Speaker electrical characteristics – electricity basics
Before we discuss the speaker electrical characteristics, we need to clear up some of the most elementary properties of electricity. I know for some of you, this will sound too basic, but this has to be explained first, before we get to the interesting stuff. The most rudimentary elements of electronics are : voltage, current and resistance.
Voltage is the potential difference between two points. To be more clear about the word “potential”, let’s explain it another way. When you look at a battery, for example, the negative pole has an excess of electrons, compared to the positive pole. So, there is a difference, a potential difference. One point is more negative than the other. The unit for measuring voltage is the volt, and it tells us how big is the electron difference between the two points. Think of it like a water pump, which gives a certain pressure, and its either off or on. If nothing is connected to it, it’s off. If you connect something to the pump, it works at its rated pressure (voltage).
When you connect an electrical conductor between two points, that has a voltage across them, current flows. This flow describes the electrons travelling from the negative pole to the positive pole. So for the battery example, if you place a wire between the negative and the positive terminal, current will flow until the number of electrons balance out. When this happens, there is no more potential difference, and the battery is dead. If we use our water pump analogy, think about a hose connected to the pump, and the current is the water flowing through the hose. The unit of measurement for current is amperes (amps for short).
Resistance is the electrical property of opposing the current flow. If we use our water pump analogy, think about a kink in the hose (similar to a resistor). That kink will bottleneck the flow of water. The exact same thing happens with electricity. The higher the resistance, the less current will flow through the conductor, considering the voltage remains the same. The unit for measuring resistance is the ohm.
We talked about the fundamental units of electricity, which also applies to the speaker electrical characteristics. Ohm’s law will show us how they interact with each other. One’s value depends on the value of the other two, and the formula is the following (also called Ohm’s law) :
E = I x R
- E – represents the voltage measured in volts. It is also known as electromotive force.
- I – represents the intensity of electron flow. It’s the current measured in amps.
- R – represents the resistance measured in ohms.
By knowing any of the 2 values you can calculate the third. Let’s take an example with a battery and a resistor, hooked up together using a wire as a conductor.
For our example, let’s say the battery is 1.5 V and the resistor is 0.5 ohms. If we use Ohm’s law, we can calculate the current. I = E / R. This means that I = 1.5 / 0.5 = 3 amps. If we double the value of the resistor, the current value will drop by half. If we double the value of the voltage, the current will double. The relationship between voltage, current and resistance is perfectly linear.
Another electrical factor that concerns audio is Power. Power is the product between voltage and current, and its unit of measurement is Watts (P = E x I). In our example, power is dissipated as heat in the resistor (the load). If we want to calculate the power dissipated by our resistor : P = 1.5 * 3 = 4.5 W.
Let’s get back to the audio world and give an example. An amplifier’s power is a product of its output voltage and current. The speaker input dictates the load. Depending on how much voltage the amplifier can produce, a certain current will be generated on the speaker’s load. As the current passes through the speaker’s voice coil, the cone will start to move and produce sound.
A perfect amplifier will double its power as the resistance is halved, because the voltage remains the same and the current is doubled. In reality this does not happen, because amplifiers cannot maintain the same voltage across a low impedance load as they can a high impedance load.
Alternating current (AC)
Up until now we talked about circuits using direct current (DC). But, there is another type, alternating current, which is the basis of all audio circuits. An audio signal is an example of AC voltage. AC voltage changes value and polarity over time, at a certain frequency. The principles that we talked earlier, including Ohm’s law, for DC circuits, also apply to AC circuits.
Let’s use the power from the wall socket as an example. The power line uses AC voltage of 120 volts at 60 Hz (standard in US). This means that the voltage changes polarity at a rate of 60 times per second.
As you can see from the illustration above, the voltage starts to rise, until it reaches the positive peak. After that, it starts to decrease until it reaches the 0 (zero) value, and it decreases even further till the negative peak. This cycle is done 60 times per second.
