How Can Type Approval Help to Control Motorcycle Noise

By Gijsjan van Blokland, Vice President Development and Outreach of I-INCE 

The sound of a motorcycle is not always pleasant to listen to. The frequent application of illegal replacement exhaust silencers (IRES) and the often observed very sporty driving style, characterized by high vehicle and high engine speeds produce peak noise levels that exceed those of regular cars and trucks with 10 to 20 dB.

A part of the solution to control the burden on the environment is improving the behavior of the riders in terms of descent usage of their bikes, a part is to be found in enforcing the technical state of the motorcycle and the exhaust system. A third part is to have a system of type approval in order to give market access to only civilized vehicles. However, even “road legal” vehicles can still produce very high noise levels due to the combined effect of relaxing sound limits and shortcomings in the test method. The test conditions are not really representative for the motorcycle operations found in reality and are not robust with respect to tampering devices, such as flaps in the exhaust, closed during type approval and open in the “noisy mode” and electronic devices that enable cycle beating and cheating [1].

Type approval of road vehicles in Europe

It is one of the objectives of the United Nation Economic Commission for Europe (UNECE) to stimulate trade between European countries and with other countries in the world.  One of the measures is to remove trade barriers between individual countries by harmonizing the technical specification of road vehicles. If a vehicle is type-approved according to the harmonized standard, then all subscribed member countries accept that vehicle, and no extra-national specification shall be enforced. Since all European Union countries subscribed to that agreement, the regulations define the noise characteristic of all motorcycles entering the market in the EU27. Next to the objective of stimulating international trade is an objective to control negative effects on the environment.

The type approval regulations of motorcycle noise are rather bulky since they cover a wide range of motorcycles, including mopeds, small motorcycles, high-powered recreational bikes, and all types of three- and four-wheelers. For this paper and for the dominant contributor of noise nuisance the focus lies on the medium to high powered two-wheelers, category L3, especially those with engine capacity > 175 cm3.

Regulation no. 41 defines the requirements with respect to sound emission of L3 categories. It defines the maximal allowed sound levels and the test procedure to establish the sound level. Limit values are given in Table I.

Measurement procedure for motorcycles 

The test procedure used to be very simple. Enter the 40 m long test section with 50 km/h in 2nd and in 3rd gear, push the pedal-to-the-metal, drive 40 m and release the accelerator. Halfway the LAmax is measured at 7,5 m distance from the center line.  

Recently this procedure is harmonized with normal daily driving conditions and this results in a procedure that is hard to explain and even harder to perform. Basically, it consists of testing the vehicle at 50 km/h under partial throttle condition in front of the microphone at an acceleration value aurban which is defined by the PMR (Power-to-Mass Ratio in [W/kg]) by:  

aurban = 1,28 * log(PMR) – 1,19 

This partial throttle acceleration aurban is constructed from a weighted average of a wide-open-throttle (WOT) pass-by and a constant speed (cruise-by) test. The selected gears during the test and consequently the engine speed (since vehicle speed is fixed at 50 km/h), is controlled since in the WOT test a specific target acceleration has to be reached, given by: 

aWOT =3,33 * log(PMR) – 4,16 

For high powered vehicles the weighting between cruise-by and WOT pass-by levels is typically 50/50, while for lower PMR’s (35-40) weighting is more like 20/80 (cruise/WOT).  

The type-approval procedure also includes an Additional Sound Emission Provision (ASEP) that states that at conditions outside of the tested range of engine speed and engine load, the vehicle’s noise emission shall not deviate from what to expect from the test values. This ASAP, unfortunately (for the environment) does not involve real driving conditions at the higher speed [2]. 

Noise source contribution during testing 

The contribution of the different noise sources of a motorcycle to the overall noise level during a WOT pass-by test and during a cruise-by test is displayed in Figure 1. Although the traces display the contribution along a 30 m path, most relevant is what happens at the moment of LAmax that typically occurs a few meters after passing the microphone. During WOT testing LAmax is nearly totally defined by the exhaust. During cruise-by exhaust, driveline and engine are at about the same level.  

The variability of some sources observed along the 30 m section can be explained by interference between direct and reflected path from tonal sources and acoustic and mechanical resonances in the drivetrain. 

