It’s a common experience that audio amplifiers can deliver signals that destroy loudspeakers. High performance domestic systems are the most vulnerable, as abuse ruggedness is not much called for.

Regardless of whether the design is domestic or professional, the fact remains that nearly every protection system taxes sonic quality, if not directly, then by stealing from the finite design and component budgets. Whether professional or domestic, there are two main causes of failure in drive-units:

1. Thermal.
Alias ‘burning out’. Applies solely to electro-dynamic drivers, including ribbon types. Caused by excess power (energy integrated over time).

When cone and compression drive-units are somewhat over-driven, the high temperature-rated adhesive holding the voice-coil wire (or foil) together, melts and deforms. If this causes the coil to rub, there will be distortion, sometimes only at higher drive levels, effectively making the driver unusable.

The wire may also fracture from chafing or impact, either at first, or eventually. Or turns can short, changing the driver’s characteristic. These fates are common in bass drivers.

If the over-drive is harder, and particularly if its onset is abrupt enough, the glue is burnt to a crisp and the conductor, if copper, may be heated to incandescence, before snapping. Large, unwanted RF signals delivered from amplifiers often have this effect. Either level of thermal failure is most common in HF drive units.

2. Mechanical.
Applies to most drive-unit types. This covers ripped cones, diaphragms and surrounds, snapped ‘tinsel’ leadout wires, and fractured voice coil wire or foil.

A loudspeaker cone attempts to move further as it is driven harder. It also attempts to move further when it is resonating, and in most enclosures, increasingly further when driven by lower frequencies.

Large excursions are a problem especially for compression and hf drivers (if driven down to their low end limit), and bass cone-drivers. If driven some way beyond the maximum linear excursion (Xmax) rating (in mm or inches), damage will ultimately result to the drive-unit, later if not immediately.

The excursion this occurs at may be specified as the Xdamage rating (again in mm or inches). For a high power bass drive-unit, Xdamage is typically 300% (3x) Xmax.

Mechanical failures can also result from the hugely high-g-forces that hf drivers are subjected to. G-forces in bass drivers can be far less yet they are commensurately stressful, in view of their higher moving mass. Mechanical damage is rare in midrange drivers.

No comments:

Post a Comment

Related Posts Plugin for WordPress, Blogger...