Two motorcycles of similar capacity and power can sound nothing alike. The difference comes from the arrangement of the cylinders and the timing of the crankshaft, not from the exhaust system alone.
Where the sound originates
An engine's exhaust note is a sequence of pressure pulses released each time an exhaust valve opens. The spacing of those pulses determines the rhythm the ear perceives.
A four-cylinder engine with evenly spaced firing produces pulses at regular intervals, which merges at speed into a smooth, high-pitched wail.
A large twin fires far less often with much bigger individual events, so each pulse remains distinguishable and the note stays low and separated.
Why crankshaft design matters more than cylinder count
Two engines with the same number of cylinders can sound entirely different depending on where the crankpins sit relative to each other.
Placing them at unequal angles makes the cylinders fire in an uneven pattern, grouping the pulses rather than spacing them evenly. That is the origin of the offbeat character certain twins are known for.
The same principle applied to a four-cylinder engine produces the deeper, syncopated note associated with crossplane designs, using ordinary cylinders in an unusual order.
What uneven firing does mechanically
Firing intervals are not only an acoustic choice. Grouped power pulses give the rear tyre brief gaps in which to recover grip, which some riders find easier to control at the limit.
Uneven firing also produces vibration that must be countered with balance shafts, adding weight and complexity.
The sound is therefore a side effect of a decision made about traction and power delivery, not the goal of it.
How the exhaust shapes what escapes
The exhaust system does not create the note but filters it. Pipe length, diameter and the volume of the silencer determine which frequencies are reinforced and which are absorbed.
Collector design matters too, because joining pipes from different cylinders lets their pulses interact before leaving the system.
This is why an aftermarket system changes the character noticeably while leaving the underlying rhythm of the engine intact.
Why noise regulation reshaped engines
Drive-by noise limits are measured under defined conditions, and meeting them has required larger silencers, more restrictive routing and catalytic converters that also damp sound.
Manufacturers have responded by tuning intake noise instead, since air drawn in through the airbox reaches the rider directly and is measured differently from the exhaust.
The result is that the sound a rider hears increasingly comes from in front of them rather than behind, which is a genuine change in how these machines are experienced.