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Measuring Motorcycle Sound Levels: A Guide to SAE J331-2020
This recommended practice establishes a comprehensive method for measuring total motorcycle sound pressure levels under full throttle acceleration and closed throttle deceleration. Unlike standards focusing solely on exhaust noise, SAE J331 captures contributions from exhaust, intake, mechanical components, tires, and road interface, providing a complete picture of real-world motorcycle sound.
🛠️ Design Insight: SAE J331 is designed to measure the total sound emitted during typical in-use operating conditions, not just the maximum sound potential. This makes it a practical tool for evaluating compliance with community noise ordinances and for benchmarking vehicle performance. It carefully differentiates between manual and automatic transmission procedures to reflect real-world operating characteristics.
Test Environment and Equipment Requirements
To ensure accurate and repeatable measurements, the standard specifies a carefully defined test site. The site must be flat, open, and free of large sound reflecting surfaces within 30 meters of the measurement area. The surface within the triangular measurement area must be dry Portland cement or bituminous asphalt concrete, with no snow, soil, or extraneous materials.
| Parameter | Specification |
|---|---|
| Microphone position | 1.2 m (±0.02 m) above ground, perpendicular to vehicle path |
| Microphone distance from vehicle path | 15 m |
| Acceleration point (before microphone) | 7.5 m |
| End point (beyond microphone) | 30 m |
| Wind speed limit | < 5.5 m/s (12 mph) |
| Sound level meter | Type 1 or Type S1A per ANSI S1.4-1983 |
| Calibrator accuracy | ±0.5 dB |
Instrumentation includes a sound level meter with fast response and A-weighting, an engine speed tachometer with ±3% accuracy, a speedometer, and an anemometer. Ambient sound must be at least 10 dB below the peak vehicle sound, and tests are only valid when wind speed is below 5.5 m/s. A windscreen may be used if it meets the frequency response criteria.
Acceleration and Deceleration Test Procedures
The standard provides distinct procedures for manual and automatic transmission motorcycles to account for different operating characteristics. Both aim to capture sound emitted during typical in-use acceleration or deceleration.
Manual Transmission Test
The vehicle must be in second gear at a constant approach speed corresponding to either 60% of rated engine speed or 50 km/h (31 mph), whichever is slower. When the front of the vehicle reaches the acceleration point (7.5 m before the microphone), the throttle is rapidly opened fully. Acceleration continues until the vehicle reaches the end point (30 m beyond microphone) or rated engine speed is attained, at which point the throttle is rapidly closed fully.
Automatic Transmission Test
Starting from a standstill at the acceleration point with the transmission in the lowest selectable range, the throttle is rapidly opened fully. The vehicle accelerates until it reaches the end point or rated engine speed. Motorcycles offering manual gear selection but functioning as an automatic in normal use are covered by this procedure.
Deceleration Test
Unless the maximum sound clearly occurs during acceleration, a deceleration test is performed. The vehicle approaches the end point from the reverse direction in the same gear used during acceleration. The throttle is closed at the end point, and deceleration continues until half rated engine speed or 15 m past the microphone, whichever comes later.
⚠️ Common Mistake: Failing to use second gear or incorrect approach speed for manual transmission tests. Always verify the gear and speed as per the standard. Also, ensure the rider mass (including equipment) is between 75 kg and 90 kg; if necessary, add ballast. Wheel slip and front wheel lift must be avoided as they can affect sound levels.
Frequently Asked Questions
Why must the test site be free of reflecting surfaces within 30 m?
Reflections from buildings, signs, or vehicles can interfere with the direct sound wave from the motorcycle, causing erroneous measurements. The 30 m requirement ensures a free-field condition above the ground plane.
What if my rider mass exceeds 90 kg?
The standard requires a combined rider and equipment mass between 75 kg and 90 kg. If the rider is heavier, the test cannot be performed as written; a lighter rider or alternative arrangement should be considered.
How many test runs are needed to determine the maximum sound level?
Sufficient runs must be performed to establish the maximum levels. Typically, at least two runs that agree within 2 dB are required to ensure repeatability. The final reported value is the highest sound level recorded.
Can I use a windscreen on the microphone?
Yes, but the screen must not affect the microphone response more than ±1 dB for frequencies from 63 to 4000 Hz and ±1.5 dB from 4000 to 10000 Hz, considering the orientation of the microphone.
