German car magazine AUTO BILD and the sound and vibration measurement company Brüel & Kjær have teamed up to perform really thorough measurements of the noise levels inside the cabin of different passenger cars. In the article Searching for The World’s Quietest Car, presenting the results of testing 14 different car models, we found a list of questions and answers about noise in cars that can be useful to gain some insights from.
1) Do bigger wheels roll louder?
“Hardly! An experiment with an Audi A3 showed that when the tyre size was increased from 225/45 R 17 to the next larger variant 225/40 R 18, the road noise only increased by half a sone*. This is hardly audible.”
* A sone is the unit of perceived loudness. In contrast to the decibel, which only measures the physically detected sound, the sone provides a more realistic evaluation of how humans experience noise.
Our take on the answer above is that this particular test was made on surfaces in Germany. If testing cars on rougher asphalts/road surfaces as in e.g. Sweden, the noise difference between different wheel sizes is likely to be larger than in this case.
2) Is a four-cylinder louder than a six-cylinder?
“Yes! Under load (when accelerating from zero to 100) a BMW 435i (with a 3.0 litre six-cylinder petrol engine), for example, measures nearly 30 sones, whereas we measured 1.5 sones more with a 428i with the 2.0 litre, four-cylinder engine. One reason, of course, is the power deficit with corresponding consequences for the transmission layout, and the different rev levels.”
3) Is a convertible with a fabric roof louder than one with a steel roof?
“Yes! A comparison of high-speed cruises shows that the fabric roof makes about twice as much noise as a fixed roof. More precisely, the MX-5 with cloth cap produces 105.3 sones, its brother with a retractable roof only 80.4 sones.”
4) Is a convertible with open roof louder?
“Yes! The moment the retractable roof is open, the wind noise increases dramatically. We could establish up to 18 sones difference.”
5) Is a petrol engine quieter than a diesel engine?
“No! At least for the Golf the following applies – a TDI behaves with more restraint during sprinting with the accelerator pedal completely depressed than its equivalent with a TSI engine. The latter achieved higher revs but resulted in more noise.”
6) Can you hear the engine at a speed of 100?
“Yes! For this example, we once again picked the Golf 2.0 TDI – its motor hums quite audibly at this speed. Only at 130 km/h and above do the wind and tyre noise drown out the TDI growl.”
7) Is a station wagon louder than a sedan?
“Yes! An Audi A3 Sportback (at a constant measured speed of 100) is actually a bit louder than the identical A3 with a boot. In fact, there is only 1.4 sones difference between the Sportback and the notchback.”
8) Is the Rolls quieter at the back than in the front?
“Yes! The quietest place in the Rolls-Royce Ghost (at a speed of 100) is at the rear to the right, even if the driver, strictly speaking, has to endure more noise – a difference of 0.9 sones. Only very sensitive people are likely to detect this.”
9) Is the rear shelf important?
“No! If there is no plate behind the rear-seat backrest, then you might think that one sound absorbing element is missing. As far as the Audi A3 Sportback is concerned, there is no measurable difference with and without the cover.”
10) Does one really only hear the clock in the Rolls-Royce?
“No! The clock is silent. If you can hear something, it’ll be the V12 and the ventilation.”
11) Are electric cars the quietest cars in the marketplace?
“No. The electric cars show that a silent motor alone is no guarantee for in-car comfort – noise is also generated by wind and tyres.”
Brüel & Kjær is using very sophisticated instruments when producing detailed in-car noise measurements, but if you yourself wish to make your own sound measurements, we would recommend that you get a decibel meter (also called a sound level meter). One great meter that we would recommend you look into is the REED Instruments R8060 Sound Level Meter with Bargraph, Type 2, 30 to 130 dB.
