We’ve talked about how speaker power handling is tested and the importance of delivering accurate test data. In the context of car audio speakers, we’ve explained that the physical size of the voice coil is a crucial element in determining how much power a speaker can handle. In this article, we’ve put together a simple, practical demonstration to show the thermal limits of a speaker.

What Defines Speaker Power Handling

Before the demonstration, we should discuss the definition of “power handling” in the context of speakers and subwoofers. Power handling describes the amount of power from an amplifier that a speaker can handle without being permanently altered negatively. This negative effect could be thermal damage to the voice coil former, the speaker’s suspension, or physical damage from excessive excursion. For example, too much low-frequency information fed into a small midrange driver might cause the voice coil former to hit the T-yoke and cause permanent deformation.

Unlike test tones, music is very dynamic. In this context, dynamic refers to a varying average level of energy. For example, a quiet passage in a song with only a female artist singing might require only a watt of power from an amplifier. When the bass guitar and drums start playing, it might take 10 or 20 watts of power to reproduce those lower frequencies. A stick hitting a floor tom drum’s skin takes less energy than strumming the lowest note on a five-string bass. The guitar sound could last several seconds, whereas the drum strike might only be a half-second. Power over time is what builds up heat in a speaker voice coil.

Cooling Capacity Analogy

A good analogy here is a car engine. For example, a Honda Civic might have a single radiator 14 inches tall and 14 inches wide with a ½-inch thick core. Conversely, a Dodge Challenger Hellcat might have a radiator that’s 25 inches wide, 18 inches tall and 1.625 inches thick. The Honda has 98 cubic inches of cooling capacity, whereas the Dodge has about 772 inches.

We know that engines are about 20-40% efficient. So, the Honda Civic, making 150 horsepower, will waste about 50 horsepower as heat under maximum load. That’s 37.3 kilowatts of heat energy. The Big Dodge can produce 700 horsepower, and assuming a similar 33% efficiency (which is likely generous), it will produce 174 kilowatts of heat.

The purpose of a radiator is to transfer the unwanted heat produced by the engine to air. If we divide the heat produced by the engine by the cubic inches of radiator area, we get 380 watts/square inch for the Honda and 225 watts/square inch for the Dodge. Given the chance that the Challenger will likely be driven more aggressively, some extra cooling capacity is cheap insurance against overheating.

Speaker Efficiency

Unfortunately, moving coil loudspeakers are notoriously inefficient. A 6.5-inch woofer might convert 0.02% of the energy from an amplifier into sound. A mid-level 12-inch subwoofer might only convert 0.25%. So, when you feed 20 watts into the midrange driver, you get the equivalent of 4 milliwatts of sound energy in the air. The rest of that power from the amplifier is wasted as heat in the voice coil and, subsequently, the parts surrounding it.

If you stop and look at different speaker designs with increasing power handling capabilities, you’ll notice that the voice coil size increases. A larger voice coil winding has more surface area. As such, the assembly can absorb more heat before failing.

For example, the Rockford Fosgate P1650 6.5-inch Punch Series speaker is rated to handle 55 watts of power. It has a voice coil diameter of 1.0 inch. The woofers in the Power Series T1650-S component set are rated for 80 watts of power handling and use a 1.2-inch diameter voice coil. The Power T3652-S set is rated for 125 watts, and the woofers have 1.5-inch diameter voice coils. So far, it all seems to make sense. An increase in diameter from 1 to 1.2 inches for a given winding height means 20% more surface area. Going from 1.2 to 1.5 inches in diameter is 25% more area. Combine this with a voice coil winding that’s likely longer, and you have significantly more heat management capacity.

Subwoofer Voice Coils

Speaker voice coils usually have a single winding of copper around the former. Subwoofers, on the other hand, can have multiple layers. Many higher-power subwoofers have four-layer voice coils, so they might be over 3 millimeters instead of a millimeter thick. This increase in size, specifically mass, further increases power handling.

The choice of voice coil former material also affects power handling. For example, aluminum has a thermal conductivity of 210 W/m-K. This means aluminum can transfer 210 watts of heat per meter of material per degree Kelvin. Copper is even better at over 400 W/m-K. On the other hand, air is a terrible conductor of heat energy at about 0.0235 W/m-K. Aramid fibers like Kevlar are also bad, at 0.04 W/m-K. If a speaker designer wants to extract heat from the voice coil winding, they might use an aluminum former. They might use an aramid glass-fiber former if they want a material that won’t heat up. Balancing physical strength, mass and thermal conductivity are all crucial in designing a reliable, high-performance speaker or subwoofer.

Let’s Compare Voice Coil Power Handling

We’ve sourced three different voice coils for this little experiment. All have relatively short windings, measuring just under 10, 18 and 20 millimeters in height. The coils have outer diameters of 26.4, 52.7 and 76.9 millimeters. The two smallest voice coils are wrapped around aluminum formers, while the larger uses two aluminum collars connected by a glass fiber backing. One collar is behind the winding, and the other is on top to connect the cone and spider. All three have two-layer windings.

