ISO 8579-1: Acceptance code for gears – Part 1: Determination of airborne sound power levels emitted by gear units

Summary

ISO 8579-1 is a specialized standard that provides a detailed procedure for measuring and reporting the airborne noise emitted by an enclosed gear unit. This standard is an “acceptance code,” meaning it is primarily intended to be used by manufacturers and customers to verify that a new or repaired gearbox meets a specified acoustic performance level. Unlike vibration analysis which focuses on structural vibration for fault detection, this standard is concerned with quantifying the overall sound power level for purposes of environmental noise control and occupational health.

Table of Contents (Conceptual Structure)

The standard is a detailed test code, outlining the precise steps required to obtain an accurate and repeatable noise measurement:

1. 1. Scope and Measurement Principle:

   This initial section defines the standard’s specific application, which is the determination of airborne sound power levels for enclosed gear units. It is explicitly an “acceptance code,” meaning its procedures are designed to verify compliance with a pre-agreed acoustic specification between a manufacturer and a purchaser. The fundamental principle is to calculate the sound power level, an intrinsic property of the source, from a series of sound pressure level measurements. This is done by taking measurements at multiple, precisely defined points on a hypothetical surface that envelops the gearbox, thereby capturing the total radiated sound energy.

2. 2. Test Environment and Conditions:

   This chapter provides strict requirements for the location and conditions of the test to ensure that the only sound being measured comes from the gearbox. It specifies that the test must be conducted in an acoustic environment that approximates a free field, meaning there are no reflecting surfaces nearby that could corrupt the measurement. An anechoic chamber is ideal, but a large, open outdoor area can also be used. Crucially, the standard mandates that the background noise from all other sources (including the motor driving the test gearbox) must be measured separately and must be at least 6 dB (preferably more than 10 dB) lower than the gearbox noise. If the background noise is too high, it will contaminate the results and invalidate the test. The gear unit must also be operated under a specified load and speed, as these conditions dramatically affect the noise generated.

3. 3. Instrumentation:

   This section specifies the performance requirements for the equipment used to conduct the test. It mandates the use of a Type 1 (or Class 1) precision Sound Level Meter, microphone, and filter set that conform to the relevant international IEC standards. This ensures a high degree of accuracy and consistency in the measurement chain. The standard also requires the use of a Sound Calibrator of the same precision class. It specifies that the entire measurement system (microphone, meter, cables) must be calibrated using this device both immediately before and immediately after the series of sound measurements. This confirms that the instrument’s sensitivity has not drifted during the test, which is essential for a valid acceptance test.

4. 4. Measurement Procedure:

   This is the prescriptive core of the standard, detailing the exact steps to be taken. It requires the definition of a hypothetical measurement surface that completely envelops the gearbox. This is typically a rectangular parallelepiped (a box shape) positioned at a fixed distance (usually 1 meter) from the reference surface of the gearbox. The standard then specifies the minimum number of microphone positions and their exact locations on this surface. For a parallelepiped surface, this is typically a set of nine points, covering the four sides, the top, and key locations in between. The sound pressure level is measured at each of these points while the gearbox is operating under the pre-agreed steady-state load and speed conditions. This multi-point measurement is necessary to accurately average the sound field and account for the directionality of the noise.

5. 5. Calculation of Sound Power Level:

   This chapter provides the mathematical formulas to process the raw data. First, the sound pressure levels (which are on a logarithmic dB scale) measured at the different microphone positions are logarithmically averaged to determine the mean sound pressure level over the entire measurement surface. This average value is then used to calculate the sound power level (Lw). The formula for this calculation takes into account the area of the hypothetical measurement surface (S). The final sound power level is a single number, expressed in dB, that represents the total acoustic energy radiated by the gearbox. This value is independent of the measurement distance and environment, making it the definitive metric for the acceptance test.

6. 6. Information to be Recorded and Reported:

   To ensure that the test results are unambiguous, comparable, and fully traceable, this final section provides a comprehensive list of all the information that must be documented in the official test report. This includes not only the final calculated sound power level, but also: a complete description of the gear unit (model, serial number, etc.); the precise operating conditions (input speed, output torque, lubricant type and temperature); a detailed description of the test environment, including a sketch of the room and the microphone positions; a list of all instrumentation used, including serial numbers and calibration dates; and the results of the background noise measurements. This rigorous documentation ensures the test’s validity and allows it to be reliably reproduced if necessary.

Key Concepts

• Sound Power vs. Sound Pressure: The standard focuses on determining sound *power*, which is the total acoustic energy radiated by the source. This is different from sound *pressure*, which is what is actually measured and varies with distance. Sound power is a more consistent metric for an acceptance test.
• Acceptance Test Code: The standard is not a diagnostic tool. It is a standardized “pass/fail” test procedure. A customer can specify a maximum acceptable sound power level in a purchase contract, and this standard provides the method to verify compliance.
• Vibro-Acoustic Link: While this standard measures airborne sound, the root cause of that sound is the structural vibration of the gearbox housing, which is in turn caused by the meshing of the gears. Therefore, high noise levels often correlate with high vibration at the Gear Mesh Frequency (GMF).

← Back to Main Index