Glossary
The terms used here
What the vocabulary of gearbox acoustics means in reports and conversations — in the sense in which I use it.
- Accelerometer
- A sensor that captures structure-borne noise as acceleration. Where it sits and how it is mounted decide which components are measurable at all.
- Acoustic test bench
- A bench quiet enough, and with a defined enough mounting, to measure gearbox noise reproducibly. Without repeatability there is no comparing one build to the next.
- Airborne noise
- The noise transmitted through air and measured with microphones. For finding causes, structure-borne noise usually says more, because it arises closer to the source.
- Bearing defect frequencies
- Orders derived from bearing geometry and speed for outer ring, inner ring, rolling elements and cage. They are what separates a bearing fault from a gear cause.
- Campbell diagram
- Amplitudes plotted over speed and frequency, where orders appear as straight lines through the origin. It shows where an order crosses a structural resonance.
- Condition monitoring
- Continuous observation in service, tracking changes on the same orders over time. The same metrics as in development, only repeated instead of taken once.
- End-of-line testing
- An acoustic check of every gearbox at the end of production, usually within seconds and against fixed limits. Those limits are only useful if they sit on orders rather than on overall levels.
- Gear mesh frequency
- The rate at which the teeth of a pair come into engagement: shaft rotational frequency times tooth count. Expressed in orders it is the gear mesh order.
- Gear mesh order
- The tooth count of the wheel in question, referenced to its own shaft. It carries the whine and is the anchor the sidebands are measured against.
- Gear quality grade
- The DIN/ISO tolerance class of a gear, which fixes the permissible deviations. A better grade limits the excitation but does not on its own guarantee a quiet gearbox.
- Gear whine
- A tonal sound that travels with speed, carried by the gear mesh order and its harmonics. The cause is usually a tooth deviation or an unfavourable stiffness distribution.
- Gearbox noise
- Any noise radiated by the gearbox or carried onwards through the structure. It splits into speed-bound parts that live on orders and load-dependent effects such as rattle.
- NVH
- Noise, vibration, harshness — the umbrella term for the noise and vibration behaviour of a vehicle or drivetrain. Gearbox acoustics is the part of it the gearbox is responsible for.
- Order analysis
- Evaluating a signal in multiples of a shaft speed instead of in hertz. An order stays the same number across the whole speed range, which is what makes it attributable to a component.
- Order spectrogram
- Like a Campbell diagram but with orders instead of frequencies on one axis. Speed-bound components stand as vertical lines, independent of speed.
- Rattle
- Broadband, irregular noise from unloaded gears striking within their backlash. It depends on torque fluctuation rather than sitting on a fixed order.
- Reference shaft
- The shaft whose angle an evaluation is referenced to. With several meshes in a train, each load-carrying mesh gets its own reference.
- Root cause analysis
- The path from a measured order to the component that produces it. It runs through the kinematic model of the gear train, not through pattern spotting in a spectrum.
- Separation quality
- A measure of how reliably two closely spaced excitations are held apart in an evaluation. A conclusion without it is a guess carried to several decimal places.
- Shaft-synchronous analysis
- Referencing the measurement to shaft angle rather than to time, normally through a tacho. Only then do speed-bound components stay sharp lines under changing speed instead of smearing into hills.
- Sidebands
- Lines one shaft order apart on either side of the gear mesh order. Their pattern says which wheel of the pair carries the deviation — the mesh order alone only says which stage.
- Structure-borne noise
- Vibration travelling through components, measured with accelerometers on the housing or at bearing seats. The path from there to audible noise runs through how the structure radiates.
- Tooth deviation
- Departure of the manufactured flank from its nominal geometry — in profile, lead or pitch. Such deviations are the most common cause of tonal gearbox noise.
- Transfer function
- The frequency-dependent relationship between an excitation and the response elsewhere. It explains why a small excitation can end up loud when the path amplifies it.