I carefully measured each coil’s impedance. The small coil is wound to a DC resistance of 6.37 ohms. The medium coil has a DC resistance of 7.07 ohms, and the smallest is 3.53 ohms. I created a spreadsheet to calculate how much voltage I should apply to each coil so that it dissipates a specific amount of power. I will start with thermal measurements with 5 watts of power, then increase to 10 watts and see how hot things get.

Speaker Voice Coil Thermal Test at 5 Watts of Power

Starting with the large voice coil, the chart below shows that the temperature rose quickly from room temperature to 125 degrees after 1 minute before settling at about 137 degrees. While that’s warm, there was no concern of damaging the voice coil winding.

The medium-sized voice coil got warmer faster. It reached 132 degrees in a minute, then tapered off to 147 degrees after three minutes.

The smallest voice coil got quite hot quite quickly. It was over 210 degrees in a minute and 288 degrees in three minutes. This isn’t enough to damage it, but that’s a reasonable amount of heat.

Speaker Voice Coil Thermal Test at 10 Watts of Power

Now, let’s repeat the test using only 10 watts of power. The large coil warmed up a bit faster, tapering off around 180 degrees. The medium-sized coil followed a similar pattern, tapering off at just over 190 degrees. The tiny voice coil temperature skyrocketed almost immediately to 300 degrees, then held around 362. This temperature is the absolute upper limit of what a voice coil can handle. Prolonged use at this level would result in damage.

Undoubtedly, you’ve seen the different power ratings for Continuous and Maximum or Music power on a speaker. Constant, steady-state tones similar to what we used for this test are very hard on speakers from a thermal perspective. If this were music with 10 dB of dynamic range, you could understand how it could handle high-power transients while cooling off during quiet moments.

Another Reason Voice Coil Temperature Matters

Before we started the testing, we measured the impedance of each voice coil. The image below shows the impedance and phase plot of the small coil.

There are a few things to learn from this measurement. First, the voice coil winding doesn’t have much inductance. The impedance only starts to increase above 1 kHz. Second, the nominal impedance is at about 3.5 ohms at lower frequencies.

After the 10-watt test, I repeated the impedance measurement. The results are below.

The impedance starts at 4.2 ohms and drops to 3.8 as the voice coil cools. With very little thermal mass, the temperature drops quickly during the measurement. While the difference between 4.2 and 3.5 doesn’t seem significant, it’s an increase of 20%.

Does this impedance increase matter? Well, amplifiers output voltage, not power. The amount of power they produce depends on the impedance of the load. If an amplifier produced 5 volts RMS, the speaker would get 7.14 watts of power when cold. Once hot, the current would decrease, and the speaker would only get 5.95 watts of power. That’s not huge, but it’s a difference of 0.79 dB SPL. Suppose your installer has agonized over dialing in a digital signal processor to deliver perfectly smooth sound. In that case, a speaker with a voice coil that heats up quickly will have less efficiency once warm, altering the balance of your audio system.

Heat Management in Car Audio Speakers Is Crucial

This experiment doesn’t consider the pole piece or top plate’s proximity to a speaker to help extract heat. It also doesn’t include any benefits from the voice coil and cone moving to create airflow. However, those features don’t significantly affect the heating or cooling rate between the voice coil sizes shown here.

If you’re looking for speakers or subwoofers that can handle the most power possible, larger voice coils can handle more heat. However, they do come with some drawbacks. We’ll look at those in another article soon. In the meantime, drop by a local specialty mobile enhancement retailer to audition speakers that will sound amazing in your car, boat, or motorcycle.

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.

As we’ve mentioned many times, adding a subwoofer is one of the best upgrades you can make to a car audio system. We know that having a shop construct a custom enclosure isn’t always in the budget. Likewise, while impressive, high-end subwoofers aren’t in everyone’s price range. With all this said, there is one item that you should spend a few dollars extra on: a ported enclosure. Let’s look at why a ported enclosure is crucial to creating a subwoofer system that will deliver the performance you likely want without breaking the bank.

Your Factory Stereo Likely Doesn’t Produce Bass Frequencies Well

Before we get into why you should choose a ported enclosure for an affordable subwoofer upgrade, we should discuss why a subwoofer is important to the performance of a car audio system. Imagine a basic stereo system with a radio and four speakers. The radio might be capable of delivering 20 watts of power to each of those speakers. If you want to play music with a lot of bass at higher volumes, the radio will quickly run out of power. When this happens, the signal from the amplifier chip in the radio will be distorted, and your music will sound garbled.

