Application of LMS Test Analysis System in Automobile PBNR Measurement

In today's world, the automobile industry has become one of the important pillar industries for the development of the national economy. In the face of the fiercely competitive automobile market, in addition to improving various performance indicators and economic indicators of automobiles, improving automobile emissions, reducing automobile vibration and noise, and improving automobile comfort have become an important aspect of modern automobile design and new technology development research. Noise, vibration and comfort are a comprehensive issue to measure the quality of automobile manufacturing, and it gives the most direct and superficial feelings to automobile users. Sound, vibration and comfort are one of the issues that major vehicle manufacturers and parts companies in the international automobile industry are concerned about. Statistics show that about 1/3 of the fault problems of the whole vehicle are related to the vibration and noise problems of the vehicle, and nearly 10~20% of the research and development expenses of major companies are spent on solving the vibration and noise problems of the vehicle. One of the main functions of the interior decoration of the car is that these parts, as a system or components, work alone and interact with each other to isolate the external sound and convert sound energy into heat energy through sound absorption to reduce the noise inside the car. Today, market competition is intensifying, and the speed of product replacement is accelerating. It has become a mainstream development to obtain useful information through competitor vehicle analysis and use it for our own benefit. Especially for domestic vehicle manufacturers, if they want to develop independently, they must master certain competitor performance analysis capabilities and study their competitors well and thoroughly. However, if the sound insulation and sound absorption measurement methods of interior parts must be studied by components, it means that the car must be purchased for disassembly, which increases the cost and time of research and development. The energy-based sound insulation and sound absorption measurement PBNR (Power Based noise Reduction) technology discussed in this article is based on a complete and undamaged prototype vehicle. Energy-based sound insulation/sound absorption test PBNR The energy-based sound insulation value PBNR is defined as the ratio of the sound power of a point sound source to the square of the sound pressure measured at a certain point, which is a function of the one-third octave frequency. It can be expressed in logarithmic form as:

Where: Πref / p2ref = 1/400.
p* is the conjugate of the sound pressure p.
(p•p*) is the mean square value of the sound pressure, or the measured sound pressure sigmoid.
Π, is the sound power of a point sound source measured in a free field.

The energy-based PBNR value can be obtained by the method of the acoustic transfer function. The acoustic transfer function is a function of the frequency of the sound pressure at the response point and the volume acceleration at the center of the point sound source. The sound power of a point sound source in a free field can be expressed by the following formula:

Here, ρ, c is the density of air and the speed of sound in air.
Qa and Qa* are the volume acceleration and its conjugate, respectively.

Therefore, Qa* Qa* is the self-regulatory value of the measured volume acceleration. Furthermore, the value of PBNR can be expressed by the following formula:

Here ap /Q is the amplitude of the measured sound pressure to volume acceleration transfer function, which can be converted into the form of a one-third octave spectrum.

Compared with the traditional sound insulation measurement (Noise Reduction) method, the PBNR method based on energy insulation/sound absorption measurement takes into account the sound insulation and sound absorption (source side and receiving side) characteristics of the system more comprehensively. For example, when measuring the acoustic characteristics from the engine compartment to the interior of the vehicle, the traditional sound insulation measurement method cannot obtain the influence of the sound absorption characteristics of the material under the engine hood and the sound insulation pad, which are often used to reduce the noise in the car, while the PBNR method takes these into account. At the same time, the PBNR method can be used to evaluate the performance of the acoustic trim of the whole vehicle without any damage to the whole vehicle. The traditional sound insulation method can only evaluate the acoustic performance of a single component or a single part, and it is time-consuming and laborious to conduct experiments without destroying the whole vehicle.

At the same time, the PBNR method is easy to implement and does not require a high level of acoustic measurement environment and measurement equipment.

The measurement of PBNR is mainly based on the transfer function method. The system flow chart is as follows: the excitation source is a volume velocity sound source, and the microphone is freely selected according to the number of actual system channels. According to the principle of reciprocity, the sound source can be placed outside the car and the microphone inside the car; or the sound source can be placed inside the car and the microphone inside the car. [page]

Figure 1 Test system block diagram

The post-processing system used in the experiment is: LMS Test.lab 6.0, which is fully functional, easy to operate and highly efficient. The data acquisition front end is LMS SCADAS. The volume velocity sound source is the LMS mid-high frequency volume velocity sound source.

