If you’ve been around the car audio scene for a few decades, then you’ll recall a time when almost all amplifiers doubled their output power when the load impedance was halved. For example, an amplifier might have been rated to produce 75 watts per channel into a 4-ohm load and 150 watts into a 2-ohm load. It wasn’t unheard of to see low-power amplifiers like the infamous Orion 225 HCCA continue to double their power, right down to 0.5 ohms per channel. Do modern amplifiers behave like this? If not, why?

Making Power from an Amplifier

Let’s start with the basics. Your amplifier is designed to boost the voltage of the signal coming from your radio. By way of an example, let’s say you have a 1-volt RMS signal and you want the amplifier to produce 50 watts of power into a 4-ohm load. The signal would need to be increased to 14.14 volts out of the amp. That’s a signal gain of just over 5 decibels.

Because car audio speakers have a relatively low voice coil impedance (4 ohms), the amplifier will also deliver a significant amount of current to the speaker. In our 50-watt, 4-ohm example, the current flowing through the speaker is 3.536 amps.

One last piece of math. We’ll call this hypothetical amplifier a Class-D design and say that it has an overall efficiency of 80% when driving 4-ohm speakers. To produce 200 watts of power, the amp will need to consume 250 watts of power from the vehicle electrical system. If there’s 13.5 volts at the amplifier power terminals, it will draw 18.52 amps of current from the alternator and battery.

Old-School Amps Were Huge

Back in the day, nobody cared if a 200-watt amplifier was 20 inches long, 12 inches wide and had to be mounted in the trunk. Today, installers want that power from a package that will fit behind the radio or under a seat. Why does the size of the amplifier matter? Well, in order for it to function reliably, the heat sink needs to be large enough to keep the amplifier cool while it produces full power. In this case, our amp needs to shed 50 watts of heat. This number is the difference between the 250 watts it consumes and the 200 watts it delivers to the speakers. Dissipating 50 watts isn’t a significant issue.

Let’s add a second speaker to each channel of the amplifier so that we have a net load impedance of 2 ohms. The amplifier will still attempt to produce 14.14 volts on each output, but it will now flow 5 amps of current to each pair of speakers. We’re up to a total output current of 20 amps from our 14.14 amp draw at 4 ohms. When you load an amp down, its efficiency drops. Let’s say this Class-D amp offers 72% efficiency when driving 2-ohm loads. If the amp is to produce 50 watts to each of the eight speakers (400 watts), it needs to draw 555.6 watts from the car. At 13.5 volts, that would entail 41.15 amps of current flowing into the amplifier. The heat sink will need to dissipate 155.6 watts of thermal energy. That’s a LOT of heat.

When designing this amplifier, the engineer will need to come up with a way for it to manage this 155.6 watts of heat without allowing the components inside the amp to overheat. If the amp has to be very small, this might be a significant problem. Large heatsinks help radiate thermal energy into the air that surrounds the amplifier. Another cooling method is to add a fan to the amplifier design. Fans can dramatically reduce the size of an amplifier and help ensure that they run at cool temperatures. If you choose an amp with a fan, make sure it flows air across the heatsink where the output and switching devices are located. Blowing air into the middle of a circuit board does almost nothing.

The last option, and one that has become quite common, is to limit the amount of current the power supply will pass. Limiting current directly limits the amount of heat energy the heatsink needs to manage. Let’s reverse-engineer how much power our amplifier can produce if we limit the heat sink’s thermal capabilities to 120 watts.

If the energy wasted by the amp is 120 watts at an efficiency of 72%, then the total power it can consume is 430 watts, with 310 watts going to the speakers. In this scenario, each channel is producing 77.5 watts of power and each of the eight speakers would be receiving 38.75 watts when everything running at its maximum output capability.

Is There Anything Wrong with Current-Limited Amplifiers?