The second issue, which might be more important, is that the four speakers in your car likely aren’t designed to reproduce bass frequencies efficiently. Typically, factory-installed speakers have relatively light cone assemblies so that they are efficient. The speakers are usually most efficient from 80 or 100 hertz and up. Part of keeping the mass of the cone assembly down is using a lightweight, short voice coil. As such, cone excursion is often limited to a few millimeters in each direction. If a speaker can’t move much air, producing bass at higher volume levels is nearly impossible.

Why a Subwoofer Upgrade Is Important

When a subwoofer is added to a stereo system, the small speakers don’t need to try to produce bass frequencies. This means the radio doesn’t have to deliver as much power to the speakers, and the speakers don’t have to work as hard. The result is that the system will sound better and play louder. If the subwoofer system is designed correctly, you’ll get much more bass output and extension. The little amp in the radio and the factory-installed speakers aren’t stressed, so they’ll be much clearer.

Speaking of power, it’s also important to remember that an entry-level subwoofer’s power handling and excursion capabilities will be limited. You want to choose the largest driver you can and use it in a properly designed enclosure to get the maximum performance. The latter is the focus of this article.

What Is a Subwoofer Enclosure?

Now, let’s talk about subwoofer enclosures. Why does a subwoofer need an enclosure at all? The primary purpose of a subwoofer enclosure is to prevent the sound coming off the back of the subwoofer cone from mixing with the sound coming off the front and canceling. If you hold a subwoofer in your hand and play music, it won’t produce any bass. If you cut a hole in a wall and mount the subwoofer into it, you’ll hear lots of bass on either side of the wall. This is similar to what happens when a subwoofer is mounted on the rear parcel shelf of a sedan. The sound coming from the back of the subwoofer gets trapped in the trunk. The sound from the front fills the cabin.

The second purpose of a subwoofer enclosure is to act as a high-pass filter. Yes, this seems contradictory to adding a subwoofer at all. Subwoofers need a high-pass filter so they aren’t damaged at extremely low frequencies. If you send an 80-hertz test tone to a subwoofer, it might move back and forth a millimeter. If we play a 40-hertz tone, the subwoofer cone would likely move 3 millimeters. If we try to reproduce a 20-hertz tone, the cone will move over 5.5 millimeters.

We scaled the graph above to display 1 millimeter of excursion at 80 hertz. That level of output requires only 23 watts of power. The enclosure used in the simulation has a volume of 20 cubic feet, rendering it useless in controlling subwoofer cone motion.

Now, 23 watts of power into a subwoofer is likely louder than you think. With that said, many likely want the bass to be louder. So, what happens if we send 100 watts to the subwoofer?

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Now we have 11.4 millimeters of excursion at 20 hertz, 5.5 millimeters at 40 hertz and 2 millimeters at 80 hertz. The subwoofer has an Xmax specification of 15 millimeters, so we’re safe, right? What if we increase the signal to the 250-watt limit of the subwoofer?

You can see by the shaded area of the red trace that there is an issue below 24.4 hertz. The subwoofer will exceed its Xmax specification of 15 millimeters if fed with 250 watts of power at any frequency at or below 24.4 hertz.

If we put the subwoofer into an enclosure, then power handling increases. Here is the excursion of the subwoofer, in orange, with it installed in a 1.0-cubic-foot enclosure.

This is why subwoofers need an enclosure. Now, the subwoofer is fine, in terms of excursion, down to 12.5 hertz. Most amplifiers will have started to decrease their output by this frequency, so we’re protected from overdriving the subwoofer.

Subwoofer System Efficiency

As you’d expect, there are benefits and drawbacks to each type of enclosure. The graph below shows the predicted free-field output of the infinite baffle simulation and our 1-cubic-foot enclosure.

It should come as no surprise that the larger enclosure predicts more output at lower frequencies. However, the red trace doesn’t consider that the subwoofer will exceed its excursion limits below 24 hertz. In reality, the subwoofer would perform similarly in both enclosures in terms of output but starts to distort earlier in the infinite baffle design.

Vented Enclosures

Now, let’s discuss why a vented enclosure is best when designing an affordable subwoofer system. First and foremost, the subwoofer might be limited in how much power it can handle. An affordable subwoofer might only deal with 250 to 300 watts of power before the voice coil might be damaged. Further, the subwoofer might only have 12 to 14 millimeters of excursion capability, rather than 18 or 20 from a high-end offering. This also limits how loudly it can play.

What if an enclosure design increased the efficiency of the subwoofer system and decreased cone excursion? No, this isn’t magic. This perfectly describes a vented subwoofer enclosure, known technically as a bass reflex enclosure. You can learn about how a bass reflex enclosure works in this article.

Let’s look at the predicted output of our 10-inch, 250-watt subwoofer in a vented enclosure (yellow) compared with a sealed enclosure of the same volume (orange).

