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Maximum Sound Level Potential for Motorcycles – An Overview of SAE J47-2020
SAE J47-2020 is a recommended practice that defines the test procedure, environment, and instrumentation for determining the maximum sound level potential of motorcycles under wide-open throttle acceleration and closed-throttle deceleration. Originally issued in 1975 and reaffirmed in 2020, this standard ensures consistent and reproducible noise measurements across different motorcycle types and test facilities.
Instrumentation Requirements 🛠️
The standard specifies precise instrumentation to guarantee accurate sound level readings. Key instruments and their requirements are summarized in the table below.
| Instrument | Specification |
|---|---|
| Sound level meter | Meets Type 1 or Type S1A per ANSI S1.4; calibrated every 2 years |
| Acoustic calibrator | Accuracy of ±0.5 dB; calibrated every 2 years |
| Engine tachometer | Steady-state accuracy of ±3% at 60% and 100% rated engine speed |
| Speedometer | Steady-state accuracy of ±3% at 100 km/h or max speed (whichever is less) |
| Anemometer | Steady-state accuracy of ±10% at 5.0 m/s |
| Microphone windscreen | Must not alter response by more than ±1 dB from 4000 to 10,000 Hz |
⚠️ Calibration critical: The sound level meter and acoustic calibrator must be calibrated at intervals not exceeding 2 years. Using uncalibrated equipment invalidates the entire measurement process.
Test Site Specification
The test site must meet strict dimensions and surface requirements to ensure acoustic consistency. Key elements include:
- A straight, level path surfaced with portland cement or bituminous asphalt concrete (dry and free of extraneous material).
- A measurement area defined by points D (15 m before microphone target) and E (15 m after) with a microphone point F located 15 m perpendicular to the vehicle path centerline.
- Microphone positioned 1.2 m ± 0.02 m above ground, with its reference axis perpendicular to the vehicle path.
- No large sound-reflecting surfaces (e.g., buildings, hillsides) within 30 m of the measurement area.
🔍 Site inspection tip: Before testing, survey the area for any temporary reflectors (parked vehicles, signs) that could compromise measurement accuracy. The ground surface must also be free from snow, soil, or gravel.
Test Procedure and Acceleration Point Determination 🔍
The core of the standard lies in correctly determining the acceleration point. Two approaches are defined based on clutch type:
Motorcycles with a Manual Clutch
To establish the acceleration point, the rider approaches the end point in first gear at a constant speed corresponding to 60% of rated engine speed. When the front wheel reaches the end point, the throttle is fully opened. The location where rated engine speed is reached (at least 7.5 m past the microphone target point) becomes the acceleration point for the test run in the opposite direction. The lowest gear that achieves this without wheel spin or clutch slippage is selected.
Motorcycles Without a Manual Clutch
These vehicles use a standing start from the end point. The lowest selectable gear is chosen, and the position where rated engine speed is reached (or the point just before the first upshift for automatic transmissions) defines the acceleration point. For continuously variable transmissions, the point where the vehicle reaches maximum speed or 100 km/h (whichever is lower) is used.
During the actual test run, the rider accelerates from the acceleration point at wide-open throttle until the end point is reached, then quickly closes the throttle. Measurements are taken on both sides of the vehicle, requiring either a second microphone point or runs in both directions.
Engineering Design Insights
The procedure is carefully engineered to capture the motorcycle’s maximum sound output under realistic acceleration conditions. Key insights include:
- Using 60% of rated engine speed as the approach speed ensures representative engine load comparable to real-world riding.
- Requiring at least 7.5 m of acceleration past the microphone target prevents premature throttle closure and ensures the sound level at rated speed is recorded.
- Trials to select the lowest gear (without wheel spin) maximize the sound level while maintaining safety and repeatability.
- The two-year calibration interval balances practical maintenance schedules with the need for high accuracy in Type 1 instruments.
Frequently Asked Questions
How is the acceleration point determined for motorcycles with a manual clutch?
The acceleration point is found by approaching the end point in first gear at a steady speed equal to 60% of the rated engine speed. When the front of the vehicle reaches the end point, the throttle is fully opened. The location where rated engine speed is reached (at least 7.5 m past the microphone target point) becomes the acceleration point for the test run in the opposite direction.
What is the required calibration interval for the sound level meter?
The sound level meter must be calibrated at intervals of no more than 2 years to ensure compliance with Type 1 or Type S1A requirements of ANSI S1.4. The acoustic calibrator also requires calibration every 2 years.
Why is the test site required to be free of reflecting surfaces within 30 m?
Large reflecting surfaces (like walls, hillsides, or parked vehicles) can cause acoustic reflections that distort the measured sound levels. The 30 m clearance ensures that only the direct sound from the motorcycle reaches the microphone, providing a true reading of the vehicle’s maximum sound level potential.
How is rated engine speed used in the test?
Rated engine speed is the rotational speed at which the engine produces maximum net brake power (per SAE J1349). It serves as the target speed during the acceleration point determination and the actual test. The motorcycle must reach this speed (or as close as possible) at the end point to ensure consistent measurement of maximum sound output.
By following the rigorous guidelines in SAE J47-2020, engineers and testers can obtain reliable sound level data that facilitates compliance, development, and comparative analysis. Always consult the latest version of the standard for complete details.
