One thing common to every car audio subwoofer enclosure is that they need to connect to the subwoofer amplifier. Most subwoofer enclosures include a terminal cup or something similar to facilitate easy connection to the subwoofers. Let’s look at subwoofer enclosure speaker wire connection options.

Guess What? Ohm’s Law

If you’re serious about car audio, Ohm’s law should be tattooed on your body – or at least, you should understand it explicitly. Almost every electrical connection requires some consideration of voltage, current and resistance. Regarding subwoofer enclosure terminals, we’ve seen many discussions where the primary consideration was power. In reality, power isn’t the issue – current is.

Let’s say you have an 800-watt amplifier powering your subwoofer enclosure. If the subwoofers in the enclosure have a net nominal impedance of 4 ohms, the amplifier must flow 14.14 amps through the wires to the enclosure. In terms of appropriately sized wiring, if your installation uses 5 feet of 16 AWG wire, you’d waste 1% of the energy from the amp as losses in the wire itself. Eight watts isn’t much and isn’t anything to worry about. Jump to 12 AWG, and the losses are only 3.4 watts. It’s still not something you’re going to lose sleep over.

These days, most car audio amplifier manufacturers offer subwoofer amplifiers designed to produce power into lower impedance loads. Let’s say your amplifier delivers 800 watts into a 2-ohm load. The amp must provide 20 amps of current to make the same power. Losses in the wiring still aren’t a huge issue. Five feet of all-copper, full-sized 16 AWG wire would convert 17 of the 800 watts into heat. Jumping to 12 AWG drops that to 6.8 watts. Still not a mess.

Let’s say your subwoofer enclosure has a nominal impedance of 1 ohm, and your chosen amp can deliver 800 watts into this impedance. We are now talking about 28 amps of current flowing through the speaker wire. That 5-foot run of 16 AWG wire would waste 34.4 watts of energy. The issue is that the power is wasted as heat. Dissipating almost 35 watts of heat is challenging, and the wire will get warm. When conductors increase in temperature, their resistance increases. That’s a whole other can of worms! If your installer uses 12 AWG wire to connect your amp to your subwoofers, the losses are down around 13.6 watts – which is pretty safe.

A quick side note about low-impedance subwoofer loads: All but two or three of the hundreds of amplifiers I’ve tested increase the distortion they add to an audio signal when driving lower impedance loads. Every amplifier I’ve tested presents a significant reduction in amplifier efficiency caused by running subwoofers at lower impedances. If you care about sound quality in any way, you’ll find a nice two-channel amp and bridge it to run your subwoofers at a 4-ohm load.

Here’s a quick chart to show how much current is required to feed subwoofers with different power levels of amplifiers and 4-, 2-and 1-ohm loads.

Any time current flows through a connection, there is going to be some amount of resistance. When current flows through a resistor, be it resistance in a connection or an actual resistor component, heat is generated. The formula to calculate the power wasted in a resistor is P = R x I x I. As the current in the equation is squared (I x I), small increases in current flow result in significant increases in the amount of wasted power and heat created.

Subwoofer Enclosure Terminal Cups and Connections

If you visit a local specialty mobile enhancement retailer and look at the prefabricated subwoofer enclosures they offer, you’ll see a variety of terminal cup solutions. Some use large binding posts that accept bare wire up to 10 or even 8 AWG. Others use small spring-loaded terminals that might accept 14 AWG wire.

The first terminal we’ll look at is the basic spring-loaded terminal type. This terminal type will be the least ideal for conducting surface area between the speaker wire from your amplifier and the connections inside the amplifier. All the current has to pass through a thin piece of stamped steel. We see people sharing images of these terminals melting all the time.

The next step up in terminal cup quality is also spring-loaded but has significantly more surface area. These terminals pass the current through a stud rather than a piece of stamped steel.

A common drawback with this type of connection is that some are designed so the clamping portion doesn’t squeeze small-gauge wire well. If the speaker wire is flopping around loose, things will get hot when you pour on the power. If the springs have enough pressure, the right size wire is used and the wire is inserted properly, these should work better than the previous option.

The last terminal cups we’ll look at are called binding posts. These are similar to the previous cup in that the wire is inserted into a hole in the center stud. A nut threads down to secure the connection rather than a spring applying pressure to the wire.

The number one issue is whether or not the nuts are tightened correctly and if they stay tight. If anything were to loosen over time, you’d end up with a high-resistance connection, which will produce significant amounts of heat.

If your car audio system uses binding posts, it’s worth checking them frequently to make sure they’re tight.