The bass reflex design is louder with the same power at all frequencies above 16 hertz. Specifically, it’s 4.8 dB louder at 30 hertz and 5 dB louder at 40 hertz. That’s like getting the same amount of output but with only 79 watts of power at 40 hertz. If your subwoofer amp is 50% efficient at this power level (which would be typical), then it only needs to draw about 6 amps of current instead of 20. That’s much easier on the vehicle’s electrical system. The reduction in current means the amp will run cooler. It also means the subwoofer voice coil won’t heat up, which reduces power compression.

Back to Cone Excursion

Is there a drawback to a bass reflex enclosure versus a sealed (acoustic suspension) enclosure design regarding power handling? At extremely low frequencies, yes. The cabinet doesn’t control subwoofer cone motion well below the tuning frequency of a bass reflex enclosure. Let’s look at the cone excursion graph of our bass reflex versus sealed enclosure.

Below 18 hertz, the bass reflex enclosure (in yellow) will have power handling issues. Depending on the music you listen to, this might be cause for concern. If you want to play the cannon blasts from the “1812 Overture” loud, the subwoofer will be mad. It will be mad if you listen to EDM or similar music with lots of infrasonic information. The solution is to set an infrasonic filter at or near 20 hertz.

Subwoofer Distortion Benefits

Aside from their dramatic increase in efficiency, there’s a second benefit to a bass reflex enclosure. As demonstrated in our series of articles about subwoofer distortion, distortion increases with cone excursion. If we look at the last graph, we can see a massive dip in excursion at the bass reflex enclosure tuning frequency. Around this frequency, most of the sound from the enclosure comes from the vent. At 25 hertz, the bass reflex enclosure subwoofer moves about 3 mm in each direction. By contrast, it’s moving 13 millimeters in the sealed enclosure. The bass reflex enclosure will produce significantly less distortion at lower frequencies if the vent is designed correctly. As such, it will sound clearer and more accurate.

Sealed Versus Ported Inexpensive Subwoofer Enclosure

If you want to purchase an affordable subwoofer system for your car or truck, you will need a subwoofer, an amplifier, wiring and an enclosure. The least expensive enclosure is going to be a sealed design. If you can manage the additional cost of a vented enclosure, the system will play louder, sound better and be more efficient. Yes, the enclosure will be a little larger, but it will be like having two subwoofers and almost twice as much power. Drop by a local specialty mobile electronics retailer today and talk with them about the enclosure options available to help you get the most performance from a subwoofer upgrade.

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 spend significant time pondering helpful content that will educate consumers about the options available to upgrade their car audio system. We’ve written dozens of articles explaining how various features and designs can improve a subwoofer’s performance, and we realized that this would be a perfect time to summarize all that education in a single document. Welcome to the ultimate guide to subwoofer performance technologies!

Subwoofer Performance Means Different Things to Different People

We’ll review several design elements that affect a subwoofer’s performance, linearity, accuracy, efficiency and cost. Some features make a subwoofer more accurate. Others make it more efficient. Most add to the cost. We’ll simply define the benefit. It’s up to you to determine whether a particular feature or design element suits your application. Do you crave having a kettle drum or low-C piano note sounding as accurate as possible? Do you want to shake the mirror off the front window? Both are valid goals for a car audio system.

Basket Materials and Design

Let’s start with the foundation of a subwoofer: the basket or chassis. A subwoofer basket aims to align the components like the motor, voice coil, spider and cone. The least expensive subwoofers will have a simple stamped basket with little reinforcement in the form of curved features. Moving up from there, baskets with embossed or rounded-over edges are much stronger and more rigid. These designs can support heaving magnet structures without the risk of bending. Finally, we have cast baskets, which are the strongest designs and can support very heavy motor structures.

The tools required to create a basket, be it stamped or cast, are expensive. If the speaker manufacturer wants to reduce the cost of developing the product, they can use an existing generic design, though it can’t copy another product’s intellectual property.

Years ago, a fad involved adding mass to stamped baskets to increase their rigidity or reduce potential resonance. Installers and enthusiasts would add layers of Dynamat to the back of each spoke. The forces on a basket are in two directions. Gravity is by far the strongest force on a subwoofer basket. Imagine the force a 10-pound magnet, T-yoke and top plate exert on the basket as your vehicle hits a pothole. If the basket is too flimsy to support this, it will bend permanently. This results in a misalignment between the voice coil and the magnet assembly. The voice coil winding will contact the top plate and damage the woofer.

Forward and rearward forces from the magnet are also pushing and pulling on the cone. In comparison, these are minute, except perhaps for some extreme power SPL products. Even then, the magnet structure is a more significant concern.

The reality is that most car audio enthusiasts perceive cast-basket subwoofers as being better purely based on their physical appearance. Many products use stamped baskets shaped to look like their cast brethren. Ultimately, proper motor structure support is the goal.

Shallow-Mount Subwoofers

With the popularity of pickup trucks came the need for subwoofer solutions that could fit into extremely shallow areas. Historically, these subwoofers sacrificed excursion capability for minimized mounting depth. Today, many shallow 10- and 12-inch subwoofers offer just as much excursion and power handling as full-size ones.

