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SAE J986-2018: Sound Level Measurement for Passenger Cars and Light Trucks
The SAE J986 standard, originally issued in 1967 and cancelled in August 2018, provided a recommended practice for measuring the exterior sound levels of passenger cars, multipurpose vehicles, and light trucks with a gross vehicle mass rating (GVMR) of 4540 kg or less. The test procedure relied on fixed initial conditions, including a specified approach speed and gear selection, and involved both full-throttle acceleration and closed-throttle deceleration runs. Although the standard is no longer active, its methodology offers valuable insights into vehicle noise characterization and the evolution of testing practices. 🔍
Test Procedure: Acceleration and Deceleration
The acceleration test is the primary mode and must be conducted first. The vehicle approaches the measurement zone at a stabilized speed of 48 km/h ±1.2 km/h. The appropriate gear is the highest numerical ratio that allows the vehicle to reach or pass the entry point of the end zone before the engine attains rated engine speed (as defined in SAE J1349). Once the front of the vehicle reaches the start point (±1.5 m), the throttle is fully depressed as rapidly as possible. The test run ends when the engine reaches rated engine speed or the front of the vehicle reaches the end of the end zone.
For vehicles with automatic transmissions that tend to downshift, the standard mandates preventing the downshift via methods such as increasing approach speed up to 60 km/h, limiting fuel supply to 95% of full load, disabling the downshift mechanism per manufacturer recommendations, or using an electronic program to lock the gear.
The deceleration test uses the maximum engine speed attained during the acceleration test as the starting approach speed in the same gear. The throttle is fully released at the start point, and the vehicle decelerates until the engine speed drops to half the approach speed or the front reaches the end of the end zone. The deceleration test is not required if prior testing shows the acceleration mode yields the maximum sound level.
🛠️ Engineering Design Insight: The test mass must not exceed 125 kg over the curb mass (including driver and equipment). This limit ensures repeatable results by minimizing variations in vehicle loading. Additionally, the acceleration test must always be performed first, as it provides the terminal engine speed data required for the deceleration test.
Critical Specifications and Common Pitfalls
The standard specifies that the sound level meter must be set to A-weighting and fast (F) exponential time averaging, conforming to ANSI S1.4-1983 or S1.4A-1985. Ambient sound levels must be at least 10 dB below the vehicle’s sound level, preferably 15 dB, and wind speed must not exceed 19 km/h. Measurements are taken on both sides of the vehicle; four consecutive readings within 2 dB are averaged per side, and the higher side average is reported.
Vehicles with multi-mode exhaust systems, multiple driving modes, or variable/dual fuel capabilities must be tested in each mode or fuel setting to capture the highest sound level. In the case of multiple systems, a sufficient number of combinations must be tested to establish the highest level combination.
| Parameter | Acceleration Test | Deceleration Test |
|---|---|---|
| Approach Speed | 48 km/h ±1.2 km/h | Maximum engine speed from acceleration test |
| Gear Selection | Highest numerical ratio that allows reaching rated speed in zone | Same gear as acceleration test |
| Throttle Action | Fully depressed rapidly at start point | Fully released rapidly at start point |
| Termination | When rated engine speed reached or front reaches end of end zone | When engine speed drops to half approach speed or front reaches end of end zone |
⚠️ Common Mistake: Failing to test all drive modes, exhaust modes, or fuel types when applicable. SAE J986 requires that each unique configuration be evaluated to identify the worst-case sound level. Overlooking a performance mode or a secondary fuel type can lead to an incomplete noise assessment.
Frequently Asked Questions
What gear should be used for the acceleration test?
The gear must be the highest numerical ratio (lowest gear) that allows the engine to reach rated engine speed (as per SAE J1349) within the measurement zone. This ensures the vehicle accelerates under maximum load while staying within its rated power band.
How can automatic downshift be prevented during the test?
If the transmission tends to downshift from the selected gear, the standard permits four methods: (a) increase the approach speed up to 60 km/h, (b) limit fuel supply to 95% of full load, (c) disable the downshift mechanism according to the manufacturer’s instructions, or (d) use an electronic program to lock the gear.
Why was SAE J986-2018 cancelled?
The standard was cancelled in August 2018 because its vehicle operating conditions and specifications were deemed obsolete for current motor vehicles. The fixed 48 km/h approach and full-throttle acceleration mode did not represent real-world driving conditions, making the test mode incompatible with in-use driving. Modern vehicles often require updated test cycles that reflect actual traffic patterns.
This article provided a concise overview of the SAE J986-2018 recommended practice for exterior sound level measurement. While the standard is now cancelled, its principles continue to inform noise testing methodologies and underscore the importance of standardized, repeatable measurement conditions in automotive engineering.
