The staff here at BestCarAudio.com look at dozens of installation photos each day and continue to see tweeters that aren’t installed in a way that will maximize the listening experience. Our goal is to help consumers get the best performance possible from their car audio system upgrades, so we’re going to perform two tests to demonstrate the importance of understanding speaker dispersion patterns. This information is vital to optimizing the angle at which tweeters are installed in a car audio system.

Let’s Talk About Speaker Directivity, Again

The term directivity describes where the sound created by a speaker goes. For the purposes of this article, we are going to talk specifically about tweeters. (We’ll follow up soon with another article about midrange speakers and woofers.) A clear understanding of these concepts for all speaker sizes is crucial to proper car audio system design and installation.

As a caveat, we want to make it clear that directivity occurs with every type of speaker from every company at every price point. Inexpensive speakers are no different from those that cost thousands of dollars. For this test, we used a 1.5-inch audiophile-grade automotive tweeter. We choose this driver because we know it has a flat response and minimal distortion.

When we talk about speaker directivity, we need to compare the circumference of a speaker to a wavelength of sound. Wavelength is calculated by dividing the speed of sound (343 meters per second) by frequency. As an example, a 1 kHz tone has a wavelength of 34.3 centimeters or about 13.5 inches. A 40 hertz tone is 8.575 meters or 337.6 inches. The length of the pipes in a church organ is calculated using this wavelength information. The same holds for wind instruments like trombones or trumpets.

For speakers playing low frequencies relative to their circumference (and proportionately, their diameter), energy radiates forward, backward, sideways, up and down. You can think of this in the same way that light radiates from a candle. Candles aren’t brighter to the side or above. The light created by a candle is a point source that radiates outward and evenly in all directions.

At extremely high frequencies, sound radiates from the speaker similarly to light from a flashlight. If you are off to the side of a narrow-beam flashlight, you won’t be illuminated. For speakers, the same thing happens. Off to the side of a speaker, high-frequency information can be attenuated by 24 decibels or more compared to being directly in front of the speaker.

Speaker Diameter Affects Directivity

The directivity characteristics of a speaker depend on the size of the speaker cone or diaphragm. Keep in mind that the advertised “size” of a woofer or midrange doesn’t describe the diameter or circumference of the actual speaker cone. For example, a 6.5-inch speaker doesn’t have a cone with a diameter of 6.5 inches. It’s usually closer to five.

For our 1.5-inch tweeter, the diaphragm has a circumference of roughly 4.712 inches, which is the wavelength of 2.865 kHz tone. Each speaker has a unitless value known as ka. The ka value is the driver’s circumference divided by wavelength. Directivity can be described as multiples of ka. For our tweeter, ka is equal to 4.712 kHz; ka = 0.5 is 1.43 kHz; ka = 2 would be 5.7 kHz; and ka = 5 would be 14.3 kHz. Above a frequency where ka = 1, the speaker becomes increasingly directional. Below this frequency, sound radiates evenly in all directions — even behind the speaker.

The graphs above represent a generalization of how the output of a speaker changes as you move off-axis from being directly in front of the driver. As can be seen in ka values of 1 or less, you would hear almost as much of that frequency when standing behind a speaker as you would being directly in front of it. By the time ka = 3, you don’t hear anywhere near as much to the speaker’s side, and it only gets worse as frequency increases.

Directivity Testing

We set the tweeter up on a pedestal and took a series of measurements at 12-degree increments. The graph below shows how the tweeter output decreases at high frequencies as the listing angle increases.

The red trace represents being directly in front of the speaker. Frequency response is within 5 dB from 1.5 kHz to above 20 kHz. Since we don’t have an anechoic chamber and are using windowed FFT measurements, these are adequately accurate results for this article.

The blue line has the microphone positioned 12 degrees to the side and directly on-center. At a meter, that’s a distance of only 20.9 centimeters or roughly 8.2 inches to the side. There are some minor changes in response up to 15 kHz of about 1 dB. At 20 kHz, the output has decreased by about five dB.

