TECH ED’S CHOICE

Car (South Africa) - - TECH - Reader Peter Pos­niak men­tioned his 1915 Pater­son has a sim­i­lar split-rim de­sign to the Con­ti­nen­tal patent cov­ered in Au­gust 2017. We’ll as­sume the sim­i­lar­i­ties end there, Peter...

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.

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