Today, farmers feed the world faster: the average farmer produces 12 times more agricultural output per hour than one in the 1950s. This is thanks to improved agriculture techniques and high-performance equipment such as tractors with GPS precision guidance, intelligent power management systems, powerful engines, high-torque transmissions, and other advanced technology.
A major part of this increased productivity focuses on enabling farmers to stay in the cab most of the day to complete large-scale field applications such as row-cropping and large livestock operations. With days that start before the sun rises and end after sunset, fatigue is a key concern. Comfortable and quiet cabs loaded with numerous features help farmers get the tough field work done as quickly and efficiently as possible.
John Deere offers a ComfortGard premium cab with advanced filtration air-conditioning, built-in refrigerator, and air-suspension upholstered seats with dual armrests. Cabs are mounted on rubber isolation mounts with acoustic trim to reduce interior sound levels to 70 dBA. A computer-controlled command center displays tractor performance and allows farmers to quickly and easily make adjustments to the hitch, hydraulic system, transmission, and other systems via a single on-screen user interface.
To boost productivity even further, many premium tractors can be driven at higher speeds on public roads—a necessity in many European countries where the farmers need to access separate fields and remote areas of the farms.
This faster operation brings about an NVH problem widespread throughout the agricultural industry: “booming” noise in the cab at high speeds. Typically found only in a narrow range of frequencies, these noise levels are generally not addressed in the coarse overall NVH limits set by legal sound-emission standards such as the OECD, EEC, and OSHA standards that tractors have met easily over the years. Nevertheless, the noise poses a challenge for tractor designers. Farmers who are accustomed to a rather quiet cab during normal operation become distracted and annoyed with sudden sound-level spikes or “booming” as the tractor accelerates on the road.
The booming noise was in the 100 to 110 Hz frequency range with sound levels increasing in extreme examples as much as 8 dBA while driving at speeds greater than 48 km/h (30 mph) on hard flat road surfaces. Numerous measurements made by John Deere showed that a combination of factors amplified the problem: resonant vibration modes defined by the cab size and shape, cab modal frequencies and associated large sheet-metal parts, and the contribution of different panels to structural sound transmission.
No single effect could be identified as dominant, however. The primary source of the problem remained a mystery, even after months of rigorous testing. Moreover, traditional sound-reduction methods such as acoustic insulation proved to be ineffective in regards to the booming noise. Typically, sound-absorbing acoustic insulation works well in abating sound above the 500 Hz frequency level but not with the low 110 Hz frequency range of booming noise. Design-wise, the cab consisted of huge acoustically transparent areas of glass and steel which could not be modified. John Deere tried design variations for the cab-mounted exhaust system as well as the windshield and roof hatch. These had little effect on lowering the noise.
To dig to the bottom of this “booming” noise and determine how best to quiet cabs at higher operating speeds, John Deere Werke Mannheim cooperated with LMS Engineering Services on a joint project to develop simulation models to predict cab booming noises.
The first step in the study was to measure tractor acoustics on a chassis dyno in a semi-anechoic room using LMS Test.Lab. Sound levels were recorded inside and outside the cab at different driving speeds, and the measurements were correlated with test track measurements. This verified what the John Deere engineers had already encountered: the noise depended on driving speed rather than engine speed, number of cylinders, or gear ratio. More specifically, the engineers found that the noise depended most heavily on the tires. The excitation frequency equaled wheel rotational speed times the number of wheel lugs.
For the particular tractor and tire type in the study, wheels with 44 lugs on a tractor traveling at 50 km/h (31 mph) produced an excitation frequency of 108 Hz—almost exactly that of the booming noise. LMS engineers reasoned that the wheel lugs acted as rotating fan blades, producing aerodynamic pulses that excited the cabin interior to resonant vibration.
To gain insight into the path of the wheel lug vibration into the cab, LMS engineers used a simulation approach to represent the acoustic behavior of the tractor. Geometry data from John Deere’s Pro/Engineer CAD system was transferred to a LMS Virtual.Lab meshing module to create a boundary element model of the cabin, wheel, and fender. This model was then used with LMS Virtual.Lab Acoustics software to predict sound pressure levels around that area of the tractor. The predictions were then correlated with actual sound measurements to validate the model.
Graphical output from the acoustic simulation indicated a “hot spot” of sound pressure with structure-born vibrations transferring the energy into the cab producing the booming noise. To test this theory, researchers removed cab parts and found that cab noise dropped by 7 dBA—virtually eliminating the booming sound. The acoustic model was then modified and simulations rerun to evaluate the noise effect of different fender sizes, shapes, and clearances.
“This hybrid approach, the use of experimental and numerical acoustic analysis, is helping us to shift NVH optimization of our tractors upfront in the development process. Moreover, it helps us to understand better the complex acoustic system we are dealing with,” said Dr. Ing. Christian von Holst, Group Leader, Suspension Systems, at John Deere Werke Mannheim.
John Krouse, Senior Contributing Editor, LMS International, wrote this article for SAE Off-Highway Engineering.