Testing Subwoofer Terminal Cups

So, just how much current can a terminal cup handle? As you can imagine, this is an excellent idea for some testing. We rounded up a few cups and connected them to the power wire on an amplifier. With a shunt resistor in series, we can accurately measure current flow. We wired the terminal cups up one at a time and slowly increased the current flow until they produced significant amounts of heat. We’ll give them an approximate current rating based on being able to handle two minutes at a specific current level. You can use the chart above to determine the amount of power the cup can handle based on the impedance of your subwoofers.

Up first was the inexpensive spring-loaded cup. We started with 5 amps of current. Not much exciting happened, and the connection generated no heat. Things stayed pretty steady as we increased current. The voltage drop also increased reasonably steadily. By 15 amps of current, the connection to the wire was producing 100 degrees F of heat. Fifteen amps equals 225 watts with the subwoofers wired to a 1-ohm load. Things started to go south at 23 amps of current, or just over 500 watts when wired to 1-ohm. The connection increased in temperature faster. At 24 amps, I could smell something, and the connection was over 180 degrees. At 25 amps, the plastic spring block started to smoke, and the connector melted. The wire is now permanently attached to the cup.

The other terminal cup I had here in the lab was a threaded binding-post style. I hypothesized that the added surface area of the center post, combined with the nut on the back securing the tab in place, would help provide less resistance and, thus, more current carrying capacity.

Once again, things seemed fine with current levels up to about 6 amps, with no measurable heat at the connection. Six amps equals 36 watts of power to a 1-ohm load. As with the inexpensive spring-loaded block, resistance increased as the current flow increased. The connection reached 100.5 degrees with 18 amps of current, which is only slightly better than the first terminal. However, this is nowhere near the performance I expected. At 28 amps of current, or 784 watts into a 1-ohm load, the entire metal assembly had melted the plastic, and the terminal started drooping.

Let’s do some quick math on my measurements during the testing. First, I measured the voltage drop from the wire connected to the terminal (just beside the connector) to the terminal strip on the back of each terminal cup. The graph below shows the blue spring-loaded cup and the orange binding post.

I used my thermal imaging camera to measure the temperature of each connection at each current level.

If we want to extrapolate some current handling limits from the day, the spring-loaded cup is good for about 22 amps of current, and the binding post cup can handle about 25 amps. These are, of course, continuous current numbers. These correlate to 484 watts at 1 ohm, 968 watts at 2 ohms and 1,936 watts at 4 ohms for the spring-loaded cup. The binding post can theoretically handle 625 watts at 1 ohm, 1,250 at 2 ohms, and 2,500 at 4 ohms.

There are a few caveats to this test. These are continuous current measurements. If you’re listening to very dynamic music, you can likely use more power. However, things don’t take long to melt at the upper limits, so be cautious in choosing your subwoofer enclosure.

Another caveat is that we used 14 AWG all-copper wire. You may experience different performance levels with larger or smaller conductors.

The Best Way to Connect to a Subwoofer Enclosure

Doing this quick test also gave us the idea to test a few quick-connect styles as suitable solutions for delivering significant amounts of power to a subwoofer enclosure. You’ll have to watch for that article soon. In the meantime, we suggest choosing an enclosure that uses multiple terminals if you’re attempting to deliver significant power at low impedances. Your local specialty mobile enhancement retailer can then run multiple speaker wires back to the amplifier to improve efficiency and provide an audio system that is as reliable as possible. Better yet, forego any connector and ask them to run the speaker wires directly from the amp to the subwoofers and seal the wire where it enters the enclosure.

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.

The concept of how car audio amplifiers distribute power to multiple speakers connected to the same channel seems simple. A little math with Ohm’s law and looking at the amplifier’s specifications should tell the tale, right? Yes and no. Things get more complicated when crossovers are involved. Don’t fret; we’re here to make everything easy to understand. Let’s dive in!

What Determines How Much Power an Amplifier Produces?

The first thing we need to understand is how amplifiers produce power. In almost all cases, car audio amplifiers increase the voltage of the incoming signal by a specific amount. The amount of increase in amplitude is called gain. Engineers specify gain in decibels. Depending on the amplifier’s sensitivity control setting, the gain might be as low as 5 or 6 dB for a small amp or 40 dB for something significant.

More gain isn’t better. That can lead to more unwanted noise added to the signal. Every source unit and amplifier combination has an ideal gain configuration that produces maximum power with as little noise as possible. More commonly, the correct gain setting ensures that the speakers connected to the amp blend smoothly with the rest of the system.

