The Trickiness of Evaluating Tests of Interior Noise Levels in Cars

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Automotive Interior Noise Measurements:Tests

For those of us who prioritize comfort and refinement in terms of the interior noise level in a car, it is great that there are various tests published.

There are way too few of them, but at least there are some out there to guide us – yet in the end, it is essential for us to test drive cars ourselves in order to assess how WE perceive the noise. It is not only the noise level measured in decibels/dB that will have an impact on us, but the actual frequency range of the noise has a tremendous effect on whether we find a certain type of noise gruesome or acceptable.

Automotive noise tests represent a great tool to help us pick out which cars within our budget range to test drive.

The tests are especially relevant as a means of comparing different cars evaluated in one and the same test, since the authors behind it will hopefully have driven the cars on the same types of road surfaces and using the same type of noise measurement methodology.

BUT I would like you to be mindful of that there are things that can skew the test results.

One such aspect is wheel size. It’s fair to generalize and say that larger wheels contribute to more road noise than smaller ones. Larger wheels mean a thinner layer of rubber between the rim and the road, which will result in more vibrations being picked up, which in turn is converted into more noise inside the cabin.

The width of the tire will also have an impact on the noise level, with narrower tires being less noisy than wider ones. Lastly, the specific tire model with its unique blend of rubber and pattern will impact the vibrations transferred from the rolling on the road.

In short, if cars being evaluated in a test are driven on different wheel sizes, tire dimensions and/or tire makes/models, then the noise results are not to be seen as directly comparable.

Here is an example to illustrate the issue:
A noise measurement study that was conducted in Sweden in 2008 placed the Renault Megane 1.6FF Touring on 5th place and the Citroen C5 V6 HDI Executive on 9th place. The Megane had 16-inch wheels on and the C5 18-inch.

The question is what would have happened if both cars had been driven on the same wheel size? On the other hand, the particular C5 model/variant tested can’t be driven on 16-inch (what guides minimum wheel size is usually the size of the brake discs), and so it is indeed tricky to conduct a test where all cars are actually driven on the same wheel size. But in order to understand the test data, this type of detail needs to be made available as part of the results.

In the case of the Swedish test referenced, all details were actually provided to the readers, but it is most common that only the decibel/dB data at different speeds are provided.

In some cases, cars tested will have laminated windows mounted – which are there to reduce wind noise – and that also will not represent a fair comparison to other cars with regular windows. On the other hand, it needs to be pointed out that some makes offer laminated windows, whereas many others don’t – so if noise-reducing windows is not even possible to order then those makes should be ‘punished’ for that. But to help us out when reading noise tests, a detail like laminated windows should be specified.

Lastly, the type of road surface may vary greatly in different markets across the world. In Sweden, where I reside, authorities have chosen to go with a very coarse/rough type of asphalt which makes our roads particularly noisy. Whereas, I know that for example the roads in Germany are generally much quieter. In the United States, I believe the road surface varies across different regions of the country; in part (or mainly?) due to the varying climate.

It can therefore be of relevance to be aware of in which country a given test was conducted to get a feel for the numbers provided. You will still be able to learn from tests in different parts of the world, but local ones in your specific market will closest reflect the conditions in which you will be driving your car after purchase.

To conclude, you can learn a lot from the results of noise studies, but please be mindful of some of the considerations described above.

And of utmost importance is to test drive yourself to see if a particular vehicle acceptably meets your requirements in terms of comfort and refinement on the roads in the area where you reside.

Please, comment below and share what the roads are like in your country.

To learn more about cars and sound/noise, get your copy of the Quiet Cars eBook – Your Guide to a Quieter Ride – plus join the Quiet Cars online community.

Image: Steve A Johnson

Quiet Cars - An Introductory Guide to a Quieter Ride (eBook and online community)
  • Wayne Brown

    Hi all.
    I live in Middelburg in the Eastern highveld of South Africa. There does not appear to be a benchmark as far as tar surface is concerned. On most of our ‘B” roads the surface consists of 8mm – 10mm size stones which creates a terrible deafening “roar” inside the vehicle cabin. Approximately 78-80 db at 120 km/h. This causes serious discomfort and after 10 or so km I have a splitting headache. The same vehicle can be driven on smooth tar and the sound level drop to around 67db. WHY WHY WHY?

    • http://magnuskahler.com/ Magnus Kahler

      Hi Wayne! Thanks for your comment. I share your frustration. My wish is for the suspension and wheel solutions of the future to be much better at absorbing vibrations instead of transferring them into the cabin, resulting in aggravating road noise. What car are you currently driving? Are there any changes you could make in terms of the size of the wheels and the type/model of tires?

      • Wayne

        Hi Magnus, thanks for your response. Apologies for belated
        response.

