Not long ago, we looked at how an increase in subwoofer voice coil temperature reduced the power delivered from your amplifier. With component temperatures easily exceeding the boiling point of water, the impedance of the voice coil can increase by more than 50%. Manufacturers that understand this go to great lengths to include technologies to help keep things cool so you can enjoy your music.

What Is a Subwoofer Voice Coil?

The voice coil is a winding of wire in the center of a subwoofer, speaker or tweeter. This winding is connected to the speaker’s terminals. When current from a radio or amplifier flows through the winding, it creates a magnetic field. The magnetic field opposes or is attracted to that of the stationary magnet in the speaker. This force moves the cone or diaphragm (that’s also attached to the voice coil former and winding) forward or rearward.

The audio signal from an amplifier or radio is an alternating current signal. This means that the voltage changes polarity, which causes the speaker or subwoofer cone to move forward and rearward. The speed at which the signal changes direction determines the frequency, and the amplitude of the signal determines cone excursion.

Heat and Resistance

A 2-ohm subwoofer rated for 500 watts of power may see as much as 15.8 amps of current flowing through the voice coil. Given the horrible efficiency of all moving-coil speakers, more than 490 of those 500 watts are converted to heat instead of sound. As such, the voice coil will get extremely hot.

When a conductor gets hot, its resistance increases. The increase in temperature is calculated by looking at the temperature coefficient. The resistance of copper increases by a factor of 1.00386 for every increase in temperature of 1 degree Celsius. Annealed copper has a temperature coefficient of 1.00393, and the resistance of aluminum increases by 1.00429 per degree.

As the voice coil gets hotter, its resistance increases and the current flowing through it from the amplifier decreases. This increased resistance results in less power being dissipated in the speaker, and the output level decreases. In short, the louder you play a speaker, the less efficient it becomes. This phenomenon is called power compression.

How Do Speaker Manufacturers Reduce Power Compression?

Keeping the voice coil as cool as possible is not only crucial to the performance of speakers but also to its longevity. The adhesives used to keep the voice coil wound together have an average maximum temperature limit of around 220 degrees Celsius. After that, the materials boil and the winding unravels, usually resulting in the assembly jamming in the magnetic gap. At this point, the driver is typically headed to the garbage bin.

Manufacturers use many cooling designs to help extract heat from the voice coil. Let’s look at a number of them.

Large-Diameter Voice Coils

One of the easiest ways to increase the thermal capacity of a subwoofer is to design it with a large voice coil winding. Quite simply, the added mass and surface area allow it to absorb and consequently dissipate heat more quickly. Think of this like boiling water on a stove. It might only take a minute to boil a cup of water in a saucepan, but it might take 10 minutes to bring a commercial stock pot of water to a boil.

Vented Pole Piece

If you look at the bottom of the speaker and find it has a mesh-covered hole in it, that’s called a vented pole piece. Air can flow in and out of the motor assembly as the speaker cone moves forward and rearward. This is one of the most common cooling designs, but it has one drawback. The rear of the speaker needs to be an inch or two away from the rear panel of the enclosure in order for air to flow in and out of the vent efficiently.

Multi-Magnet Designs

Over the years, several speakers have been designed with multiple stacks of compact magnets rather than one or two large ceramic units. The space between the magnets allows hot air to escape from the outside of the voice coil.

Spider Plateau Venting

The area of the basket to which the spider is attached is called the spider mounting plateau. Including vents in the basket below this lip allows hot air to escape from the top edge of the voice coil. It can also help improve the subwoofer’s linearity by preventing the air in this space from compressing or rarefying at high excursion levels.

Vented Voice Coil Formers

Another way to allow air to flow around the voice coil and motor assembly is to add vents to the voice coil former itself. These vents can work similarly to spider plateau vents to help pressurized air from under the spider or the dust cap escape.

Vented Reinforcing Rings

The point at the base of a subwoofer cone where the spider and voice coil are attached is often referred to as the triple-joint. This is not only an area of significant stress, but it can get very hot. JL Audio created its Vented Reinforcement Collar (VRC) to reinforce this connection and allow air to flow between the components to improve longevity.