As mentioned, amplifiers increase voltage. Feeding a 1-volt signal from your radio into an amplifier might increase that by 10 dB to 10 volts. That voltage goes to the speaker connected to the amplifier. If the speaker has a nominal impedance of 4 ohms, you can use Ohm’s law to calculate how much current will flow through the speaker. We can calculate current by dividing the output voltage by the load (speaker) resistance. In this configuration, 10 divided by four is 2.5 amps.

You can also calculate how much power the speaker gets. To calculate power with voltage and resistance values, square the voltage and then divide that by the resistance. In this example, 10 squared is 100. One hundred divided by four is 25. So, 10 volts applied to a 4-ohm speaker results in the speaker getting 25 watts of power. Another option is to multiply current by voltage to calculate power. In this example, we have 10 volts and 2.5 amps of current, which equals 25 watts.

With a lower impedance speaker, as you might find with a subwoofer configuration, the amplifier’s power production increases because more current flows through the voice coil. Interestingly, this happens without any adjustment in gain settings. Let’s run the same math with a 2-ohm speaker. The speaker will have 5 amps of current flowing through it (10 volts divided by 2 ohms) and will get 50 watts of power (10 squared divided by two).

We can change the impedance an amplifier sees in several ways. A single 4-ohm speaker will, of course, present a 4-ohm load. Two 4-ohm speakers wired in parallel present a 2-ohm load. Read this if you need a reminder about parallel wiring. You could wire two 2-ohm speakers in series to get a 4-ohm load. You could wire four 1-ohm speakers in series to get 4 ohms. There are some performance-related drawbacks to wiring speakers in series. Primarily, the inductance values add and can dramatically affect midrange performance. It’s best to avoid series wiring when possible.

How Passive Crossovers Affect Amplifier Power Production

So far, this has all been relatively simple. However, we’ve assumed that our speakers act as a fixed-value load of 4 or 2 ohms. The reality is that they aren’t. Because audio signals are alternating current waveforms, and our speakers have characteristics like resonance and inductance, the impedance the amplifier sees varies with frequency.

Impedance is the term used to describe the opposite of current flow in a circuit with alternating current. It’s similar in concept to resistance but much more complicated to calculate and manipulate, as phase is also an issue.

The graph below shows the impedance of a 6.5-inch component speaker set with different settings on the passive crossover network.

The first thing to notice from the graph above is the spike at just over 60 Hz. This spike represents the resonant frequency of the woofer in the speaker system. The impedance at the resonance peak is close to 16 ohms. The amplifier produces much less power at this frequency, but the speaker becomes more efficient. Resonance implies that a small amount of energy produces significantly more output. If the speaker is well-designed, which is the case with the Rockford Fosgate T3652S woofer, the frequency response around the resonant frequency will match that of the higher frequencies.

The other bump in the impedance graph is around the crossover point between the tweeter and woofer. This impedance rise likely results from underlapping the woofer and tweeter crossover points. Ultimately, the system measures well in this area. This impedance bump is of no concern so long as the amplifier you have chosen has excellent output voltage stability in terms of load impedance.

Passive Crossovers and Amplifier Power Production

Now, it’s time to introduce alternating current signals and passive crossovers.

The purpose of crossovers is to limit the frequency range where a speaker plays sound. A low-pass crossover allows a speaker to play up to a chosen frequency. To remember how they work, keep this in mind: A low-pass filter passes audio frequencies lower than the crossover point. Above that frequency, the speaker produces less sound as you move farther from the crossover point. We use low-pass crossovers on subwoofers as we don’t want them playing midbass or midrange frequencies. A low-pass crossover on a midrange driver attenuates output where the tweeter takes over.

The other type of crossover is called a high-pass. This type of filter blocks low-frequency sounds. We use high-pass crossovers on midrange drivers and tweeters. An infrasonic filter, often mistakenly called a subsonic filter, is also a high-pass filter. These filters work the same way in attenuating output as you move away from the crossover point. Here’s how to remember how high-pass filters work: A high-pass filter passes audio frequencies higher than the crossover point.

In both cases, crossovers are not a brick wall. They don’t stop all information above or below the crossover frequency. The rate at which the sound gets quieter is called the crossover slope. For this article, we’ll talk about simple first-order -6 dB/octave passive crossovers. In real-world applications, these filters aren’t steep enough (don’t attenuate fast enough) to provide adequate filtering. Nevertheless, they are perfect for explaining power distribution when we connect multiple speakers to an amplifier.

The response graph above is a screenshot from the ARC Audio ARC DNA DSP software suite’s Graph option. These high- and low-pass crossovers are set to 500 Hz to make them easy to see.

How Passive Crossover Components Reduce Speaker Output

To add crossovers to speakers, a technician will wire a non-polarized electrolytic or a mylar foil capacitor in series to create a high-pass filter. An inductor in series with a speaker creates a low-pass filter. It doesn’t matter if the capacitors or inductors are on the positive or negative lead to the speaker.

