By Alberto Behar, Ryerson University, Toronto, Canada
Here is a very common situation: you are in your front yard, by your swimming pool, enjoying a good book. Nice, sunny day, a pleasant breeze. Quiet. Suddenly a harsh noise makes you jump out of your seat and a rush of adrenaline makes all kinds of disorders inside your body. You look around for the source of this disturbance and by the time you are up looking, the source is gone. If you are close to the street, you may smell a whiff of gas. The culprit is gone in a second and again all is quiet.
It also happens on the highway. You are driving nicely, aware of who is around and not being prepared for anything unusual, when, suddenly, there is this thunder out of the blue sky and this motorcycle speeds up avoiding cars left and right and disappearing from your sight and your hearing in no time.
So, big deal, just a motorcycle speeding away, his driver enjoying the feeling of omnipotence, like being King of the Universe.
Is this a problem with the motorcycle or of the driver?
Let’s examine a little but what’s going on. A motorcycle is a bicycle most often driven by a one-cylinder, two or four-stroke engine. In this internal combustion engine (ICE) the combustion of the fuel with an oxidizer (usually air) is ignited by a spark plug. The expansion of the high-temperature and high-pressure gases transforms chemical energy into useful mechanical energy that drives the motorcycle.
So, how do manufacturers control noise? They use silencers that are so efficient, that if properly maintained make the noise insignificant!
So, why motorcycles are noisy? Very simple: their owners choose to replace the manufacturer’s silencer with so-called “after-market” silencers that reduce very little the noise or modify the spectrum so that they sound “powerful”! Some users even leave the engine without any silencing device. Prof. Fuchs in the Argentinean city of Cordoba once conducted a survey of the motives behind removing or modifying the silencer. The results were multiple, but the underlying motive was to make them visible! (Or shall we say “important”?).
So, here we have a perfect environmental annoyance generator, very seldom, unfortunately, controlled by the authorities…Now, what about the driver of the motorcycle?
Is there a potential hearing hazard?
Due to the duration of the phenomenon, this noise can be classified as “annoying” and there should be no hazard for permanent noise-induced hearing loss to the casual receiver. This, of course, does not apply to people involved in motorcycle maintenance who are exposed for an entire work shift, 5 days a week.
Another exception is for those attending motorcycle racing events. There too noise levels and the duration of the exposure can generate hearing hazards to the attendees.
Now, what is the situation of the person riding the motorcycle? Obviously, he is exposed to the engine noise all the time the engine is going on. Also, he is much closer to both the engine and the exhaust (silenced or not). The result is higher noise levels for longer time periods and potentially hazardous situations.
Surprisingly enough, engine/exhaust noise is the only hazard motorcycle rides are exposed. There is another source, also serious, consisting of the aerodynamic noise created by the turbulent airflow around the helmet— the so-called wind noise. Several studies were done using essentially similar techniques: a miniature microphone is placed at the rider’s ear under the helmet and sound levels are measured in various riding conditions. All these studies show excessive wind noise around the helmet – about 90 dB(A) at 60 km/h and increasing linearly when plotted again the log of speed, to reach 110 dB(A) at 160 km/h!!! (1)
Temporary threshold shift has been reported after only 1 hour of high-speed riding as a subjective complaint of tinnitus. After long periods at high speed, riders commonly report other non-specific complaints such as fatigue, headache, and even disequilibrium.
What about the attenuation of the helmet
Does the helmet provide some attenuation to the rider? The answer is “no”. Modern helmets offer very poor low-frequency sound attenuation. There is also a phenomenon of resonance at 250 Hz. The source is a turbulent boundary layer, vibrating against the outside of the helmet shell, with its maximum sound energy focused between 250 and 500 Hz. The best protection for the rider is still the use of earplugs.
Broadly, there are two types of motorcyclists—amateur and professional. Professional riders can be further subdivided into racers, dispatch riders, and police motorcyclists. Motorcycle noise hazard is basically dependent on the speed and duration and frequency of the rides (their noise exposure level)
Regarding the annoying side of the noise, the obvious measure is better policing, something that has proven to be inexistent or inefficient. Switching to electric bikes in the future appears as the only viable solution.
(1) McCombe, A., J R Soc Med January 2003 vol. 96 no.1 7-9