As we’ve demonstrated, limiting current controls the maximum amount of power an amplifier can produce in order to ensure that it doesn’t overheat when pushed to its limits. Are there any drawbacks to a design like this? Not really. Simply, you don’t get as much power when you load the amplifier outputs down further. This design decision isn’t directly detrimental to sound quality, distortion or the addition of noise to the audio signal.

If you’re shopping for an amplifier for your vehicle, drop by your local specialty mobile enhancement retailer and ask them about the best solution for your system design and budget.

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

It’s time to dispel another unsubstantiated car audio myth! This time, we’re talking about claiming that you can’t ground an amplifier directly to the battery in a vehicle. Perhaps it would add to the clarity of the statement to say that many inexperienced installers think the ground wire for the amp has to go to the chassis of the vehicle. Many of these statements also add, “Connecting directly to the battery will cause noise.” All of these statements are nonsense. Let’s check them out.

How Electricity Flows

Whether you subscribe to the conventional current theory that electricity flows from the positive side of a power source through a load and back to the negative terminal, or to the electron theory that holds that electrons flow from negative to positive, what matters is that an electrical circuit is a loop. Your load, be it a light bulb, radio or amplifier, needs to have two electrical connections. Power flows from the electrical source through the load and back to the source again. The current in both conductors is equal.

Our vehicles have two power sources: the battery and the alternator. The battery is there to start the vehicle. It feeds a high-torque electric motor that spins the engine when you turn the ignition key to start. Once the engine starts, the alternator takes over the operation of the vehicle. Technically, you can remove the battery from the car and it will continue to run. Don’t try this, though, as many new vehicles monitor current flow in and out of the battery to control the alternator’s function. Once the engine is running, the alternator recharges the battery.

Having an Amplifier Installed in Your Vehicle

In the “good old days,” installing an amplifier in a car or truck involved having your installer run a large-gauge wire from the battery’s positive terminal through a fuse holder or circuit breaker to the positive terminal of the amplifier. The wire from the amplifier’s ground terminal would be bolted to the chassis of the vehicle.

Until about a decade ago, most cars and trucks were built with steel, and the chassis or unibody components were spot-welded together. As a result, there was often enough surface area to allow current to flow from the ground of the amp back to the negative terminal of the battery or alternator.

What’s Wrong with a Chassis Ground?

A few things could go wrong when using the chassis as a ground point for a high-power amplifier. First, the connection to the chassis needs to be secure and reliable. Second, your installer needs to brush or grind away any paint, rustproofing or undercoating from the metal before they drill a hole to make the connection. Third, the chassis may not be a direct electrical return path to the battery.

The ground connection needs to be secure. Because the cable is of a reasonably large gauge, it can experience moderate loads during acceleration and braking and from the vibrations associated with regular driving. This stress can loosen the connection and cause intermittent behavior or damage.

You’d think removing paint, primer and coatings would be easy, but the number of poor grounds we see is amazing. In some cases, especially with cars that are painted silver, some primers and corrosion-resistant coatings can look similar to bare metal.

In many modern vehicles, lighter metals such as aluminum are used in vehicle construction. These materials don’t conduct as well as steel. What’s more concerning is that many new vehicles are assembled with high-strength adhesives and even two-sided tapes from companies like 3M, Dow Chemical and Henkel. These materials are not electrically conductive.

Ohm’s Law in 20 Seconds

Whenever current flows through a device or conductor with resistance, a voltage is produced across that device. In the case of a piece of wire, the resistance is typically low, so very little voltage is wasted. When the current flow is significant, we can see several volts across the wire, and it will heat up. To calculate the voltage drop across a resistance, multiply the current flow (in amps) by the total resistance of the device (ohms). For example, 2 amps of current flowing through a 4-ohm resistor results in 8 volts being produced across that resistor.