We aren’t going to go into the math of how capacitors and inductors act as crossovers in this article. We covered capacitors here and inductors here. Consider these articles prerequisites if you want a detailed understanding of how these devices function. In short, the caps and coils increase their impedance at and beyond the crossover point.

The graph below shows the impedance of a 4-ohm speaker wired in series with a capacitor and a second speaker wired in series with an inductor.

We chose the values for the capacitor and inductor in the graph so that the crossover point is at 500 Hz. At this frequency, the impedance of the passive filters is the same as the speaker impedance or 4 ohms. The result is that the amplifier “sees” an impedance of 8 ohms and produces half as much power as it would without the capacitor in the circuit.

This second graph shows the power the amplifier produces for each speaker network based on the load impedance. You can see that the amplifier delivers 12.5 watts into both at the crossover point of 500 Hz. If we sum the power into each speaker, the amplifier still produces 25 watts.

A properly designed passive crossover network performs two tasks: Primarily, it serves as an acoustic filter between two speakers. The most common application in car audio systems is attenuating the output of a midrange speaker or woofer where a tweeter starts playing. The same network would include a high-pass filter so the tweeter wouldn’t play low-frequency information. The second task is to prevent an amplifier from seeing the impedance of two speakers at once. Some companies don’t execute this second criterion well, resulting in upset amplifiers and difficulty with system design.

Amplifier Power Distribution Takeaways

The inspiration for this article on speaker power distribution stemmed from a discussion about how much power an amplifier produces when driving a midrange speaker and a tweeter. Many people mistakenly thought that since the woofer and the tweeter have a nominal impedance of 4 ohms, the amp would see them as a 2-ohm load. Without crossovers, this assumption would be correct. However, the tweeter wouldn’t last long without a crossover to protect it from being over-powered and over-driven.

If you aren’t sure how power will be distributed among the many speakers connected to your car audio system, drop by a local speciality mobile enhancement retailer. They can help design, install, configure and calibrate a sound system you’ll enjoy!

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.

What is a custom car audio subwoofer enclosure? Does it need to be wrapped in leather or vinyl? Should it be made with fiberglass? Does it need acrylic windows? Is LED lighting a necessity? The short answer is no to all of these questions. Let’s delve into what makes a subwoofer enclosure custom and why it’s the best way to upgrade the bass in your car audio system.

Subwoofer Enclosure Volume Matters

How large does a subwoofer enclosure need to be? The answer to that depends on the subwoofers you want to use. Thinking that way puts the cart before the horse, though. The best way to design a subwoofer system is to tell the product specialist you’re working with how much space you’re willing to allocate to the enclosure. They can take a series of measurements, do some calculations and suggest a subwoofer or subwoofers to deliver the best performance based on the available air volume. No matter what the manufacturers tell you, cramming large subwoofers into small enclosures results in poor performance. You’ll get more deep bass from a single driver in an optimized enclosure than a bunch of larger drivers crammed into an undersized design.

Part of designating the space available for your subwoofer enclosure should include considerations about accessing storage or a spare tire. The last thing you want is to be stranded on the side of the road because part of your stereo has trapped a spare or blocked access to the vehicle battery. Before you tell the shop how much space they can use, look under the trunk floor to determine what you might need to get to. Make some notes so you can share that information with the shop.

Space Optimization Is Key

The number one factor that defines a custom subwoofer enclosure is that it optimizes the available space in the vehicle. Let’s say you want a vented enclosure with two 10-inch subwoofers. Most 10-inch subwoofers on the market work very well in about 1 cubic foot of air space. So, this enclosure would need a net volume of 2 cubic feet plus the displacement of the drivers and the vent. Let’s use a pair of ARC Audio X2 10D2V2 10-inch subwoofers for this simulation. With 1 cubic foot each, plus a 4-inch diameter round vent, the enclosure needs a net internal air volume of about 2.15 cubic feet.

The person designing the enclosure for these subwoofers should optimize it so that it intrudes into the cargo area of the trunk as little as possible. Therefore, it should use the full width and all the available height to make it as shallow as possible. If we have 40 inches of width and 15 inches of height, the enclosure would need to be 8.625 inches in depth. These measurements assume the enclosure is a rectangle with no angled rear panel. If we wanted the rear panel to have a 20-degree angle, the depth at the top would shrink to about 6 inches. That gives us two more inches of usable cargo space.

Both designs are custom enclosures if finished in a durable carpet that matches the cargo area. That’s it. Nothing fancy or exotic is required to make this a custom solution. The customization aspect is that the enclosure is optimized for your vehicle and uses the available space efficiently.

