主题：Power handling and heat dissipation

modifed: For transducer driver mechanisms, talking about "Power handling and heat dissipation",thanks a lot! i want to learn this from your help

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In any inefficient device,the wasted power almost always showw up as heat and the lodspeaker is no exception.Most of energy loss appears as heat in the coil.Most manufactures rate their loudspeakers by the amount of power they can eurvive,measured in Watt,under certain conditions,without sustaining damage. This is known as power holding figure and is absolutely meaningless.

The power-handling figure is not the power actually dissipated. In fact it is the power taht would be dissipated by a resistor having the same resistance as the normal impedance of the speaker.As a driver unit is a comples or recative load as well as being temperature dependent,the actual power delivered by the amplifier will be substantially less than the power delivered to an ideal resistor.

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In driver units which offer high power-handling,the designer is backed into a corner by the survival requirement.The cone will have to be heavier to survive the mechanical ordeal,and so the speaker will be less efficient.This means that more heat will be developed,which in turn means that the coil temperaure will rise and the coil will expand in diameter by 0.4 mm. This requires a greater clearance in the outer pole piece, raising the reluctance of the gap and reducing the BL product,further reducing efficiency. Consequently high-power handling speakers are seldom very efficient and therefore need to consume that hight power and suffer more from thermal compression.

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For quality reasons, moving-coil drivers must be designed with effective cooling measures so that the coil temperature excursion is minimized. The coil can shed heat by radiation and by conduction. Conduction is limited because the coil former is attached only to the cone and the spider. Most spiders are thermal insulators, as are most cone materials, with the notable exception of aluminium cones,which can contribute significantly to coil colling if the attachment of the coil former to the cone is designed as a thermal path.

 

The ultimate cooling method is ferrofluid, a magnetic liquid is retained in the magnet gap by the magnetic field itself and conducts heats to the pole pieces. Ferrofluid became available around 1974, and consists of a colloid of magnetic particles (Fe₃O₄) of about 0.01 mm in diameter suspended in a low vapour pressure diester based on ethylexyl-azelate. The use of ferrofluid does result in some viscous damping due to shearing in the fluid.

Ferrofluid is generally designed to saturate at flux levels considerably below those present in the gap so that the reluctance of the gap is not affected. It is compatible with most coil, adhesive and former materials, except for paper and rubber-based materials. Capton absorbs some of the carrier but without ill effect. The cooling effect is typical in the tweeter.