Proper Amplifier Grounding

Feeding your high-performance amplifier with the power it needs means delivering as much voltage as possible to the power terminals. If the amplifier draws a significant amount of current, then your installer will need to use large-gauge conductors to prevent voltage losses due to the resistance in those conductors. This same logic applies to the ground return path. A test performed several decades ago by one of the engineers at JL Audio in Miramar, Florida, showed that most vehicle chassis have a resistance equivalent to a piece of four AWG cable. Modern vehicles are much worse.

Let’s say you have a good quality 1,400-watt amplifier like the ARC Audio ARC1000.4 DSP we tested a few months ago. This amp produced 1,406 watts of power (at 1% distortion) when provided with 13.37 volts at the power terminals. It drew 137.1 amps of current to deliver this power (at an impressive 76.7% efficiency). If your vehicle had a 16-foot run of all-copper, CTA-2015 compliant, four-AWG power cable to feed the amp, there would be a voltage drop of 0.588 volts across the length power cable. The ground return path (assuming it has the same resistance) would result in a similar loss of voltage at this current level. Your electrical system would need to be able to supply the battery with 14.546 volts and have a spare 137 amps of current capacity available. For a full-sized sedan or truck, these numbers are quite reasonable. So, you might be able to get away with using the chassis as a ground – if you know it’s all-steel from front to back.

When Grounding Goes Wrong

Now, what happens when we run into a vehicle where the chassis isn’t a good ground? Perhaps it only has the current-carrying capacity of a piece of 12-AWG wire. At low to moderate volume levels, our amplifier won’t draw much current. If the amp is a modern Class-D design, it will likely have a driver IC that includes low-voltage support for start-stop vehicles. If so, the amp won’t shut off even if the voltage drops below 6 volts for a moment.

Let’s run the math again. We want about 1,400 watts and will attempt to draw 137 amps of current. With a chassis with the current-carrying capacity of a 12-AWG conductor, 3.74 volts would be lost across our chassis connection. The amp will likely stay on, but it certainly isn’t going to produce 1,400 watts when it only sees 10.8 volts. In our experience, aluminum and bonded-construction chassis have even more resistance, and the voltage drops are significantly worse.

The Parallel Ground

Going back to the testing done by the folks at JL Audio, they propose that your installer run what they refer to as a parallel ground. The installer can bolt the ground lead from the amp to the chassis, then run a conductor of the same size as the power leads to the battery’s negative terminal.

In this scenario, the worst-case condition is that the new ground wire carries all the current from the amp, and you have a minimal voltage drop. More likely, some current will also flow through the chassis to the battery, and this results in less voltage being wasted than if it wasn’t grounded at the rear of the vehicle.

Stop The Noise

We can’t fathom where the myth about grounding to a battery and getting noise came from. Perhaps there was an installation that had a ground loop of some sort. Ground loops can (and do) happen when the chassis is used for grounding.

Let’s dispel this myth with one sentence. Think of all the marine audio systems and stereos in Corvettes that you’ve seen over the years. Fiberglass certainly can’t be used as a ground return path. These vehicles don’t have any noise problems associated with how power is distributed.

When it comes time to have an amplifier installed in your vehicle, you can improve its efficiency by providing it with as much voltage as possible. This means minimizing voltage losses due to power cable or ground return path resistance. Upgrading to larger or better wiring and having the installer at your local specialty mobile enhancement retailer run a parallel ground is a great way to ensure that your system will function reliably.

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 professional audio system installation, be it for a church, dance club, home theater or your car, requires DSP calibration. Your smartphone, laptop and smart speaker use digital signal processors to optimize the sound you hear from the tiny speakers. Digital signal processors aren’t magic, and they aren’t complicated. They offer a reliable and straightforward way to calibrate what you hear so that voices and instruments sound the best they can.

Signal Processing in Car Audio Systems

If you look at any radio in the dash of a car or truck, you’ll see it has bass and treble controls. The treble control adjustment lets you fine-tune the output of high frequencies to calm speakers that are bright or add some sizzle when things seem dull. The bass control is often used when the speakers in an audio system don’t produce enough bass on their own. While this can lead to the addition of a lot of distortion if the volume is turned up, it remains a popular tweak. Nobody hesitates to adjust these controls when they get a new radio.

