tire. One way to test this sensitivity is to compare the level changes associated with changing speed. This can be achieved by comparing the ISO coast-by noise results with the cavity noise results over a range of speeds.

The experimental setup for this measurement was as for the normal ISO standard coast-by noise test, with the exception that a TCM was fitted inside each tire. On the outside of the vehicle – an SUV in this case – a small magnetically attached antenna was placed on the body near each wheel. Inside the vehicle, the four cables from the antenna were connected to a recording system. As the vehicle began each run the recording was started; at the end of the run the recording was stopped. The excess recording, approximately 10 seconds in duration, before and after a 20m ISO section, was of interest as it showed how each tire’s cavity modes were excited through the acceleration phases. The data for speeds from 72-90km/h is presented in Table 1. Unfortunately, as is often the case, two data points are missing due to high background noise at the ISO coast-by microphone.

The data in Table 1 demonstrates that the internal TCM level does indeed follow the far field radiated noise trend that is measured at the ISO-defined far field microphone position. The TCM level is therefore responding, as does the PBN level, to increasing speed and in the case of a tire, increasing speed results in increasing force input at the contact patch. The noise level measured inside the tire is unaffected by background noise and therefore allows testing to be carried out on days where the wind is too strong for an ISO test. Additionally, once the correction factor from ISO surface to a local road surface is established, it is possible to obtain ISO PBN levels by driving on the local road. This will save test time, enabling faster and more economical tire development cycles, while offering the additional benefit of easy data acquisition for running over non-ISO surfaces. This gives tire manufacturers the potential to develop tires that perform well on both ISO and regular road surfaces.

The spectral content within the tire cavity, for a progression in pass-by speeds from 74km/h to 91km/h, shows a relatively uniform increase in each third octave band with increasing speed, (see Figure 8).

Figure 8: TCM spectral content @ 74, 77, 82, 86 & 91km/h, showing increasing Awt level with speed

 

In-vehicle noise
Vehicles propelled by electric motors have no combustion noise to mask road and tire noise. Should the trend towards lighter structures and deteriorating road surfaces continue, then road and tire noise will become ever more apparent to the occupants of the vehicle. Reduction in road and tire noise, particularly for speeds < 50km/h, is likely to become a focus for both tire and vehicle manufacturers over the next 10 years.

In-vehicle noise (cabin) levels associated with tires will be made up of two principal components: firstly, the primary tire cavity mode and possibly other resonance effects of tire and wheel; plus the tread noise exciting a response in the vehicle structure and airborne transmission.

The TCM data can be successfully correlated with microphones positioned within the cabin to reveal when and how tire noise enters the cabin. Initial research indicates that the phase of the primary cavity resonance within each of the four tires has an important bearing on the level measured in the cabin.

The recording of two microphones at the driver's and/or passenger’s head position while also gathering the TCM data from each wheel requires an expansion from 4 to 6+ channels. Allowance should be made for an engine tacho to allow power train related noise to be screened out of the analysis. For the time history shown the driver needed to make some adjustment to the speed to hit the target speed of 86km/h at the entry gate for the ISO coast-by test run. The time at the entry gate was 4.2 seconds and the time leaving the gate was 6 seconds at which point the driver again changed speed. These adjustments to brake and/or throttle are significant because they put additional energy into the tires. The overlay of the summed and high pass filtered data (100Hz to get rid of the powertrain noise) is shown in Figure 9. The summed TCM data shows a very strong series of resonant peaks just before the entry between 3.6 and 4.2 seconds and again at 7.5 & 8.5 seconds.


Figure 9: SPLs @ 86km/h, starts at solid cursor and finishes at dotted cursor

The time domain data exhibits a strong visual correlation between the summed tire waveform and the summed ear waveform. The strong implication from this analysis is that when the four tire’s primary cavity mode is simultaneously generated and in phase, then the primary cavity resonance appears strongly in the cabin. This should therefore show up strongly in the frequency domain where a modulation of the 177Hz primary cavity tone should appear in the cabin when it goes high at all four tires. The results of a joint time frequency analysis, otherwise known as Wavelet analysis, does indeed reveal such a correlation (see Figures 10a & b). In Figure 10b it is possible to see the strong powertrain line looping in and then out at time point two seconds at a frequency of 105Hz.

Figure 10a: Time/Frequency analysis of summed TCM showing primary cavity mode at 177Hz

Figure 10b: Wave response at driver's ear

 

Conclusions
The work reported here shows how tire cavity sound pressure levels can be used to predict both the far field radiated noise and cabin noise levels. In addition, the often difficult and time consuming task of road noise route tracking through the vehicle structure can be simplified using TCM data. Using TCM data it should be possible for OEMs to both specify and match tires to vehicles enhancing customer satisfaction.

A further intriguing possibility exists that the ISO coast-by noise testing for tires, required by EU legislation, might be achievable using TCM technology. This would allow far higher utilization of test tracks where weather and background noise can limit available test time.

With thanks to Cooper Tire & Rubber Company Europe Ltd for the use of facilities and permission to publish this article. For more information, please email alan@baysystems.ltd.uk

 

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