The application of PBNR technology in the whole vehicle comparison test

is based on the PBNR theory. The whole vehicle sound insulation and sound absorption test was conducted on representative models of four brands in the same competitive field in the market. The experiment assumed that the sound source came from different external locations of the vehicle, such as the engine compartment, the tire-ground contact area, the underside of the vehicle body floor, and the exhaust pipe position.

Figure 4 Sound insulation and absorption performance of the floor area/driver's ear

Figure 5 Sound insulation and absorption performance of engine compartment/driver's ear

Figure 6 Sound insulation and absorption performance of tire contact area/driver's ear [page]

Figure 7 Exhaust tail pipe/passenger ear sound insulation and absorption performance

From the measurement results, it can be seen that despite the different brands, vehicles of the same level have similar sound insulation/sound absorption characteristics for sound sources from different parts, and the change trends of the sound insulation/sound absorption in the one-third octave band spectrum are basically the same. The sound insulation/sound absorption characteristics of the floor area and the tire ground contact area to the driver's ears are two broken lines with 1000Hz as the inflection point, which decrease successively with the decrease of frequency; the changes below 1000Hz are faster, and the changes above are gentle. The reason is that the sound insulation/sound absorption characteristics below 1000Hz are mainly due to the sound insulation (quality control) characteristics of the system, and the sound absorption characteristics contribute little below 1000Hz. The sound insulation/sound characteristics above 1000Hz are the result of the combined effect of the sound insulation and sound absorption characteristics of the system, and the sound absorption coefficient of the material changes very little with frequency, which makes the sound insulation/sound absorption change slowly in the frequency band above 1000Hz. The sound insulation/sound absorption characteristics from the engine compartment to the driver's ears show uniform changes. This is because the sound insulation pad on the front firewall plays the main acoustic role in this transmission path, and the sound insulation pad is mainly a sound insulation part without any sound absorption characteristics. The sound insulation and sound absorption characteristics from the exhaust pipe to the rear passenger's ears basically change evenly in other frequency bands except for a trough. This trough is because the weak link of this transmission path is from the tail pipe through the rear windshield directly to the rear passenger's ears. At the trough, since this frequency corresponds to the "matching frequency" of the glass, the sound insulation characteristics are weakened.

In order to verify whether the PBNR test results can reflect the noise performance of the actual vehicle during driving, two of the vehicles were equipped with the same tires for road noise tests, and the results are shown in Figure 8. Comparing Figure 8 with Figure 6, it can be found that the two are similar. Under low-speed sliding conditions, tire noise is the main sound source, so the sound insulation and sound absorption characteristics from the tire ground contact area to the driver's ears reflect the level of actual road noise in the car. In Figure 6, the sound insulation characteristics of car #2 from 500 to 2000 Hz are higher than those of car #3, and the road noise inside the car #2 is lower than that of car #3. The other frequency bands are also basically consistent.

Figure 8 The sound spectrum characteristics at the driver's ear measured on the road

Conclusion

Compared with traditional sound insulation and sound absorption test technologies, the energy-based sound insulation and sound absorption test technology is simple, easy and fast to test, and can save R&D funds without disassembling the whole vehicle. It is very suitable for the analysis of the characteristics of competitors' whole vehicles, which is of great significance for us to improve our independent development level. In addition, the PBNR test results can also be used to verify the statistical energy (SEA) model for simulation analysis and prediction, and improve the accuracy of simulation analysis. At the same time, the PBNR results can also be used as subsystem requirements and provided to subsystem integration suppliers as a basis for system development.

References
1. LMS Engineering Services, Mid High Frequency Volume Acceleration Source, E-MHFVVS, 2004.
2. J. Zhu, Q. Zhang, et al., ”Power-Based Reduction Technique and Its Application to SEA Modeling,” InterNoise 2002, Dearborn, Michigan, 2002.