When it comes to car audio subwoofer enclosures, the two most popular options are sealed or vented. As far as which design is best for your vehicle, let’s see if we can clear up some misconceptions and stereotypes. As often happens, some trade-offs accompany each decision. Consider this article the master reference for choosing the right subwoofer enclosure solution for your application.

Why Does a Subwoofer Need an Enclosure?

Let’s review a few key factors about subwoofers (and speakers in general). First and foremost, the primary purpose of a subwoofer enclosure is to prevent the sound that’s coming off the back of the speaker cone from mixing with the sound coming from the front. If these two mix, they cancel each other out almost perfectly. If you’ve ever held a subwoofer in your hand without an enclosure while it’s playing, you’ll know it doesn’t produce much sound.

Second, an enclosure acts as a mechanical high-pass filter that limits low-frequency output. Why do we need to limit bass from a subwoofer? As frequency decreases, cone excursion increases dramatically to produce an equivalent output. In fact, for every halving of frequency, cone excursion doubles.

The simulation above shows the predicted cone excursion (in millimeters) of an audiophile-grade 10-inch subwoofer without an enclosure. This is a great driver, and it has an Xmax specification of 19 mm. As such, at frequencies below 22 hertz, when driven with 500 watts of power, the distortion would skyrocket. If we increase the power to the subwoofer to 750 watts, that frequency increases to 28 hertz. At a drive level of 1,000 watts, the driver will reach its Xmax limit at 33 hertz.

Many subwoofers don’t have this much excursion capability, so we need to limit the distance the cone can move. We install the subwoofer in an enclosure so that the air in the enclosure combines with the suspension of the driver to limit cone motion. More specifically, we are adding the stiffness of the air spring in the enclosure to the stiffness of the subwoofer suspension (spider and surround) to make the net system stiffer. Here’s the predicted cone excursion of this driver in the manufacturer-recommend 0.6-cubic-foot enclosure.

This second graph shows that the driver’s excursion is limited to about 11.7 millimeters when driven with 500 watts. Excursion increases to only 16.5 millimeters at the lowest frequencies when fed 1,000 watts. In this enclosure, cone excursion is no longer an issue.

The trade-off for limited cone excursion is a decrease in output capability. The graph below shows the predicted frequency response of our subwoofer system in the infinite baffle simulation and the small sealed enclosure.

Below 47 hertz, the infinite baffle driver becomes more efficient. For example, at 25 Hz, it’s 3.6 dB louder in the infinite baffle.

Subwoofer Cone Excursion and Distortion

More output seems ideal, as long as we are below the Xmax limit, right? Well, yes and no. Every moving coil speaker produces more distortion as cone excursion increases. In addition, variations in suspension compliance (the inverse of stiffness) and magnetic field strength mean that the cone may not track the input signal accurately at high excursion levels. Given the above considerations, we want to limit cone excursion whenever possible. As such, more or larger diameter subwoofers in a system can improve sound quality, as long as each is in a correctly designed enclosure.

Let’s add the bass reflex (also known as ported or vented) enclosure to the mix. A vented enclosure is similar to our sealed enclosure, except it has a tube (or square, triangle or rectangle) with a specific length and area. The vent is a Helmholtz resonator. What’s that? Have you ever blown across the top of a bottle of pop (OK, soda) to hear it hum? That’s a Helmholtz resonator. The resonant frequency is lower if you drink some of the pop and blow again. This is because you’re exciting the air in the chamber, and it resonates at a specific frequency. Helmholtz resonators are used on the intact ducting and exhaust systems of cars to cancel out resonances in the system.

In a vented subwoofer enclosure, the vibration from the subwoofer cone causes the column of air in the vent to resonate. At a specific frequency, called the tuning frequency, the resonance in the vent is maximized. As a result, the vent now acts as the primary sound source for the enclosure, and output from the subwoofer cone itself is minimal. Here’s the cone excursion graph of our audiophile-grade 10-inch subwoofer in a 1-cubic-foot vented enclosure that has a vent tuned to resonate at 33 hertz.

