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How a loudspeaker's impedance influences fidelity Many loudspeaker designers, reviewers, hobbyist, and consumers, fail to recognize some basic points about how a loudspeaker should be measured and what effects the amplifier can have on tests and measures as well as the musical performance, timbre, bandwidth, presence and so on. This FAQ addresses the most basic electrical relations between amplifier and loudspeaker. The Article does not attempt to detail the more complex dynamic behavior of the loudspeaker system, thermal changes at the transducers motor, loudspeaker cable influences, environmental conditions, nor how an audio power amplifier’s design will react to these dynamic impedance variables. 2.8 Volts equal 1 Watt, right? Only for true 8 Ohm loudspeakers. Ohm’s law, power, and SI units of measure: V=IR I=V/R R=V/I W=IV I=√W/R R=V2/W Efficiency = output / input Electric potential is Voltage V, current is Amperes I, resistance is Ohms R, power is Wattage W. Loudspeakers are generally reactive AC devises. Power factors and impedance differentials between amplifier and loudspeaker must be considered. Solving for power, Watts not Voltage, is essential for understanding relationships. Power in Watts is current times Voltage. Phase angles can be ignored in basic loudspeaker testing but does factor in more complex dynamic behavior modeling. Without the correct understanding of basic power transfer, a complete detailing of the system and device under test, measures and data cannot be accurately correlated into observed fidelity. It should also be pointed out that without a basic understanding of test system, device and procedure, techno marketing can easily manipulate the tests, data, and you. The amplifier is a major factor in how a loudspeaker system performances, both technically and sonically. If testing the efficiency of a driver in an infinite baffle, correctly mapping the impedance across its bandwidth, then at least distilling the string of points into a nominal number is essential. You cannot simply pump in 2.83 Volts and call it. Data acquired this way has little meaning. Once impedance is known, power and system efficiency can then be tested and the results can be useful. Application: basic, using a nominal impedance reference. The Zu260FR series of driver measures 12 Ohms nominal as used in the Druid loudspeaker system. Remember, the driver’s measures and performance are in part determined by the acoustic impedance system (box, horn, baffle...) they are used in. To measure efficiency, also referred to as sensitivity, at 1 Watt, we solve for current as Wattage and resistance are known, then Voltage, all measures being at the loudspeaker’s input. First solve for Amperes; square-root of Wattage over resistance (ignoring phase) which equals 0.289 Amps. Then take Amps times resistance to solve for Voltage and we arrive at 3.47 Volts input. So a 12 Ohm load requires 3.47 Volts at input to reach 1 Watt. An input of 2.83 Volts into a 12 Ohm load yields 2/3 Watts. Nearly all modern tests and measures on loudspeakers simply input 2.83 Volts (assumption of an 8 Ohm standard) which has almost no lay correlation to actual transduction efficiency and power. If all loudspeakers had a nominal impedance of 8 Ohms then a 2.83 Volt input would be fine and does in fact result in a nominal 1 Watt of input power. And with 4 Ohm nominal loudspeakers we get; 2.83 Volts input equals 2 Watts at input. Again, this example is basic but gives a real idea of how much power is being soaked and how much work is being done. Understanding power transfer dynamics will hint to why power amplifiers have such a huge impact on the playback systems timbre, dynamic range, bass response, presence, treble, how loud it sounds, and so on. Remember, the reactivity of a dynamic driver is dramatically effected by the loading model (box, horn, baffle...) and the necessity for measuring the device as a complete loudspeaker. This also reveals how the exaggerated “sensitivity” measures are being generated by the majority of brands. Now that we understand the basic relations between impedances voltage and current we can now approach how a given power amplifier might behave and influence the tone, power, and presence of playback. |