Motor Cooling Designs

Allowing air to flow around the magnet assembly is crucial to helping manage temperatures in a subwoofer. Where many drivers add rubber boots to provide a clean and tidy appearance, those devices can insulate the magnets and reduce their cooling efficiency. Leaving the magnets as open as possible to the air in the enclosure helps them stay cooler longer.

Vented Cone Designs

Some subwoofers feature vents in the underside of the cone beneath the dust cap. These vents relieve pressure and allow hot air to escape. Some companies use a composite component to attach the cone to the voice coil and include vents in that design.

Front-Mounted Motor Assemblies

Subwoofers like the ARC Audio SW Series have their motor assembly mounted to the front of the cone. These compact designs use high-temperature neodymium magnets instead of large ceramic magnets to maintain magnetic field strength in a small package. Having the motor on the outside of the woofer allows heat to dissipate efficiently.

Enclosure Design Affects Power Handling

The type of enclosure that your subwoofers are installed into can significantly affect their ability to handle power. In an acoustic suspension (sealed) enclosure, the air inside is heated by the speaker and has no way to escape. This same logic applies to enclosures that use passive radiators. We’ve seen instances where wiring has been scorched, and stickers on the back of the subwoofer have fallen off.

A single-tuned bass-reflex enclosure provides the ability for the air to be exchanged with the outside air. This helps keep the subwoofer cooler. If you’re having a bandpass enclosure constructed, ask that the motor assembly be installed on the side with the vent. This will increase power handling and reduce power compression.

Keep Your Subwoofers Cool for Better Performance

If you are a bass head or compete in car audio SPL competitions, keeping your subwoofers as cool as possible is a good idea. It might not be insane for a competitor who has to compete in many rounds to run ducts from the climate control system to the motor assembly and allow the air conditioning to cool things off.

For those of you who play music loudly for long periods of time, drop by your local specialty mobile enhancement retailer today and ask them about subwoofers with cooling features that can handle the performance levels you have in mind.

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 can’t count the number of times we’ve heard people say that listening to music over Bluetooth A2DP doesn’t sound as good as if you played a WAV file directly from a USB memory stick. So we fired up the Sony XAV-AX7000 multimedia receiver that we reviewed in late 2020 and made a series of tests to take a look at this. The results might surprise you.

Testing Bluetooth Audio Quality

While we performed at least a dozen comparisons between the quality of direct WAV file playback and that of an audio signal streamed from a nearby computer to the radio, one test in particular stood out to provide meaningful information. We’ll look at some basics first, then show where there was a big difference.

Our test setup used a Sony XAV-AX7000 multimedia receiver with the front preamp outputs feeding into our RME Babyface digital interface. The Babyface Pro is a crucial part of the testing we do. It provides exemplary noise and distortion performance to capture audio and test signals without adding significant noise or distortion. To serve as our reference, we played our composite audio test file from a USB memory stick connected to the Sony radio, then captured the output using the Babyface. Next, we repeated the process, but this time we used a Bluetooth 4.0 USB dongle connected to our test PC and played the audio file from the computer to the radio, then captured the output, once again using the Babyface. Because many of the test signals are quite high in amplitude, we monitored the radio’s output using our oscilloscope to ensure that the signal was never clipped.

We tested both configurations for frequency response using sweeps, pink noise and white noise. We looked for the addition of unwanted ringing using impulse tones. We analyzed harmonic distortion using pure test tones and evaluated intermodulation distortion using CCIF test tracks. We’ll show you the results, including analysis of the original digital audio signal compared to that of the two recordings.

Bluetooth Frequency Response

If someone were to have asked us whether Bluetooth can handle the same audio bandwidth as an uncompressed stereo 44.1kHz sample rate, 16-bit audio file, we’d have bet money that the answer was no. So what did our tests show? It most certainly can.

The two graphs above show the frequency response of our digital test file in red, the output of the radio playing the WAV from a USB memory stick in yellow and the output when the audio was streamed over Bluetooth in blue. The slight low-frequency attenuation below 20 Hz is common to both sources, and more importantly, both can reproduce audio up to 20 kHz without any issue. Call us both pleased and surprised.