Even if you’ve chosen the very best speakers on the planet and had them installed in optimum locations in your vehicle, the confines of the car or truck itself wreak havoc on what you hear. Reflections from the windshield and side windows, the floor, roof, seats and dash all combine at the listening position to dramatically change the perceived frequency response of the audio system. It’s not abnormal to see peaks and dips of more than 20 dB across the usable audio range. Left uncorrected, these variations in frequency response detract from the realism of your audio system.

For about 15 years, vehicle manufacturers have used amplifiers that include electronic crossovers and multi-band equalizers to fine-tune the audio systems they deliver to customers. Even with inexpensive speakers, these systems offer smooth frequency response. However, they may still not have enough bass or play loudly enough, which is why there’s an aftermarket car audio industry.

What Is a Car Audio DSP?

A car audio DSP is nothing more than a set of very accurate multi-band equalizers, adjustable electronic crossovers and signal delay processors in a single compact and efficient package. Unlike the bass and treble controls on your radio, these equalizers have as many as 40 bands of adjustment available to let your installer fine-tune the speakers’ output accurately across the entire audio frequency range. If there’s a peak at 1 kHz, it can be tamed. If there’s a dip at 350 Hz, it can be boosted.

The crossovers in a DSP are part of setting up your audio system. You don’t want deep bass to be sent to your door speakers if you have a subwoofer in the system, right? Your installer will set filters that ensure that only the correct information is sent to each speaker in the system. When done properly, each speaker will sound better and be able to play louder.

Signal delays are a tricky subject. In essence, they are used to compensate for different path lengths between the speaker installation location and the listening position. How these delays are configured depends on the goals of the system. If there will only ever be one person in the car, then everything can be set relative to the driver’s listening position. If the system is designed such that everyone in the car needs to be able to enjoy the music, then the settings change dramatically. Let the product specialist and installer at the shop you’re working with know your expectations before they design your audio system.

The Right Tools for the Job

Two tools are an absolute necessity for setting up a car audio digital signal processor: a real-time audio analyzer and a way to measure path lengths between speakers. An RTA is a sound-level meter that shows the volume or output of an audio system at different frequencies. For example, your installer will be able to see how loud 1,000 Hz is relative to 800 Hz and 1250 Hz. Most RTAs divide the audio spectrum into 30 or 31 bands. This is called a 1/3-octave RTA. There are 10 octaves between 20 Hz and 20 kHz, so we have three measurements in each octave. It’s impossible to calibrate a DSP efficiently and accurately without an RTA.

You can download an RTA application for your smartphone if you want to see how they work. With that said, to set up an audio system, a microphone with a flat audio response from 20 Hz to 20 kHz is required. In short, don’t use your smartphone to try to adjust a DSP.

Your installer will also need a way to measure path lengths. Across the industry, installers use a multitude of DSP calibration processes. Some folks use a tape measure, while others use impulse audio tracks. As long as their process is repeatable and predictable, you’re in good hands.

What Is a Simple Car Audio System?

We titled this article the way we did because many audio enthusiasts believe that including a DSP in an audio system is reserved only for the fanciest and most elaborate designs. The truth is, if you have a set of speakers in your car, your system is a candidate for proper calibration. In addition, if you add a subwoofer, then there’s even more reason to include one in your audio system upgrade.

Years ago, processors were expensive. Now, options from companies like Rockford Fosgate, ARC Audio, Audison, Axxess and many others are quite affordable. When you factor in the cost of the DSP, an hour or so of labor and a few extra RCA cables, this single component offers the most significant and cost-effective upgrade you can make to improve the performance of your car audio system. So drop by your local specialty mobile enhancement retailer today and ask to audition a car audio system that’s been calibrated with a DSP. We expect you’ll be impressed.

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.

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.