Car Audio

Awhile ago, a client asked us what you get when you buy a “better” headunit. The usual answer is that you get more features and improved functionality. Those of us with a drive for the ultimate in sound quality and realism from our mobile electronics systems choose products in hopes of them receiving improved sound quality. Beyond frequency response and noise, what else makes one radio sound better than another?

We invited two head unit specimens to the Best Car Audio test bench for a little head-to-head battle. The first subject is a modern multimedia station that is equipped with navigation, smartphone integration and the general Bluetooth goodness. The challenger is a veteran, but premium, CD receiver. It has never heard of MP3 files, doesn’t understand the concept of satellite radio and thinks Bluetooth is the result of eating blueberry pie. However, in its day, it was one of the best. We do not need to mention names here, but we will call this the bench battle of features versus performance.

The Equalizer – Premium Test Equipment

Measuring the frequency response of a car audio product is relatively easy if you can feed a known signal into the device. When it comes to measuring a signal source, that is responsible for generating the signal, so all we can do is measure the output. We have a high-end digital interface on our bench. It offers a flat frequency response from 5 Hz to 92 kHz with a tolerance of 1 dB. The signal to noise ratio is an amazing 116 dBA and distortion is specified at less than 0.00032%. These specifications exceed those of both source units we are going to test.

Frequency Response Measurement

Testing the frequency response of a source unit requires some trickery. We have developed a reliable method that has proven itself time after time. The image below shows the frequency response of the reference signal. It has a slight incline in the high-frequency region, but everything is within a tolerance of about 1 dB. We converted this 192 kHz, 24-bit test track down to the CD standard of 44.1 kHz and 16 bit. Response to 22.05 kHz remains ruler-flat.

Frequency Response Results

Before we get into the results, we want to explain how to use the measurement graphs. The test track uses random noise as part of the test procedure. We process that after the test is complete. What you want to observe is the trend of the charts. A small peak or valley is not an anomaly in this scenario. Average the curve in your mind to see the overall trend.

The CD receiver: We played our test track from a standard CD audio file to make sure both source units were given the same information. The frequency response of the unit showed a typical response. The high-frequency filter kicks in around 17 kHz, which is normal for consumer products.

The DVD receiver: We played the same CD in the DVD receiver to see how it responded. The manufacturer of the DVD receiver has included a 2 dB boost on the top end that starts at 7 kHz and peaks at just over 2 dB at 15 kHz. The high-frequency filter response is similar to that of the CD player.

The high-frequency boost is not a big deal in terms of how a system sounds. Most of us have some high-frequency attenuation in our hearing, so this helps put some of the sizzle and air back into our music. It would be worth checking whether the source unit output clips when a 0 dB 15 kHz tone is played. This article is not a product review, so we will save that for someone else to tackle.

Bring the Noise

Our next challenge for the new versus old shootout is a little more technical: We wanted to see how each unit performed regarding background noise. This test is often reserved for lab environments, but can quantify the effort put into the component selection and system design.

For this test, we used a 1 kHz test tone recorded at -90 dB relative to full scale. Because the two source units have different pre-amp capabilities, we adjusted them so the output of the 1 kHz tone was equal in amplitude. This would best depict the noise imposed on the signal.

The CD receiver: We can see that the background noise relative to the signal is very quiet. There is a little bump at 60 Hz that was created by the power supply on our test bench. Otherwise, the test was impressive.

The DVD receiver: The background noise relative to the 1 kHz is 10 to 15 dB louder than that of the CD receiver at higher frequencies. There are also some spurious harmonic distortions in the output signal, mostly above 1 kHz. The large bump in noise in the low-frequency region could be caused by our 60 Hz 120 volt power supply causing some harmonics. The bandwidth is really wide, so it is hard to determine for sure.

A Distorted Perspective

We decided to repeat the test with a full-amplitude 1 kHz sine wave to see what harmonic content would be created at higher internal levels. The limits of the FFT analysis in our software starts to show up here. The flat horizontal line on the left of the chart and the angled line on the right are due to the analysis software and don’t represent noise.

The CD receiver: We noted two small harmonics at 14,750 and 165,000 Hz, with a slight harmonic at 12,000 Hz. Otherwise, the signal was very pure.

The DVD receiver: Harmonic distortion was clearly present at 1 kHz intervals starting at 2 kHz. It is worth noting that the first resonance is 45 dB quieter than the reference signal. If you were just playing the test tone, you might be able to hear it, but only just barely.

The Grand Finale – Intermodulation Distortion

Testing for intermodulation distortion is, well, mean. Out intermodulation distortion test is comprised of a CD test tone with 19 and 20 kHz sine waves played simultaneously. The spectral response of the test track can be seen below.