Bluetooth Crosstalk Testing

Crosstalk measures how much audio signal is leaked from one stereo channel to an adjacent channel. In these tests, the left channel contains a 1 kHz test tone, and the right has complete silence. To keep the graphs understandable, we separated each source.

We can see no leakage of the test signal from the left to the right in our test signal.

When streamed over Bluetooth, the Sony radio produced output in the right channel 86.5 dB quieter than what was played in the left. This is an outstanding performance.

When our crosstalk test signal was played from the USB memory stick, the right channel had an output that was 76 dB below the left channel. This isn’t quite as good as the Bluetooth stream but is still considered very good performance.

Bluetooth Harmonic Distortion Testing

Harmonics are multiples of a fundamental frequency. For example, the first harmonic of a 500 Hz test tone is 1 kHz, and the second is 1.5 kHz, the fourth is 2 kHz and so on. Harmonic distortion is the addition of information at multiples of the original signal. Because this mimics how we hear sounds in nature, such as instruments and voices, harmonic distortion isn’t all that unpleasant until the levels become significant.

To test the harmonic distortion characteristics of Bluetooth versus a WAV file, we recorded a 1 kHz test tone at -30 dB in both audio channels. The results of our testing are shown below.

The results of the distortion testing are complicated and need thorough analysis. Looking at the red trace, we see a pure 1 kHz reference tone with no other information visible. The blue trace recreates the 1 kHz tone faithfully but adds harmonic distortion at 2 kHz, 4.5 kHz and 6.5 kHz. The 2k trace is normal harmonic distortion. The 4.5 and 6.5 aren’t normal as they aren’t integer multiples of 1 kHz. You can also see a pair of peaks on either side of the 1 kHz tone at 900 Hz and 1,100 Hz. This distortion is more common when testing for intermodulation distortion and can, along with the 4.5 and 6.5k tones, sound unpleasant.

Looking at the yellow trace, we see the addition of typical harmonic information. There is information at 3 kHz and 5 kHz. While ideally we’d have no additional information, this is typical and quite acceptable.

Bluetooth Intermodulation Distortion Testing

The last test we’ll look at is designed to show distortions that aren’t linear. Where harmonics are multiples of individual frequencies, intermodulation distortion can add or subtract the difference between two simultaneous frequencies.

The test we used was known as the International Telephonic Consultative Committee (CCIF) intermodulation distortion test. As this committee no longer exists, the test standard has been adopted by the International Telecommunications Union (ITU) and is now known as the IMD (ITU-R) test.

This evaluation involves playing two tones simultaneously – in this case, 19 kHz and 20 kHz. The difference between 19 and 20k is 1 kHz, so poor performance would show the addition of information at 1 kHz. The test will also demonstrate how unwanted information is added at multiples of 1 kHz on either side of the 19 and 20k test signals.

The test results aren’t absolute as they show that the different sources deliver significantly different distortion characteristics. The red trace is our test stimulus – two tones at 19 and 20 kHz, both at the same level. The yellow WAV file trace shows the typical behavior we expect to see when analyzing an audio device. There’s some signal added at 1 kHz, and more at 18 and 21 kHz. However, the levels are quite acceptable for a consumer product.

The blue trace is much more confusing to unravel. While there is some unwanted information at 1 kHz, the groups of four peaks at 2k to 3.5, 7.5 to 9k and 10 to 14k are simply abnormal. Thankfully, they are low enough in amplitude (peaking at -75 dB from the reference signals) that they don’t destroy the listening experience.

Keep in mind that audio signals contain thousands of frequencies, all played at the same time. Harmonics and intermodulation sums, differences and products are created for every single one of those frequencies. The more distortion there is, the less lifelike your listening experience will be.

Conclusions on Bluetooth Audio Quality

We would have said the measured performance of Bluetooth streaming versus the playback of a WAV file would vary more. Many variables can affect performance. Considerations for which Bluetooth codec is used depend on the phone or device you are using and its software. For this test, the Bluetooth dongle we chose uses the Qualcomm CSR8510 A10 chipset, which offers Bluetooth 4.0 +HS connectivity. The specs for the Sony radio include Bluetooth 3.0 and support for the A2DP protocol version 1.3. As always, the performance of your specific combination of car radio and audio source may vary.

