Enjoy the silence while you’re driving

Silence is golden, as the saying goes. Noise can be an irritant, including in cars. Find out how Škoda’s developers are trying to eliminate car noise.

7. 2. 2023

Even the best and most modern cars make a variety of sounds and noises. And that includes electric cars, which are very quiet compared to an internal combustion engine. This is particularly noticeable as the speed increases, when tyre noise and aerodynamic noise generally get louder. At a speed of around 100 km/h upwards, you mainly hear aerodynamic noise. These are areas that the developers at Škoda have been focusing on for a long time, of course, but they’ve recently become even more important.

Filip Nováček from Škoda technical development and a dummy head fitted with microphones

Noise caused by airflow is the subject of a discipline called aeroacoustics. “We test cars in an aeroacoustic tunnel where air is blown at them as if they were driving. We can change the air speed and, if necessary, turn the car to see the effect of a side wind,” says Filip Nováček from Škoda’s technical development department. He adds tunnel measurements have an advantage over measuring on a regular road in that there are no other external influences interfering with the data, so each measurement is almost exactly reproducible. “We can try out various alternatives and be confident that we are measuring the effect accurately,” he explains. For many acoustic tests and measurements Škoda has its own facilities and laboratories, but for aeroacoustic testing it uses specialist sites across Europe.

Measuring heads that listen

How does this kind of test actually take place and what does the workplace look like? Aeroacoustics experts measure an entire “finished” car. “We need to hear what’s going on in the interior,” explains Nováček, explaining why models are not used. The “test track” is a closed air circuit with a soundproof wind tunnel, which allows the sounds caused by the airflow to be measured. The car is placed in a soundproof measuring chamber, and inside the car engineers place a “measuring head” on each seat. These are special models with built-in microphones that simulate how a human hears. Acoustic cameras, which are dozens of tiny microphones linked to a video camera, are then pointed at the car from outside. This combination makes it possible to identify in the image where any sound is coming from.

How does aeroacoustic testing take place?

The test model is immobilised in the test area. The noise in the room is measured using microphones placed in a soundproof chamber. A nozzle adjoining the room accelerates the airflow so that the air entering the test chamber is truly uniform and does not swirl unnecessarily. The airflow in front of the nozzle is unified by rectifier screens. Conversely, the air behind the test room needs to be slowed down, this is done by a diffuser. In the corners of the tunnel, there are guide vanes that absorb all the noise but above all guide the airflow in the right direction. Finally, the heat generated in the aeroacoustic tunnel is removed by a cooling surface located opposite the fan.

A powerful fan propels the air for the test. Unlike in a conventional wind tunnel, however, this fan is located further away from the test area to minimise the effect of its noise on the measurements. The circulating airflow is directed by guide vanes located at each of the four corners of the closed tunnel circuit. These absorb noise and guide the air in the desired direction. Before entering the measuring area, the air flows through something called rectifier screens, which unify the airflow and eliminate swirling. Next there is a nozzle that accelerates the airflow, while reducing swirl and ensuring uniformity. This accelerated air enters the test area. Before the air completes the cycle and is again driven by the fan, it passes through a cooling surface so it loses the heat generated in the cycle.

Various insulation materials reduce noise in a car’s interior to varying degrees.

Deliberately generated sounds

The developers at Škoda are not just looking at acoustics in terms of airflow, though: that’s just one discipline. For example, the Czech carmaker has two state-of-the-art acoustic measuring chambers with cylindrical dynamometers, which allow a car’s noise in general to be measured while simulating driving on different surfaces. Here, not only is powertrain noise measured (including exhaust noise in cars with internal combustion engines), but also other sounds that the car and its individual parts make when driving. Škoda also carries out driving tests in all kinds of climatic conditions, on its own test polygon and around the world.

Here, too, the advent of electric cars is making the work of acousticians perhaps even more important. “Internal combustion engines mask a lot of sounds, but electric cars are so quiet that even the slightest noise will stand out. This increases the acoustic requirements placed on all the parts of the vehicle. Drivers may be unnecessarily distracted by unfamiliar noises and may even think there is something wrong with the car when everything is in fact working perfectly. This can sometimes have dangerous consequences,” explains Filip Nováček.

Filip Nováček in the acoustic chamber

In most cases, the developers try to eliminate noises, but with engines or exhausts, for example, it’s also about tuning the sound in a pleasing way. And with electric cars and plug-in hybrids, there is even deliberately generated sound. That’s the sound these cars make when driving at low speeds, when they are otherwise dangerously quiet for pedestrians. The Enyaq iV family of electric cars also has its own original sound that alerts pedestrians to the moving car. Developing a sound like this is a real science.