Basket Cooling Features

The design of a subwoofer basket or chassis can affect its performance and longevity. A prime example is venting under the spider mounting plateau. The top plate can get warm, so allowing hot air to escape from the top surface can improve power handling. Many stamped subwoofers use vents stamped into the basket, while cast basket designs may include either slots or a design that uses a perforated plastic spacer for the spiders.

While we’re on the topic of cooling, Rockford Fosgate includes its Inductive Damping Heat Sink on its Power Series subwoofers. This finned copper element mounts on top of the top plate and improves heat transfer for enhanced power handling.

Magnet Materials

One of the heaviest components of a subwoofer is the magnet. The least expensive quality options might have a 10-ounce magnet, while some high-power options might have ceramic magnets weighing over 25 pounds. On its own, the weight or even the size of the magnet isn’t a feature that directly correlates to quality. However, a stronger magnet can improve efficiency. A larger magnet can help with power handling.

Most subwoofers use ceramic magnets, which are also known as ferrite magnets. These magnets are relatively simple to produce and not too expensive. They can put up with the high temperatures that large subwoofers have to endure. Rare earth magnets, made from neodymium, are much smaller for a given magnetic field strength. Depending on their quality, they can lose some of their magnetic strength when exposed to high temperatures.

We’re seeing more speakers and shallow-mount subwoofers using neodymium magnets inside the voice coil winding. This reduces size and weight at the expense of increased cost.

Speaker Wire Terminals

We should talk about electrical terminals while we’re on the topic of subwoofer baskets. The least-expensive option is a non-conductive cardboard or plastic tab with a pair of spade connectors attached. Your installer can crimp connectors onto the speaker wires and slide them onto the terminals.

Moving up, we find spring-loaded terminals. These are usually attached to a plastic tab or integrated into a custom-tooled mounting solution. While these are a seemingly simple design element, they cost a little more in terms of the raw components and the expense to tool the injection molding for the mounting solution.

At the extreme end of the terminal world are fully custom connection solutions. They can include unique spring terminals or terminal blocks with set screws. Once again, invoking these designs adds cost to the subwoofer as the parts are often one-off designs unique to that brand. When massive amounts of current need to be delivered reliably, these are often the best choices.

Mounting Gaskets and Hardware

It might seem like nothing, but including a mounting gasket with a subwoofer is essential. The seal between the enclosure mounting surface and the underside of the subwoofer basket is crucial to noise-free operation and maximum performance. Many subwoofers, even some that cost a lot, don’t include a mounting gasket. Ideally, the gasket should come already installed on the subwoofer. Yes, this process adds manufacturing expense but saves time and cost during installation.

Once again, something that seems insignificant but can save time during installation is for a subwoofer manufacturer to include mounting hardware. A little plastic bag with eight mounting screws that are sized perfectly for the subwoofer in mind will make getting your subwoofer system up and running quickly a breeze. Yes, it adds a few cents to the cost of the product, but it saves your installer time searching for the ideal hardware.

Subwoofer Grilles

As an extension of the discussion about mounting solutions, we should talk about subwoofer grilles. Protecting a subwoofer from damage when it is exposed is crucial to ensuring that it will last. If an object runs into the cone, it can be permanently damaged. Many subwoofers include perforated mesh grilles, or the manufacturer offers them as accessories.

Whether a manufacturer-provided grille is used or you have your installer construct a protection solution for your subwoofer, make sure it won’t interfere with operation. If the surround rubs on something, it will be damaged. This interference can also cause distortion.

Vented Pole Piece Designs

The most popular cooling feature has been a vented pole piece. This design element involves an open channel that extends from the base of the T-yoke upward to underneath the dust cap. This vent allows hot air to escape from the inside of the motor assembly.

There is a situation where a pole piece or T-yoke vent can be an issue. For subwoofers designed for shallow mounting solutions, leaving room behind the subwoofer for air to enter and exit a pole-piece vent requires a gap between the back of the enclosure and the deepest portion of the motor structure. Some companies forego this vent to minimize mounting depth requirements.

Surround Designs and Materials

Let’s talk about subwoofer surround designs and materials. The least expensive subwoofers will use foam surrounds. These function well, but they can dry out and crack or degrade after a few years of exposure to the sun. A better solution is a synthetic rubber. Pure rubber also has issues with UV degradation, so materials like Santoprene or IIR-butyl are better options. It’s crucial, however, that the material doesn’t change dramatically in cold weather. The subwoofer won’t perform properly if the surround becomes stiff when temperatures go below freezing.

If you’ve ever stopped to consider the motion of a subwoofer cone relative to the shape of a surround, you’ll recognize that features built into the surround might improve linearity. Because the inside of the surround is smaller, it can, even at a microscopic level, compress laterally as the cone moves inward or stretch when it moves outward. Many subwoofers use ridges in their surround designs to improve linearity at high excursion levels.