Bottom line, we honestly expected a disaster, and that’s not what the testing showed. This is excellent news for folks who enjoy streaming music from their smartphone to their radio. If this sounds appealing, drop by your local specialty mobile enhancement retailer today to learn about the connectivity and Bluetooth streaming options available for your vehicle.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

A comment came up the other day that it was unwise to use all channels of a four-channel amplifier to power a pair of subwoofers. Not only is this incorrect, but this practice opens up several unique system designs and installation possibilities, especially where high-power requirements are necessary. Let’s dive in!

Four-Channel Amplifiers Are Amazing!

In terms of application flexibility, investing in a four-channel amp is a great choice. It can be used to power a set of front and rear speakers while maintaining fader capability. It could also run a set of midrange drivers from one pair of channels and tweeters from the other pair. When combined with a digital signal processor, this is a recipe for a fantastic soundstage in the front of a vehicle.

Another popular application for a four-channel amplifier is to power an entire audio system that includes a subwoofer. The front and rear speakers can be wired in parallel to the front channels, and the subwoofer can be bridged to the rear channels. This design maintains subwoofer level control functionality from the source unit or includes level control on the amplifier.

Many car audio retailers that install marine audio systems will use a four-channel amplifier to power a pair of wakeboard tower speakers. This application typically provides more power to the speakers at significantly less cost than an equivalently powerful two-channel amplifier. This is also a popular application for four-channel motorcycle amplifiers where only two speakers are in use. We’ll add that it’s worth being very cautious with this sort of application. Most compact four-channel amplifiers can produce more than 140 watts of power when bridged. This is enough power to damage almost any two-way speaker, no matter what the power ratings say.

Many amplifiers include a bridging switching that combines the left and right inputs to produce a mono signal for use when driving a single speaker. Finally, a select number of four-channel amplifiers also include a dedicated two-channel switch. With this switch enabled, the left input is routed to one pair of channels in mono and a right input is routed to the other pair.

Four-Channel Amplifiers and Subwoofers

Let’s get back to the original statement about using a four-channel amplifier on subwoofers. First, is there anything wrong with using a subwoofer on a multichannel amplifier? Short answer – no. As long as the amplifier offers frequency response that extends low enough (and all of them do), they are ideal for a subwoofer. You’ll find that most multichannel amplifiers have the option of high- or low-pass filters on both sets of channels.

The key to powering subwoofers is to optimize the drivers’ impedance with the amplifier’s output capabilities. Most four-channel amplifiers make the most power, with each channel driving a 2-ohm load or when both sets of channels are bridged into a pair of 4-ohm loads.

Using a Four-Channel Amplifier with Subwoofers

There are a few ways that a four-channel amp can be used to power subwoofers in a car audio system. Your installer can wire a 4-ohm subwoofer to each pair of channels in a bridged configuration. A similar option would be to wire a dual 2-ohm voice coil subwoofer to each pair of channels with one coil on each.

If you have a single, dual 4-ohm voice coil subwoofer, and it can handle the power the amp will produce, then your installer can wire one voice coil to one pair of bridged channels and the other coil to the other pair.

Are There Issues with Bridging Amplifiers?

Any time there are separate sensitivity controls for the channels driving a single speaker, care must be taken to ensure that the voltages produced are equal. Your installer can use a multimeter in its AC Volts mode along with a test tone to match things perfectly. Of course, this applies to two-channel amplifiers with individual sensitivity controls in bridged applications as well.

Other than that, all amplifiers operate linearly, so any increase in the signal that’s equal on one channel will be proportionate on all channels set with the same sensitivity levels and crossover frequencies.

If you have a multichannel amplifier where many channels will connect to a single speaker, make sure that the crossovers are set to function identically on all channels. This is more crucial than variances in voltage as crossovers not only affect output level, they can alter the phase relationship between signals and cause unwanted cancellation. Imagine one part of the voice coil trying to push the cone upward while the other winding is pushing it down. The results will be a dramatic decrease in output in a frequency range where it’s not desirable.

Don’t Overload Me!