We can see that cone excursion is dramatically increased around the tuning frequency of 33 hertz. It increases slightly at 60 hertz, but in this design, that’s inconsequential. What does matter is that the vent acts like a hole in the enclosure at low frequencies, and cone excursion increased dramatically below 27 hertz. If we want to maximize the output of the system, the use of an electronic infrasonic filter will be necessary at 25 hertz.

What’s the benefit of our vented enclosure, then? Here’s the predicted output graph.

As you can see, we gained an impressive 6.5 dB of output at 40 hertz for the same input power. We’d need to drive the sealed subwoofer with 2,235 watts to produce the same output. For many reasons, including the risk of fire, that won’t work.

Sealed vs. Vented – Enclosure Size

In the case of this example, the sealed enclosure has a net internal air volume of 0.6 cubic foot. Our vented enclosure is 1 cubic foot. Translated into dimensions, the outside dimensions of the sealed enclosure, constructed of ¾-inch MDF, would be (as an example) 12 by 12 by 11.75 inches. The vented enclosure would need to be 12 by 12 by 18.2 inches. That’s an increase in length of more than 50%. If you need a small enclosure to fit in a specific space, sealed might be your only option.

Sealed vs. Vented – Efficiency

Comparing the two enclosures above clarifies that the vented design is significantly louder at all frequencies above 18.5 hertz. So, if you’re looking for the most output from a system with a small amplifier, then a vented enclosure is the best choice. The vented enclosure is the best choice if you’re after the loudest system.

Sealed vs. Vented – Sound Quality

When it comes to outright sound quality, choosing your enclosure is more complicated. We’ll need to start by looking at what happens when we put these enclosures into a vehicle. The graph below shows two traces for each enclosure. The lower trace of each color is the free-field predicted response, and the second trace includes an approximation of the response of the system in a car or SUV. This in-car response information is based on data that Boston Acoustics included with one of their drivers in the BassBox Pro simulation software I use. I’ve seen in-car graphs from other sources that are similar, so this is adequate for our purposes.

As you can see, at low frequencies, based on the provided information, a significant amount of boost is added. It’s on the order of more than 20 dB SPL below 30 hertz. What looked like a smooth, flat response from the vented enclosure now has a prominent peak from 30 to 45 hertz. What looked like somewhat limited output from the sealed enclosure appears reasonably flat.

Here’s the answer to choosing sealed or vented for sound quality. If the system doesn’t have an equalizer to flatten the response, then a sealed enclosure would be better. If the system does have an equalizer, then choose a vented enclosure. Why choose the vented design when there is an EQ? Well, you can flatten the response and dramatically reduce the power required from your amplifier to hit a target response curve. More importantly, cone excursion will be decreased dramatically with the vented enclosure so that less distortion will be added to the sound produced by the subwoofer.

One quick note: For the last statement to be true, the vent in the enclosure needs to be designed and executed correctly. That’s a topic for an entirely different article.

Sealed vs. Vented – Infrasonic Performance

Many people really like deep bass. I’m not talking about 25 or 30 hertz; I mean 10 to 15 hertz bass. The kind that you don’t hear but feel in your back and behind. If that’s your cup of tea, then a sealed enclosure might be the better option for your car audio system.

Sealed vs. Vented – Limited Xmax Subwoofers

If you want to have an enclosure constructed for a subwoofer with limited excursion capability, you might want to consider the vented design. This might be an entry-level subwoofer with a short magnetic field or a shallow-mount subwoofer.

Sealed vs. Vented – Enclosure Construction Cost

This one will be up to the specialty mobile enhancement retailer you’re working with. The cost of constructing a vented enclosure is likely higher than for a sealed design. With that said, the performance benefits may offset this cost. You might want to read our article about choosing subwoofer sizes as a single 12 in a vented enclosure might outperform two 10-inch subs in a sealed enclosure. The net cost should be much less. Talk to the product specialist you’re working with and have them do some simulations with the drivers you have in mind.

Pick Your Priorities, Then Pick Your Enclosure

There you go – a whole slew of reasons why you might pick a sealed enclosure or a vented one for your car audio subwoofer. Depending on your application and expectations, there isn’t a clear winner. Make a list of what you want from a subwoofer, then cross-reference those criteria with the answers above. If you reach a stalemate, prioritize your criteria and repeat the process. Your local specialty mobile enhancement retailer should have no problem delivering a solution that will sound great based on that list.

