Speed limit 30 – less noise?

Kurt Heutschi, a senior scientist in Empa's Acoustics / Noise Control lab, explains what noise does and why we perceive sound so differently.
Artificial noise: In Empa‘s «AuraLab,« a test subject assesses her perception of rail travel noise. Image: Empa

Mr. Heutschi, can noise actually be measured?

Noise is the term used to describe unwanted sound. In this context, unwanted means that it depends on a person's judgment whether a sound signal is noise or not. A sound measuring instrument cannot make this classification, i.e. noise is not measurable.

We perceive a buzz saw as annoying, but a rushing stream as pleasant. Why is that?

When evaluating sounds, humans tend to perceive natural sounds as less annoying or even as pleasant and enriching. Technical sounds tend to be rated as more unpleasant. The buzz saw is particularly extreme because its sound contains a very distinct tone, that is, a specific, dominant sound frequency. Street noise is more of a hissing sound, i.e., less annoying than the buzz saw at the same sound intensity. It is interesting to note that personal attitudes also have an influence on the evaluation. If, for example, I have shares in a company producing or operating wind turbines, their noise bothers me much less, since it signals that I am earning money.

Is traffic noise more acceptable in a city?

I think we have accepted in society that our need for mobility leads to noise. If we feel that this noise is unavoidable, we are much more likely to accept it. Where our acceptance quickly vanishes, however, is with posers, i.e., when cars are intentionally driven at extra loud levels. This does not satisfy a need for mobility, but rather personal passions are acted out.

How does the speed limit of 30 km/h compare to the speed limit of 50 km/h in terms of noise?

It depends on the type of vehicle, i.e. car, delivery vehicle or truck, plus the driving style also plays a role. But on average, a car emits about 5 dB less noise at 30 km/h compared to 50 km/h. This can be seen by comparing the maximum noise levels, i.e. when the vehicle passes at the shortest distance. However, if we look at the total amount of noise while a vehicle is passing us – or a resident – then the noise is reduced by only about 3 dB at speed 30. The reason for this is that the slower vehicle is significantly quieter, but it also takes longer to pass us.

Do we perceive noise differently during the day and at night?

The reduction of 5 dB in the maximum level plays a role primarily at night, since potential sleep disturbances particularly depend on the maximum level. During the day, when many vehicles are on the road, a constant noise level builds up – you could call it a sound carpet. At 30 km/h, this sound carpet is 3 dB lower than at 50 km/h. This is a noticeable improvement, but nowhere near as noticeable as at night. Of course, a 30 km/h zone only has an effect if everyone adheres to the speed limit. Even a few speeding drivers would generate large and highly annoying noise peaks.