We’ve talked about the benefits of not running amplifiers into low impedances in terms of their efficiency and output voltage regulation. The discussion about a four-channel amplifier and subwoofers behind this article revealed that someone put two subwoofers on one pair of channels and none on the other. The application would have been better served in terms of sound quality and performance with even distribution of the subwoofers, even if the amp were to make slightly less power. Remember, you need to double the power produced by the amplifier to increase the system’s output by 3 dB. Since it’s unlikely that overloading one set of channels resulted in double the power production, this was an unwise decision.

All this stuff comes down to knowing the features and intended implementation of the products suggested for a specific application. Choosing the “right” amplifier for a particular task can mean balancing the use of an existing product for an upgrade or offering a client the most power for their dollar in a newly designed system. Ultimately, the success or failure of an upgrade relies on everything – including product selection, integration and configuration – being on point.

This sort of thing is what makes shopping for an expert installer so challenging. When it’s time to upgrade your car audio system, start by visiting the specialty shops in your area. Ask questions about their suggestions so that you can understand their philosophy. If it seems counterintuitive when they suggest using a four-channel amp in a particular application, don’t fret. They might be trying to help.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

In a recent online discussion, someone stated that a car audio amplifier couldn’t be power-hungry. After the eye-twitching passed and our heart rates settled, we knew this was a perfect topic for our series on car audio myths. Let’s look at how amplifiers, specifically subwoofer amplifiers, make use of the power from your vehicle’s electrical system.

What Is Amplifier Efficiency?

There’s no electronic device on the planet that is 100% efficient. If the electrical system in your car supplied 12V and 100 amps of current to an amplifier, that would be 1,200 watts. That amplifier can’t convert 100% of that energy into power to drive a speaker. Yes, the Rockford Fosgate T15kW amplifier had a bank of ultra-capacitors built in that allowed it to produce more power than it consumed for a few seconds. We’re talking about normal amplifiers in this article.

Calculating amplifier efficiency is relatively easy if you have the right tools. First, you need to measure the power going into an amplifier. Since a DC voltage powers amplifiers, a voltmeter at the amplifier terminals and a current clamp or current shunt resistor can give you the information you need. Next, you need to measure the power coming out of the amplifier. Measuring AC signals can be complex because the current and voltage might not be in phase at all frequencies. A tool like the D’Amore Engineering AMM-1 works great because it takes the phase relationship between current and voltage into account in its power calculation. Once you have the two numbers, you can divide the power coming out of the amp by the power going in, multiply by 100, and you’ll have the percentage efficiency.

Examples of Amplifier Efficiency

A good example of an efficient amplifier would be the Rockford Fosgate M5-1000X1 that the BestCarAudio.com gang reviewed earlier in 2021. When driving a 1-ohm load, the amp produced 975 watts at 1% THD+N. The supply voltage was 13.75 at the amp terminals, and the amp was consuming about 98.5 amps of current. The efficiency was 72%.

Another great amplifier is the ARC Audio ARC 1000.4 that was also reviewed earlier in 2021. This four-channel amp produced 1,406 watts when driving a pair of 4-ohm resistive loads. The amp was fed with 13.37 volts and consumed 137.1 amps of current. The efficiency was 77%.

Both of these companies have world-class engineering and design teams behind the products they sell. Sadly, this isn’t always the case. Here’s an example from the other end of the spectrum. Our friends at Of Sound Mind Labs tested a monoblock amplifier that was rated for just over 1,000 watts. It produced 934 watts at 1% THD+N when fed with 13.6 volts. Current consumption was 119.4 amps for a calculated efficiency of only 57.6%. Look at the Rockford specs. The M5-1000X1 made 50 more watts while drawing 20 amps less current.

In short, it’s easy to say that the last amplifier in this example is indeed quite power-hungry. So, where does the extra power go? That energy is converted into heat. The efficiency was so bad that the amplifier shut itself down because of thermal overload after running for only two minutes at full power when connected to a 1-ohm load.

Here’s another way to think about amplifier efficiency. The power going into the amp is either output to the load or is wasted as heat. In our two examples of good amplifiers, the Rockford Fosgate amp would need to dissipate about 380 watts of energy as heat. The ARC Audio amp is in the same ballpark with 420 watts of heat. Our unfortunately power-hungry amp is wasting 687 watts as heat. It’s no wonder it overheated so quickly.

