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.

In any discussion about understanding sound, the unit of decibels will undoubtedly become part of the conversation. Unlike almost all other units of measurement, the decibel is not a linear scale. That is to say, 1 decibel (also written as dB) is not one-tenth the amplitude or strength of 10dB. In this article, we’ll explain how the decibel scale works and present some reference information to help you understand how the decibel scale works.

What is Sound?

Sound is a vibration of air molecules that vibrates our eardrums. The eardrum passes these vibrations through to the middle ear through tiny bones called ossicles. The inner ear has a shape similar to that of a snail shell and contains microscopic hair cells that convert these vibrations into minute electrical signals. These signals are transmitted to the hearing nerve and subsequently to our brain. Each inner ear contains roughly 18,000 hair cells, all of which are said to fit on the head of a pin. Once a hair cell is damaged, it never grows back or repairs itself.

Understanding the Decibel

The decibel unit was created in the 1920s by Bell Telephone Laboratories to describe losses in communication cables used in early telephone systems. The original unit was MSC (Miles of Standard Cable) and was the loss of signal in 1 mile of cable at a frequency of 795.8 Hz that was equivalent to the smallest perceivable attenuation detectable to the average listener.

The Decibel and Sound Level Measurement

When discussing sound levels, the proper format is to use the unit dB SPL, dB(SPL) or dBSPL. The reference for any statement is the sound pressure as compared to 0dB. 0dB is defined as the perceived sound of a mosquito at a distance of 10 feet from the listener.

Because dB SPL expresses a ratio, sounds can be quieter than 0dB. Imagine if you will, you are in the space where the sound created by that original mosquito was measured. If we take away the mosquito, the space will be quieter. How much quieter depends on other sources of noise. Electrical noise created by lighting and noise caused by heating and cooling systems all contribute. If we eliminate as many noises as possible, the room will get quieter and quieter.

According to Guinness World Records, the quietest place in the world in 2012 was an anechoic test chamber at Orfield Laboratories in Minneapolis. The sound level in this room was measured at -13dBA. In October 2015, a team of engineers at the Microsoft head office in Redmond, Washington, smashed this record with measurements taken in the anechoic chamber in Building 87. A team of independent specialists measured a noise level of -20.35 dBA. The room is not only completely isolated from all sources of noise and vibration, but the walls are lined with large acoustic foam wedges design to absorb sound.

At the opposite end of the sound spectrum we have 191 dB SPL. This is the sound level where the air is pressurized to 1 Bar or 1 atmosphere. Linear sound cannot exist above this level because the low-pressure side of the wave reaches an absolute vacuum. There are louder noises (such as nuclear explosions), but they are examined as pressure waves rather than sounds.

All Sounds Are Not Perceived Equally

The human ear is not sensitive to all sounds equally. In 1933, the results from research into how our ears perceive different frequencies was published. Researchers Fletcher and Munson released a set of human hearing sensitivity curves that are based on frequency and amplitude. The curves were created by playing a pure 1 kHz tone and a tone at a different frequency alternately. The amplitude of the 1 kHz tone was adjusted until participants felt the level of the two were equivalent. The adjustment level was recorded and they moved to another frequency.

In 1937, similar testing was done by Churcher and King, but the results differed a great deal from the Fletcher Munson charts. Researchers Robinson and Dadson repeated the testing in 1956 with newer equipment. The resulting measurements were accepted and defined the ISO 226 normal equal loudness-level contours. These remained the standard until 2003 when new testing further revised the graphs.

What the curves tell us is that our hearing is most sensitive around 2 to 3 kHz, depending on amplitude. We are less sensitive to high-frequency information around 10 kHz and 150 Hz by about 20dB. We are increasingly less sensitive to sounds below 150 Hz, but this phenomenon decreases as volume increases.

How We Perceive Sound

Many statements about sound levels get thrown around the industry. Let’s talk about and clarify a couple of the most common.

3dB is twice as loud. No. No, it isn’t. A change of 3dB represents a doubling or halving of acoustic energy. It takes an amplifier twice as much power to produce a tone at 73dB as it requires at 70dB. The reality is, most listeners can just barely perceive a change in level of 3dB at all audible frequencies.

If 3dB isn’t twice as loud, what is? Based on extensive testing, it is agreed that a change in level of 10dB is considered to be twice or half as loud.

A Listening Test

Just for fun and education, below is a series of test tones to demonstrate our ability to detect differences in amplitude. These tests are created to make the differences as easily perceivable as possible.

The tones involve a sine wave at a frequency of 1 kHz recorded at a starting level of -10dB from the full scale in a 44.1 kHz, 16-bit uncompressed .wav file format. The amplitude (volume) of the waveform is decreased at one-, two- and three-second marks by varying amounts. For most, discerning the 1dB per step decrease is easy. Many will be able to detect the 0.5dB decrease per step. The 0.25dB decrease per step is difficult to hear.

Track 1

1 kHz, decreasing in amplitude by 1.0 dB at one-second intervals

Track 2

1 kHz decreasing in amplitude by 0.5 dB at one-second intervals

Track 3

1 kHz decreasing in amplitude by 0.25 dB at one-second intervals

Now, based on your results, does this test disprove the above statements about 3dB and 10dB differences? Not at all. As mentioned, the tests are designed to make the perception of level changes very easy. If you were to listen to a song, then play the same song again five minutes later after adjusting the volume up or down by 0.5dB or 1dB, most people wouldn’t be able to detect the difference.