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.

Ah, the oh-so-complex world of amplifier sensitivity control configuration. One would think that there would be a scientific process that would ensure that an amplifier could be set perfectly every time. But in reality, many criteria affect where a sensitivity control is adjusted. The topic of gain overlap pertains equally to source units as amplifiers. What is it? Why do we need it? Let’s find out.

Amplifier Sensitivity Settings

The purpose of the sensitivity (or gain) setting on an amplifier is to allow it to be matched with a variety of source options. For example, if you have a 100-watts-per-channel stereo amp and a radio that can produce 2 volts of output on the RCAs, the amplifier needs to have more signal gain than if the radio made 5 volts of output. However, the maximum undistorted power output remains at 100 watts no matter where the sensitivity control is configured.

A few things to think about as we dive deeper into this discussion. In 99% of cases, technicians use a 0 dB track at a specific frequency to set sensitivity controls. If the amplifier is powering a subwoofer or feeding a full-range signal to a speaker, this method should do a good job of preventing any clipping of the outputs. However, if the amp is used with a high-pass filter to power a set of midrange speakers, there’s an entirely different procedure to find an optimum setting.

Second, music isn’t always recorded at the loudest possible level. Modern music is close, though. Let’s look at a few tracks to get an idea of this concept.

First is the amplitude-based analysis of “Galway Girl” by Ed Sheeran.

As you can see, the song is recorded at a reasonably high volume and maintains a high average volume. Having a look at the statistics shows us that the maximum recording level is -0.09 dB, very close to the maximum possible level of 0 dB. The average level for the song is -9.65 dB, as shown below.

Let’s look at another track. This time we’ll analyze “Easy on me” by Adele.

Not surprisingly, this song doesn’t appear quite as loud – that is, until the drums come in at 1:27 into the song. You can see just how much her voice is compressed to the maximum level of -0.20 dB. The average level for this track is a little lower at -12.28 dB.

Let’s go back a few decades and see how music was recorded before the “loudness wars” resulted in produces and engineers boosting levels to make voices stand out on the radio. Here’s “Hungry Like the Wolf” by Duran Duran.

This track dramatically represents how the average loudness of modern songs has been boosted. You can see lots of black space below the 0 dB peak.

The peak level for this track remains high at 0 dB on the right channel, but now our average level is way down around -20 dB. In terms of how loud the song seems, this would be 8 to 10 dB lower than something modern.

Last and certainly not least, let’s look at “Brothers in Arms” by Dire Straits. It shouldn’t be news to anyone listening to this album that it has a low recording level. Or does it?

As you can see in the statistics below, the average RMS level of this track is way down around -24 dB. If you want this loud, you’ll need to turn up the volume a little more. Keep in mind, though, the maximum recording level is still high a -0.20 dB.

Introducing Gain Overlap

From a purely scientific standpoint, all of the recordings analyzed above have a very similar maximum recording level. As such, if your audio system is set up to just clip with the volume at full, it should be fine. However, in reality, we might want to be able to turn the volume up a little higher than full, so we can make quiet songs loud. This ability to turn the volume up higher is gain overlap.

Let’s say we want the average level of Duran Duran to be the same as Ed Sheeran; we need about 8 dB more gain in the system. That sounds simple enough, right? Your installer can increase the sensitivity control such that a lower input voltage will drive our 100-watt amplifier to produce full power.

All fine and dandy, right? What happens when our favorite modern song starts to play on the radio, and we crank the volume? Now we have 8 dB extra gain, and the amplifier is driving hard into clipping, adding tons of distortion. The music will sound terrible, and the additional high-frequency content (caused by clipping the outputs) can and likely will overpower the tweeters in the system and damage them.

Let’s take a look at a modern source unit. We have the Sony XAV-9500ES Mobile ES receiver set up on our test bench from its recent Test Drive Review. The built-in amplifier is configured with a typical amount of gain overlap. Playing a 0 dB test tone, the output of the amplifier reaches full power when the volume control is 44 out of 50. Add six more “notches” to make things good and loud. There is 6 dB of gain overlap in this particular radio on the built-in amplifier.