By contrast, if the shop has a pre-built enclosure that’s 34 inches wide and 13 inches tall, it would need to be 11.375 inches deep. Would it work? Yes. Would it sound the same? Yes. Might it save you money versus having an enclosure built specifically for your application? Maybe. Will you have the most space to fit your groceries, sports equipment, luggage or beer? No, not at all.

Here are a few examples of custom enclosures designed to deliver great bass while taking up as little space as possible.

More Custom Subwoofer Enclosure Options

Now, there is a next level of custom subwoofer enclosure beyond a square or slanted-back prism. You might have a significant amount of room inside a spare tire or behind a trim panel in the trunk that can be used for an enclosure. Once again, the choice of drivers for these applications depends on the available space. Just because you can physically fit a 12-inch subwoofer inside a spare tire enclosure doesn’t mean that’s the choice of driver that will produce the most low-frequency output or deliver the tightest bass. A single 10-inch subwoofer might play louder at lower frequencies. An 8-inch subwoofer in a vented design will likely be even louder. Once again, the shop you’re working with should calculate the available volume and suggest a subwoofer based on that information.

Vehicle-Specific Enclosures

Many companies offer off-the-shelf subwoofer enclosures designed for specific vehicles. These enclosures are typically optimized for a specific location in the vehicle and may use a combination of stack-fab or fiberglass construction. With the benefits of mass production, these custom enclosures can make adding an optimized bass solution more affordable than having a shop create a one-off solution. You’ll still need an expert to run all the wiring and configure and calibrate the electronics.

Net Audio in Wichita Falls, Texas, offers this 2019+ Ram 1500 Crew Cab bass reflex subwoofer solution.

Musicar in Portland, Oregon, offers a variety of BMW OE-Look subwoofer upgrades, including this enclosure for F32/F83 coupes with a Morel 10-inch subwoofer.

Audio Designs and Custom Graphics in Jacksonville, Florida, has a complete line of Phantom Fit enclosures, including this one for 2015-22 Mustangs.

MTI Acoustics in College Station, Texas, offers application-specific subwoofer enclosures like this one for Jeep Gladiators.

Upgrade Your Car Stereo with a Subwoofer System Today

As we’ve shown, there doesn’t need to be anything fancy or exotic about a custom subwoofer enclosure. The enclosure needs to be constructed to be specific to your needs. You can certainly go for something flashy if you want. However, we prefer to stick with a simple, well-constructed enclosure and opt for a subwoofer that includes technologies that make it more accurate and linear. No matter your goal, drop by a local specialty mobile enhancement retailer today to find out what they can build to deliver great bass in your car, truck or SUV.

Lead-In Image: Thanks to Perzan Auto Radio in Upper Darby, Pennsylvania, for the photo of this enclosure they constructed for a client’s 2023 Bentley Continental GT Azure. The enclosure features a pair of JL Audio 10W6v3 subwoofers and matching SGR-10W6v2/v3 grilles. The client can still access the space under the trunk floor without moving the enclosure.

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.

Car audio upgrades are much more complex than setting up speakers in your living room. They are, unless all you have in the system is a set of coaxial speakers in your doors. Your installer has to consider crossover slopes and alignments if you have front and rear speakers and a subwoofer. They must also understand how the settings interact at the crossover frequency. Let’s take a nerdy look at crossover slopes and alignments, and how they sum together.

Why Do Speakers Need Crossovers?

We primarily use crossovers on speakers to protect smaller drivers from damage from over-excursion and overpowering. Say your car audio system has a set of 1-inch tweeters in the dash or the doors. Those small drivers can’t produce bass frequencies with any efficiency, and they can only a handle few watts of power. Yes, we know they say 80, 100 or 150 watts on them. But that’s their rating with pink noise referenced to bass frequencies. When you filter out everything below 3,000 Hz, all that’s left of a 100-watt signal is about 0.26 watt for the tweeter. Feeding it a sine wave at 100 watts will destroy it in about a second.

The second reason we use crossovers is to prevent excursion damage. Can you imagine sending 100 watts of deep bass information into a 3.5-inch dash speaker or a PA-style midrange? With only a few millimeters of excursion capability, these small speakers will be pushed well beyond their linear excursion limits, adding significant distortion to the output. Think of it like mechanical clipping. It sounds terrible and can damage the suspension and voice coil. High-pass crossovers are the ideal solution for preventing the above issues.

Low-pass crossovers are needed to ensure that the output from the speaker system comes from a single source. You would never want a midrange driver playing up to 5 kHz when the tweeter is playing from 3 kHz and up. The goal of the low-pass crossover on a midrange or woofer is to keep the transition from one speaker to the other smooth and transparent.

