Bangkok--5 Apr--Ford Motor
? Ford engineers aimed to not only reduce noise in Ford Focus’ cabin but also do deliver the right sound quality that gives appropriate feedback of the driving situation
? The team reduced combustion noise and increased refinement on the diesel engines while enhancing the sporty character of the petrol during full acceleration manoeuvres
? Focus delivers reduced wind and road noise thanks to an increased stiffness in the components of the body structure and their attachments
The all-new Ford Focus has been tested in some of the most extreme environments on the planet to ensure impeccable performance, reliability and dependability wherever it is sold. It offers pinpoint handling, razor sharp performance and class-leading efficiency.
Yet all of this would be wasted if the cabin were noisy, uncomfortable and filled with squeaks and rattles. This is where the car’s Noise, Vibration, Harshness (NVH) team comes in, based at Ford's Merkenich Technical Centre in Germany.
"Our aim is not only to reduce noise, but also to design the right sound quality," explains Dr. Martin Hildebrandt, NVH expert in the Focus team. "We don't want the driver to feel completely isolated, but we do want to create a pleasant environment combined with an appropriate feedback of the driving situation. We reduced wind and road noise and we put a lot of work into the quality of the sound coming from the engine."
Improving Engine NVH
Petrol and diesel engines have different sound characteristics and pose different challenges. For the diesel units, the team's main aim was to reduce combustion noise and increase refinement, while for petrol engines the aim was to enhance the sporty character of the engines during full acceleration manoeuvres while retaining smoothness and quietness.
"We have made a number of changes in terms of refinement,” says Dr. Hildebrandt. “We have significantly improved the whole sound and absorption package, for example the dashboard insulation and headliner absorption. We also introduced an NVH-optimised engine cover, newly developed exhaust systems and reinforced hoses to reduce unwanted turbo noises. For the diesel engines, we revised the fuel injection system with multiple injection cycles, as this significantly reduces impulsiveness, or what is often referred to as diesel knocking.”
"We do a test where we pressurise the vehicle with air and detect any leakage caused by tiny gaps. This helps us optimise the sealing of the body. We also look at metrics like 'Speech Intelligibility' which is a measure of how easily passengers can communicate with each other during driving with a certain background noise. For the diesel engines, the new Focus is eight per cent better than the current model in terms of speech intelligibility. This is a significant improvement."
A key element of the Ford DNA is the sporty character of the Ford cars. Creating a pleasant sound quality is not just about reducing the noise level, but also improving the character of the sound and giving the driver appropriate feedback.
Dr. Hildebrandt explains: "In the current Focus, the petrol engines sound very refined, but we wanted to add more of an impression of power, to mix this refinement with a sportier character. So we added a symposer to the engine.”
“This device takes pressure pulsations from the intake system charge duct, amplifies them and transfers this sound into the cabin. We can tune the output coming from the symposer in order to create the most natural sound impression.”
Reducing Wind and Road Noise
Another key contributor to the quality impression is wind noise. The wind noise loudness was tested in an acoustic wind tunnel, simulating driving at various speeds. The team focused on avoiding any local wind noise spots caused by aspiration (where the pressure difference between the interior and exterior causes air to pass through the body seals in certain areas) or by turbulences. Wind noise in the all-new Focus is significantly lower than both current Focus and its competitors — at 130 km/h, loudness levels in new Focus is three sones lower than in the current model.
"We have used what we call the 'sashed door' concept, which has also been applied on Mondeo,” said Dr. Hildebrandt. “This means the doors are stiffer, which leads to less deflection and less noise at all speeds. We have also increased the thickness of the glass and added a special acoustic layer to the windscreen. The door mirrors and A-pillar are specially shaped to reduce noise and the wiper system has also been optimised to create minimal disturbance of the air flow."
Road noise is also greatly reduced in the new Focus. This is tested on various types of road surfaces at different speeds, including at 50 km/h on a road made of small cobbles.
"A highly efficient way to reduce road noise is through changes to the body structure," explains Thomas Mueller, manager, Body Structures, Ford of Europe. "We have optimised body-to-chassis attachment point stiffness, increased overall body stiffness, increased chassis component stiffness, optimised panel stiffness and we have optimised the deadening layer on the body panels. The amount of airborne noise emitted from the tyre patch has also been significantly reduced."
Compared to the current Focus, the changes are remarkable. In the suspension, for instance, the front top mount attachment points are 50 per cent stiffer, the rear subframe cross member attachment points are 75 per cent stiffer and the rear top mount attachment points are 45 per cent stiffer. The overall torsional stiffness is improved by around 15 per cent compared to the current vehicle.
Mueller continues: "By increasing the stiffness of these components and their attachments, we are able to reduce lateral deflection in specific areas of the vehicle. This greatly reduces noise. We have also used more ultra-high strength steels than in the current model and we have used new construction techniques to increase the strength and efficiency."
The B pillar uses a unique construction. Made from tailor-rolled boron steel, its thickness varies according to the strength that is needed in certain areas. The transition between these areas is smooth and this gives a balanced deformation behaviour. As well as increasing stiffness, this construction method also reduces weight and improves material utilisation.
"This new Focus is the most refined Focus to date," says Dr Hildebrandt. "It is quiet when you want it to be, but it also has a great sporty sound character. Our aim is for the driver to leave the car at the end of the journey unstressed, but also to have enjoyed the drive."