The CD receiver: When you are looking at an intermodulation comparison test, you are looking for frequency content that wasn’t in the original file. In the case of the CD receiver, we can see a very small bump at 1 kHz. This is significant because it is the difference between 19 kHz and 20 kHz. This would be considered an excellent result. A few other spikes show up at 3,500, 4,500 and 9,500 Hz, but they are still quite low – peaking at -93 dB relative to the test signal level of –15 dB.

The DVD receiver: We cannot really explain what happened here. There is 1 kHz content only 20 dB down from the 19k Hz and 20 kHz tones. Then harmonics upon harmonics of this up to 21 kHz. This test shows why some source units sound accurate and pure, while others do not.

Are Headunit Features Worth the Trade-off?

A few months ago, we published an article about harmonic distortion. That served to establish the basic understanding of how distortion creates content that wasn’t present in an original audio file. Our intent in this comparison is not to put down the modern DVD receiver, but to show what happens when manufacturers forego the bells and whistles and simply focus on all-out performance.

The same tests that apply to these source units are also common in amplifiers and speakers. We will subject an amplifier to the same mean and nasty tests in the coming months.

Don’t ever let price, perception or age dictate how you think a product sounds. Work with your mobile electronics specialist retailer to listen and compare for yourself. You will be amazed at what you hear.

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 all know that the performance of different makes and models of speakers and subwoofers has a dramatic difference in how they sound. When it comes to the installation of subwoofers, the choice of enclosure plays a huge role in the sound of the subwoofer system. In this article, we look at some of the benefits of custom vented enclosures and solutions available.

The Factors that Determine Performance.

Three criteria have the most effect on performance: air volume, tuning frequency (for bass reflex enclosures) and construction. Every subwoofer has its electro-mechanical characteristics. Subsequently, each subwoofer needs its very own enclosure design. With that said, some relatively standard designs offer good performance across a wide variety of speakers.

The difference between having an enclosure that “works” and one that sounds amazing is all in the design. Have your mobile electronics retailer confirm that the air volume and tuning frequency match your application before you purchase. (We will explain that happens when the air volume or tuning frequencies are “off” in another article.)

Wait, Why Vented Enclosures?

While there are benefits and drawbacks to each type of enclosure, a vented (bass reflex) enclosure will offer excellent efficiency and low-frequency extension for most people. These enclosure characteristics reduce the amount of power we need to send to the woofer, and therefore reduce the chances of overpowering and damaging the woofer.

Off-the-shelf Enclosures

Many car audio retailers offer subwoofer enclosures that have been mass-produced. These enclosures vary dramatically in construction quality and design. Some companies tune their enclosures relatively high to increase efficiency at higher frequencies while sacrificing low-frequency output and power handling. The materials used to build these enclosures also vary in quality.

Almost all of these off-the-shelf enclosures are made of MDF. That said, the density of the material varies a great deal. Some MDF is very soft in the middle, offering less resistance to panel deformation. Enclosures made of this soft-center MDF are also prone to having the mounting hardware strip when your installer goes to mount the subwoofer.

These days, efficient use of space is increasingly important for auto sound enthusiasts. An off-the-shelf enclosure may not maximize the available space in the storage area of your vehicle. Enclosure manufacturers try to balance the dimensions of the enclosure against the airspace requirements of the intended driver and the space available in the average vehicle.

Construction Methods

Many enclosure manufacturers claim to use a rabbet joint where two panels meet. Unlike a butt joint, a stepped rabbet joint will increase the surface area of the connection by about 50%. Enclosure assembly typically makes use of a generous amount of glue. Once glued, the panels are held together with brad nails to allow the glue to set up. The additional surface area provided by the rabbet connection results in a stronger joint and a reduced chance of air leaking.

Vent Considerations

If a vent (or port) is designed and constructed properly, a vented enclosure can produce less distortion than a sealed (acoustic suspension) enclosure. To reduce distortion and meet these goals, the vent must have adequate surface area and be designed in a way that the air entering and exiting the vent will couple well with the air in the listening environment. Several subwoofer manufacturers design and construct their enclosures. These stand out from the crowd as having excellent vent designs.

Enclosures with no radius or taper on the vent end are more prone to noise. This sharp edge on the vent can create chuffing or other noises as air rushes over its edges. For a 3-inch or larger vent, a simple 3/4”-inch radius on the vent edge is just barely enough to be beneficial. A much larger radius offers more benefit in allowing the air inside the vent to decelerate properly.

Your Enclosure May Need Bracing

If you are looking for the best possible performance from your subwoofer system, then every effort possible should be made to ensure that each panel of the enclosure is as rigid as possible. One way to add strength to an enclosure is for the manufacturer to install bracing. Bracing connects opposite panels to each other to reduce flexing. Panel vibrations can, in extreme cases, cause glue joints to fail.