The dark green trace now has the microphone located 24 degrees off-axis. Overall output is down another dB or two between 3kHz and 15kHz, but the output at 20 kHz is down 13 dB. As we move farther and farther to the tweeter side, the output decreases more and more.

Think back to the math we did to calculate that ka = 1 frequency of 2.865 kHz. All the frequency response measurements are nearly identical at that frequency and below, and they start to separate more and more at multiples of that frequency. It’s almost like the math works!

Option 2 – The Tweeter Bounce

It’s not always physically easy to position a tweeter so it aims equally at the driver and passenger. One option in these instances is to install the tweeter near the base of the windshield and point it upward. The sound from the tweeter will bounce off the glass and radiate into the listening area.

While this sounds ideal, there are drawbacks. In this configuration, you effectively have two sound sources with different path lengths. This difference in path lengths will cause a certain amount of constructive and destructive interference called comb filtering.

The graph below shows the response of our audiophile tweeter at zero degrees in red, at 48 degrees in blue and measurement at 90 degrees off-axis in green. The fourth measurement is still at 90 degrees, but we have added a large piece of glass in front of the tweeter and angled it at 45 degrees. This last trace is in gray and shows us the measured response off of the glass.

Bouncing the tweeter output off our windshield analog, the response above 4 kHz is as good as if the tweeter were installed on-axis directly with the listening. But, unfortunately, everything comes with a price. We now have a dip of about 8 dB at 2.7 kHz. Yes, we could fix that with an equalizer, but that would require driving the tweeter with more than six times as much power around that frequency range. We could raise the crossover point to 3.5 kHz, but now we might have frequency response problems with the high-frequency information from the midrange speaker.

The Takeaway on Aiming Tweeters

If you want to hear all the music your stereo reproduces, then it would be ideal to be within plus or minus 15 degrees of on-axis with the tweeters in your car audio system. Any effort you might have put into reproducing the highest audio frequencies with better amplifiers or high-resolution audio players is lost if you are beyond that angle. You can also explore how your system behaves with the tweeters in the corners of the dash. You’ll sacrifice a bit of stage width and may have to deal with some frequency response issues at lower frequencies than sail panel locations, but it might require less fabrication.

We’d never argue that installers and technicians have different tuning and installation methods. With that said, and in spite of their best efforts, they can’t change the laws of physics. Choosing a tweeter location that has both of the tweeters pointed at the middle of the car between the driver and passenger’s head will ensure both can enjoy extended frequency response with good symmetry from both sides of the vehicle. Drop by your local specialty mobile enhancement retailer today to discuss how they can aim the tweeters in your car or truck for the best performance.

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.

It seems do-it-yourself installers have taken a liking to our discussion about wiring multiple subwoofers to a single amplifier. The popularity makes sense, since many car audio enthusiasts aren’t familiar with series and parallel wiring. A question came up on Facebook a few weeks ago asking if it was better to wire a pair of dual voice coil subwoofers in series with their coils in parallel or vice versa. We thought it might be fun to turn that question into yet another experiment and dive deep into the options of series-parallel wiring.

Wiring Dual Voice Coil Subwoofers

Unless your installer has access to some very low impedance subwoofers, most installations will see the subwoofers you have purchased wired in parallel. For example, say you’ve chosen a pair of 10-inch subs with dual 4-ohm coils. In that case, your installer could wire all four coils in parallel to present a 1-ohm load to a monoblock subwoofer amplifier. What if you’re after a solution that will offer the best sound quality possible, and you’ve chosen a two-channel Class-AB amp to power your subwoofers? In most cases, these amplifiers want to see a 4-ohm load when bridged. It’s not difficult to wire a pair of dual 4-ohm subwoofers to present this load, but there are a couple of options.

Series the Coils’ Woofers on Each Driver, Parallel the Subwoofers

Your installer’s first option is to wire the voice coils on each subwoofer in series. For our dual 4-ohm subwoofers, this wiring configuration would add the voice coil impedance on each driver to produce a nominal 8-ohm load. Next, your installer would wire each subwoofer in parallel with your amplifier to create a 4-ohm load.