Crossover Slopes and Output Attenuation

Crossovers don’t stop all sound reproduction above or below a specific frequency. For example, a tweeter still produces audio information at 2 and 2.5 kHz when set up with a 3-kHz high-pass crossover. If you look at the graph below, you’ll see a -6 dB/octave high-pass crossover set at 3 kHz.

If we analyze the information carefully, we can see that the output from this ARC Audio digital signal processor would be at -3 dB at the crossover frequency of 3 kHz. The frequency set in the software can be called the crossover frequency or the knee frequency. Looking farther to the left, the output decreases with frequency. The difference between 3 kHz and 2 kHz is 6 dB. The difference between 2 kHz and 1 kHz is another 6 dB. The rate at which the output gets quieter is called the crossover slope. In this example, it’s -6 dB per octave. This is also known as a first-order crossover.

Digital signal processors have made it easy for installers to set crossovers quickly and accurately. Entering a value into software is much more precise than turning a knob on an amplifier. Those knobs are connected to potentiometers (adjustable resistors) that are notoriously inconsistent. Some companies used resistor networks instead of potentiometers on their electronic crossovers, like AudioControl’s infamous 24XS.

The image above shows our original first-order, -6 dB/octave high-pass filter in white. The gray trace is a -12 dB/octave second-order filter. The green trace is a third-order, -18 dB/octave filter. Finally, the violet trace is a fourth-order, -24 dB/octave high-pass filter.

The benefit of steeper filters is improved signal attenuation at lower frequencies. The first-order filter was down 10 dB at 1 kHz. This amplitude means the speaker would get 1/10 as much power as it does through the pass band (frequencies above the crossover point). The second-order filter results in the signal at 1 kHz being -19.2 dB down. That’s close to 1/100 the power at 1 kHz compared to above 3 kHz. The third-order filter is down 28.7 dB, and the fourth-order is -38.2.

A second benefit of steeper filters is that the range of frequencies where the output comes from both drivers simultaneously is much smaller.

Crossover Alignments

So far, we’ve been looking at a crossover with a Butterworth response. Originally, crossovers were constructed using capacitors, inductors and resistors. Balancing the attenuation rate while delivering flat performance through the pass band (the range of frequencies you want to hear) was tricky with off-the-shelf passive components, even in electronic circuits. Butterworth filters offer moderate roll-off rates but deliver smooth response through the pass-band. They also have an output level of -3 dB at the crossover frequency. This level at the crossover frequency is a crucial consideration that we’ll circle back to later.

Another commonly available crossover type is called a Bessel alignment. Bessel filters offered the best group delay, whereas the Butterworth had the smoothest pass-band response characteristics. These are also popular in audio systems. We’ll get into a deep discussion of group-delay another time. For now, think of it like “timing issues.” Bessel filters are very similar to Butterworth in that they have a -3 dB level at the crossover frequency. Bessel filters are only available in even-order alignments, so second-order -12 dB/octave or fourth-order -24 dB/octave in most systems.

The last filter we’ll talk about is called the Linkwitz-Riley. This is another filter option that’s only available in even-order alignments. Technicians designing electronic circuits can create a Linkwitz-Riley (LR) filter by combining two Butterworth filters. So, a second-order LR (LR2) is two first-order Butterworth filters added together in series. An LR4 is two second-order Butterworth filters. The key benefit of the Linkwitz-Riley filter is that the output is at -6 dB at the crossover frequency.

The image above shows a Butterworth alignment in white, a Linkwitz-Riley alignment in gray and a Bessel alignment in green.

Speakers and Signal Summing

Two identical speakers playing the same signal at the same amplitude, at equal distances from the listener, will produce 6 dB SPL more output than a single speaker.

Producing smooth frequency response through the crossover region is crucial for configuring and calibrating car audio systems. If the crossovers you’ve chosen have a -3 dB level at the knee frequency (the frequency set in the software), then the output of the two speakers sums to produce a bump that’s +3 dB in amplitude. This is a problem. We don’t want bumps in frequency response anywhere in the system. The high- and low-pass signals sum flat if your installer uses a Linkwitz-Riley filter at -6 dB at the crossover point. As a result, the system is much easier to equalize, and there’s a reduced overlap range between the two drivers.

If you look at most car audio amplifiers with built-in crossovers, you’ll find that entry- to mid-level models offer -12 dB/octave Butterworth crossovers. As you move up in the model ranges, you might find they have -18 or -24 dB/octave filters. Very few amplifiers with built-in electronic crossovers offer Linkwitz-Riley alignments.