Too Long; Didn’t Read: Can Car Audio Amplifiers Be Power Hungry?

The answer is an unequivocal yes. Sadly, the companies that don’t publish efficiency specifications are often some of the worst offenders. If your amp runs hot, it’s wasting power. It if overheats when played loudly, it’s wasting a LOT of energy. Every amplifier will get warm, but if you can’t leave your hand on it, then there may be better options available. Drop by your local specialty mobile enhancement retailer today to find a high-quality amp like the Rockford Fosgate and ARC Audio solutions we mentioned in this article.

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.

Every so often, someone asks if it’s a good idea to upgrade their factory-installed stereo system by swapping out the original subwoofer with a new one. We aren’t talking about building a new enclosure and adding a dedicated amplifier. They want to put a different 6-, 8- or 10-inch in the factory location and connect it to the OE amplifier. The answer to whether or not this is a good idea depends on a lot of variables. Let’s dive in and analyze this in more detail.

Potential Issue One: Speaker Impedance

Many factory-installed subwoofers have non-standard impedances. The driver may have dual 1-ohm coils or a single half-ohm winding. Thankfully, there are hundreds of different subwoofers on the market. If your installer has the tools to check the original sub, a solution with the same impedance might be an option. One caveat is that if you’ve damaged the woofer and need a replacement, measuring the impedance might not be an option. You can’t guess at this, and online resources like Mitchell1 ProDemand don’t typically include specifications for the woofer impedance.

Potential Issue Two: Subwoofer Dimensions

There’s always the question of “Will it fit?” Most aftermarket subwoofers are physically deeper than their factory-installed counterparts. This extra depth often results from a large magnet assembly and deeper baskets that allow for increased excursion. If there’s a solution with the right impedance and there’s enough depth in the enclosure, then your installer can move on to issues three and four.

Potential Issue Three: Enclosure Design Parameters

For some unknown reason, many original equipment subwoofers are installed in small sealed enclosures. Perhaps the reduced physical size of the enclosure helps to save weight in the vehicle – a crucial consideration for vehicle manufacturers. A vented enclosure would offer better efficiency and more extension. Nevertheless, the enclosures are what they are. If you plan to replace the speaker used in the enclosure, your installer needs to calculate the volume or design of the enclosure and verify that the proposed replacement will work in that space.

Potential Issue Four: Frequency Response and Efficiency

Though a bit of a generalization, most aftermarket subwoofers are designed to provide good low-frequency extension and do so by sacrificing some upper-bass output or overall efficiency. Suppose your installer were to find a woofer of the correct impedance that would fit and work with the provided enclosure. In that case, there’s the risk that the output characteristics of the new driver may not blend with the rest of the speakers in the audio system. Sure, you might increase your output by a few decibels at 40 or 45 Hz, but the system might have a dip in the response at 70 or 80 Hz where it blends with the speakers reproducing midbass frequencies. It’s far more likely that an aftermarket woofer will be less efficient and will need a dedicated amplifier to reach the same output level. You’ll get more deep bass, but you’ll need more power.

How To Upgrade an Audio System with a Factory-installed Subwoofer

If you want to be guaranteed that the money you’ll invest will be worthwhile, then you need to include an amplifier and preferably some sort of equalization solution with your upgraded subwoofer. Assuming the sub you have in mind will fit and perform well in the enclosure, the new amplifier will let your installer adjust the bass level to suit your listening preferences. The equalizer for the subwoofer can then be used to compensate for frequency response in the audio signal produced by the factory amplifier or variations in output from the new driver you’ve chosen.

It may be simpler to use a different enclosure. The odds are good that the optimized design of a vented enclosure of the correct volume and tuning frequency will result in much deeper and more accurate bass production.

Drop by your local specialty mobile enhancement retailer today to find out how you can upgrade the subwoofer in your car, truck or SUV. If space is at a premium, they may be able to use a factory-installed subwoofer enclosure or construct a new enclosure that fits into the same space.

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.