Vehicle Acoustics

4-wheel roller test bench with pass-by measurement hall.

Research and testing center of the Vehicle Acoustics

The new laboratory building for vehicle acoustics houses a state-of-the art measurement hall for exterior noise with a 4-wheel roller test bench. The construction of this test facility in an official and independent research institution offers the unique possibility for an interdisciplinary research approach at a pre-competitive stage. Therefore, vehicle acoustics has not only become a core competence of the Department of Acoustics of Fraunhofer IBP but also a high-quality center for analyses and testing.

Description of the Test Facility

© Fraunhofer IBP
© Fraunhofer IBP

The test facility allows a realistic drive operation of automobiles and light trucks with two-wheel and four-wheel drive on the roller test bench with four individually driven rollers and high-precision control for acoustic and vibration measurements inside and outside the vehicle.

The test facility is used for simulated pass-by measurements, where exterior noise data of a vehicle are collected at a defined distance. This kind of measurement, which is statutory for the registration of any new vehicle, is usually performed at outdoor test sites. By simulating the pass-by in a semi-anechoic room, providing sound conditions comparable to the open, these measurement can be performed at any time independent of weather conditions. Due to the measurements compliance with tefulations can be checked already during the stage of development allowing early response.

The 4-wheel roller test bench, where each wheel is individually controlled, allows all kinds of vibration and acoustic investigations of the vehicle. The noise contribution of vehicle components can be determinded and analyzed by methods like uncoupling or bar (impulse) tests as well as by means of modern methods of noise analysis and sound design. This follows the intention to avoid the occurrence of unwanted rattling or humming in the vehicle and to achieve the desired sound character of the vehicle in terms of sound quality by target-oriented influencing of the noise components. Moreover, important fields of research in vehicle acoustics are the improvement of driver concentration by means of lower sound levels in the vehicle interior, the enhancement of speech intelligibility of mobile phones and conversations in the vehicle, voice control and audible warning signals as well as the development and testing of acoustic components.

Measurement Facility

© Fraunhofer IBP
© Fraunhofer IBP
© Fraunhofer IBP

The interior dimensions of the measurement facility are approx. lenght x width x height = 25 x 18,9 x 6 cbm, access is provided by a gate with an opening of approx. 3 x 3 m2 for light trucks. An additional door connects the measurement facility and the adjacent control room. The ceiling, walls, doors and gates are lined with absorbers developed by the Fraunhofer IBP, the asymmetrically structured absorber and broadband compact absorber, in such a way that a semi-anechoic field with 40 Hz lower cut-off frequency is present in the measurement facility. The background noise level for the measurement facility amounted to 25 dB(A), at maximum room ventilation to 30 dB(A) and with additional maximum vehicle cooling system to 55 dB(A).  The floor is hard-walled and equipped with a plastic coating. The ventilation systems supplies adjustable air flows of up to 40.000 cbm per hour. The routing of the air is effected by slots in the frontal wall and ceiling edges. HVAC allows conditions of 23°C ± 5°C at full-load operation and normal outdoor temperatures from -10°C to 30°C. On each of the two long walls 30 microphones are installed at a distance of 7.5 m to the test bench center for the measurement of simulated pass-by noise. Openings in the floor are installed in the center of the microphone paths and close to the vehicle for housing the front-ends of the acoustic data acquisition system. The floor has cut-outs at the microphone paths to accommodate microphone cables.

Roller Test Bench

The testing technology consists of a 4-wheel roller test bench with individually driven rollers having a diameter of 75 inches and a perimeter of 6 m. The roller width is 550 mm and the exterior wheel track 2200 mm. The rear pair of rollers is infinitely variable for a wheel base from 2200 mm to 4000 mm with a positioning precision of ± 1 mm. All rollers can be adjusted to a common zero point. The test facility is suited for vehicles of a total weight from 800 kg to 4000 kg at a maximum axle weight of 2500 kg. The vehicles can be tied up by ropes, rods or at the wheel hubs.

With an input power of 300 kW per roller the test facility is suitable for a drive speed of up to 320 kg/h and provides a constant tensile force of 7500 N up to 120 kg/h so that an acceleration of almost 1 g is achieved. High-precision control guarantees an exact synchronization of the rollers. Load cycle change tests, where electronic automotive safety systems like ABS and ESP can have an influence on the test bench control system, require a maximum deviation of the speeds between the rollers of the front and rear axles of 0.5 km/h at maximum acceleration and of 0.2 km/h at moderate accelerations or constant speeds. This is to ensure that these auxiliary programs are not activated. For low-speed tests with bars mounted on the rollers of one axle the deviation must not exeed 1 mm to avoid double pulses.

Parameters controllable from the control room and partly from the vehicle by means of a remote control comprise distance, speed, acceleration, tensile force, mass simulation and driving resistance simulation. The variables are individually controllable at all four rollers.

Pallet System

A pallet system, which totally replaces the test facility cover, is used for an efficient workflow in the test facility and thus rapid and reproducible exchange of vehicles. The vehicle is fixed on the pallet, prepared and moved to the test facility by means of air cushions. The pallet is set in there and fixed instead of the cover. Two different types are used: One pallet for exterior noise measurements with roller openings and otherwise completely closed surface with reverberant coating as well as pallets for vehicle investigations of all kinds with groove rails and an opening for the pit. The pallet for exterior noise measurements is lifted to the ceiling of the measurement facility for storage by motorized devices. In the process, the absorber lining is opened and closed afterwards by motorized wings. The other pallets can be moved with the vehicle to the preparation rooms and be stored there.