Use Your Power for Good, Not Evil

So, why design or configure an audio system so that you can easily push an amplifier to the point that it distorts? We’ve discussed the technical reason already: To play quiet audio sources at the maximum output level of the amplifier. Does having gain overlap built into a system mean you can potentially damage it? Yes. Absolutely 100%, yes. As such, this means that the system operator needs to take some responsibility for how loudly it’s played. Translated, that means you have to know when you’ve reached full volume in terms of the amplifier’s output capabilities. Your installer should be able to tell you what “full volume” is for normal modern recordings, just like the 44/50 on the Sony radio. Be honest with yourself; if you aren’t going to be able to curb your enthusiasm, ask the technician working on your installation not to include any overlap.

What if you ignore our suggestion and just crank the volume? How hard is your amp going to try to work? For example, a sensitivity setting with 6 dB of overlap would make the amp try to produce 400 watts of power if you maxed out the volume with a track recorded at 0 dB.

How To Avoid Distortion and Play Your Music Loudly

So, what’s needed to design an audio system where the amplifiers can’t distort? The short answer is money. If you want to feed 50 watts of power to your speakers, but have the system configured with 6 dB of gain overlap, then buy a 200-watts-per-channel amp. If you want to provide your subwoofers with 500 watts of power, choose a 2,000-watt amp. Financially, this doesn’t work, does it? A good 500-watt monoblock amp might cost $650. A 2,000-watt amp of the same caliber might cost $1,500-2,000.

Of course, while our wallets might not like the suggestion above, that’s not the only problem. The speakers in your car or truck won’t be capable of handling four times their rated power for very long. For example, if you have a subwoofer rated for 500 watts but feet it 2,000 watts for more than a few seconds, the voice coil is likely to be damaged. Likewise, the suspension components likely aren’t designed to provide the increase in excursion that 2,000 watts of power would command.

If you want your system to play at extreme volume levels, then you’ll want more speakers or subwoofers in the system. If a set of good quality 6.5-inch component speakers and a 10-inch sub aren’t loud enough, consider adding a second set of speakers and a second subwoofer. You can also double the system’s power, so each driver works equally hard. Pushing a low to moderately rated speaker beyond its capabilities will sound bad and likely damage it. Also, overdriven speakers shouldn’t be covered under the manufacturer’s warranty. That’s not a design or component failure; it’s abuse.

When you’re at a local specialty mobile enhancement retailer discussing your audio system, be honest about your expectations. If you can’t afford the system you want, wait until you can. Purchasing less than you’ll be happy with is a good way to damage the speakers or subwoofers.

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 while ago, we talked about why dashcam resolution is crucial to capturing details in the event of an accident or near-miss. The topic of dashcam image sensor resolution appears to have become a defining factor in the sales pitch from many brands. But is resolution the ultimate decision-maker when it comes to quality? Let’s take a look at some considerations.

Do You Need a 4K Dashcam?

Having a dashcam in your vehicle is important if you want to protect yourself from staged accidents, fraud or false accusations. Most basic cameras have image sensors that offer 1920×1080 resolution. That’s enough information to know whether a light was red or a car actually stopped at a stop sign. With solutions like these, you’re looking for general information about a situation.

What if there’s a hit-and-run accident, or you see someone driving dangerously and want to report it? In those cases, capturing identifying information about the vehicle and driver is paramount. Having a higher-resolution camera with a 2K 2560×1440-pixel or preferably a 4K 3840×2160-pixel image sensor dramatically increases your chance of being able to read a license plate and provide that information to the authorities.

Does a 4K Dashcam Guarantee You’ll Capture a License Plate?

We’ve seen several retailers and dashcam brands promoting 4K solutions as a guarantee that you’ll be able to see a license plate. Be wary of that type of statement. If a car passes perpendicular to you, the plate will likely only be visible for a few frames of video. The same applies if it’s coming toward you. Once the vehicle is close enough for the camera to make out the letters and numbers, it will pass by very quickly. Having a high-resolution camera increases your odds of seeing a tag, but nothing guarantees you’ll capture it.

