Rorty Zorst

During this year’s Abu Dhabi Desert Challenge, where I was marshalling all week, I got to thinking about motor vehicle noise. It’s obvious that racing cars and bikes produce a lot more noise than normal road vehicles.

Bikes on the Desert Challenge may not exceed 115dB(A) at 2 metres when inspected at the scrutineering just prior to the event. That’s very loud indeed, and it was just as well the test that was being carried out on each competitor just a few metres from where I was working only took a couple of seconds. It’s a lot louder than normal road vehicles, for which I suspect most of us are grateful.

Quoting numbers of decibels is actually pretty meaningless without specifying the distance from the source. The further away you are, the quieter the sound. By way of example, in the United States, the Federal limit from the EPA for motorcycle noise is 80dB(A) at 50 feet. Some high-school physics tells us that this is the same as 109dB(A) at 20 inches, or 98dB(A) at 2 metres. So when New Hampshire specifies a limit of 106dB(A) at 20 inches, this is actually fairly close to the EPA limit. The whole issue of distance is deliberately obfuscated by manufacturers of after-market horns: “138dB(A),” the blurb says, “at 100mm.” That’s 112dB(A) at 2 metres. It’s still a lot louder than the comedy horns fitted by most vehicle manufacturers, but less than the noise made by Marc Coma and his buddies on the ADDC.

Here’s the thing. There are plenty of people who believe that motorcycles alone are responsible for antisocial vehicle noise. If you live in JBR in Dubai, or the Pearl in Doha and you’re not deaf as a post, you’ll have been disturbed by the late-night antics of the Inadequate Silencer Owners Club. And some of these are indeed motorcycles. I wonder how many are performance cars? One solution, the one adopted by Sharjah Police in January this year, is to ban motorcycles from Sharjah’s main roads after 10pm. This, instead of ticketing the driver of every antisocially-loud vehicle.

Typical government reaction is to legislate for ever more stringent noise limits. Manufacturers comply with larger, heavier, more complicated, and more expensive exhaust systems. This in turn encourages an increased proportion of motorcyclists to switch to after-market systems. Reasons include saving weight and cost; another is to make the machine sound like a motorcycle and not a sewing machine. I contend that if the legal limits were set at a more easily achievable level, fewer bikers would replace their stock systems.

My own machine has a manufacturer’s plate stating “95dB(A) at 4400rpm.” It doesn’t specify a distance. When I bought an after-market silencer (yes, I too dislike the gigantic 28lb bazooka dangling off my bike), one feature that I liked was that the manufacturer was extremely candid about how noisy it was. 93dB(A) at 20 inches – the EPA test. This is an identical result to the OEM system, and less than what's on the manufacturer's plate and what's engraved on the original silencer. The “race” performance full system from the same exhaust system company produces 99dB(A) at 20 inches, which is significantly louder but still below the EPA limit.

For cars, the limit set by the European Union from 2012 is 71dB(A). I’ve been unable to find the distance, nor the speed, nor even the pavement surface. All these factors are significant. You experience around 80dB(A) from traffic when you stand on the kerb 5m from a busy road. I wonder, given this 71dB(A) limit, why a Mustang, or a Lamborghini, or a Ferrari all seem to be allowed, in standard manufacturer’s trim, to be significantly louder than pretty much any stock motorcycle? I guess most cars are very quiet compared with motorbikes, and most bikes are pretty quiet. Bikes are invisible anyway, so it’s generally assumed that all bikes: the ones that Mr Joseph Public notices, are loud.

EDITED 21 April. I've discovered a source for how motorcycle noise is measured in Europe. It's EEC Directive 78/1015/EEC. The test, essentially, is to accelerate the bike over a 20m distance in second or third gear between two microphones 15m apart. I found a research paper that compared the results of the European Union test with a static test. It's here. In short, the static test produced sound pressure levels that were up to 13dB(A) higher than the EU drive-by test, with almost all results falling within two standard deviations of the drive-by mean. A factory standard Honda Fireblade produced 83dB(A) in the drive-by test and 95dB(A) in the static. That's equivalent to 83dB(A) at 2m.

I’ve tabulated some sound pressure levels below, all sucked out of the intertubes and corrected to measurement at 2m, unless noted otherwise:-

Noise source              SPL at 2 metres

EU limit for cars         71dB(A) distance N/A

My bike at 5000rpm        81dB(A)

Inside A340 cattle class  85dB(A) distance N/A

Race exhaust for GTR      87dB(A)
Kerbside of busy road     88dB(A)

Inside London Tube train  94dB(A) distance N/A

EPA limit                 98dB(A)

Performance horn         112dB(A)

Diesel truck             114dB(A)

ADDC Moto                115dB(A)

The situation with dodgy after-market exhausts has improved over recent years. Manufacturers of many systems now dyno-develop them and get them certified as road legal. Beowulf in UK and Staintune in Australia, for example. The days of every chancer with a pipe bender and a supply of two-inch stainless steel tube are almost over.

Oh, and lest we forget, these are maximum noise levels. If you don’t ride around at maximum, wide-open throttle, your machine will be a lot quieter. Everyone, surely, owes it to common decency to keep the noise down when leaving at the crack of sparrowfart or getting home after a session of midnight oiling.

And then there are Harley-Davidsons. Mysteriously,these machines are “expected to be loud” and, with the exception of residents of JBR who have to be up for work at 6am, are tolerated by everyone including the vehicle inspectors at registration time, whereas riceburners have to sound like wristwatches…

Found on a HD forum:
Stock muffler 98dB(A)  (the EPA limit, measured at 2 metres)                          

Aftermarket 107dB(A)  Jeez…

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