If we are concerned with speaker electrical characteristics, an audio signal is also an AC signal. To simulate the above example, you would have to play a pure 60 Hz tone, and you will get the exact same sine wave. However, music is made by a large number of frequencies and the audio signal will be a complex AC voltage.
In the audio world, the inductance of the voice coil plays a large role on the design of the speaker Now we are getting deeper into the speaker electrical characteristics. To get more of an idea on how inductance works, let’s try to explain electromagnetic inductance first.
Electromagnetic inductance is the relationship between moving electrons and magnetism :
- Moving electrons produce a magnetic field around the conductor.
- Moving magnetic field makes electrons flow through a conductor.
For example : Imagine you have a coil of wire (the conductor) and a magnet that moves through the coil. This induces a voltage across the wire. As long as the magnet is moving, voltage is generated. Depending on the direction the magnet is moving, polarity of the voltage will be different.
Let’s take the other approach and apply a voltage through the coil. This will create a magnetic field around the coil and will act like a magnet. The magnetic field is intensified by increasing the number of spools on the coil. Doing so, we have created an inductor, which has a property called inductance.We measure inductance (L) in henrys.
Speaker electrical characteristics
How the speaker works is derived from this principle discussed earlier. The voice coil is surrounded by a permanent magnet, which is fixed. When you apply voltage to the coil, it generates a magnetic field, which interacts with the magnetic field of the fixed magnet. Because the audio signal is AC voltage, the coil will move up and down, depending on the polarity of the voltage. The cone of the speaker follows the movement of the coil and it produces sound.
If we look closely on how the voice coil of the speaker operates, we can see an interesting phenomenon. As the AC current flows through the coil, it creates a magnetic field around it, which expands and collapses, following the alternating fashion of the current. This constant changing magnetic field induces a voltage across the coil. This happens because we satisfied the requirements for electromagnetic inductance : a conductor, a magnetic field, and a relative motion between them. The motion comes from the expanding and collapsing magnetic fields. This induces a voltage across the coil which opposes the applied voltage, reducing current flow. This opposition of current flow is called inductive reactance. The higher the alternating frequency, the higher the inductive reactance and so the opposition of current flow.
- Inductive reactance : XL = 2πfL
- f – Frequency in Hertz.
- L – Inductance in Henrys.
As you can see the inductive reactance is directly proportional to frequency. That’s why when you look at a speaker impedance chart, you will see a rise in impedance as frequency goes up. That’s because the inductive reactance opposes more and more as frequency increases.
There are two types of reactance : inductive reactance and capacitive reactance. So, the other reactive element (besides inductance), is what we call capacitance and it’s marked by the letter C. These two elements are very important when it comes to speaker electrical characteristics. Similar to how inductance works, capacitance also reacts to AC. Unlike the inductor, which becomes more reactive at higher frequencies, the capacitor reacts the other way around. The capacitor acts like a resistor which increases its resistance as frequency decreases. At DC, the capacitor has infinite reactance and looks like an open circuit.
Capacitive reactance : XC = 1 / (2πfC)
- f – Frequency in Hertz.
- C – Capacitance in Farads.
These are the most basic speaker electrical characteristics. You need these to understand things down the road. The capacitive and inductive properties are the building blocks of passive crossovers. Since they are more reactive depending on frequency, they’re usage is to filter low frequencies for the tweeter and high frequencies for the woofer. Placing capacitors, inductors and resistors in a smart fashion can result in great crossover designs. Also, these properties stay at the core of a speaker design. The inductance is given by the voice coil and the capacitance is given by certain mechanical factors of the speaker. They give the speaker a certain amount of damping, which is critical for speaker performance, especially when it comes to transient response. An article about impedance will follow soon. Learn more about speaker electrical characteristics here.
- Basic Electricity (Dover Books on Electrical Engineering) 2nd Edition by Bureau of Naval Personnel (Dover Publications, 1970).
- The Audio Expert: Everything You Need to Know About Audio by Ethan Winer (Focal Press, 2012).
- The Complete Guide to High-End Audio 5th Edition by Robert Harley (Acapella Publishing, 2015).
- Image source : link.