You’ll find a few companies that stitch the surround to the cone. This connection is less likely to fail than the connection to the basket. So, while it can’t hurt, it’s often a cosmetic feature rather than one that improves reliability.

Cone Materials

A more controversial topic when it comes to subwoofer features that improve performance is cone material. Subwoofers are designed to play frequencies below about 100 hertz. As such, issues like cone and dust cap resonance aren’t the concern they are with midrange speakers. The only requirement is a subwoofer cone that can handle the forces exerted on it without being damaged. As such, a 10-inch woofer with a half-inch of excursion ability doesn’t need to be as strong as a 15-inch woofer that can move over 3 inches. So, polypropylene, paper, carbon, layered composites or even metals perform the same, as long as they can handle the forces exerted by the motor assembly. The same applies to dust caps, whether they cover the center of the cone or its entire surface.

There are some environmental considerations in terms of cone materials. Pressed paper cones could absorb humidity in extremely damp climates without a moisture barrier. Better designs will include a clear coating. All materials and designs should use materials that don’t degrade under prolonged UV exposure. Drying, cracking or fading are signs of ultraviolet degradation. If you aren’t sure about a product, ask to compare a new unit that was freshly removed from the package to one that has been on a display board for a while.

Designs That Increase Cone Area

Several companies have developed subwoofers that fit a specific dimension but include features significantly increasing cone area. Let’s look at three of the most popular examples.

First, Rockford Fosgate’s Vertical Attach Surround Technique moves the outer edge of the surround closer to the outside edge of the driver. In some applications, this can increase the driver’s effective cone area by as much as 25% with no changes to the mounting requirements.

Another area-optimized subwoofer would be the JL Audio W7 series. These drivers use a unique surround design that attaches to the outer edge of the basket after it’s secured to the enclosure. In keeping with the 8-inch theme, the 8W7AE-3 has an Sd specification of 245 square centimeters.

The king of effective radiating area for a given driver size goes to square subwoofers. Made popular by KICKER, their L-Series woofers make better use of the available space on the front of an enclosure. For example, the L7T82 8-inch woofer has an Sd value of 269 square centimeters.

Voice Coil Sizes and Winding Layers

The voice coil is the heart of any subwoofer. If it fails, you won’t hear anything. Voice coils are nothing more than a length (or two or four) of wire wrapped around a former. The wire might be pure copper, copper-coated aluminum or aluminum. The choice of material doesn’t affect power handling or sound quality but might be chosen to add resistance to hit a target impedance.

The size of a voice coil affects performance in the context of power handling. A 2-inch winding that’s an inch tall won’t dissipate as much heat as a coil that’s 4 inches in diameter and 2 inches tall. The physical volume on the coil winding is a key contributor to power handling. If it overheats, the varnish on the wire will melt, and the coil will short or unravel.

While additional windings add more thermal mass to the voice coil, they have a negative effect of increasing the inductance of the winding. Added inductance opposes changes in the flow of alternating current. If the inductance is high enough, it can affect the midbass performance of a subwoofer. So balancing size, power handling, mass and midbass performance is crucial to the design process.

One design element that can improve voice coil performance is the use of flat-wound conductors instead of round. The flat conductors eliminate the gaps between round conductors to strengthen the magnetic density of the winding. This can translate into increased driver efficiency. There is also increased mass for a given set of outer dimensions. This can be beneficial or detrimental, depending on the goals for the subwoofer and the rest of the materials chosen. One thing is certain: Using a flat-wound voice coil will increase the cost of the subwoofer.

Voice Coil Former Materials

Another area that can affect the reliability of a subwoofer is the material used for the voice coil. We’ve seen everything from cardboard tubes (not unlike a paper towel roll) to anodized aluminum or synthetic materials like Kapton. These materials have different physical properties in terms of heat management and mass. The most efficient car audio subwoofers might convert two percent of an amplifier’s energy into sound. The rest is wasted as heat. As such, the temperature of a voice coil former might reach hundreds of degrees. It needs to remain dimensionally stable and rigid for the subwoofer to function correctly. Different materials help extract heat from the voice coil winding. Aluminum that’s been anodized black is a popular choice for high-power handling designs.

Some voice coils have vents at the top between where the winding ends and the coil attaches to the cone. These vents allow hot air to escape from inside the winding and improve power handling.

Voice Coil Winding Length

If you want a subwoofer that will play loudly, especially at low frequencies, the cone assembly needs to be able to move forward and rearward linearly. This specification is called Xmax. It describes the mathematical cone motion limit by subtracting the top plate’s height from the voice coil’s height and dividing it by two. So, if a driver has a 2-inch-tall winding and a 0.5-inch-thick top plate, the Xmax specification would be 0.75 inch or just over 19 millimeters.