What Other Features Affect Dashcam Image Quality?

If you’re shopping for the best camera solution, there are other considerations beyond camera resolution. For example, if you’re driving at night, then a camera with an image sensor that’s optimized for low-light levels will dramatically reduce noise in the image and help make objects clearer.

High Dynamic Range (HDR) processing is another feature that helps to brighten dark image areas and tone down parts that might be overexposed so that you can see details. In reality, this is contrast compression, but it makes a big difference when bright light sources like car headlights or the sun are in the image.

Camera Speed Matters!

If you want to increase your chances of capturing details, one essential feature to look for is a higher frame rate. If you look at how video is captured, it’s essentially a series of still images. When we play the pictures back in quick succession, we see the perception of moving objects. Most dash cameras record at a rate of 25 or 30 frames per second. This means there are 25 or 30 still images recorded for every second of the video. This framerate is very similar to what we see on broadcast television (29.97 frames per second).

If you want to capture those split-second moments when a license plate might be visible, look for a camera that offers 60 fps recording. These cameras take 60 images for every second of video they record, doubling your chances of seeing something crucial. Yes, it will use the storage on the memory card faster. However, storage space isn’t a concern unless you need to go back a check something from a few hours earlier. Plus, large-capacity micro-SD cards aren’t overly expensive.

Image Compression and Video Codecs

Another factor that affects image quality is the video compression settings. Most cameras store videos in MPEG format to make the files compatible with popular computers and smartphones. There are several different settings that the camera manufacturer can configure to determine how much information is stored and how much is discarded to reduce file size. Compared with uncompressed video files, the MPEG format can reduce file sizes by six to 26 times. Some cameras offer options to make global adjustments to compression settings. Again, we suggest using as little as possible to help capture details.

Another advancement in video compression came with the move from the H.264 compression standard to the newer, more efficient H.265 format. The H.265 format offers more video file compression while claiming the same image detail. People will argue the efficacy and fine details with any lossy compression format. Nevertheless, H.265 is worth looking for in your next camera purchase, though its ultimate performance depends on how the manufacturer configures the system.

Wi-Fi Connectivity

While it isn’t a feature specifically related to the image-capturing capabilities of the camera, Wi-Fi connectivity is also an important option. If your dashcam doesn’t have Wi-Fi, then you’ll need to remove the memory card and connect to a computer to view what it’s recorded. If you see something happen and want to share it with the police in short order, being able to connect your phone and download a file in a minute will let you show a video or send a screenshot right away.

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Pick the Right Dash Camera for Your Needs

Having any dashcam is better than none at all. With that said, the frustration of looking at a blurry or pixelated image when trying to extract information is frustrating. We’ve been there. Drop by your local specialty mobile enhancement retailer today to find out about the dashcams they have available with the options and performance features that will meet your expectations.

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 too long ago, we took a look at how speaker-level line output converters worked. We discussed the difference between passive and active devices and explained what features to look for. If you have a vehicle with a premium factory-installed sound system, those interfaces might not be adequate to provide your amplifier with a signal that will sound good with new speakers. So let’s look at some of the intelligent car audio speaker-level interfaces available and why they’re the best choice to upgrade your stereo.

Factory-Installed Audio Systems are Complex

While modern factory-installed sound systems might not use the best speakers and are often limited in power delivery, the signal processing included in them can be pretty complex. Companies like Bose, Panasonic and Harman (the brand behind JBL, Infinity, Lexicon, Mark Levinson, Bang & Olufsen and Harman Kardon) know a LOT about optimizing their products to sound the best they can. So while an aftermarket system might sound better with larger subwoofers, better speakers and more powerful amplifiers, these companies and their technicians are masters when it comes to system configuration and calibration.

A prime example of the capabilities of these companies is the Mark Levinson system in the Lexus LS. There are 23 speakers, including four in the ceiling. The system has a state-of-the-art upmixer that takes a stereo audio track and creates left, center, right, left side, right side, left rear, right rear, front overhead and rear overhead channels. The signals going to each channel are unique, so the system reproduces audio as though you were in a live venue with the stage in front of the listening position. In addition, the system has 2,400 watts of power, so it gets good and loud.