        I am 1.76m tall and weigh 65kg to give you an idea of my
        size. I have a few vehicles in my household and I drive them all from time to
        time and will share the experiences relating to consumption, comfort, noise and
        NVH. (all driven on the same stretch of road) As a car enthusiast, I have also driven many
        other vehicles and will comment where relevant.

        My current vehicle (soon to be passed on to 18yr
        old daughter) is a 2007 Hyundai Accent 1.6 GLS HS (high spec) with cruise control and
        tow bar as added options. Purchased new in August 2007 it now has 219000km on
        the odo. (the only non-maintenance item replaced was the electrical ignition
        switch at 198000km costing ZAR400.00) An extremely reliable transport appliance!
        However it is not a very comfortable car to sit in longer than 20 minutes.
        Bouncy and soft rear suspension, I have uprated spring for rear, shocks
        replaced at 215000km with Gabriel Gas Ryders. I get cramps in my ankle, knee
        and bum, and get serious lower back pain from driving this car. I have
        redesigned and repositioned the throttle pedal to suit my right foot. The
        interior sound levels are high, with excessive road/tyre noise filtering into
        the cabin. In 2011 I completely stripped the vehicle down (dashboard, console,
        seats, carpet and roof lining) to spray the components black (it was a hideous
        light tan colour) and to “soundproof” the car. I used a compressed sponge,
        white upholstery filling and bitumen roof sealer. The entire floor pan and boot
        was given 5 coats of the bitumen sealer, and sponge placed behind all interior
        trim panels, the firewall, the rear footwells and the in the boot panels. All
        the door panels were given sponge and upholstery filler treatment. The exercise
        lowered the sound levels slightly, but it still ineffective on rough tar. I
        changed from the original 175/65/14 steel rims and tyres to 195/55/15 alloys
        and tyres. I have just put on a new set of Dunlop SP6060 tyres hoping to lower
        the sound levels. It is slightly quieter and softer on the road but I was
        really hoping for more.

        I have considered Dynamat but the wife is
        not too keen on spending more money as my last cash outlay and efforts did reap
        the desired results. I opt to buy another vehicle and pass this car to my
        daughter! Hard driving (fast acceleration and 160kph) returns 11.5 km/l and
        good (slow acceleration and 110kph) returns 13.5 km/l. Noise=78-80db @ 120kph,
        58-60db @ 60kph

        Current daughter’s vehicle. 2004 Chev Aveo 1.5
        LT hatchback. Very comfortable, good handling car with good suspension. Spacious
        front and rear, comfortable front and rear. Excellent city runabout, easy to drive
        and park. Changed 185/60/14 steel rims and tyres to alloys and 195/50/15
        Bridgestone MY01 tyres. The most deafening car I have driven to date! At 60kph
        it is as noisy as a BMW 320D at 180kph!) Hard driving (fast acceleration and
        160kph) returns 10 km/l and good (slow acceleration and 110kph) returns 13 km/l.
        Noise=85-90db @ 120kph, 72-76db @ 60kph

        Has a Sony stereo with amp, Rockford
        Fosgate speakers to overcome the road noise.

        Wife’s’ car. 2010 Fiat Punto 1.2 Active (at the
        time it was the cheapest new vehicle in the Fiat showroom) Very comfortable,
        good handling car (for its narrow tyres 165/80/14) with good suspension.
        Slightly more firm than Chev. Spacious front and rear, comfortable front and
        rear. Excellent city runabout, easy to drive and park. The engine and tyre
        noise is fairly high, but is NOT tiring or offensive. This is a lovely car to
        drive and have found it to be more comfortable than a 1989 Mercedes Benz 200
        (W124 series) After a 200 km trip I had
        no aches or pains, and could still easily have driven further. In town
        returns (good driving) 16.5 km/l and
        highway 110km/h returns 19km/l (not a fast, powerful car – top speed is
        160km/h) Noise=75-80db @ 120kph, 58-60db @ 60kph. The engine noise is audible,
        but is not a bother. It is a pleasure to listen to, which confirms your theory
        that is the pitch of the noise which becomes tiresome, not only the noise
        itself.

        Sons car 2011 Fiat Bravo 1.4 T-Jet Sport. Firm,
        sporty suspension which is ideally suited to smooth roads. Excellent handling,
        brakes and dynamics. 225/45/17 Michelin Primacy tyres. Quiet inside. Only
        vehicle with lumber support adjustment. No left foot rest. NVH good.
        Noise=74-76db @ 120kph, 54-56db @ 60kph.

        I am in the process of searching for a newer (2010-2013)
        used vehicle that is fairly quiet and comfortable to drive, yet also
        economical.

        Regards

        Wayne

        • http://magnuskahler.com/ Magnus Kahler

          Hi Wayne! Thanks a lot for sharing your detailed experience of the different vehicles in your household. The Chev Aveo with a noise level of 85-90 dB at 120 kph sounds pretty bad. I was surprised by the equivalent numbers of the Fiat Bravo.

          Pls, let us know how things go in your search for a newer used vehicle.