Application-specific Subwoofer Enclosures

We have talked about some of the benefits and some of the drawbacks of an off-the-shelf subwoofer enclosure. The alternative is to have your mobile electronics retailer design and construct an enclosure to fit your vehicle. Maximizing usable storage space can be achieved by having your fabricator fit the enclosure snugly to the sides of the vehicle. Many vehicle-specific enclosures combine different construction techniques. Flat panels, fiberglass, stacked-panel fabrication and more can allow an installer to make amazing use of every cubic inch of your storage space.

Maximize Bass Custom Vented Enclosures

One of the first and most beneficial additions than can be made to a factory audio system is a subwoofer system. When it is time to take that first step, visit your local mobile electronics retailer. They will be happy to show you what is available for your vehicle, and what they can create. We know that no matter what you choose, you will be pleased with the dramatic results.

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.

For almost as long as there have been car audio enthusiasts, they have been adding amplifiers to their vehicles to increase the power available to drive their speakers. More power means we can turn the volume up higher without distortion. When it came to connecting amplifiers to speakers, the first power boosters connected to the speaker wires of the radio. Now, modern head units offer dedicated RCA preamp outputs to make things easier.

Manufacturers specify how much voltage their source units can produce on these preamp outputs. Most radios offer at least 2 volts of signal, but some offer 4 volt, 5 volt or even 8 volts. Does this preamp voltage matter? Does more voltage make your system louder? Read on to find out.

Signal Chain

When we install an aftermarket radio in your dash and connect it to the amplifier, we have to make at least one adjustment to ensure everything will work properly. We refer to this step as “setting the gains.” This procedure involves matching the output voltage limit of the source unit with power production capabilities of the amplifier to ensure that both achieve maximum output when the volume is cranked all the way up.

Adjusting an amplifier’s sensitivity is, in theory, a simple process. That said, there are many things to take into consideration. How loud is the source material recorded? Does the head unit distort at full volume? Has anyone adjusted the radio’s tone controls or equalizer? Does the amplifier have any tone controls? If any one of these criteria are overlooked, the sensitivity adjustment may not be accurate.

What is the drawback to an improperly configured sensitivity control? If it is set too low, then you cannot get all the power available from the amplifier to your speakers. If the sensitivity control is set too high, then you can easily distort the output of the amplifier. A second side effect of adjusting the sensitivity control too high is that you increase the noise produced by the amplifier. Nobody wants to hear a hiss in the background of their music, so setting things properly is critical.

What Does High Preamp Voltage Do?

Some intensive research among several of us “old” car audio enthusiasts revealed that there were even a few twin-shaft cassette receivers with high-voltage preamp outputs. Some sales and marketing folks decided that more voltage meant more volume. In the early ’90s, several head unit manufacturers started marketing their radios as having voltage preamp outputs. If nothing else changed, sure – in theory, more voltage means more output. That said, if you swap from a 2 V source unit to a 4 V, and then readjust the sensitivity control on your amplifier down to compensate for the extra voltage, the maximum output level should stay the same.

If more voltage does not make your system louder, what is the benefit of this extra voltage? The answer is a reduction in gain of the amplifier and, thus, a reduction in potential noise. If your amplifier is set up to produce full power with a 2 volt signal and has a subsequent signal to noise ratio of 85 dB, then it is not unreasonable to expect that the noise would reduce by about 3 dB when we turn the sensitivity down by the same amount.

To the Test

We set up a premium consumer-grade amplifier on the lab test bench. It had an S/N Ratio specification of 89 dB when producing 1 watt of output and connected to a 4 ohm load. This is a pretty good rating these days. We set the amp up to produce 1 volt of output with a 1 kHz sine wave at a reference level for our function generator at -30 dB relative to full signal. We then took a long frequency response measurement.

The next step was to reduce the output signal of the function generator by an arbitrary amount – we chose 6 dB. We readjusted the sensitivity control of the amplifier so the output level was once again 1 volt and took another long frequency response measurement.

The results of the two measurements are shown in Figure 1.

In Figure 1, you can see the large spike in frequency at 1 kHz on the right side of the screen. We zoomed in to fill the screen with as much information as possible below this frequency. The gold line shows the background noise produced by the amplifier with the sensitivity control at the lower of the two settings. The green line shows the background noise when we increased the sensitivity of the amplifier by 6 dB.

Figure 2 shows the difference in noise level at 100 Hz. Not surprisingly, the difference in noise is just under 6 dB.

Conclusion

When it’s time to go shopping for a new source unit, among the dozens of cool connectivity features, options for display technologies and different brand names, paying attention to specifications is still important. If you are planning to add an amplifier to your mobile electronics system, make sure your source unit can produce at 4 volts of output on the preamps. This extra voltage will allow your installer to reduce both the sensitivity controls on your amplifier and the background noise level of the system.

