TECH ED’S CHOICE
CABIN-NOISE MEASUREMENT
I read CAR’S article (May 2017) on cabin noise with interest. However, I was disappointed with your decision to add only idle noise to your road tests. Who cares about idle noise? How much time do we spend in a car idling compared with an hour’s trip on the freeway or even a 10-hour journey on holiday?
What I find most annoying is the amount of cabin noise in some cars when travelling at speeds in excess of 60 km/h. If you are going to add noise levels to your road tests, it would be much more useful to me to know the noise level of all cars, say at 80 km/h, tested on the same road in both directions on a fairly calm day.
I assume that most of the noise comes from tyres on the road surface, wind and rear axle noise, and some from the engine bay. I would definitely be interested in a car with less road noise so I can hear the radio or the people in the back seat more clearly. Therefore, the figures from a moving vehicle would be much more useful. BRUCE ABREY Port Elizabeth We share your frustration. The initial point of the sound-measurement test was to determine a noise reading taken at speed. Unfortunately, as explained in that technical article, this is not possible as for the following reasons: 1. The human ear can accurately discern a sound pressure level difference of only 2 DBA, and the difference of most cars in the same segment at the national speed limit is less than 2 DBA. 2. The noise perception of the human ear does not only take pressure level into account, but also the frequency distribution. The V8 purr of the Mercedes-amg S63 might be a more pleasing sound at a higher pressure level than the clatter of a diesel engine, even if it is at a lower pressure level. The study of human-ear sound perception is ongoing and complex. 3. The climatic conditions on the day of testing have a massive influence on the sound level. You are correct that tyre – and especially wind – noise are the main sound generators at speed. Wind (from any direction) can easily add more than 2 DBA to a reading, meaning that cars that aren’t tested on the same day in the same conditions can’t be compared regarding interior sound level.
Therefore, the only current sound test with repeatable results is the idle test that we conduct in our closed test garage under controlled conditions. We disagree that the idle test result is completely pointless, as many commuters spend a lot of time in their vehicles stationary in stop/start traffic each day.