We’ll revisit the decibel in future articles and explain how different rating curves affect the numbers we read when looking at audio equipment noise measurements and specifications. Until then, we hope you enjoyed this article and the test tracks.

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.

There is nothing worse than turning up your music only to hear your car speakers or subwoofers rattle and buzz because they are damaged. Well, there is something worse: You could turn up the volume and hear nothing at all. We hear stories about people damaging their car speakers all the time. In almost every case, the issue is over-powering them because of unwise adjustments to the sound system. In this tongue-in-cheek article, we’ll discuss the five fastest ways to blow your car speakers. Let’s be clear: We don’t want you to damage your speakers and, more importantly, your hearing. The reality is, this is a list of five things NOT to do to your car stereo system. We hope you enjoy!

1. Turn Up the Gains on Your Amps

When a mobile electronics specialist installs an amplifier in your vehicle, the gain control (also called the sensitivity control) should be adjusted so that the amp will produce its maximum power when the volume on the source unit is turned up all the way. Some installers provide a little extra range on the volume so that quiet recordings can still play loudly. This is called gain overlap.

The amount of power your amplifier produces is fixed. That is to say, no amount of knob-turning, button-pushing or amp-gain-tweaking will allow it to produce more power. Turning up the gains on your amp only causes the amp to produce full power with a lower input voltage from your source unit. There is no benefit to this, and depending on your system, could introduce more background noise.

If you think your system doesn’t play loudly enough or seems to get too loud with only a little turn of the volume, go back to your installer and have him or her check the settings on the amp while playing the music you enjoy.

2. Crank the Bass Boost!

Perhaps the most dangerous control on an amplifier, besides an improperly set gain control, is the bass boost control. In all cases, this single-band equalizer increases the output of the amplifier around a specific range of frequencies — usually in the 40 to 50 Hz region. What the control doesn’t do is increase the maximum available power from your amp. If your audio system is configured to produce full power with the volume on your radio turned up all the way, turning up the bass boost on an amp or processor will cause the amp to distort at the frequencies that you have boosted. It won’t make the system play any louder.

If you turn the bass boost up 10dB, then you need to turn the gain control by an effective 10dB to keep everything equal. Perhaps it’s easier to leave it alone?

3. Wire the Amplifier To Below-Spec Impedance

If you have multiple subwoofers with dual voice coil designs, a variety of options are available to wire them to your amp. The voice coils can be wired in series, in a series-parallel configuration or all in parallel. The maximum amount of power an amplifier produces is dependent on the voltage and current provided by the amp. Lower load impedances will typically cause an amp to produce more current and consequently more power. With that said, there is a limit. All amplifiers have a minimum load impedance rating. This means the manufacturer has designed the amp for a specific current limit that won’t over-tax the power supply transformer and the power supply and output switching devices.

Changing the way your subs are wired to something that is beyond the specification of your amplifier may allow it to produce a little more power, but in the case of most amplifiers, all it does it make the amp run much hotter because the efficiency is reduced. If your amp was producing 1,000 watts and rewiring it made an extra 50 or even 100 watts, well, that difference is almost inaudible.

4. Adjust the Tone Controls or EQ on Your Radio

If your radio has an equalizer or simple bass and treble controls, turning them up will make different frequencies of your music louder relative to others. With that said, it won’t make a properly configured and tuned audio system play any louder. Just like the bass boost on an amp, equalizers and tone controls affect the signal level at specific frequencies.

Another common problem with adjusting equalizer controls in a source unit is the ease of distorting the output signal. The preamp signals from radios are rated for a specific amount of voltage, usually 2, 4 or 5 volts RMS. Turning up the tone controls on the deck could cause the signal coming from the radio to distort and make your music sound horrible.

5. Buy the Wrong Amplifier

All speakers and subwoofers have power ratings. In almost all cases, this rating is the amount of power that the speaker can manage from a thermal standpoint. You see, speakers are notoriously inefficient. More than 95 percent of the energy fed into a speaker is converted to heat. If you feed a woofer 100 watts of power, 95 watts go into heating the voice coil and motor assembly and less than 5 watts are converted into acoustic energy.

If you buy an amplifier that produces more power than a speaker or subwoofer is rated to handle, you will overheat the voice coil assembly, and it will fail.

At the opposite end of the spectrum, having too little power can also cause problems. Let’s say you have a coaxial speaker rated for 70 watts of power and you are using an amplifier rated for 50 watts. You’d think that you are pretty safe, right? If you push that amplifier to the point that its output signal reaches clipping, the amp will produce a great deal more high-frequency energy. The additional energy can cause the tweeter to heat up and possibly fail.

Another consideration about amplifiers is that most can produce 150 percent to 200 percent of their rated power as extra energy when pushed into clipping or distortion. So, a 50-watt amplifier can easily produce 75 watts of distorted power and still damage that 70-watt speaker.

Make sure you have enough power to enjoy your music at the listening levels you want without having to push an amp to the point of distortion.

If you have any questions about purchasing the right products for your mobile audio system, visit your local mobile electronics specialist retailer. They will ensure you get the right solutions that are configured so your car audio system sounds great and will last for years.

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.