Setting Electronic Crossovers

Let’s discuss setting a crossover on an amplifier between a subwoofer and the door speakers. In almost all instances, assuming the door speakers can play loudly and at midbass frequencies, the optimum crossover point is usually 80 Hz. Regarding the crossover slope, you want it to be as steep as possible, up to -24 dB/octave.

If a technician eyeballs the crossover options on an amplifier and tries to set them both to that frequency, we run into several problems. First, the actual crossover frequency is likely quite different than the labels on the amp chassis because of variances in the potentiometer inside the amp.

Next, even if the labels were perfect, unless the electronic crossovers have a Linkwitz-Riley alignment, the system’s output will have a 3 dB bump at the crossover frequency. We must underlap the crossovers when they are Butterworth or Bessel alignments. This makes using a real-time analyzer the only accurate way to set this type of electronic crossover.

Adding a high-quality digital signal processor to the system is a more straightforward and predictable solution. Your installer can select Linkwitz-Riley filter alignments and make precise crossover frequency selections. Of equal importance, they can then use a calibrated microphone to adjust the frequency response of the system to compensate for reflections and resonances in the vehicle. Drop by a local specialty mobile enhancement retailer today to discover the digital signal processors that are available to upgrade your system.

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’s a common belief that an audio system with more subwoofers will produce more bass. This statement can be 100% true or completely false. Why might it be false? A subwoofer’s output depends heavily on enclosure design. Let’s look at two examples where the output of one subwoofer is more than two.

The Most Bass for Your Dollar

If you spend any time searching the countless car audio groups on Facebook, you’ll see dozens of photos of under-seat subwoofer enclosures for pickup trucks. Many of these enclosures have three or four subwoofers in them. If they are 8-inch subwoofers, this might work well. If they are 10-inch subwoofers, they’re likely somewhat cramped for space.

When we’re talking about subwoofer systems, the size of the enclosure relative to the parameters of the subwoofer itself determines performance. You could have a shop build a cube that measures 12 by 12 by 12 inches and mount a 10-inch subwoofer on all six sides. It would look cool, but it would likely sound terrible!

How much bass a subwoofer produces depends on how far the cone moves forward or rearward for a given amount of power. Professionals use enclosure simulation software like BassBox Pro or Term-Pro to model how a subwoofer will behave in different enclosure designs. These software packages can simulate acoustic suspension (sealed), bass reflex (vented) and various bandpass enclosure designs.

Someone with experience needs to analyze and interpret the information provided by the software simulations to determine whether the design is suitable and safe for the subwoofer with the chosen amplifier. These software packages, on their own, don’t calculate the perfect enclosure for any application. They’re like a spreadsheet: They work with the electromechanical parameters of the subwoofer and the provided enclosure information.

Let’s talk about acoustic suspension enclosures, which are the simplest to understand and predict. When a subwoofer is installed in an acoustic suspension enclosure, the compliance of the air in the enclosure combines with the compliance of the driver’s suspension to form a spring system. Compliance is the reciprocal of stiffness. Or, put another way, a rubber band is more compliant than a pencil. A large amount of air is very compliant, and a small amount of air isn’t when we’re talking about compressing it. More specific to subwoofer enclosure simulations, it’s easier to compress the air in a large enclosure than in a small one.

When a subwoofer is installed in a very small enclosure, the resulting system is not very compliant. It will take significant power to move the subwoofer cone at low frequencies. Why does the enclosure size have a more significant effect at low frequencies? For each decrease of one octave, a subwoofer cone has to move twice as far to produce the same output. For example, if a subwoofer moves back and forth 2 millimeters to produce a specific output at 60 Hz, it has to move 4 millimeters to produce that same output at 30 Hz. If the speaker is in an enclosure that limits how easily the cone moves, it will produce less output for a given power input.

Since we aren’t installing subwoofers for midbass, installing any subwoofer or woofer in a small enclosure means limiting how much bass the system produces. This low-frequency limiting is one of the reasons we use enclosures. Without an enclosure, the subwoofer would bottom out when driven with moderate power.

The graph below shows the predicted frequency response of the ARC Audio X2 10D4v2 subwoofer we reviewed recently in three different enclosures. The red trace represents a sealed enclosure with a net internal air volume of 0.663 cubic foot. The yellow trace shows the predicted response of the subwoofer in an enclosure with only 0.45 cubic foot of space. Finally, the green trace is the response with the subwoofer in an enclosure with 1.0 cubic foot of space.