Unfortunately, the motor structure is just one component in determining the usable excursion limit of a subwoofer. The second, perhaps of more importance, is the position where the driver becomes nonlinear. This could be caused by the spider or surround reaching its limit or when the voice coil starts to leave the gap. The Xmax10 specification is where a subwoofer reaches a distortion level of 10%. This is considered the upper usable excursion limit.

Bumped Bottom Plate

Part of designing a subwoofer with significant excursion capability is ensuring that the voice coil former doesn’t smash into the bottom plate (or T-yoke). If you’ve ever heard this, you know it sounds like a small hammer hitting a bench vice. In drivers with aluminum formers, it can roll over the end of the former and cause it to jam up in the gap when it tries to move upward. Subwoofers with fiberglass or Kapton formers can crack, causing the assembly to change shape and bind.

Using a bumped bottom plate is a straightforward design element that helps prevent this condition. Rather than being in line with the magnet bottom, a recess is cut or molded to add additional space. Subwoofers with bumped bottom plates were “cool” in the ’80s and ’90s, but it’s a common feature now. This feature is more likely found on a subwoofer with a single magnet than one with double- or triple-stacked designs.

Triple-Joint Reinforcing Designs

The joint where the voice coil, spider and cone are glued together is often called the triple joint. In many subwoofers, all three components are at the same point, while others have the spider mounted lower down the former. The exact location of everything depends on the design. As we’ve mentioned, the woofer cone needs to be strong enough to withstand the forces applied to it where it meets the former. Many companies will include injection-molded components at this joint to increase gluing surface area and add strength.

Pressure Relief Features Improve Linearity

A high-power subwoofer may be capable of moving forward or rearward over a range of more than 2 inches. As such, if pressure or a slight vacuum were to build up in the area, the driver would be less linear at high volume levels. Venting under the spider mounting plateau and the dust cap can prevent pressure changes, making the subwoofer more linear when pushed hard. Some drivers even have vents in the cone under the dust cap to release air pressure.

Suspension Component Selection

We’re starting to talk about implementing components that affect the Thiele/Small parameters of a subwoofer. There is no perfect subwoofer. Some are designed to be well-damped, while others have some intended resonance to increase output. The spider’s stiffness is the main component in determining the Equivalent Compliance (Vas) for a given subwoofer size. A stiff or narrow-diameter spider will typically result in a smaller Vas number. A soft or large-diameter spider will result in a higher Vas value. The compliance also affects the driver’s Qms or mechanical Q. Of course, no single parameter on its own serves as the final determining factor in the suitability of one subwoofer over another for a specific application.

A subwoofer’s Qts value (Total Driver Q) considers the electrical and mechanic Q factors. Low-Q drivers will have a smoother response, as the driver is less likely to resonate after the signal is removed. A high-Q driver might produce a little more bass, but this comes at the cost of a slightly less accurate sound with more ringing or overhang. Subwoofers designed to be more efficient and offer a little boom will usually have a somewhat higher Q, between 0.5 and 0.65. Low-Q drivers would be in the 0.3-0.4 range. Drivers with Qts values over 0.7 are typically designed for infinite baffle applications.

Some subwoofers use dual spiders to help keep the cone and motor assembly from rocking at high excursion levels. Implementing dual spiders usually increases Vas and Qms, but two compliant spiders might improve longevity. If the designer knows the cone will remain centered in the gap, they can tighten the gap to the top plate, which will improve efficiency.

Linear Versus Progressive Spiders

Two spider geometries are commonly available for car audio subwoofers. Linear spiders exert similar amounts of resistance to motion through a range of excursion levels. Progressive spiders have more compliance (move more easily) at lower excursion levels and then provide lots of control when the cone moves a lot. In reality, a spider will have a specific compliance curve that depends on the materials used in construction and the overall shape. The spider should prevent the voice coil former from bottoming on the back plate. While it seems counterintuitive, most high-end subwoofers have progressive-rate spiders. These days, “small-box” subwoofer designs are popular. As such, the compliance of the air in the enclosure plays a more significant role in the control cone excursion than the suspension. There are, of course, exceptions.

Tinsel Lead Management

Tinsel leads provide a flexible electrical connection between the subwoofer terminals and the voice coil winding. Finding tinsel leads sewn loosely to a spider is common practice to control their motion. There should be enough flexibility so the lead doesn’t affect compliance but enough control to prevent the leads from slapping into the cone. Other manufacturers will coat the leads in plastic or rubber and cut then to a very tight length tolerance to ensure that they don’t make noise. Tinsel leads floating around without proper management can short together or make a ticking noise as they bump into the cone.

In the early ’90s, a popular subwoofer line used carbon fiber woofer cones. The uncoated tinsel leads would run into these electrically conductive cones and, in extreme cases, burn a hole through the cone. The design also resulted in damaged amplifiers.

