Lower displacement, fewer cylinders – the downsizing of engines has led to the powerful engine sound being lost. Thanks to sound modules from MANN+HUMMEL, drivers are once again able to hear this sound.

The gruff roar of a lion, the sonorous bellowing of a stag, the deep growl of a bear, the vicious snarl of a wolf – when Matthias Alex talks about the sound of cars, it doesn't take long before he starts making references to wild animals. This is exactly how a sports car should sound – rough, wild, and powerful. "When the driver steps on the gas pedal, he doesn't just want to feel the acceleration, he wants to hear the growling, snarling, and bellowing sounds his vehicle makes," says Alex. The sound engineer knows exactly what he is talking about. As engine sound team leader, he and his team develop modules to help drivers hear these sounds inside their cars.

In the MANN+HUMMEL sound lab in Ludwigsburg, alternate pairs of beige-colored foam wedges project horizontally and vertically from the ceiling and walls in a space measuring around 700 square feet. They absorb all noises and prevent echo. Everything in here sounds dull, as if it were muffled by cotton wool. To ensure that sounds are not distorted by vibration, the entire room is isolated on giant springs.  

The silver Ford Mondeo on the roller device in the center of the room is lashed to the front towing lug using strong belts. They prevent the vehicle from breaking free to the side during acceleration. Claus Riedel prepares the demonstration vehicle for sound tests. Prototypes and vehicles at the development stage shortly before series production are normally tested here.

Of course, we are not allowed to see them. In this case, Riedel prepares a rental car, placing microphones in front of the engine compartment and in the interior at the level of the driver's headrest. He places a model head with built-in microphone ears on the passenger seat. This strange companion picks up sound precisely where it would be perceived by humans – at a potential passenger's ear level.

Growling, snarling, and bellowing

Matthias Alex prepares for the upcoming tests in the anteroom. The sound engineer starts up his laptop, connects it to the technical equipment to the left and right of the desk and sets the lowest engine speed, top speed, and acceleration time – in this case 50 seconds – for the roller test bench. He will later be able to see the sound that the car's engine produces visualized as colored resonance curves on his screen.  

The 49-year old has been with MANN+HUMMEL since 1994. After spending a short time working in the design department he made the move to acoustics. Since 1999, he has been responsible for the engine sound team, which is made up of two mechanics and six engineers. You could say that this makes him the most important composer at MANN+HUMMEL. His job is to create engine sounds that live up to the wishes and expectations of car manufacturers and buyers. In expert circles this is referred to as sound design.  

While Claus Riedel continues to prepare the car and checks that the setup is correct with acoustic engineers Jürgen Tilg and Andreas Link, Alex explains what sound design involves. The quest to find the "right" car sound has taken on an even greater significance in recent years due to the downsizing of engines. Engines must deliver the desired performance from ever smaller displacements. Turbochargers, which deliver additional performance, have a high pitched, unpleasant sound and therefore have to be damped. As a consequence, the resonant sports car sound is lost.

Acoustic engineers counteract this effect using sound modules such as symposers and soundpipes. MANN+HUMMEL recognized the need for this technology at an early stage. At the turn of the millennium, the company began developing the first symposer – which was eventually used in series production in the Ford Focus ST. It was a big success. The sound in the compact speedster was so well received by the technical press and the industry as a whole that almost all sports car manufacturers soon wanted to use it in their fast cars. This success also led to accolades for MANN+HUMMEL. The car manufacturer's management team were so delighted that they awarded MANN+HUMMEL the European Technical Achievement Award for 2006.

The sound modules developed by MANN+HUMMEL are passive in nature. They amplify the sound that the engine generates itself. The symposer works directly at the source. It records pulsations from the engine close to the throttle valve where they have not yet been altered, amplifies them and conveys the filtered sound to the driver’s seat. Drivers then get that sporty sound feedback they are after. An advantage of this sound transfer is that the vehicle hardly emits any additional noise, as it is known to be the case with sports silencers. Therefore, it is easier to comply with legal noise regulations.  

The soundpipe, which was also developed by  MANN+HUMMEL, works in a similar way. However, in this case a vibrating membrane is used as a sound amplifier, which boosts the pulsations of the engine like a drum. This principle is suitable only for naturally aspirated engines.

