In our ongoing series of articles about car audio electrical theory, we are going to introduce the concept of alternating current power sources and signals. Understanding the basics of AC is crucial to understanding how a mobile audio system works. This article uses a lot of references to the electricity delivery systems used in our homes and offices to help establish a basic understanding of AC circuits. We’ll build on this foundation in this and subsequent articles to help form an understanding of the complexities of AC systems.

The Difference Between AC and DC

The voltage produced by the electrical system in our vehicles is called direct current. The electrons flow in one direction from one terminal of the battery to the other (except when we are recharging the battery). While there are changes in the voltage level as we add loads to the circuit, or when the alternator starts recharging the battery, the direction of current flow to the electric and electronic devices in the vehicle never changes.

Conversely, the power supplied by your local electric company to drive the lights and appliances our homes and at work is called alternating current. It has this name because the flow of electrons changes direction 60 times a second. Yes, this sounds weird. Who would want their power to go back and forth? Don’t fret; we’ll explain it all shortly. Just keep reading.

Power Loss in Transmission Wires

Researchers believe that the first electrical power source was a clay pot that contained tin plates and an iron rod. If filled with an acidic solution like vinegar, a voltage would be produced on the metal terminals. The belief is that this first battery was created more than 2,000 years ago. All batteries are direct current power sources.

Using electricity to do work started to become popular in the late 1800s, and as such, the need to deliver electricity to homes and offices became necessary. The problem with delivering power over long distances is voltage loss in the wires because of their resistance.

As we know from Ohm’s law and the power calculations we have recently discussed, the power in a circuit is directly proportional to the current and voltage (P = I x V) in the circuit. Power is also proportional to the square of current in the circuit relative to the resistance (P = I^2 x R). If we can transmit power with more voltage and less current, less power is wasted in the transmission wires.

Adoption of Alternating Current

A significant benefit of alternating current power supplies in commercial and residential applications is that it is easy to change the relationship between voltage and current using a transformer. A transformer is a device that uses magnetic fields to increase or decrease the voltage to current ratio. For example, an ideal 2:1 transformer would convert 10 volts and five amps of AC to five volts and 10 amps.

George Westinghouse is credited with the popularization of the delivery of AC power to homes, thanks to being awarded the contract to supply power to light the 1893 World’s Fair Columbian Exposition. Westinghouse used transformers based on patents he purchased from Lucien Gaulard and John Dixon Gibbs. Gaulard and Gibbs invented the transformer in London in 1881.

The output of a generator in a nuclear, coal or hydroelectric plant is 20 to 22 kilovolts. This voltage is stepped up to between 155,000 to 765,000 volts using a transformer for distribution around the state or province. Most of the high-voltage towers you see along the highway or in clearings have around 500,000 volts flowing through the three power conductors.

Each city or portion of a city will have some type of electrical substation where the electricity from these high-voltage lines is stepped down to lower voltages for distribution around different neighborhoods. These voltages are usually in the 16kV range to maintain an adequate level of transmission efficiency over these short to moderate distances. Transformers in enclosures at the side of the road or installed underground convert that voltage to the 120V feeds that run to the electrical panels in our homes.

By way of an example, let’s look at 1 mile of 8 AWG stranded cable. According to the American Wire Gauge standard, 1 mile of 8 AWG copper wire will have a maximum resistance of 3.782 ohms and an ideal resistance of 3.6 ohms.

If we want 5,000 watts of power delivered through this mile of cable, there will be some energy lost to the resistance in the cable. If we transmit our power at 240 volts, there will be 20.83 amps of current flowing in the cable. With a resistance of 3.6 ohms, the cable itself causes a loss of 1562.5 and we lose 75 volts across the cable. Clearly, low-voltage signal transmission over long distances doesn’t work.

If we increase the voltage up to 16,000 volts, the power loss in the cable drops to 0.3125 watts and we only lose 1.125 volts to the cable.

High-voltage transmission lines are how electric companies can deliver megawatts of electricity over long distances with minimal power loss. At 500,000 volts, we can transmit 1 megawatt of electricity over 100 miles and lose only 720 volts. That’s 0.144 percent!

OK, enough about the relationship of AC power and voltage. Let’s talk about audio systems.

A First Look at Audio Signals

Unlike the 60Hz AC waveform that feeds our homes, audio signals contain voltage information that mimics the changes in air pressure that we would perceive as sound. In most cases, sounds are recorded using a microphone that works in the opposite way a speaker does. Sound energy moves a small diaphragm that includes a coil of wire. The coil of wire moves past a fixed magnet. The motion of the coil through the magnetic field induces a voltage in the wire. The distance the diaphragm moves determines the amplitude of the voltage signal. Louder sounds produce higher voltages.