As you can see, the ARC Audio subwoofer produces more bass from a larger enclosure for a given amount of power. This is true of all subwoofers. When driven with 200 watts of power, the 1.0-cubic-foot enclosure would produce 98.9 dB SPL output (in a free-field measurement) at 30 hertz. The 0.663-cubic-foot enclosure produces 97.6 dB of output at the same frequency. Finally, the 0.45-cubic-foot enclosure produces 95.2 dB of output at 30 Hz.

Let’s look at this data from another perspective. Consider how much more power it would take for the smaller enclosures to play as loudly as the larger designs. We will reference 200 watts of power into the 1.0-cubic-foot enclosure. The 0.663-cubic-foot enclosure would need 272 watts of power at 30 hertz to produce the same output. The 0.45-cubic-foot enclosure needs a whopping 469 watts to match the 30-hertz output of the large enclosure. Think about how much hotter the sub would get and how much harder the amplifier and vehicle alternator would have to work to produce the same output.

 

What if we look at this from the opposite perspective? If we provide the ARC Audio subwoofer with 200 watts of power in the small 0.45-cubic-foot enclosure and it produces 95.2 dB of output, how much less energy would be needed to match that output from the larger enclosures? The answer is that the 0.663-cubic-foot enclosure is just as loud with only 148.4 watts of power, and the 1.0-cubic-foot enclosure would only need 85.5 watts to produce 95.2 dB of output. As you can see, cramming a subwoofer into a small enclosure is counterproductive in terms of efficiency.

Are More Subwoofers Always Louder?

Now let’s talk about multiple subwoofers and whether or not they are always louder. Most car audio enthusiasts think adding a second subwoofer increases the output of a system by 6 dB SPL. This statement is true under a specific set of conditions. Let’s say we have a single subwoofer in a 0.663-cubic-foot enclosure, and a 200-watt amplifier powers it. If we want to use two subwoofers, each driver needs 0.663 cubic foot of airspace. We also need an amplifier that can provide a total of 400 watts. If we meet these conditions, the system’s maximum output will increase by 6 dB SPL. If we have double the airspace but only 200 watts to share between the drivers, the output increases by 3 dB SPL.

The graph below shows a single X2 subwoofer in 0.664 cubic foot of space in red and a pair of those subwoofers in 1.326 cubic feet in teal. The total power is 200 watts for each simulation.

What happens if we ask our installer to cram both subwoofers into a 0.664-cubic-foot enclosure?

The graph above shows that the subwoofer system produces less bass with two drivers sharing the 0.663-cubic-foot enclosure and 200 watts (total) than with a single driver (in red). Proper subwoofer enclosure design is crucial to maximizing car audio system efficiency. If we doubled the power when adding the second sub, it would be louder, but maybe only by 2 to 2.5 dB.

Ported Subwoofer Enclosure Solutions Add Efficiency

What if you want the most bass output for our investment? What enclosure should you use? The answer depends on how much space you have in the vehicle. Let’s say we have room for two subwoofers in an acoustic suspension enclosure with a net volume of 1.324 cubic feet. This is a large enough enclosure to ensure that the drivers play loudly at low frequencies, right? Sure, but is this the most efficient use of our money? Guess what? No, it isn’t.

If you have the shop you’re working with design and construct a vented enclosure using the 1.324 cubic feet of space and a single subwoofer, the system will produce significantly more bass. Two drivers in an acoustic suspension enclosure with a volume of 1.324 cubic feet, sharing 200 watts, will produce 102.9 dB SPL at 35 hertz. A single driver in a 1.324-cubic-foot bass reflex enclosure would deliver a mind-blowing 107.8 dB of output at the same frequency. That’s 4.9 dB more output. Your sealed enclosure would need 618 watts of power to reach the same output level. Chances are, the subwoofers wouldn’t appreciate receiving that much power.

Does Adding More Subwoofers Make My Car Audio System Play Louder?

So, let’s answer the question, “Does adding more subwoofers make my car audio system play louder?” The answer is yes if your enclosure design has double the air volume every time you double the number of subwoofers. Your system will play 6 dB SPL louder every time you double the number of drivers in this scenario.

Unless the enclosure was grossly oversized, adding more subwoofers to a given volume is unlikely to increase low-frequency output. This is why it’s crucial for the shop you’re dealing with to model the enclosure options so that you get the most bass for your investment. In most cases, especially for an under-seat truck enclosure, a single driver in a bass reflex (vented) enclosure produces significantly more low-frequency energy than two, three or even four drivers in an acoustic suspension design. Drop by a local specialty mobile enhancement retailer today and talk with them about your goals for your subwoofer system upgrade. If they know how to optimize enclosure designs with simulation software, the chances are that you’ll get the best bang for your buck, bass-wise!

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.