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.

In the car audio world, terms like subwoofer box rise and power compression are relatively familiar to those who push their sound systems to the limit. So, after extensive research (three minutes) and a detailed planning session (another two minutes), we decided that it would be worth doing an experiment to explain and quantify these elements.

Let’s Talk About Speaker and Subwoofer Efficiency

If you’ve read the technical articles on BestCarAudio.com, you know that speakers of all shapes and sizes are notoriously inefficient. At the highest levels, maybe 2% of the power coming from your amplifier is converted into sound. The rest of that power is wasted as heat. For a high-performance 6.5-inch midrange/midbass speaker, efficiency is around 0.33%. For a 12-inch subwoofer of the same caliber, 0.27% isn’t abnormal. Enthusiasts often misunderstand that a massive subwoofer with a heavy cone, stiff suspension and a large-diameter four-layer voice coil is often less efficient. Efficiency numbers of 0.15% or less are common for some of these big, beastly subs.

Where we see the best efficiency specifications from speakers are in those designed for concerts. The so-called Pro Sound drivers trade low-frequency output capabilities for increased output at higher frequencies. Most 6.5-inch woofers are designed to play down to 75 or 80 Hz without issue in car audio systems. However, drivers designed for concerts typically have 50% to 25% of the cone excursion capability and a higher resonant frequency, so they can only play down to 150 to 200 Hz. The trade-off is an increase in output of 6 dB or more with the same amount of power. Using speakers like this in a car audio system can be tricky as you’ll still need a driver to cover the midbass range from 80 to 200 Hz.

Resistance Is Futile

Every electronic component or piece of wire has some amount of resistance. These resistances can be very small, measuring in the decimal milli-ohm range for a yard of power wire. For example, the voice coil on a speaker or subwoofer might be wound from wire as large as 18 gauge. For tweeters, the wire can be as small as 46 gauge.

The speaker’s impedance is determined by the size of the wire, the material it’s made from (copper, aluminum or copper-clad aluminum) and the length of the wire. To reach a target impedance of 4 ohms or similar, the engineer designing the coil assembly balances these characteristics along with the target mass for the completed assembly.

Let’s consider one last concept before we get into our discussion about power compression and box rise. As any conductor heats up, resistance increases. Pure copper has a temperature coefficient of +0.393% per degree Celsius. If a piece of wire measures 4 ohms at room temperature (20C/68F), it increases to 4.0393 ohms if the temperature rises 30C/86F. Aluminum has a temperature coefficient of +0.4309% per degree Celsius.

If 2 volts of signal is applied to a 4-ohm speaker, the amplifier delivers 2 watts of power. If the impedance of that speaker increases, the current from the amplifier decreases. If the impedance increases to 4.039 ohms because the coil is now at 30 degrees, the power dissipated in the speaker drops to 0.9903 watts with the same 2 volts applied.

Let’s Look at Power Compression

If you followed the math in the previous paragraph, you’ll quickly realize that heat is the enemy of a speaker. As the voice coil heats up, its resistance increases. As the resistance increases, the power the amplifier delivers to the speaker decreases. To maintain the same output level, you need to increase the power from the amp. The additional energy required to achieve the same output level results in more heat being generated in the voice coil. If you are good at recognizing patterns, you’ll already notice that we are in an endless loop that can quickly approach the thermal limits of the speaker.

Let’s Measure It!

We borrowed a Kicker L7 cone and spider assembly from Lee Mattason at Burlington Radioactive. First, we’ll use our Smith and Larson Woofer Tester 2 to measure the coil’s impedance. Then, we’ll start heating the coil assembly and take additional impedance measurements to see how the impedance changes.

The chart above shows the impedance of the voice coils of the Kicker L7 sub wired in series at different temperature points. At room temperature (17 degrees Celsius), the coil had an impedance of 3.975 ohms at 10 Hz. With the temperature of the coil at 107 degrees, the impedance had increased to 5.08 ohms. As a note, we could smell a faint whiff of the voice coil adhesive with the temperature at 120 degrees, so we didn’t increase it further. This sample needs to go back to the store and be put on display for consumers to see. Damaging it is not an option.

Let’s say we have a subwoofer amplifier rated to deliver the full 900 watts that this sub is rated for. When that amp produces that 900 watts into the 3.975-ohm load, it will be delivering 59.81 volts to the coil. It only takes a second or two for the coil to heat up well past our arbitrarily-chosen 107-degree mark at that power level.

We can now recalculate the power going to the sub with the voice coils at 107 degrees with 59.81 volts from the amplifier and the new impedance of 5.08 ohms. That works out to 704.18 watts. The sub’s output should decrease by 1.07 dB based on simple math, assuming the cone assembly was operating linearly. If you need the same level of output that was provided by 900 watts when the speaker was cold, you now need an amplifier capable of producing 1,151 watts. Then, the sub will get much hotter, much faster, and the impedance will increase even more.

Every single speaker on the planet behaves the same. The voice coil impedance increases as temperature increases. It doesn’t matter the brand, the size of the speaker or its design. Thankfully, small speakers like tweeters rarely see much more than a watt or two of power, so their fragile, tiny voice coils don’t heat up quickly.

A Good Reason to Choose High-Power Speakers

If you’re shopping for speakers for your car, truck, boat or SUV and plan to play them loudly, then choosing a solution with good power handling capabilities becomes as crucial as the sound quality of those speakers. On the other hand, if you are basshead to who competes in SPL competitions and you want to get the most power from your amplifiers so that your system can achieve the highest sound level numbers, then do everything you can to keep the subwoofers cool before your run. In either case, your local specialty mobile enhancement retailer can help you choose speakers, amplifiers and subwoofers that will deliver impressive performance and excellent reliability.

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.