Below is a picture of an audio waveform as seen on an oscilloscope. The person speaking said the word audio.

Understanding Power in Alternating Current Circuits

The basic concept of power in an AC circuit is the same as for a DC circuit, but some calculations need to be completed before we can apply Ohm’s law. We’ll look at the 120V, 60Hz residential power supply to explain the math in the simplest of terms.

To measure power, we need to look at the amount of work completed over a given period. In the case of a light bulb plugged into an outlet, the filament doesn’t care which direction current is flowing, but the amount of light and heat created depends on the amplitude of the voltage supplied. The work done by the bulb is calculated by the number of electrons that flow through the bulb for a given amount of time.

To determine the work done by an AC voltage, we need to calculate the value of that signal that does the same amount of work as a DC voltage. This value is called the RMS or root mean square value and is 1/sqrt 2, or 0.70711 for sine waves. For our 120V power feed coming out of the wall, 120V volts is the RMS voltage. The peak voltage is about 167.7 volts. To be clear, the value of 0.70711 only works for a sinusoidal waveform. The RMS value of a square wave is 1.0 and for a symmetrical triangle wave is 0.577.

By definition, the RMS AC voltage can perform the same amount of work as DC voltage of the same value.

The image below shows a single cycle of a sinusoidal waveform. The peak voltage is 167.7 volts, and the two orange lines define the RMS value of 120V.

Basic Understanding of Alternating Current Sources and Signals

For this article, the takeaway is that the audio waveforms on the preamp and speaker wires in our stereo system are alternating current signals. In the next article, we will discuss the concept of frequency and amplitude in more detail.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

We’ll continue our introduction to the basics of car audio electrical theory by talking about wiring loads in series and parallel. Understanding the characteristics of each wiring option and how it relates to power delivery and current consumption is crucial in choosing the right speakers for your sound system. All reputable mobile enhancement retailers know the basics of series and parallel wiring by heart and can help you get the right combination of speakers or subwoofers to ensure optimum performance from your sound system.

Electrical Circuit Review

At this point, you should be familiar with the basic concept of wiring a load to a power source. In our cars, this could be something as simple as plugging a USB phone charger into the center console or having your installer integrate an amplifier into the electrical system in your vehicle.

The most basic of electrical circuits has a single power source and a single load. The two devices are connected together with the positive terminal of the source connected to the positive terminal of the load and likewise for the negative terminals. Current flows from the power source, through the load and back to the opposite terminal of the source.

Wiring Loads in Parallel

Any device we wire to the electrical system in our cars and trucks is considered to be wired in parallel with other loads. The positive connections all go to the same source of electricity, and the ground connections are all effectively connected to the same terminal of the battery.

The first and most important characteristic of loads wired in parallel is that the voltage across all of those loads is equal.

Knowing this makes it easy to calculate the current through each load using the equation I = V ÷ R. We can also calculate the power dissipated by each load using the equation P = V^2 ÷ R.

In the diagram above, we see two loads connected to a common 12-volt power source. Load 1 has a resistance of 20 ohms and Load 2 has a resistance of 15 ohms. Using the equations above, we can calculate that 0.6 amp of current flows though the 20-ohm load and 0.8 amp flows through the 15-ohm branch. Likewise, the 20-ohm branch dissipates 7.2 watts of energy and the 15-ohm branch dissipates 9.6 watts.

The power source needs to provide a total of 1.4 amps of current to the circuit.

Calculating the Resistance of Loads In Parallel

An important part of understanding parallel loads and how they affect the power drawn from the supply is a required understanding of how to calculate the net resistance of multiple loads in parallel.

The formula to calculate the total resistance multiple loads wired in parallel is 1/Rt = 1/R1 + 1/R2 + 1R3 and so on, until you have included all the loads.

For our 15- and 20-ohm loads in the example, the math would be: 1/Rt = 1/20 + 1/15, or 1/Rt = 0.05 + 0.06667. This works out to 1/Rt = 0.11667 which works out to 8.571 ohms.

There are a few shortcuts you can take to calculate resistance when multiple loads of the same value are used. Look at the following circuit:

In this circuit, all four loads are 8 ohms. We can do the math and see that the net resistance is 2 ohms. Where all the loads in the circuit are the same, we can simply divide the resistance of each by the number of loads.

So, 1/8 + 1/8 + 1/8 + 1/8 = 8 ÷ 4 = 2

Please remember, this only works when all the load resistances are identical.