Parallel Each Woofer’s Coils, Series the Subwoofers

The second option is to wire the voice coils on each subwoofer in parallel, then wire the two subwoofers in series with each other to the amp. Each subwoofer would have a net impedance of 2 ohms, and wiring those loads in series would present our amplifier with a 4-ohm load.

However, our subwoofers aren’t resistors. We talked about the reactive characteristics of speakers and subwoofers not long ago in this article. Since we’re dealing with inductance and capacitance along with the resistance of the voice coil, is there a chance that the two wiring options present different results in terms of performance? Let’s see what happens!

Let’s Do a Test!

We have a pair of 10-inch subwoofers that we’ve meant to install into the sound system in our office. Yes, we need the ability to reproduce 25 Hz with authority while watching Cleetus McFarland, AvE, Project Farm and bigclivedotcom on YouTube. OK, maybe we don’t NEED it, but we want it. As we described in our examples above, the subwoofers have dual 4-ohm voice coils, so they’ll be perfect candidates for our experiment.

First, we measured the Thiele/Small parameters of one subwoofer using our new Clio Pocket 1 portable measuring system. Next, we measured the sub with its voice coils wired in series and then again with the coils wired in parallel to generate the data below.

Not surprisingly, the mechanical characteristics like resonance frequency (Fs) and compliance (Vas) didn’t change. As we expected, the electrical measurements like DC resistance, electrical Q, and inductance (at 1 kHz) did change.

Electromechanical Series-Parallel Wiring Results

Next, we wired the subwoofers together in the configurations we showed in the two diagrams at the beginning of the article and repeated the measurements to see what, if anything, had changed.

We’ll start by saying that the differences are minimal. The reality is, the system will work just fine wired either way. With that said, there are signs that wiring the coils on each sub in series and wiring the subwoofers in parallel is slightly beneficial.

If you look at the chart above, the DC resistance of the C-S W-P configuration is a little lower, as is the driver’s total Q (Qts). A lower Q-factor can mean less resonance at Fs and a more accurate bass response.

Sometimes an experiment yields earth-shattering results. Other times, the outcome is subtler or less controversial. Most professional installers wire coils in series and those subwoofers in parallel. If you need a hand choosing suitable subwoofers for your car audio system or help in wiring them, drop by your local specialty mobile enhancement retailer today.

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 were recently at an event that was hosting a mobile audio system SPL competition. If you aren’t familiar with this sort of thing, it’s a contest to see who has the loudest stereo. We aren’t talking about cranking the dial on the factory radio. Instead, some of these folks build steel and concrete reinforced subwoofer enclosures and feed tens of thousands of watts to their subwoofers.

As you can imagine, delivering that much power requires significant electrical system upgrades. Many of these creations have multiple alternators and huge banks of batteries. If you’ve upgraded your car or truck with an amplifier that can produce hundreds of watts to your speakers, there’s a wiring upgrade called the Big Three that can improve the efficiency of your electrical system. Let’s look at what this upgrade is and why it’s a great starting point for a high-power car audio system.

How Automotive Charging Systems Work

The first thing you need to know about delivering power to your amplifier is that most of the energy will come from your alternator, not the battery. The battery is there to start the car. Once a vehicle is started, the alternator, which is mechanically driven by the engine, provides power to replenish the battery for the next start. The alternator also runs the computers, ignition system, fans and lights in your vehicle. Once you’ve started the vehicle, you could remove the battery and everything would, under normal conditions, operate just fine.

This scenario changes if you add a high-power amplifier to your vehicle. Say you have a 1,000-watt amplifier and you want to deliver that full amount of power to a few high-power subwoofers. At 13.8 volts, most amplifiers of this type will consume about 100 amps of current. If you have the engine running, headlights on and a climate control system in operation, your car or truck may be using 30 or 40 amps of current to power those systems. If the alternator is only rated to produce 80 amps, there certainly won’t be 100 amps left for your stereo.

