How To Measure Exhaust Sound

By describing exhaust pipe installations we provide useful data so that the reader-and potential customer-can make an informed decision about the product. Information on price, product specifications, and dynamometer performance numbers are easily obtainable facts and standardized enough for valid comparison. Information on appearance and sound, however, is usually subjective and left to the eye or ear of the beholder. Nevertheless, recent attention on noise levels of motorcycles suggests that we might include a more defined measurement of sound in our descriptions.

For our purpose, we will treat sound level, measured in decibels (dB), the same as horsepower and torque, not as absolute but as relative to a familiar instance, usually stock. By providing a dimension of sound for both a stock exhaust and for a modified exhaust, we give the reader another point with which to gauge comparative performance.

We have based our sound test loosely on the SAE International (SAE) J1287 Measurement of Exhaust Sound Levels of Stationary Motorcycles Standard. We chose it because SAE designed this test procedure to measure exhaust noise, not total vehicle noise. The SAE Standard establishes precise test procedures, environment settings, and instrument specifications for use in conducting a sound measurement.

The SAE Standard stipulates that the sound meter microphone be located 0.5m behind, at the same height as the exhaust outlet, and at a 45-degree angle to the motorcycle's line of travel. The sound level is recorded during steady-state engine operation at half the engine's maximum rated rpm. We used a RadioShack Digital Sound Level Meter (Model: 33-2055, $49.99).

Our first test was on an '07 Road King Custom-first the stock pipes, then with a set of Hard Krome T-REX 3-inch True Duals. At half throttle the stock pipes yielded a docile 98 decibels while the T-REX produced a rumbling 116 decibels.

The products and procedures discussed in our articles are for demonstration, off-highway, or competition use only. EPA, competition, or relevant land-use sound regulations should be consulted. Use on public roadways may violate federal, state, or local laws imposing sound limits. Consult your local DOT regulations for more information.

Appropriate ear and hearing protection is advised.