Wiring Loads in Series

The second option in terms of wiring loads together is to wire them in series. The schematic below shows two loads wired in series with a voltage source.

In a series circuit, the current through all the loads is the same. The voltage drop across the loads is dependent on the total current flowing in the circuit at the value of the individual load resistance.

Another trait of series circuits that makes them very easy to work with is that the total circuit resistance is equal to the sum of all the loads. The equation is Rt = R1 + R2 + R3 and so on until all the loads are considered. For our example with the 15- and 20-ohm resistors, the total resistance in a series circuit would be 35 ohms. The current through the circuit is calculated using the I = V ÷ R equation, which would be 12 ÷ 35, or 0.343 amp for this circuit.

To calculate the voltage across each load, we can multiply the current times the resistance for each value from the V = I x R equation. The voltage across R1 is 6.857 volts and the voltage across R2 is 5.143 volts. Not coincidentally, the sum of these two voltages is equal to our supply voltage of 12 V.

In automotive applications, the problem with wiring loads in series is that the total power supplied to the circuit depends on the resistance of each component in the circuit. This makes predicting results for dynamic loads very difficult. Where we do occasionally wire loads in series is when we connect subwoofers to an amplifier or in the rare occasion we are using passive crossover components with a speaker.

Series-Parallel Wiring for Subwoofers

Let’s use the example of an amplifier designed to produce its rated power into a 4-ohm load. If we want to connect a single subwoofer to the amp, it should have a nominal impedance of 4 ohms. Depending on the brand of subwoofer you are looking at, you may have a single voice coil 4-ohm sub available, a dual 2-ohm configuration or a dual 8-ohm.

If you choose a dual 2-ohm woofer, the voice coils will need to be wired in series before the positive and negative connections are attached to the amplifier. If you use the dual 8-ohm sub, the coils need to be wired in parallel.

What if we want to wire multiple subwoofers to a single amplifier channel? In this case, the net impedance still needs to be 4 ohms. You can use a pair of single voice coil 2-ohm subs or a pair of dual 4-ohm subs. The pair of 2-ohm subs would be wired in series and then to the amp. The dual 4-ohm subs would have their individual voice coils wired in series, then the two subwoofers would be wired in parallel to the amplifier.

You will note that we switched the power source in this diagram to an AC source. You can think of that as your amplifier. We didn’t want anyone calling us out for suggesting that you connect your subwoofers to your battery.

You can continue wiring multiple subwoofers in simultaneous series and parallel loads until you run out of trunk space, so long as the net results keeps the amp happy with a 4-ohm load.

Choose the Right Subwoofers for Your Amplifier

Understanding the basics of series and parallel wiring is instrumental in ensuring you get the right subwoofer combination for your amplifier, or the right amplifier for your choice of subwoofers. Your local mobile electronics specialist retailer can help ensure you get the right solution for your application and install it so that it sounds great. In the next car audio electrical theory article, we will introduce the concept of alternating current.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

In our ongoing series of articles about car audio electrical theory, we are going to introduce the concept of alternating current power sources and signals. Understanding the basics of AC is crucial to understanding how a mobile audio system works. This article uses a lot of references to the electricity delivery systems used in our homes and offices to help establish a basic understanding of AC circuits. We’ll build on this foundation in this and subsequent articles to help form an understanding of the complexities of AC systems.

The Difference Between AC and DC

The voltage produced by the electrical system in our vehicles is called direct current. The electrons flow in one direction from one terminal of the battery to the other (except when we are recharging the battery). While there are changes in the voltage level as we add loads to the circuit, or when the alternator starts recharging the battery, the direction of current flow to the electric and electronic devices in the vehicle never changes.

Conversely, the power supplied by your local electric company to drive the lights and appliances our homes and at work is called alternating current. It has this name because the flow of electrons changes direction 60 times a second. Yes, this sounds weird. Who would want their power to go back and forth? Don’t fret; we’ll explain it all shortly. Just keep reading.

Power Loss in Transmission Wires

Researchers believe that the first electrical power source was a clay pot that contained tin plates and an iron rod. If filled with an acidic solution like vinegar, a voltage would be produced on the metal terminals. The belief is that this first battery was created more than 2,000 years ago. All batteries are direct current power sources.

Using electricity to do work started to become popular in the late 1800s, and as such, the need to deliver electricity to homes and offices became necessary. The problem with delivering power over long distances is voltage loss in the wires because of their resistance.

