Noise hell or sweet music

For those who use the car not only as a means of transport, but also as a mobile office, the level of noise is crucial to making the day bearable

BY JOHANNES HARTKOPF-MIKKELSEN, BORSEN.DK

Low noise levels are an important feature of any company car, allowing the driver not only to conduct telephone conversations and listen to the radio while on the road, but also ensure that he reaches his destination in comfort.

That’s why the magazine "Firmabilen" ("The Company Car") decided to measure the noise levels of four obvious company-car candidates. The measurements were made in cooperation with Brüel & Kjær, who as suppliers and developers of noise measurement equipment to, among others, the automotive industry, are experts within this field.

Wind and road are crucial

Common to all four cars is the fact that their engines are practically silent at a constant speed of 130 km/h. There is, however, a great difference between how much wind and road noise finds its way into the cabins of the different cars, just as there is a big difference in how the noise is perceived.

Measurements show that there is a difference of 1.9 dB between the test’s quietest car – the BMW 316d saloon – and the test’s noisiest car, the Ford S-Max 2.0 TDCi 140. It might not sound like much, but a 6 dB increase in noise levels corresponds to twice the experienced noise levels.  Converted into percentages, this means that there is 25% more noise in an S-Max going at 130km/h than there is in a 316d travelling at the same speed.

The Citroën C5 2.0 HDi 140 Tourer has a noise level of 71.5 dB when travelling at 140km/h, just marginally higher than the BMW 316d saloon. However, it is quite clear that there is a big difference in how the decibels sound when listening to the recordings of the two cars’ noise levels in succession.

The wind and road noise entering the BMW cabin has much lower frequency than that entering the C5. That means that the noise in the BMW is deeper and more bass-like, whereas the high-frequency noise in the C5 is perceived as being more treble-like.

"The question of which is the most pleasant – the high- or low-frequency noise – is a subjective matter," says Alun Crewe, Marketing Director at Brüel & Kjær and responsible for carrying out the noise measurements and analyses for this test.

He says that individual tests have shown that Europeans and Americans generally prefer low-frequency noise, while Asians prefer high-frequency noise.

In general, the results of the noise measurement analyses reflect how easy it is to have a conversation with a passenger or via a hands-free telephone. However, the outcome is not a foregone conclusion, because if there can be a difference in the noise within a car – high- or low-frequency – there can also be a difference in how much the cabin noise disturbs the frequencies of normal human speech, which are those most audible to the human ear.

In the BMW 316d, the driver can hear 61.9% of the sound coming from the co-passenger’s voice when the co-passenger speaks normally and they sit one metre apart. In the Ford S-Max the result is 53.3%, while the Peugeot 508 and the Citroën C5 are somewhere in between the two at 55% and 58.5%, respectively.

Camouflaged engine noise

One of the biggest challenges facing diesel engines is the intrusive and uninspiring engine noise, whereas the petrol engine can have a soft and desirable sound. "The exploding sounds of a four-cylinder diesel engine remind one a lot of the raw sound of a machine gun," says Alun Crewe.

In order to establish the different characteristics of the different car engines, we measured the noise levels while accelerating in third gear.

From idle, it seems that the BMW 316d engine has the roughest, rawest sound of all the test cars, whereas the engine noise remains pleasantly in the background when the car is at speed. The measurements also show that the BMW engine, looked at in isolation, has quite an unpleasant sound  map, but luckily for the car, the noise is counterbalanced by the wind and road noise, with the happy  result that the noise is not perceived as unpleasant.

The Ford S-Max and Citroën C5 models used for the tests have the same 2.0 litre diesel engine with 140 horsepower. The engine is the result of a cooperation between Ford and PSA (Peugeot and Citroën), and feels the same in both cars when it’s a question of acceleration capability and power delivery. On the other hand, under acceleration, the Citroën C5 engine has a more rumbling and intrusive sound.

Analyses carried out by Brüel & Kjær show that, when you isolate engine noise from wind and road noise, the engine noise of the Ford S-Max is less of a rumble but rougher than the Citroën C5. This can be due to the difference in how the engines are mounted, or the differences between the respective windscreens and bulkheads between the cabin and engine – areas that are typically responsible for allowing noise into car cabins.

However, this can mostly be blamed on something other than the engine. Because the Ford S-Max has more wind and road noise than the Citroën C5, the engine noise of the S-Max is, quite simply, better camouflaged than that of the C5.

The Peugeot 508’s 1.6 litre diesel engine is also the result of the cooperation between Ford and PSA. The engine can be clearly heard under acceleration with high revs, but of all the engines tested, this little diesel engine sounds most like a petrol engine. And that hardly makes for unpleasant noise.