Furthering our discussion about car audio electrical theory brings us to a discussion of how the flow of electricity through a conductor creates magnetic fields around the conductor. Understanding the relationship between current flow and magnetism is crucial to understanding how a speaker works.

History of Electromagnetism

The first documented correlation between electricity and magnetism came from Gian Domenico Romagnosi, a 19^th-century Italian legal scholar who noticed that a magnetized needle moved in the presence of a voltaic pile (the predecessor to a battery). Hans Christian Ørsted observed a similar occurrence in April 1820. He was setting up materials for an evening lecture and noticed that a compass needle changed directions when he connected a battery to a circuit. Neither Romagnosi nor Ørsted could explain the phenomenon, but they knew there was a defined relationship.

In 1873, James Clark Maxwell released a publication titled A Treatise on Electricity and Magnetism, which explained the presence of four effects:

1. Electric charges attract or repel one another with a force inversely proportional to the square of the distance between them: Unlike charges attract, like ones repel.
2. Magnetic poles (or states of polarization at individual points) attract or repel one another in a manner similar to positive and negative charges and always exist as pairs: Every north pole is yoked to a south pole.
3. An electric current inside a wire creates a corresponding circumferential magnetic field outside the wire. Its direction (clockwise or counter-clockwise) depends on the direction of the current in the wire.
4. A current is induced in a loop of wire when it is moved toward or away from a magnetic field, or a magnet is moved toward or away from it; the direction of current depends on that of the movement.

What Causes a Magnetic Field when Electricity Flows?

Electricity is the movement of electrons into and out of a conductor. One electron enters the end of a conductor, bumps into another electron, and so on until a different electron leaves the other end of the conductor and enters the load.

Because there are effectively more electrons in the conductor when current is flowing, the balance of negatively charged electrons to positively charged ions is upset and thus causes an imbalance in the magnetic field around the conductor.

We could devote thousands of words to explaining how atoms work. But in short, the core of an atom has a core of positively charged protons with a bunch of negatively charged electrons circling this core. When there is no flow of current, an atom doesn’t have a magnetic field because the quantity and path of the electrons around the protons are balanced. When we bump an electron out of an atom and into another, the atoms become imbalanced and thus produce a net magnetic field.

An Explanation On a Larger Scale

When electricity flows from the positive terminal of a battery to the negative, a magnetic field is created around the conductor. If you look at the image below, you will see the direction of the magnetic field relative to the flow of power.

In schools, this is often referred to as the right-hand rule. If you wrap your right hand around a conductor with your thumb extended upward in the direction of current flow (putting positive below your hand and negative above), your fingers point in the direction of the magnetic field.

Keep in mind that for audio signals, the polarity of the current changes from positive to negative in the same way that the vibrations produced by someone talking or playing an instrument pressurize and rarefy the air to produce sound.

How Magnetism Makes a Speaker Work

Conventional moving coil loudspeakers use a coil of wire (called a voice coil) and a fixed magnet. The electricity from the amplifier flows through the voice coil and creates a magnetic field. The polarity of the magnetic field pulls the voice inward or pushes it out in an amount proportional to the strength of the magnetic field.

The diagram below shows the force exerted on the voice coil with the current flowing through the positive half of the audio waveform.

This diagram shows the force exerted on the voice coil with the current flowing through the negative half of the audio waveform.

As the polarity of the current reverses, so too does the force exerted on the voice coil, which is attached to the speaker cone through the voice coil former.

Magnetism Isn’t Always Beneficial

Regarding speakers, we rely on and need magnetic fields for them to work. With that said, magnetism doesn’t always work in our favor.

If there is a large amount of current flowing through a conductor, there will be a strong magnetic field around that conductor. If we place another conductor in that magnetic field, a voltage will be produced across the second piece of wire.

In our vehicles, many devices such as fans, sensors, the alternator, lighting control modules and computers create magnetic fields containing high-frequency noise. When an improperly shielded interconnect passes through one of these fields, it can pick up that noise and produce a voltage on the conductor. This phenomenon is why it’s important for your installer to run the interconnect cables and often the speaker wires in your car away from sources of electrical noise.

Consult Your Local Mobile Electronic Installation Experts

When it’s time to upgrade the sound system in your vehicle, visit your local specialized mobile enhancement retailer. They have the training and experience to ensure your new audio system will sound great and be free of unwanted noise!

In our next article, we are going to talk about inductance and capacitance and how those characteristics affect high-frequency electrical signals.
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 discussion of car audio electrical theory, we need to discuss some of the characteristics of alternating current signals. These points of discussion include the concept of amplitude and frequency. Understanding the concept of frequency is crucial to developing an understanding of how the components in our audio systems work.

The Concept of Signal Amplitude

Thankfully, we are going to start off easy with a discussion of signal amplitude. When it comes to the ability of an AC signal to do work, just as with a DC power source, more amplitude (or level) means that more work can be done.

In a DC power source, the amplitude is fixed at a certain level. In our cars, this level is around 12 volts. In our homes, the voltage at the wall receptacle is 120V. High-power devices like an electric stove, a clothes dryer or an air conditioner are typically powered by 240V to reduce the amount of current required to make these devices operate.

When we want to reproduce sound, we need to supply an audio signal from an amplifier to the voice coil of a speaker. Ignoring the design limitations of a speaker, supplying more voltage results in the cone moving farther and thus producing more sound.