As we know from Ohm’s law and the power calculations we have recently discussed, the power in a circuit is directly proportional to the current and voltage (P = I x V) in the circuit. Power is also proportional to the square of current in the circuit relative to the resistance (P = I^2 x R). If we can transmit power with more voltage and less current, less power is wasted in the transmission wires.

Adoption of Alternating Current

A significant benefit of alternating current power supplies in commercial and residential applications is that it is easy to change the relationship between voltage and current using a transformer. A transformer is a device that uses magnetic fields to increase or decrease the voltage to current ratio. For example, an ideal 2:1 transformer would convert 10 volts and five amps of AC to five volts and 10 amps.

George Westinghouse is credited with the popularization of the delivery of AC power to homes, thanks to being awarded the contract to supply power to light the 1893 World’s Fair Columbian Exposition. Westinghouse used transformers based on patents he purchased from Lucien Gaulard and John Dixon Gibbs. Gaulard and Gibbs invented the transformer in London in 1881.

The output of a generator in a nuclear, coal or hydroelectric plant is 20 to 22 kilovolts. This voltage is stepped up to between 155,000 to 765,000 volts using a transformer for distribution around the state or province. Most of the high-voltage towers you see along the highway or in clearings have around 500,000 volts flowing through the three power conductors.

Each city or portion of a city will have some type of electrical substation where the electricity from these high-voltage lines is stepped down to lower voltages for distribution around different neighborhoods. These voltages are usually in the 16kV range to maintain an adequate level of transmission efficiency over these short to moderate distances. Transformers in enclosures at the side of the road or installed underground convert that voltage to the 120V feeds that run to the electrical panels in our homes.

By way of an example, let’s look at 1 mile of 8 AWG stranded cable. According to the American Wire Gauge standard, 1 mile of 8 AWG copper wire will have a maximum resistance of 3.782 ohms and an ideal resistance of 3.6 ohms.

If we want 5,000 watts of power delivered through this mile of cable, there will be some energy lost to the resistance in the cable. If we transmit our power at 240 volts, there will be 20.83 amps of current flowing in the cable. With a resistance of 3.6 ohms, the cable itself causes a loss of 1562.5 and we lose 75 volts across the cable. Clearly, low-voltage signal transmission over long distances doesn’t work.

If we increase the voltage up to 16,000 volts, the power loss in the cable drops to 0.3125 watts and we only lose 1.125 volts to the cable.

High-voltage transmission lines are how electric companies can deliver megawatts of electricity over long distances with minimal power loss. At 500,000 volts, we can transmit 1 megawatt of electricity over 100 miles and lose only 720 volts. That’s 0.144 percent!

OK, enough about the relationship of AC power and voltage. Let’s talk about audio systems.

A First Look at Audio Signals

Unlike the 60Hz AC waveform that feeds our homes, audio signals contain voltage information that mimics the changes in air pressure that we would perceive as sound. In most cases, sounds are recorded using a microphone that works in the opposite way a speaker does. Sound energy moves a small diaphragm that includes a coil of wire. The coil of wire moves past a fixed magnet. The motion of the coil through the magnetic field induces a voltage in the wire. The distance the diaphragm moves determines the amplitude of the voltage signal. Louder sounds produce higher voltages.

Below is a picture of an audio waveform as seen on an oscilloscope. The person speaking said the word audio.

Understanding Power in Alternating Current Circuits

The basic concept of power in an AC circuit is the same as for a DC circuit, but some calculations need to be completed before we can apply Ohm’s law. We’ll look at the 120V, 60Hz residential power supply to explain the math in the simplest of terms.

To measure power, we need to look at the amount of work completed over a given period. In the case of a light bulb plugged into an outlet, the filament doesn’t care which direction current is flowing, but the amount of light and heat created depends on the amplitude of the voltage supplied. The work done by the bulb is calculated by the number of electrons that flow through the bulb for a given amount of time.

To determine the work done by an AC voltage, we need to calculate the value of that signal that does the same amount of work as a DC voltage. This value is called the RMS or root mean square value and is 1/sqrt 2, or 0.70711 for sine waves. For our 120V power feed coming out of the wall, 120V volts is the RMS voltage. The peak voltage is about 167.7 volts. To be clear, the value of 0.70711 only works for a sinusoidal waveform. The RMS value of a square wave is 1.0 and for a symmetrical triangle wave is 0.577.

By definition, the RMS AC voltage can perform the same amount of work as DC voltage of the same value.

The image below shows a single cycle of a sinusoidal waveform. The peak voltage is 167.7 volts, and the two orange lines define the RMS value of 120V.

