Two motorcycles with similar power outputs can sound entirely different, and riders describe the difference as character. It is produced almost entirely by the spacing of the firing events.
Sound is a record of the firing order
Each combustion event releases a pulse of exhaust gas. What a listener hears is the rhythm of those pulses and the pressure waves they set up in the exhaust system.
An engine that fires at perfectly even intervals produces a smooth, continuous note. One that fires unevenly produces a pulsing sound with an audible gap.
Cylinder count matters only because it influences how those intervals are arranged, which is why two engines with the same number of cylinders can sound quite unalike.
Twins can be arranged for very different rhythms
A parallel twin with both pistons rising together fires at even intervals and sounds comparatively smooth, though it vibrates strongly because the pistons move as one mass.
Offsetting the crank pins changes everything. A ninety degree offset produces two closely spaced firing events followed by a longer gap, which is the source of the characteristic uneven beat.
V-twins achieve a similar effect through the angle between the cylinders, and the resulting pulse spacing is a large part of why the layout has such a devoted following.
An inline four smooths everything out
Four cylinders firing at even intervals across two crankshaft revolutions produce a pulse every half turn, which the ear perceives as a continuous note rather than separate beats.
Because the pistons move in opposing pairs, primary forces largely cancel, so the engine is smooth enough to rev far higher than a twin comfortably can.
Higher revolutions raise the fundamental frequency of the exhaust note, which is why the layout produces a rising wail rather than a thump.
Traction is affected as well as sound
Uneven firing delivers torque in distinct pulses with brief pauses between them, and there is evidence that those pauses allow a tyre to recover grip at the limit.
This reasoning drove the adoption of irregular firing intervals in racing engines, where the aim was rider feedback and traction rather than sound.
The audible character came along as a by-product, and manufacturers now design for it deliberately because buyers respond to it.
Why the difference is worth preserving
Emissions and noise regulation push exhaust systems towards heavy silencing, which flattens the differences between engines and removes much of what riders recognise.
Manufacturers respond by tuning intake resonance and airbox design, since intake noise is easier to shape than exhaust noise under the same limits.
The underlying rhythm survives regardless, because it comes from the crankshaft rather than the pipework, and it remains the clearest signature of a machine's layout.