The additional current the amplifier wants will come from the battery, albeit at a lower voltage. You will likely find that an amplifier rated for 1,000 watts at 14.4 volts can only produce about 800 watts at 12 volts without clipping the output signal and adding significant distortion.

How To Make the Most of Your Alternator

The first step in optimizing the electrical system in a car or truck is to reduce waste. In this case, waste is the power converted to heat because of resistance in cables and connections. First, have a look at your alternator and battery. How large are the wires running to and from them? Unless you have a big truck with a high-output alternator, chances are that the wiring is 4 or maybe even as small as 6-AWG.

Please make no mistake about it, the company that built your car or truck deliberately chose the smallest, lightest and least expensive wire that would allow the vehicle to function reliably. They had no intention of providing conductors that could handle two to three times what the alternator was rated for.

The Wiring Upgrades that Make the Difference

The Big Three upgrade involves adding to or replacing the existing conductors with larger, high-quality wires. The upgrades in the Big Three are:

• The run from the alternator chassis to the negative terminal of the battery.
• The run from the output of the alternator to the positive terminal of the battery.
• The ground connection from the negative terminal of the battery to the chassis of the vehicle.

Depending on your vehicle, there may be other wires that can be upgraded at the same time. For example, if there is a main power distribution box under the hood, running new wire to it can help provide more voltage to the factory-installed electrical and electronic components in your vehicle.

Another upgrade you may want to make at the same time is replacing the battery terminals. Some factory-installed terminals are notoriously flimsy. If you’re demanding significant current from the battery, then optimizing each component along the way only makes sense.

Is the Big Three Enough?

In 2012, we wrote an article about upgrading the stock electrical system wiring in modern cars and trucks. Even though it was a decade ago, new vehicles at the time were being constructed out of materials like QuietSteel and aluminum, and some companies had started using adhesives instead of spot welds to bond unibody panels together. All of these advances in vehicle assembly work against our desire to deliver large amounts of current to an amplifier, especially when we want to use the body as the ground return path.

Power delivery from the positive terminal of the alternator is as important as the ground connection to the chassis and battery. If the current can’t flow through both, you are wasting energy by transforming it into heat. Upgrading and adding grounds with the Big Three is a start. If you are serious about optimizing your electrical system, then you’ll want to add a ground wire from the battery or alternator directly to your amplifier. Back in 2012, we called this the Big Four.

Full disclosure: This wasn’t our idea. The first we heard of running a dedicated ground wire was when we were checking out Precision Power’s fleet of Chevrolet Suburban demo vehicles back in the ’90s. The topic came up again when Ford introduced QuietSteel in the F-150 vehicles, and companies were having problems with amplifiers failing because of low voltages. As a solution, JL Audio suggested adding what they called a parallel ground. This wire would connect to a conventional ground point in the rear of the vehicle and run to the battery’s negative terminal or the alternator. A benefit of the parallel ground is that it offers less resistance than a dedicated ground wire or using the chassis. As a result, current can flow through both paths, reducing the voltage drop and improving system performance.

Optimize Your Vehicle Electrical System

If you’ve purchased a high-power amplifier for your vehicle, talk to your local specialty mobile enhancement retailer about upgrading all the wiring in the electrical system. Implementing an upgrade like the Big Three or, better yet, the Big Four will allow the amplifier to produce more power and allow the system to play louder with improved reliability and efficiency.

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.

When it comes to expressing the quality of an audio system component, the amount of distortion it adds to a signal is the defining factor. In this article, we’ll talk about how much distortion each part of an audio system adds – so that you can target your budget for the greatest benefit. We’ll rank four categories of audio equipment based on the amount of distortion they add to your music. Ultimately, our suggestion is to optimize your upgrade budget by focusing on the products that will deliver the most significant improvement in perceived audio quality.

What Is Audio System Distortion?

Distortion is the addition of unwanted information to an audio signal. In minimal amounts, distortion can be difficult to perceive. However, above a level of about 0.5%, distortion is audible and can change the music’s tonal balance and perceived dynamics.