Sound branding

For car manufacturers, the sound modules offer the opportunity to give their sports cars more acoustic drive. In addition, they help to fine-tune the engine sound or create an engine sound that matches the car brand. This process of sound branding has become increasingly important in the last 10 to 15 years. "We have developed vehicle sounds to match specific car brands and even individual models," says Alex. And not only for sports versions. Sound modules are also used in off-road vehicles, sports coupes, luxury limousines, and even in small cars.  

The sound of an engine is influenced by many factors – every engine and every vehicle sounds different. The engine installation position and angle as well as material characteristics all have an effect on the ultimate sound. Meeting the customer's demands as precisely as possible requires a great deal of experience. When it comes to acoustics, the air intake systems in the engine compartment are important for MANN+HUMMEL. This is because the sound design is always linked to the pipes and filters to which the sound modules are connected. The filters are expansion chambers and as such are acoustic boxes, which also have a damping effect. The further upstream they are installed along the intake path, the quieter the vehicle will be – and vice versa.

The employees at the sound lab work on the sound of one vehicle for between two days and a week.

"The measurements are performed in third gear at full throttle, because this is the critical driving mode for the intake pressure," says Alex, as he issues the command "Start measurement" to Riedel by microphone. Riedel presses the gas pedal until the maximum engine speed is reached after 50 seconds in third gear. After another 30 seconds or so have passed, he issues the command "Stop measurement".

Riedel takes his foot off the gas. Alex analyzes the frequencies on his monitor and decides whether he needs to make any changes, such as replacing a resonator, for example. The process then begins again. The team conducts up to ten measurements, depending on the scope of conversion involved. They generally have between two days and a week per vehicle before they present the results to the customer. If no further readjustment is required, the sound module can then go into series production.

From the concept phase onwards

Testing symposers in the actual engine is part of the late development phase of the car. Alex and his team already work in close cooperation with the customer in the concept phase. In this phase, computer simulations are mainly used, in which a preliminary design of the symposers and soundpipes is created. "Sound has much to do with subjective perception," says Alex. That is why the subsequent test in the car is so  important. The concept phase involves extremely close consultation with the design department. Alex has regular meetings with Tobias Wörz, Lead Product Engineer Technical Plastic Parts.

Material is a critical factor

Wörz is also the link to the customer when it comes to installation space in the vehicle. The sound is often determined by just a few inches of space. Although symposers have a standard main body, they vary significantly in terms of hole pattern, connecting geometries, and materials and have to be adapted to the respective engine and engine compartment.  

One of the key challenges in developing the sound modules is building in the required emotion – and the subjective perception. In some cases it may be necessary to change the sound shortly before the production launch because the customer's management team wants a different engine sound. "Acoustics, along with the look and feel of a car, are key sales arguments," says Alex. The sense of pride that you get from being directly involved in such an emotive and therefore important product is immense.

Different sound preferences

In this context, it comes as no surprise that car manufacturers attach so much importance to achieving the perfect sound. Although there has been very little change in terms of sound character in recent years, it is clear that a car's throaty sound and sportiness are closely linked to the volume. "It is not possible to keep making cars quieter, as somewhere along the line the emotion will be lost," says Alex. Tastes differ hugely in this respect. The sound engineer has to take different preferences into account, particularly between America, Europe, and Asia. In North America, for example, the typical rumbling sound of an eight-cylinder engine is extremely popular. European manufacturers would have difficulty with such a large-capacity sound. In Europe, the six-cylinder sound is used a great deal. The engines should have a somewhat "dirty" sound. "A too clean sound isn't sporty," says Alex. It's a completely different story in Asia, where more screeching sounds are preferred.

Global team

MANN+HUMMEL is a leading supplier of intake solutions. "The specific competencies that we bring in our team along with knowledge of acoustics are our expertise in the area of filters and pressure loss and our understanding of production and manufacturers," says Alex. "We can draw on this wealth of experience."  And this happens worldwide, because MANN+HUMMEL has acoustic engineers working at almost every development location. The one thing they all have in common is the fact that they were all trained in Ludwigsburg. In addition, all in-house test benches worldwide have been developed and certified by MANN+HUMMEL. This ensures that the level of sound-specific expertise developed by Matthias Alex and his team is the same at every location.

Technician Claus Riedel drives the test vehicle through the main gates and into the still empty sound lab. The room measures some 65 square meters and is fully equipped with foam wedges.
The stops at the rear tires additionally fix the front-wheel drive test vehicle in position.
Just as the driver hears it: the acoustic dummy with microphones installed in its “head”.
In the technology room of the sound lab, Matthias Alex logs the test on the computer and adjusts the visualized sounds.

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