Visit your local mobile electronics specialist retailer today for more information on which source units have high-voltage preamp outputs.

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.

For decades, automobile manufacturers have installed relatively large rear speakers in their vehicles. The stereotype of a set of 6×9 speakers sitting on the parcel shelf of a Monte Carlo is as “car audio” as it gets. As the industry has evolved, the importance of rear speakers has diminished. Are rear speakers necessary? Do they offer a benefit? Are we just holding on to a bad habit? Read on to explore more about rear speakers.

Why are They So Big?

Automakers used to take advantage of the opportunity to put speakers in the rear of their vehicles because those speakers could be larger that the ones in the front of the vehicle. Consider a third-generation Camaro or Firebird. They had a set of 4×6 speakers on the dash and a set of 6×9 speakers in the rear sides. Likewise, a fourth-generation Monte Carlo had 3-1/2-inch speakers in the dash and 4×10-inch speakers on the skinny little rear deck.

In most cases, a speaker with more cone area will play louder for a given amount of power. Larger speakers are capable of producing more bass. We recently saw a factory audio system with a 9-inch door speaker, and 6x9s in a pickup truck and SUV doors are very common.

What is Our Goal?

The trend in the last decade for both factory and aftermarket car audio systems has been to recreate a live performance as realistically as possible. This shift in design has introduced us to the need for proper speaker placement and the use of larger speakers in the front of the vehicle. The use of digital signal processors to set up crossovers, signal delay and equalization for each speaker location has further improved on our ability to create what is known as a soundstage in our vehicles.

A soundstage refers to a perceived source location for our music. In a perfect world, our music would appear to come from far out in front of us and provide a good sense of width and depth, excellent focus, and realistic ambiance – just like a live performance. Automobile manufacturers have to balance cosmetic design versus optimum speaker placement, and speaker placement often loses the fight.

What is Your Goal?

How do you like to enjoy your music? Do you attend live performances? Do you listen to a two-channel system at home? Do you usually use headphones? Are you into surround sound? Your answers to these questions can help your car audio specialist retailer design a system that will sound the way you want.

If you like live performances and two-channel audio, then it’s most likely that you would enjoy a system with a soundstage that is out on the dash of the vehicle. In this application, rear speakers are not very important, and in most cases, the money saved by not purchasing and installing rear speakers at all will allow you to buy better front speakers.

If you listen to headphones, then you may want the sound from your system to wrap around you evenly. You may enjoy a system that makes you feel as if you are in the middle of the sound. An equal amount of sound will come from in front of you as behind you. In a system like this, using similar or even identical speakers in the front and back can help produce great results.

If you watch a lot of movies in surround sound, then you need to work with your retailer to decide what is right for you. In movies, the side and rear channels are usually reserved for special effects and ambiance. You may want to go without rear speakers in a mobile audio system. Very few sources include surround information, and currently there are no aftermarket surround sound reproduction solutions, although some are coming.

Are You a Purist?

In pure, raw technical terms, you want as few speakers in your system as possible. Additional sources of sound can result in interference patterns called comb filtering. An ideal system would offer full-range left and right signals, and – because we typically can’t use large speakers in our doors – a subwoofer to fill in the bottom few octaves.

The placement of the speakers can have a dramatic effect on the sound of the system. From a perspective of tonal accuracy, having the midrange speakers in the doors works well. This location typically produces a very wide soundstage. A door location does not often result in a soundstage that appears to emanate from far out in front of you. To achieve that, you may need to use a smaller midrange speaker with a tweeter in a dash or A-pillar location.

When possible, try to ensure that tweeters are mounted within 20 degrees of being on-axis to the listening position.

Who’s Back There?

Another consideration for rear speakers is whether you have passengers in the rear of the vehicle. If you have children or friends in the back, then it would serve them well to have speakers to enjoy. Yes, they will be able to hear the dash or front door speakers, but not as clearly as if they had something close by. It would not be unreasonable to design an audio system that allowed you to turn the rear speakers on and off, depending on your needs.

Rear Speakers as Rear Fill

For systems designed with a focus on the front soundstage, rear speakers are often operated at a much lower relative level than the front. Some people call this technique “rear fill.” The intent is to provide a sense of spaciousness to the listening environment. Employing rear fill speakers splits the divide between systems with or without rear speakers. The relative level of these speakers is critical. Too much can start to pull the soundstage rearward.

There is no right or wrong when it comes to a preference for the location of your soundstage. Whatever you enjoy is the right choice. Your local mobile electronics specialist retailer can work with you to design a system that sounds the way you enjoy. Drop in today and ask if they have a demo vehicle that you can audition. That’s a great way to decide what solution is best for you.

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.