Sometimes a gearbox problem can be easily identified by the sound it makes during operation. Conversely, other situations exist for which “normal sounding” and “abnormal sounding” are simply too ambiguous to be of any diagnostic value.

By understanding what sound is and applying some fundamental techniques, it's possible to clearly quantify the sound that a gearbox or gearmotor generates. The measured value, by itself, may not be used as a diagnostic tool, but may provide significant clues to the OEM, gearbox manufacturer, or even the operator regarding a unit's condition. This may provide a starting point for root cause analysis and, ultimately, corrective action.

Basic review: The science of sound

The study of sound is a complex science. Careers have been made and volumes written specifically regarding the science of sound. In short, sound is defined as the back-and-forth vibration of particles in a medium (such as air, water, or a solid) that are created when a traveling wave passes through the medium. By virtue of this back-and-forth motion, a pressure wave is created, which depends on the amount of air pressure fluctuation that the sound source creates. In other words, when something vibrates, it causes the particles near it to vibrate as well. These particles likewise bounce into and away from each other, forming high and low pressure waves that form the sound we hear when these waves reach our ears.

Three elements make up sound: a sound source, such as a gearbox; a medium — typically air; and a sound receptor — the human ear, for example. Note: In contrast with the scientifically defined term of sound, noise is simply defined as undesirable sound.

Sound pressure primer

Sound pressure is usually expressed in units of Newtons per square meter (N/m²), also known as Pascal or simply Pa. The lowest sound pressure humans can hear is approximately 0.00002 Pa. At the other end of the scale, sound pressures of approximately 20 Pa are discomforting.

Given this broad spectrum in which sound pressure can be perceived by the human ear, it is more convenient to refer to sound pressure levels when addressing questions of sound.

Sound pressure level L_p can be defined with mathematical logarithms that compare a sound pressure against an established sound pressure standard:

where p = Sound pressure of the wave

p₀ = Standard reference sound pressure — 0.00002 Pa = 20 µN/m²

The stated value for p₀ is used because it is near the lower limit of human audibility. While mathematically dimensionless, the value is referred to as a decibel (dB), named after physicist Alexander Graham Bell.

Weighing scales referred to as the A, B, and C scales approximate what the human ear hears. Typically in industry, the A weighing filter is applied in sound pressure level equipment and discussion. This is because people hear high frequency sounds better than low frequency ones, and the applied A weighing factor deemphasizes the lower frequencies from a reading. When representing results gathered from sound measurements, it is common to include the scale setting that was used to measure the data: For example, dBA represents a sound pressure level measured with the A weighing factor applied.

The table below details some common sound pressure levels along with corresponding sound pressures.

Many companies offer specialized equipment designed to measure sound pressure levels; the size, function, and cost of this equipment varies widely. In an industrial environment, it is common to use handheld meters for such evaluations. Often called decibel (or simply dB) meters, they provide a simple, portable, and economical way of measuring sound pressure levels. Many are available with built-in filters that allow the measurement to be displayed on the A, B, or C weighing scales.

As with any sensitive piece of equipment, calibration is vital, so specialized sound calibrators should be used with the meters to ensure accuracy.

Understanding gear sounds

Gearboxes and gearmotors generate operational sounds in a variety of ways: Gear mesh, bearing rotation, lubricant splash, oil pump operation, and the interaction of the gearbox within the machine structure all contribute to the overall sound generated by a gearbox during operation.