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.
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