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.