Motorcycle Exhaust dB Killers: What They Do and When to Use Them
A dB killer — also known as a baffle or noise insert — is one of the most misunderstood components in the aftermarket exhaust world. Many riders fit an aftermarket exhaust, toss the dB killer in a drawer, and never think about it again. This is often a mistake, particularly for UK riders who use their bikes at track days. Here's a clear explanation of what dB killers do and when you actually need one.
What a dB Killer Actually Does
A dB killer is a perforated insert that fits inside the silencer's end-can, reducing the effective diameter of the exhaust outlet. By forcing exhaust gas through a smaller, more tortuous path before it exits, the insert absorbs sound energy — particularly in the mid-to-high frequency range that the human ear finds most intrusive.
The noise reduction is typically 3–6 dB depending on the insert design, the silencer it's fitted to, and the engine it's attached to. This sounds modest, but decibels are logarithmic — a 3 dB reduction represents half the sound intensity. In practical terms, fitting a dB killer takes an exhaust that sounds aggressively loud at idle and in traffic to something that's loud but tolerable.
The Effect on Performance
Restricting exhaust flow with a dB killer does affect engine performance — but the magnitude is often overstated. On a typical modern motorcycle with a quality aftermarket exhaust, fitting a dB killer reduces peak power by approximately 1–3bhp, with the greatest effect at high rpm where exhaust scavenging matters most. For road use, this is entirely irrelevant. For track use, most riders won't feel a 2bhp difference, though championship racers optimising for lap times will prefer to run without.
UK Track Day Noise Limits
Most UK circuits operate static noise tests at the start of each track day session. The limit varies by circuit and session type:
- Quiet track days: typically 98–100 dB, measured statically at 0.5m from the exhaust outlet at 3,500–5,000rpm
- Standard track days: typically 102–105 dB by the same method
- Race meetings (club level): often 105 dB static or a drive-by standard
The most common limit encountered at UK circuits is 102 dB. Many popular aftermarket exhausts on certain bikes exceed this without a dB killer — particularly open-ended race systems and some higher-end slip-ons on V-twins and large-displacement triples.
Static vs Drive-By Measurement: The Difference
The static test (the standard for UK track days) measures sound at a fixed point with the bike stationary and the throttle blipped to a specified rpm. Drive-by testing (used in competition and for road type-approval) measures peak sound as the bike passes a microphone at a set distance and speed. These are different measurements — a bike that passes a static test may fail a drive-by test, and vice versa.
Road type-approval (E-mark) uses drive-by testing. Track day noise limits use static testing. An exhaust with an E-mark for road use does not guarantee track day compliance — always check the silencer's static dB rating against your circuit's limit.
How to Know if Your Exhaust Needs a dB Killer for Track Days
Most quality aftermarket silencers have their approximate noise level published by the manufacturer. If your exhaust is listed at 102–105 dB without the dB killer, you'll likely need one for quieter track day sessions. Test this at home before your track day using a decibel meter app (they're imprecise but indicative) held at 0.5m from the outlet at 4,000rpm.
Road Use: Is a dB Killer Required?
For UK road use, your exhaust must not exceed the noise level for which your bike was type-approved. If your aftermarket exhaust is not E-marked and you're running without a dB killer, you could fail a roadside noise check. All E-marked aftermarket exhausts are sold with dB killers as standard for this reason — the E-mark noise test is conducted with the insert fitted.
Browse our full exhaust range — all our exhausts include dB killers where applicable. See also our guide to track day noise limits in the UK.