There are a number of answers, but the two most important are impedance and sensitivity. First, impedance[1]. Impedance is resistance that varies with frequency. The term nominal means average, this is not only possible but quite likely that significant impedance variations will occur throughout the speaker's frequency range.
Indeed, a variation as wide as 3 to 40 Ohms is possible. The amount of variation, in addition to how low or high the range, determines how difficult the speaker is to drive and thus defines the role of the amplifier. The amplifier must be able to deal with these impedance variations, producing the amount of power necessary to drive the speaker at any frequency. If the amplifier is not capable of dealing with the impedance swings, audible distortion occurs.
Ideally, an amplifier should be a constant voltage source. That is, for a given input signal, the amplifier should produce a constant voltage across the speaker terminals whatever the load.
If we connect a 4 Ohm speaker, halving the original load, the same 20 Volts would now produce watts, and further, watts into 2 Ohms. From this example, we clearly see that each time the load resistance is halved, the amplifier should ideally double its output. This high current capability is especially important if the loudspeaker impedance dips into a very low range. The continued doubling must stop at some point, the progression cannot go on forever and, if carried too far, could end in disaster.
Either the power supply will run out of current and fail to maintain the amplifiers output wattage, or worse, go beyond the capability of the output devices, creating excessive heat and eventually destruction of the transistors. Even a speaker with a nominal 8 Ohm rating can fall below 4 Ohms at certain frequencies. If the current reserves of the amplifier are not sufficient to sustain its output wattage into low impedances, the unit will "run out of gas" sonically, at the time when the extra power is needed most.
This helps to explain why a very high quality 50 watt per channel amplifier may sound less strained than another unit rated at watts per channel. To calculate the power of any particular component in a circuit, multiply the voltage drop across it by the current running through it. For instance, if current flows at a rate of 10 amps while voltage is 10 volts, then the circuit dissipates power at a rate of W.
The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts V. The rate of flow of an electric charge, measured in amperes or amps. When one coulomb of charge moves past one point in once second, current is said to flow at a rate of one ampere. Current flows from negative potential to a positive potential through a load.
The rate at which work is done. Moreso, it can be essential in troubleshooting them when things go wrong. While there are a great many electrical terms, four fundamentally important ones are Ohms, Volts, Amps and Watts.
Ohms law tells us that Ohms are equal to volts divided by amps, while Watts law tells us that Watts are equal to Volts x Amps. Using these two basic formulas, we can calculate to find 2 unknown values if we know any other 2.
Named for Georg Simon Ohm, the Ohm is a unit of measurement for the resistance in an electrical circuit. A device with greater resistance will have a higher ohm value associated with it. Many times, we see ohm values for loudspeakers. Therefore, an 8 ohm loudspeaker presents more resistance to an amplifier than a 4 ohm loudspeaker does. Voltage is the electrical potential of the circuit. It is present whether electricity is flowing or not.
A good example of this is a battery, or an outlet.
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