It’s become clear that most people think distortion is only present in an audio system when a speaker is overdriven or an amplifier starts to clip. While that level of distortion is very audible, unwanted harmonic and intermodulation distortion is added even at moderate listening levels. For example, a low-quality amplifier providing less than a watt of power to a speaker can add almost 1% distortion to the signal. A poorly designed speaker or one operating outside of its intended frequency range can easily add 3-5% distortion, and a subwoofer driven at moderate levels can be well over 10%.

No. 1 – Speakers and Subwoofers

If the paragraph above wasn’t enough of a hint, let’s make it clear: Speakers and subwoofers are notoriously non-linear. Let’s say you’re feeding a 100 Hz sine wave into an inexpensive woofer. If the design has less compliance (more stiffness) in the forward direction relative to the rearward, the driver’s output will be reduced for half of the waveform. That’s distortion. If the driver has changes in inductance relative to forward or rearward motion, there will be a reduction in output over one half of the produced audio waveform. Distortion is also possible from resonances in the cone, surround and dust cap that add significant unwanted energy.

You need to know that ALL speakers add more distortion as they are driven to higher excursion levels. Aside from not using a properly designed or constructed enclosure for your subwoofers, or trying to play the woofers in your doors at frequencies below 80 Hz, louder means less quality.

When shopping for car audio upgrades, look for speakers that include distortion-reducing technologies. We’ve talked about copper and aluminum shorting rings, copper caps and flat progressive spiders. To minimize power compression (a reduction in output as the speaker is driven at higher levels), you’ll need to choose a solution that balances cooling features like a large-diameter voice coil with acceptable high-frequency performance.

Last but certainly not least, the product specialist you’re working with needs to design, install and calibrate your audio system properly. Proper speaker mounting adapters and spacers made from weather-resistant materials are a starting point for installation. Choosing driver sizes that offer good directivity performance, so your music sounds great from both sides of the vehicle, is another important criterion. Finally, setting crossovers, equalization and signal delays to create a realistic listening experience is crucial. Getting any of these steps wrong will reduce your enjoyment of your favorite music.

No. 2 – Amplifiers

If you’ve read our articles on distortion, then you know that we have a passion for testing amplifiers. At their worst, amplifiers add maybe 1/10th of the distortion to an audio signal that a low-quality speaker does. This doesn’t mean that they aren’t in a solid second place in terms of where you should invest in an audio upgrade.

Amplifiers are supposed to amplify a signal – nothing more, nothing less. They should output what you feed to them without adding warmth, brightness or changes to overall tonal balance.

Low-quality or poorly designed amplifiers can add distortion because of the crossover between the positive and negative output devices. In addition, they can exhibit frequency response issues from poorly designed bass boost and filter circuits. Amplifiers with high output impedance can be affected by changes in load impedance.

A second consideration for amplifiers is noise. If you turn your car stereo system on and hear a lot of hiss, it’s likely from the amplifier. Every amplifier adds a little background noise. Whether noise affects your enjoyment depends on just how much is added. The signal-to-noise ratio lets you know what’s going on behind the scenes.

A few quick tips for buying high-quality amplifiers: As far as Class-D amplifiers have come, they are best suited to powering woofers and subwoofers. The output filter networks can get a little fussy at higher frequencies. As such, Class-AB amplifiers are better for midrange and high-frequency duties. Second, the physical size of an amplifier significantly affects how much noise is added to the output. Small amplifiers are typically noisier. Third, watch how specifications on distortion are provided. The ANSI/CTA-2006-C standard requires that distortion and noise numbers be published when measured at an output level of 1 watt into a 4-ohm load. Measurements at other levels may be misleading.

No. 3 – Source Units

We’ve tested several source units lately, and though their effect on sound quality isn’t as dramatic as the speakers and amplifiers you choose, there are measurable differences from one model to another. While amplifiers have to deal with high voltages and significant current delivery, the signals inside a radio or multimedia receiver are pretty small. These low levels mean that noise from a power supply can affect what you hear.