Additionally, a stationary (or fast-idle) test is performed with the microphone 0,5 m from the exhaust(s) at an angle of 45° and an engine speed of 50% or 75% of the rated engine speed. Such a test is meant to enable roadside inspections. The relation between the fast-idle test and the level recorded during the accelerated pass-by test is nearly non-existent, as is shown in Figure 2.  

Figure 1: noise source ranking during a type approval test. Vehicle: L3 motorcycle of 800 cm3 and PMR>50. Exhaust is the dominant source during the accelerated (WOT) test while the engine and the driveline are the major systems in the cruise-by test. Source: [2].  

How can type approval help to control motorcycle noise 

It is clear that type of approval cannot restrain the drivers from opening their gas handles and enjoying the high engine speeds and accelerations that will even make regular bikes become very noisy. But apart from this, there are many areas where type approval regulations can make a difference.  

It can regulate the sound emission of the new bikes entering the vehicle fleet. The present R41 limit values seem to be lowered relative to the former EURO4 levels, but that is mainly an effect of the introduction of the new testing procedure. The present levels imply the same technical status as the former levels. The limits have effectively not changed since the nineties and technology are available to lower limit values with 2 dB. In [2] the Costs and the Benefits of tightening limits were investigated and it was concluded that a 2 dB lower limit is not only technically feasible but would bring two times more than it will cost. 

The ASEP in the present type approval system is limited with respect to the driving conditions found to be causing the majority of annoyance. Extending the speed range would certainly be beneficial for controlling the emitted noise outside of the tested conditions. 

Improve standstill test  

It is a shared view from all parties involved, motorcycle manufacturers, environmental authorities, experts, the public, and even the drivers that the sound characteristic of the bike is determined by the choice of exhaust. The manufacturers since their investigations show that the exhaust is dominant in the type approval test procedure, the authorities because their noise monitoring stations record high peak levels, not explained by vehicle or engine speed, the public because they experience unusual loud bikes and the drivers since they increase their acoustic presence by ordering the right IRES from the internet shop.  

Enforcement of “loud pipes” is hampered by the low representativity of the fast idle test. As is shown in Figure 2 a motorcycle that meets the (former) 80 dB limit can have stationary sound levels between 80 and 110 dB. Even when a 5 dB margin is applied during a roadside test, a vehicle with an official stationary level of 85 dB(A) testing at 95 dB(A) will be fined while a fellow driver with 105 dB(A) may be considered perfectly legal.  There clearly is a need for a more representative stand-still test for roadside checks. 

Nevertheless, you better take care that the fast idle level of your bike is ≤ 95 dB when you want to drive specific scenic roads in the Alps this summer since the government of the Austrian Bundesland of Tirol applies the fast-idle test level to control noise nuisance along their roads [4].   

Figure 2: Stationary versus. acceleration test results (L3 vehicles with PMR > 50). The indicated type approval limits refer to the former test procedure that did not include cruise-by conditions. Source: [2]
Figure 3: Roadside check of a motorcycle noise compliance. The picture was taken from [5]

Wrap-up of findings 

Motorcycles are not a large contributor to overall equivalent traffic noise levels, but they represent significant annoyance due to their high peak levels and their interference with the pleasantness of rural areas on sunny weekends.  

The European type approval regulations do limit the sound production of motorcycles, but they do not represent the state of art anymore. There is room for a further 2 dB lowering of limit values. Such lowering will cause extra costs but the benefits for society will be much larger [2].   

The Additional Sound Emission Provisions and the stationary soundcheck in the type approval procedure are considered inadequate to control the emitted noise during aggressive driving and to enforce illegal silencers. It is estimated that those two will increase average motorcycle fleet emission by more than 10 dB [3]. This has to be addressed.  


[1] Dittrich, M, Papadimitriou, G., Steven, H Ntziachristos, L., Durampart, M., “Developments in Regulations for Sound Emission of L-category vehicles”, Proceedings Euronoise 2018, paper nr 174. 

[2] Papadimitriou, G., Ntziachristos, L., Durampart, M., Dittrich, M., Steven, H., “Study on Euro 5 sound level limits of L-category vehicles”, November, European Union 2017, doi:10.2873/7388 

[3] “Motorcycle noise: the curious silence, a report by the motorcycle industry”, International Motorcycle Manufacturers Association, Geneva 


[5] ACEM, “Transport sustainability, sound emissions, and noise: A collective responsibility for a shared responsibility”, 2021.