If our amplifier is producing 1 volt rms of signal to a speaker with a nominal impedance of 4 ohms, then the speaker is receiving 0.25 watts of power (calculated using the equation P = V^2 ÷ R). If we increase the voltage to 2 volts, the power at the speaker is now 1 watt ((2×2) ÷ 4). If the voltage increases to 10 volts, the power is now 25 watts.

If we were to look at the two signals described above (1Vrms and 2Vrms) on an oscilloscope (a device that shows voltage relative to time), you would see the following:
Just a reminder: The RMS value of a sine wave is 0.707 times its peak value. In the case of these waveforms, the peak values would be 1.414 and 2.818 volts.

The Concept of Frequency

Signals Containing Multiple Frequencies

Let’s step back a bit and look at the fundamentals of analyzing the frequency content of a signal. The graph you see below shows a single 1kHz signal.

The “stuff” you see at the bottom of the screen is noise. Every signal contains some amount of noise. For this graph, we can see that the 1kHz signal is recorded at a level of 0dB and that the loudest noise component is almost 170dB quieter. This low amplitude makes the noise level irrelevant.

What can be difficult to understand is that a signal can be, and often is, made up of many different frequencies. This graph shows an audio signal that contains 1kHz and 2kHz signals.

Almost every audio signal we hear comprises an infinite number of frequencies. The relative level of these frequencies is what makes one person’s voice sound different than another’s or makes a piano sound different than a guitar.

These two frequency response graphs show a piano and a guitar both playing Middle C with a frequency of 256 Hz.

The red line represents the response of the guitar, showing a peak at 256 Hz, a strong harmonic at 512 Hz and an intermodulation peak at 768Hz.

The green line shows the frequency response of a piano playing the same 256 Hz middle C note. It has significantly more harmonic content with harmonics and intermodulation peaks above and below the fundamental.

Audio Measurement Waveforms

Two waveforms are commonly used to test audio equipment and audio signals. The first is called a white noise signal. This signal includes random audio signals at all frequencies up to the cutoff of the recording medium (in this case, 22.05kHz or our 44.1kHz sampling rate WAV file). Each frequency is the same in terms of amplitude. We can use this signal along with a real-time analyzer to measure the frequency response of audio components.

Here is the frequency response plot of a white noise signal:

Another important signal is called pink noise. We use this signal when measuring the frequency response of a speaker. Unlike white noise that contains signals at equal levels at all frequencies, pink noise has an equal amount of signal energy per octave. When looked at in the frequency domain, the level decreases at a rate of 10dB per octave as frequency increases.

When you play pink noise through a set of speakers and measure the response with a microphone, you will be looking for a flat waveform.

Frequency Response of a Loudspeaker

Let’s take a high-quality, 6.5-inch coaxial speaker with a specified efficiency of 89dB when supplied with pink noise at a level of 2.83V and measured at a distance of 1 meter. A value of 2.83 volts happens to work out to 2 watts using the P = V^2/R equation.

While this specification works when we feed the speaker a pink noise signal, it doesn’t tell us how loud the speaker is at a specific frequency. For that, we need a frequency response graph.

This frequency response graph shows us how much sound energy this speaker will produce when driven by a pink noise signal.

This particular driver has a gentle dip around 1kHz, some emphasis in the mid-bass region between 80 and 150Hz and a gently rising response above 2kHz to improve off-axis performance. In a car, this speaker sounds amazing!

The Bonus Signal – A Square Wave

OK, strap on your space suit, thinking cap or whatever will help you understand the following. We are going to look at a square wave. A square wave is a waveform that combines harmonics (multiples) of a fundamental frequency to create a waveform of a specific shape. The waveform appears to have two values, one high and one low. It’s for this reason that people incorrectly assume that these are Direct Current (DC) levels.

The formula to create a square wave is made up of multiple odd-ordered harmonics of the fundamental frequency. If you have a 30Hz square wave and look at it in the frequency domain, you can see these harmonics.

When an amplifier is pushed beyond its output voltage limit, it creates a square wave. There is no DC content in the signal, but it IS full of high-frequency harmonic content.

Using an Excel spreadsheet created by Alexander Weiner from Germany, here are six graphs that show how a square wave is created by adding odd-ordered harmonics to a fundamental signal. For a perfect waveform, we need an infinite number of harmonics.

The yellow line shows a single sine wave with no harmonics.

The yellow waveform adds the third harmonic of the fundamental frequency.

The yellow waveform adds the third and fifth harmonic of the fundamental frequency.

The yellow waveform adds the third, fifth and seventh harmonic of the fundamental frequency.

The yellow waveform shows the 100 odd-ordered harmonics as well as the fundamental frequency.

In this graph, we have the fundamental frequency and 256 odd-ordered harmonics added together.

If you have ever wondered why tweeters seem to the be the first to fail when an amp is driven into clipping or distortion, the reason is the addition of high-frequency information to the audio signal. Where we might have been feeding one or two watts to a tweeter with music, a square wave or a waveform containing significant harmonics contains a great deal more high-frequency information.

We hope this wasn’t too much to information for a single article. Understanding waveform amplitude and frequency content are crucial to any discussion of a mobile audio system. In our next article, we are going to discuss the flow of electricity through a conductor and the associated magnetic field that is created.

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.