Upgrading these audio systems is complicated. It requires careful planning and accurate measurement of the audio signals that will be used for the upgrades and testing of signal routing for different frequencies and audio sources. This isn’t something you’d want to undertake in your driveway.

Intelligent Line Output Converters

In a more conventional factory-installed audio system, upgrading is less involved but still complicated. Crossovers, equalization, all-pass filters and signal delays are expected on systems with door and dash speakers in the vehicle’s front. Thankfully, a few companies have created line-output converters with built-in digital signal processors that can undo these features and provide a reasonably wide bandwidth signal that’s phase-coherent – like you’d get from an aftermarket source unit.

When the technician upgrading your audio system completes the wiring, these processors require a calibration step. Most include an audio track to be played through the system while the software analyzes the audio signals being produced. Once the measurement step is done, they analyze the information and configure the digital signal processor to undo equalization and signal delays, and combine signals from multiple channels. Once complete, your installer can move on to configuring your new system.

What Happens if You Don’t Undo Signal Processing?

Some upgrade methods work from what the OEM audio system offers. In those instances, you’ll need a digital signal processor with dedicated inputs for each channel from the factory amp. In addition, the system will need to use similar drivers and stock locations. For example, suppose you want your installer to build new A-pillar pods or use speakers that operate in the frequency range where a crossover existed in the factory stereo. In that case, intelligent line-output converters (or a DSP with these features) are necessary.

Let’s look at a simple example of combining two audio signals where one has been delayed from the other by a few milliseconds. This would be akin to using an active line-output converter to combine door and dash speaker signals.

The graph above shows the frequency response of our original signals in red. I applied a 0.6-millisecond delay to one signal, then combined the two to produce the orange trace. As you can see, the frequency response is a disaster. That curve is what’s known as a comb filter, as it looks sort of like a comb you’d use to straighten your hair before work or school. Signal delays are just one of the reasons why it’s crucial for the installer working on your vehicle to understand what’s happening in the time domain and the frequency domain for each channel of your sound system.

Choose Your Upgrades Wisely

If you’ve read this and think, “I have no clue what they are talking about,” that’s 100% OK. This is where you need to choose a professional mobile enhancement retailer to help you upgrade your car audio system. You’ll want to ask them what test equipment they have to measure the frequency and phase response of the factory-installed audio system before you let them work on 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.

It seems like everything to do with car audio installations has something to do with managing voltages. For starters, your electrical system needs to produce enough voltage to keep your radio and amplifier going. And amplifiers need to increase the voltage to drive speakers. When adding an amplifier to a factory-installed audio system, your installer will need to measure the voltage that the radio or amp produces. Chances are, they’ll need to use a line output converter to reduce that voltage so it’s compatible with a new amplifier. Let’s look at how these converters work and some of the options they include to make upgrading your car audio system easier.

What Is a Line Output Converter?

These simple integration devices go by several names. They’re sometimes called high-to-low or hi-lo converters, speaker input adapters or line level converters. Their task, however, is relatively simple. They take an audio signal intended to drive a speaker and lower the voltage so that it can be connected to the RCA preamp input on an amplifier or signal processor.

Most amplifiers want to see a maximum input voltage of 4 to 6 volts. Beyond the rated maximum input voltage, the signal can overdrive the input circuitry and cause clipping and distortion. Yes, you can clip the input to an amplifier with too much voltage.

Even a modest car radio can produce about 6.5 volts (peak to peak) output on the speaker wires. A small amplifier rated at 45 watts can deliver 13.4 volts. A subwoofer amplifier integrated into a factory-installed audio system could easily produce more than 30 volts.

How Do Line Level Converters Work?

There are two common types of converters on the market. The least expensive incorporates small audio transformers to reduce the voltage. The input winding on the transformer might have two or three times as many turns as the output, lowering the voltage by 50 or 60%. These devices are often passive in that they don’t require a power and ground connection to function.