Shorting Rings and Copper T-Yoke Caps

Several options are available to help reduce changes in inductance based on subwoofer cone position. When a subwoofer moves outward, less of the voice coil might be inside the pole piece. As such, its inductance goes down, and higher-frequency performance improves. The opposite happens when the woofer cone moves rearward, and the midbass response worsens. A designer can add a shorting ring under the top plate or a copper cap on the T-yoke to reduce the overall inductance of the subwoofer coil and make these changes in inductance less significant.

Subwoofers with inductance management can play higher frequencies better, making them sound tighter and more dynamic. These drivers are also typically easier to blend with the midbass speakers in the front of the vehicle as the driver’s and the filter’s overall response is more predictable. Induction management is necessary for any subwoofer to be considered a high-end, sound-quality driver. Shorting rings can be made from aluminum or copper, with the latter being more expensive.

Dual Voice Coil Designs

Many subwoofers have two voice coil windings and two sets of external terminals. These windings can be wired in series, parallel or driven by different amplifier channels. Combining the suitable subwoofers with a particular amplifier will ensure that the system can play loudly and efficiently. From a performance benefit, dual voice coil subwoofers don’t sound or work better than their single coil brethren. They may, however, provide flexibility in optimizing available amplifier power.

Voice Coil Series or Parallel Switches

Getting the wiring on dual voice coil subwoofers right can be tricky for amateurs. Some companies include switches or plugs that make selecting between series and parallel wiring easy. The drawback to these features is an inability to feed each voice coil from a different amplifier channel. It’s rarely an issue, but it does come up with low-quality, non-bridgeable amplifiers.

JBL has a feature on some higher-end subwoofers called Selectable Smart Impedance that allows a single driver to present an amplifier with a nominal 2- or 4-ohm load, which can be selected with a chassis-mounted switch. SSI uses three 6-ohm voice coils to allow this level of flexibility. Of course, SSI is a patented design, so no companies outside the Harman group can use it without proper licensing.

ANSI/CTA-2031 Compliance

Just as the power specifications for car audio amplifiers got out of hand, so too have some subwoofer specifications. The ANSI/CTA-2031 standard describes testing criteria for car audio speakers and subwoofers. These specifications include continuous power handling, sensitivity, frequency response, excursion limits and dimensions, to name a few. Companies that confirm that the information they publish about their speakers complies with this standard have invested significant time and effort into ensuring that you get a quality product that functions precisely as described.

Suitable Thiele/Small Parameters

The last topic we’ll discuss is the suitability of a specific subwoofer for a particular application. Attempting to use a large speaker in an undersized enclosure is a recipe for weak bass and high distortion. When deciding which subwoofer will sound best for your car audio system, work with a reputable mobile enhancement retailer with the tools and experience to measure, model and design a subwoofer solution that will deliver the performance you want. We suggest working backward from the available space in the vehicle and then choosing subwoofers that will provide the most possible low-frequency output in that space. If your goal is maximum SPL, the criteria are similar, but rather than tuning the enclosure for maximum extension, it will be tuned higher or more efficiency.

For someone to design an enclosure, they need specifications that allow software applications like Term-PRO or BassBox Pro to simulate the frequency response, power handling and vent performance at different drive levels. These specifications are called Thiele/Small parameters. Each value describes a distinct characteristic of the cone assembly, suspension components and voice coil windings. With accurate data, an expert designer can create something that will perform well in your vehicle.

Thiele/Small parameters all work together to describe a subwoofer. As such, no single one stands out from the others as needing to be a specific value. As they say, there can be many ways to reach a destination. The right combination of design elements can provide impressive efficiency, but this often comes with a loss of low-frequency extension. Nobody, and we mean absolutely nobody, has found a way to defy the laws of physics regarding subwoofer design.

It’s crucial to note that the accuracy of the specifications is tied directly to the quality control standards of the manufacturer. If there are variances in cone mass, suspension compliance or adhesive application, the resulting subwoofer will behave differently from another sample. Consumers underestimate the importance of proper quality control during manufacturing.

Get Help Choosing the Best Subwoofer for Your Needs

Choosing the best subwoofer for a specific application depends on many variables. You might be on a budget and select something inexpensive with a foam surround, stamped steel basket, polypropylene cone and a single ceramic magnet to warm up a factory-installed audio system. Alternatively, you may want a subwoofer that can play as loudly as possible, so you’d choose something with a much larger double-stacked magnet assembly, a cast basket, enhanced cooling features, dual flat-wound voice coils and a strong cone to put up with more than 1,000 watts of power and high excursion levels. Lastly, you may want a driver with features like a shorting ring and copper T-yoke cap, a flat-wound voice coil, a progressive spider, a rubber surround and a compliant suspension to deliver highly accurate bass with very low distortion.

Adding a subwoofer to your car audio system is one of the best upgrades to improve its performance. Whether inexpensive, fancy or loud, this guide will help you work with a local specialty mobile enhancement retailer to pick a product that offers an excellent balance of value and performance to suit your expectations.

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.