| Electrical Characteristics Of A Dynamic Loudspeaker |
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Electrical characteristics of a dynamic Loudspeaker are expressed as a curve of the loudspeaker's Electrical Impedance versus Frequency . A loudspeaker is an electro-mechanical transducer and uses a voice coil to activate the diaphragm or cone. The moving system of the loudspeaker (including the cone, cone suspension, spider and the voice coil) has a certain Mass and Compliance . This is most commonly likened to a simple mass suspended by a Spring that has a certain Resonant frequency at which the system will vibrate most freely. This frequency is known as the "free-space resonance" of the speaker and is designated as '''''Fs'''''. At this frequency, since the voice coil is vibrating with the maximum peak-to-peak Amplitude and velocity, the Back-emf generated in the coil is also at its maximum. This causes the electrical impedance of the speaker to be at its maximum at ''Fs'', shown as '''''Zmax''''' in the graph. For frequencies just below resonance, the impedance rises rapidly as the frequency approaches ''Fs'' and is Inductive in nature. At resonance the impedance is purely Resistive and beyond it as the impedance drops, it behaves Capacitively . The impedance then reaches a minimum value ('''''Znom''''') at some frequency where the behaviour is fairly (but not perfectly) resistive over some range. This impedance is close to, but usually slightly different from the quoted impedance. Beyond the ''Znom'' point the impedance is again largely inductive and continues to rise gradually. The frequency ''Fs'' and the frequencies above and below it where the impedance is ''Zmax''/√2 are important in determining the loudspeaker's T/S Parameters which are used to design a suitable enclosure for the loudspeaker. Note that ''Fs'' itself is one of the T/S parameters of the loudspeaker. LOAD IMPEDANCE AND AMPLIFIERS The variation in loudspeaker impedance is a consideration in Audio Amplifier design to ensure reliability. There are two main factors to consider when matching a speaker to an amplifier. Minimum Impedance This is the minimum value in the impedance vs. frequency relationship, usually somewhat (perhaps 10-25%) higher than the DC resistance of the voice coil as measured by an ohmmeter. Minimum impedance is important bacause the lower the impedance, the higher the current must be at the same drive voltage. The output devices of an amplifier are only rated for a certain current level, and when this is exceeded the device will eventually fail. Impedance Phase Angle Impedance variations of the load with frequency translate into variation in the Phase relationship between the amplifier's output Volt age and Current . For a resistive load, usually (but not always) the voltage across the amplifier's internal output devices is maximum when the load current is minimum (and the voltage is minimum across the load) and vice-versa, and as a result the power dissipation in them is minimum. But due to the complex and variable nature of the loudspeaker load and its effect on the phase relationship between the voltage and current, the current will not necessarily be at its minimum when the voltage across the output devices is maximum - this results in increased power dissipation which manifests as heating in the output devices. The phase angle varies the most near resonance in moving coil loudspeakers. If this point is not taken into consideration during the design of the amplifier, the amplifier may overheat causing it to shut down, or worse, cause thermal destruction of the amplifier's output devices. REFERENCES
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