Back in car audio’s heyday, we had CD units like the Sony Mobile ES CDX-C90, Clarion ProAudio DRZ-9255 and Alpine’s F#1 Status DVI-9990E. Of course, the days of high-end CD players are gone, but there are still a few options for a high-quality source unit.

Picking a good head unit is tough. You’ll likely choose between something mainstream with good performance or an upgrade to something designed to offer better sound quality. For example, we used to suggest 24-bit Burr-Brown D/A converters. Now, those are in almost every multimedia receiver. Our advice is to choose a product that has the features you want and is clearly designed to deliver improved audio quality.

No. 4 – Digital Signal Processors

Putting the DSP family at No. 4 in our list might puzzle anyone who understands how DSPs work and why they are an absolute necessity in a premium car audio system. Let’s make it clear that you need a DSP to equalize and filter the signals going to your speakers for them to deliver realistic performance. As such, a DSP is one of the most important components in an audio system. Its ranking in this article is based on whether or not you need to allocate extra funds to purchase a product in each category to yield better performance. So, having a DSP is a requirement; buying the fanciest one on the market won’t make a huge difference to what you hear.

Having said that, there are subtle differences, especially between entry-level and premium solutions. For example, most digital signal processors use similar DSP chips to perform the audio adjustments. As these are handled in the digital domain, the differences in transparency are more about the analog circuitry and the conversion process.

One tip: Don’t let yourself be fooled by claims about support for high-resolution in a DSP. In most cases, the analog-to-digital and digital-to-analog converters are noisier when running at higher frequencies. Stick with solid performance and ensure that you have an expert configure the system, and you’ll be delighted.

Choose Your Car Audio Upgrades Wisely

Here’s one last thought on buying high-quality car audio equipment: Every brand on the market is competing for your dollar. Some of them offer excellent products that result from hours, if not years, of designing and testing to deliver the best possible performance. On the other hand, some products are just expensive while delivering mediocre performance. So make sure you take the time to audition several options from different specialty mobile enhancement retailers near you.

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.

A while back, we took a quick look at how alternating current (AC) signals in our car audio systems work. While the amount of work done by an AC voltage source can be the same as that of a direct current (DC) source, the changes in current flow direction add complications. In this article, we’ll dive a little deeper into AC signals and explain why a fixed-value resistor can’t directly simulate a speaker.

Ohm’s Law – AC and DC Current Flow

Ohm’s law is a constant for both DC and AC circuits. When 1 amp of current flows through a resistor with a value of 1 ohm, then 1 volt of electric potential will be produced across that resistor. If the resistor value is increased to 2 ohms, and we apply 1 volt across it, then 0.5 amp of current flows through it. The triangle below shows the three ways we can calculate voltage, current or resistance if we have two of the other variables.

In an AC circuit, we know that the current flow switches back and forth. In home and commercial electrical systems, the voltage is in the shape of a sinusoidal waveform. The work done by the voltage or current is an average level of the signal. For a sine wave, the average level, called the RMS level, is 0.707 times the peak voltage referenced to the ground.

Alternating Current Signals Are Complicated

When current flows through a conductor, it creates a magnetic field around that conductor. This phenomenon is known as Oersted’s law, named for Danish physicist Hans Christian Oersted, who discovered this relationship in April 1820.

In a DC circuit, if we have current flowing through a coil of wire, the magnetic field created in that coil opposes changes in current flow. If we remove the voltage source, the current will continue to flow, even if only for a few milliseconds. This phenomenon is why many automotive relays have a diode on the bottom of a socket. The diode shunts the reverse-polarity current spike that results when we remove the voltage source. As a result, the magnetic field in the relay coil collapses and produces a voltage spike. This spike can damage the circuitry driving the relay or cause arcing in a switch.

