Lesson 4: Frequency Reactive Components.



Until now the components we've discussed have exactly the same characteristics at all frequencies within the audio band. Now we're going to look at components that react differently at different frequencies. These fall into one of 2 categories according to their principle of operation.

The first category operates on the principle of Static Charge. We have all seen the effects of combing our hair with a plastic comb and seeing the hair attracted to the comb, or walking on woollen carpet and feeling a light electric shock when we touch the door knob. These are the effects of static charge. As a negative Capacitors. charge is built up on one side, it attracts positive charges on the other, and if the attraction is strong enough a spark will jump between the two sides and the most common of example of this is of course lightning. Components that use this principle are called "Capacitors", or their original name was "Condensers".

Capacitors are essentially 2 plates separated by an insulator (a substance that does not conduct electricity). Commonly they are 2 sheets of aluminium foil separated by a sheet of plastic. The larger the plates, the more charge the capacitor can hold. There are several types of capacitors and you can research them if you choose, but at this point we will only be looking at their effects in a circuit.

Direct Current electricity can not pass through a capacitor. There is however an initial current that flows into the capacitor as it is charging, but once it is fully charged the current dies away to nothing. Alternating current, on the other hand, is continuously changing and therefore current appears to flow as the capacitor is continually charging and discharging. If the frequency of the A/C is slow enough or the capacity of the capacitor is small enough and the capacitor is able to fully charge before the A/C reverses direction then once again the current will die away to nothing until the polarity of the A/C wave crosses zero and changes direction. What this means is that capacitors like A/C, and the higher frequency the more it "disappears". The lower the frequency, the more it looks like a resistor, until when the frequency reaches zero, current stops completely (after the initial charge period).

The second category operates on the principle of Magnetism. I'm sure we all remember from our school science classes that there is a clear relationship between electric current and magnetism. A piece of wire wrapped around an iron nail and connected to a battery creates an electro-magnet that loses all magnetism when the battery is disconnected. Coils.

Another attribute is that if another coil of wire is placed in the magnetic field created by the first, then a current can be measured in the second coil whose voltage varies in synchronism with the variations of the first. This principle is used in electrical "transformers". Just a note of clarification: this principle only works when the field is varying. When the field created by the first coil stays at a constant level nothing is induced into the second, unless there is a third magnetically influenced element introduced into the field (for example, like a steel guitar string vibrating in the magnetic field of an electric guitar pickup. Guitar pickups usually contain permanent magnets, but I'm just using the example to demonstrate how a third magnetically influenced element can create variations in a constant magnetic field).

A closer look at the process shows that it takes time for the magnetic field to build up when the current is switched on, and to decay when the current is switched off. This has to do with the transformer principle happening within neighbouring windings of the same coil. Unfortunately, the voltage induced in the neighbouring winding is actually in the reverse direction to the original current that is trying to create the magnetic field. Of course, the original current is stronger than the induced current so, eventually the field builds up to its maximum strength, at which point the growing field is no longer growing, so the induced current is no longer being induced, and so, the field remains stable until the original current is switched off.

Once the original current is switched off the field immediately starts to decay. Again, there is a current induced as the field changes. This field is in fact in the same direction as the original current which tends to retard the decaying effect, thereby increasing the decay time of the magnetic field. There is a name for this induced current that affects the magnetic field characteristics, it is called "Back EMF", which stands for Electro-Motive-Force.

Now, to return to the topic of the lesson. Coils are simply pieces of wire wrapped in circles, therefore they pose little resistance to Direct Current. Once the D/C has been switched on, the field goes through the growth with its back EMF controlling the amount of current as described above, but once the field reaches its maximum the current also reaches its maximum and the circuit stabilises and remains constant until switched off.

Alternating current is a different story. Because A/C is continually changing, it is constantly subject to the back EMF it creates. In fact the slower the A/C changes (ie, the lower the frequency) the less a coil opposes it. The faster it changes (ie, the higher the frequency) the more it resists the flow of current, simply because it does not have enough time to build up the field. At a certain frequency, determined by the "inductance" of the coil, the resistance increases to infinity, and current stops flowing. Passive Crossover.

The Real World:


The best way to demonstrate the two frequency reactive components is to look at a simple passive crossover network, as found in almost every speaker system containing different size speakers. The small speakers (tweeters) get sent only the high frequencies (treble), the biggest (woofers) get the lowest frequencies (bass), and the middle size speakers (no nickname) get a band in the middle (midrange).

The component that allows only high frequencies is the capacitor, so choosing the appropriate value capacitor will allow only frequencies above a certain point to pass through to the tweeter. A coil will allow only low frequencies through to the woofer, and a combination will set the upper (coil) and lower (capacitor) limits of the band of midrange frequencies.



We're now ready to move on to the next lesson. Click here to learn about Diodes and Transistors, or click here to return to the Electronics for Sound Techs index page.