Basic Understanding of Alternating Current Sources and Signals

For this article, the takeaway is that the audio waveforms on the preamp and speaker wires in our stereo system are alternating current signals. In the next article, we will discuss the concept of frequency and amplitude in more detail.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

For several years, the Best Car Audio team has provided articles on the features, functions and benefits of all manner of car audio products and services. Well into our third year, it’s time to go back to the basics and talk about the fundamental theories of electricity and how they relate to our car audio systems. Grab something to drink, get comfortable and enjoy: It’s time to get our learn on about the basics and Ohm’s Law!

What is Electricity?

In its most basic of terms, electricity is a group of charged electrons that can be used to do work. The electricity in our cars comes from two sources: the battery and the alternator. After the battery is used to start the car, the alternator recharges the battery and provides electricity to run the fans, lights, electronic circuits and computers that keep our cars running.

Terminology: Voltage

To understand electricity, you need to understand a few terms. The first we will talk about is voltage. Voltage is a unit of measurement that quantifies the difference in electrical potential between two points. The SI unit of measure is volts and is represented with a capital V.

Once again, in relation to our vehicles, we have a 12V electrical system. More specifically, a fully charged car battery will rest at 12.6 volts for a conventional lead-acid design. Some AGM batteries will rest at 13.0 to 13.2 volts.

You can look at a voltage as electrical pressure. Imagine a water tank sitting on a table. If you connect a hose to the bottom of the tank, gravity will push the water out of the house. If you get a bigger tank, there is more pressure pushing the water out of the hose. So, more voltage is akin to more pressure.

Terminology: Current

It’s important to know the quantity of electricity moving through a circuit. We use the SI unit ampere to quantify the volume of electrons moving in a conductor. The original definition for the ampere involved quantification of the magnetic force created between two infinitely long parallel conductors (wires). While this is a valid definition, it’s never used in schools or any training. A simpler explanation is that 1 amp of current is equivalent to 6.2415093 × 10^18 elementary charges moving through a boundary over a period of one second. An elementary charge is the is the electric charge carried by a single proton.

Using our water analogy, the current flowing in an electrical circuit is similar to the amount of water flowing in a pipe or hose. The amount of water flowing in that hose would be measured in gallons or liters per minute. A higher number means there is more pressure pushing on the water. So, back to our electrical terminology, when more voltage is present, more current flows through our circuit.

Choosing a specific example in an automotive example is tricky because vehicles differ dramatically in their electrical needs. With that said, most new cars and trucks have an alternator that can provide between 65 and 120 amps of current to power different devices. Batteries also vary a great deal as do the ratings available to quantify the amount of current they will provide. Most batteries have a capacity of 60 to 80 amp-hours. This rating descibes a battery’s ability to supply 1 amp of current for 60 to 80 hours before being considered depleted. Sadly, the equation cannot be reversed. A battery can’t supply 60 to 80 amps of current for an hour due to limits in the chemical conversation process.

Terminology: Resistance

Resistance is the description of the opposition to the flow of current in a circuit. We use the SI unit ohm to quantify this value. Unlike voltage and current, the symbol used to represent resistance is the uppercase Greek letter omega: Ω. More resistance in a circuit reduces the ability for electrons to flow and thereby decreases the number of amps flowing.

In our water and barrel example, pinching the hose would increase resistance and reduce the amount of water that flows. In an electrical system, the size of the conductors we use to wire circuits and the design of the circuits themselves determine how much resistance is present.

Terminology: Ohm’s Law

Thankfully, in simple circuits, the relationship between voltage, current and resistance is linear. When we have more voltage available, more current flows for a given resistance. Likewise, less resistance in a circuit causes more current to flow for a given voltage. Ohm’s law is a simple mathematical equation that allows you to calculate any of the three values, provided you know two others.

The three equations are:

Voltage = Current x Resistance Current = Voltage ÷ Resistance Resistance = Voltage ÷ Amperage

V = I x R I = V ÷ R R = V ÷ I

Understanding Ohm’s law is the most important factor in working with and understanding electrical circuits.

Ohm’s Law Examples

One of the most common expressions used in teaching people about Ohm’s law is as follows: In a circuit with one volt of potential and a resistance of one ohm, one amp of current will flow. Seeing the relationship between this statement, we can calculate that for a fixed resistance, two amps of current will flow if we increase the voltage to two volts. Said another way, as the voltage potential applied to a circuit increases, the current through the circuit will also increase, as long as the resistance remains constant.

That’s it for our first lesson on car audio electrical theory. In the next lesson, we’ll talk about how we can use electricity to do work and discuss the equations used to quantify this work as power.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.