Audio Signals Are Complicated

The current flowing to a speaker (or more specifically, a tweeter) may change direction as often as 20,000 times a second. All speakers (that we will worry about) use a voice coil that creates a magnetic field that makes the cone move. This same magnetic field opposes changes to the current flow direction. As such, we have more opposition to AC flow than we would for the same amount of DC current.

If we connect a digital multimeter to a speaker, the meter applies a tiny DC current to the voice coil. The number on the meter screen tells us the DC resistance in voice coil winding.

For AC circuits, we need to measure impedance. The Oxford Dictionary defines impedance as “the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.” Since we skipped over it, reactance is the opposition to AC current flow caused by inductance or capacitance. For example, the voice coil in a speaker will act as an inductor at mid to high frequencies.

Let’s look at a sizeable 6.5-inch woofer. If I use a high-quality digital multimeter to measure the resistance, we get a reading of 3.7 ohms.

If I want to know how the speaker opposes the flow of AC current, I need to feed it an AC signal and measure the opposition to current flow at any frequency relevant to the application for the driver. For this article, I’ll use the Smith & Larson Woofer Tester 2. This device can measure DC resistance and inductive and reactive capacitance at any frequency up to 20 kHz. The result is a plot of impedance (AC opposite to current flow) along with a phase plot that tells us whether the load is capacitive or inductive.

If we look closely at the impedance measurement, we can see that the woofer has an impedance of about 30 ohms at a frequency of 42 Hz. This is the driver’s resonant frequency (Fs) and represents the point at which the least amount of current produces the most output.

At higher frequencies, the inductance of the voice coil becomes the primary opposition to AC current flow in the speaker. The Woofer Tester 2 measured the inductance of this driver as having a value of 0.827 millihenry. You can see that the impedance starts to rise at an exponential rate above about 250 Hz. By the time the drive frequency is at 20 kHz, the impedance is approximately 54 ohms.

Driving a woofer or subwoofer is one type of challenge for an amplifier. This task requires significant amounts of power. The load can also be heavily reactive, meaning current and voltage may not be in phase with each other.

Driving a set of component speakers is an entirely different scenario. Even the most minute changes in output level can be readily apparent to the listener. Since speakers can present significantly varying impedances based on frequency, the voltage and current supplied by the amplifier will change as well. These variances in load impedance can result in small changes in output level as the amplifier’s circuitry interacts with the load to create a voltage divider. In systems where there is no equalization, especially in home audio systems, these variances can change how the listener perceives the music.

We created this complex reactive load to challenge amplifiers that cross the BestCarAudio.com test bench. We take frequency response measurements into all the rated loads for the amp and a measurement when connected to this simulated reactive load. Better amplifiers exhibit smaller changes in output level relative to changes in load impedance.

The graph above shows the effective frequency response of an amplifier when fed a 4-ohm resistor (in red) and 2-ohm resistor (violet) and our reactive test load (in black). As you can see, the output of the amp varies depending on what it’s connected to. This phenomenon is more common with Class-D amplifiers than with Class-AB designs.

Why Is Impedance Important to Car Audio Systems?

There are a few takeaways from this article. First, if you measure the resistance of a speaker with a multimeter, you are getting a general feel for its ability to pass current. For example, if the meter measures 3.7 to 4.2 ohms, then you have a nominally 4-ohm speaker.

However, if you intend to design passive crossovers, you need to know the exact impedance around the crossover frequency to choose the correct values. While very few people create passive crossovers these days, we do see many people suggesting that those circuits are interchangeable between different brands, makes and models of speakers. Nothing could be further from the truth. You can’t assume a crossover designed for Brand A speakers will function adequately or sound right with Brand B.

Another consideration about AC signals surfaces when measuring power in a reactive load (like a speaker). The opposition to changes in current means that current peaks lag behind voltage peaks in inductive loads. If you want to measure power from an amplifier when connected to a speaker, you need to measure current and voltage simultaneously. We don’t mean by using a voltmeter and current clamp and taking peak readings. You need to measure both using something like the D’Amore Engineering AMM-1. Those “clamped” power readings the SPL guys talk about are meaningless.

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.