How Loud Is a Motorcycle? Decibel Levels Explained

"How loud is a motorcycle?" usually means one of two things. A neighbour asking is thinking about the sound radiating outward, which is what type-approval regulations measure with a microphone at the roadside as the bike accelerates past. A rider asking is thinking about their own ears, where wind noise around the helmet dominates above town speeds.

Those two numbers are not versions of each other. A machine that comfortably passes an 80 dB pass-by test still exposes its rider to well over 100 dB(A) at motorway speed, because the pass-by test does not measure — and is not trying to measure — what happens inside a helmet at 70 mph.

The most detailed dataset we found is from the Institute of Sound and Vibration Research at the University of Southampton, presented at Internoise, combining on-road measurements with wind-tunnel work.

Two findings stand out. First, the rate of increase: above about 25 m/s (roughly 56 mph), level rises around 15.5 dB per doubling of speed — far steeper than engine noise alone would give. Second, the character: the on-road and wind-tunnel results together attribute the dominant contribution at speed to airflow around the helmet rather than to the machine.

The earplug results are worth reading twice. With plugs fitted, the study measured 74 dB(A) at 22 m/s and 80 dB(A) at 32 m/s — below the level where sustained exposure becomes a concern, at speeds where the unprotected figures are far higher.

This is also the mechanism that makes helmet marketing claims hard to evaluate: the sound reaching the ear is generated by turbulent airflow at the helmet's edges and vents, so fit and aerodynamics matter as much as material or padding.

In the United States, 40 CFR Part 205 Subpart D sets noise-emission standards for new street motorcycles: 83 dB for the 1983 model year and 80 dB for 1986 and later, with moped-type machines limited to 70 dB. Off-road motorcycles are limited to 83 dB then 80 dB up to 170 cc and 86 dB then 82 dB above 170 cc. All of these are measured with the standardised pass-by procedure in the regulation's appendix, not with a meter held wherever you like.

In Europe and much of the rest of the world, UN ECE Regulation 41 governs type approval. It uses a moving pass-by test and also records a stationary level measured close to the exhaust, which is noted in the vehicle's documents so an enforcement officer can compare a suspect machine against its own approved value.

SAE J2825 is the standard behind most roadside enforcement of aftermarket exhausts in the US: it defines a stationary test at a fixed microphone position and engine speed, reporting the maximum fast A-weighted level. It is a procedure, not a limit — the limit is set by whichever state or city adopts it.

Because pass-by testing requires a test site and a procedure, most enforcement in practice uses a stationary test such as SAE J2825, and most numbers quoted in conversation come from neither. A reading taken a metre behind a tailpipe, a roadside pass-by measurement and a rider-ear figure can all be honest measurements and still differ by 30 dB or more.

If you want to compare two exhausts meaningfully, the comparison has to hold the method constant: same position, same engine speed, same meter, same weighting. Otherwise you are comparing methods, not exhausts.

There are two reasons a phone or a browser is a poor instrument for this. Inside a helmet, the microphone is in a turbulent airflow and measures its own wind noise as much as the sound field. Outside, a pass-by measurement requires a known distance, a known speed and a defined site.

What our browser meter can do honestly is relative comparison on the same device: whether one exhaust reads higher than another at the same standing position, or whether earplugs change what your own microphone picks up. Uncalibrated it reports A-weighted signal level in dBFS rather than sound pressure, and even calibrated it is an estimate, not a type-approval instrument.