Text: Rumble, as its descriptive name suggests, is a low, rumbling noise produced by poorly built turntables or changers. It is caused by vibrations of the turntable mechanism that are picked up by the cartridge along with the signal on the record. In quality turntables, rumble is minimized by the use of properly balanced drive motors, shock mounts, and often by an elastic transmission between the motor shaft and the turntable rim (usually in the form of a plastic belt) that filters out motor vibration before it reaches the stylus. Turntable rumble is measured, in decibels, against a standard level tone played on a test record. The minimum requirement for high fidelity, as defined by the National Association of Broadcasters, is -35 db, meaning that the rumble must be 35 db softer than the test tone. (Note that unless turntable rumble figures are specified as being derived according to the NAB standard, they cannot be compared with one another).

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Source: 103