The second and most popular converter adds circuitry to provide a low-impedance output to the new amplifier. These devices require a power, accessory and ground connection to function. They can also serve as a line driver to increase the output voltage relative to the input. If you have a modest source unit that can only provide 1.5 or 2 volts of output on the preamp, adding a line driver to bump that voltage to 4 or 5 volts will let your installer turn down the sensitivity control on your amplifiers to improve the signal-to-noise ratio of your audio system.

What To Look for When Shopping for a Line Output Converter

If you’re in the market for a quality line output converter, you’ll want to know how much voltage it can accept on the speaker-level inputs and how much it can increase or decrease that signal, and you’ll need to know the output impedance on the preamp side. Most good-quality converters can accept up to 40 volts on the inputs and have an output impedance of no more than 200 ohms, though lower is better.

You’ll also want to check the frequency response of the device. Entry-level transformer-based converters may not pass deep bass or high-frequency audio information as well as the active units. Accordingly, a frequency response spec of at least 10 Hz to 40 kHz with a tolerance of 1 dB is a good benchmark.

Since these are audio signal processors, noise and distortion specifications are also worth checking. A total harmonic distortion (THD) spec of no more than 0.05% is good and noise should be quieter than 110 dB.

Remote Turn-On Detection Features

One of the most common features of a line output converter is providing an amplifier turn-on output signal. Let’s say you’re having a subwoofer amplifier added to a factory-installed sound system. There likely won’t be an easily accessible wire that goes to 12V when the radio turns on. Many output converters have several ways to detect when the radio is on and produce this trigger. First, they can monitor the speaker wires for voltage. Once it detects an audio signal, it turns itself on and generates the remote output. The drawback of this option is that the unit might be fooled into turning on when a car door is closed. If the vehicle is relatively airtight, closing a door or the trunk can momentarily pressurize the interior, causing the speakers to move. When that happens, they produce a voltage and sometimes this tricks the converter.

The second way these devices can trigger an output is to monitor the input connects for a DC voltage on the speaker wires. For example, most radios use a speaker output device configuration called BTL, or bridge-tied load. There will be a few volts on the speaker wires when the radio turns on. The converter will sense this voltage and activate the output. If the source in your vehicle works this way, this is the best option for your installer to use.

Bonus Line Output Converter Features

Many line output converters come with additional features. One of the most common is a remote level control. If you’re having a subwoofer added and want to adjust its volume relative to the rest of the system, this is a great option.

Another popular feature is an equalizer. Your installer may find that the lowest audio frequencies from the factory source are attenuated. Adding a little boost to that missing information is a great way to deliver bass with good extension and impact.

Many of the better processors include speaker load simulators. Most Class-D amplifiers used in factory audio systems need to see a speaker connected to their outputs to function. As such, if your installer is adding an amplifier to drive those speakers, a relatively low-impedance load needs to be added to the speaker wires.

Channels and Signal Summing

The last topic we should discuss is understanding how many channels are needed for the line output converter. As we mentioned, typically, these are used when adding a subwoofer amplifier to a factory-installed source unit. That doesn’t mean that they aren’t also common for adding an amp to drive front and rear speakers. Many branded audio systems (like Bose, JBL, Fender, Infinity, Lexicon, Mark Levinson and B&O) that come with new cars and trucks are easily upgraded using multi-channel line output converter interfaces. Your installer can feed the output of the converter to a digital signal processor and new amplifiers, speakers and subwoofers.

Many multi-channel line output converters can sum signals together from multiple inputs. These days, using this feature is a risky proposition unless your installer has confirmed that the audio signals are in phase at the crossover frequency. For example, let’s say the front speakers in your car include a woofer in the door and a small midrange speaker in the dash (what many call a middler). If there is signal delay applied to the woofer, summing the signals together in a line output converter can result in the audio signal having an unusable frequency response. The summing circuits on these devices work perfectly, but the signals coming from a factory amplifier may not be compatible. So everything has to be tested. We’ll talk about signal summing processors in another article soon.

If you plan to have an amplifier added to a factory-installed audio system, chances are you’ll need a line output converter. It’s even more likely that you’ll need one that can provide a remote turn-on signal for that new amp. Drop by your local specialty mobile enhancement retailer to find out about the solutions that are compatible with your vehicle to deliver great sound.

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