There’s a common belief that an audio system with more subwoofers will produce more bass. This statement can be 100% true or completely false. Why might it be false? A subwoofer’s output depends heavily on enclosure design. Let’s look at two examples where the output of one subwoofer is more than two.

The Most Bass for Your Dollar

If you spend any time searching the countless car audio groups on Facebook, you’ll see dozens of photos of under-seat subwoofer enclosures for pickup trucks. Many of these enclosures have three or four subwoofers in them. If they are 8-inch subwoofers, this might work well. If they are 10-inch subwoofers, they’re likely somewhat cramped for space.

When we’re talking about subwoofer systems, the size of the enclosure relative to the parameters of the subwoofer itself determines performance. You could have a shop build a cube that measures 12 by 12 by 12 inches and mount a 10-inch subwoofer on all six sides. It would look cool, but it would likely sound terrible!

How much bass a subwoofer produces depends on how far the cone moves forward or rearward for a given amount of power. Professionals use enclosure simulation software like BassBox Pro or Term-Pro to model how a subwoofer will behave in different enclosure designs. These software packages can simulate acoustic suspension (sealed), bass reflex (vented) and various bandpass enclosure designs.

Someone with experience needs to analyze and interpret the information provided by the software simulations to determine whether the design is suitable and safe for the subwoofer with the chosen amplifier. These software packages, on their own, don’t calculate the perfect enclosure for any application. They’re like a spreadsheet: They work with the electromechanical parameters of the subwoofer and the provided enclosure information.

Let’s talk about acoustic suspension enclosures, which are the simplest to understand and predict. When a subwoofer is installed in an acoustic suspension enclosure, the compliance of the air in the enclosure combines with the compliance of the driver’s suspension to form a spring system. Compliance is the reciprocal of stiffness. Or, put another way, a rubber band is more compliant than a pencil. A large amount of air is very compliant, and a small amount of air isn’t when we’re talking about compressing it. More specific to subwoofer enclosure simulations, it’s easier to compress the air in a large enclosure than in a small one.

When a subwoofer is installed in a very small enclosure, the resulting system is not very compliant. It will take significant power to move the subwoofer cone at low frequencies. Why does the enclosure size have a more significant effect at low frequencies? For each decrease of one octave, a subwoofer cone has to move twice as far to produce the same output. For example, if a subwoofer moves back and forth 2 millimeters to produce a specific output at 60 Hz, it has to move 4 millimeters to produce that same output at 30 Hz. If the speaker is in an enclosure that limits how easily the cone moves, it will produce less output for a given power input.

Since we aren’t installing subwoofers for midbass, installing any subwoofer or woofer in a small enclosure means limiting how much bass the system produces. This low-frequency limiting is one of the reasons we use enclosures. Without an enclosure, the subwoofer would bottom out when driven with moderate power.

The graph below shows the predicted frequency response of the ARC Audio X2 10D4v2 subwoofer we reviewed recently in three different enclosures. The red trace represents a sealed enclosure with a net internal air volume of 0.663 cubic foot. The yellow trace shows the predicted response of the subwoofer in an enclosure with only 0.45 cubic foot of space. Finally, the green trace is the response with the subwoofer in an enclosure with 1.0 cubic foot of space.

As you can see, the ARC Audio subwoofer produces more bass from a larger enclosure for a given amount of power. This is true of all subwoofers. When driven with 200 watts of power, the 1.0-cubic-foot enclosure would produce 98.9 dB SPL output (in a free-field measurement) at 30 hertz. The 0.663-cubic-foot enclosure produces 97.6 dB of output at the same frequency. Finally, the 0.45-cubic-foot enclosure produces 95.2 dB of output at 30 Hz.

Let’s look at this data from another perspective. Consider how much more power it would take for the smaller enclosures to play as loudly as the larger designs. We will reference 200 watts of power into the 1.0-cubic-foot enclosure. The 0.663-cubic-foot enclosure would need 272 watts of power at 30 hertz to produce the same output. The 0.45-cubic-foot enclosure needs a whopping 469 watts to match the 30-hertz output of the large enclosure. Think about how much hotter the sub would get and how much harder the amplifier and vehicle alternator would have to work to produce the same output.

What if we look at this from the opposite perspective? If we provide the ARC Audio subwoofer with 200 watts of power in the small 0.45-cubic-foot enclosure and it produces 95.2 dB of output, how much less energy would be needed to match that output from the larger enclosures? The answer is that the 0.663-cubic-foot enclosure is just as loud with only 148.4 watts of power, and the 1.0-cubic-foot enclosure would only need 85.5 watts to produce 95.2 dB of output. As you can see, cramming a subwoofer into a small enclosure is counterproductive in terms of efficiency.

Are More Subwoofers Always Louder?

Now let’s talk about multiple subwoofers and whether or not they are always louder. Most car audio enthusiasts think adding a second subwoofer increases the output of a system by 6 dB SPL. This statement is true under a specific set of conditions. Let’s say we have a single subwoofer in a 0.663-cubic-foot enclosure, and a 200-watt amplifier powers it. If we want to use two subwoofers, each driver needs 0.663 cubic foot of airspace. We also need an amplifier that can provide a total of 400 watts. If we meet these conditions, the system’s maximum output will increase by 6 dB SPL. If we have double the airspace but only 200 watts to share between the drivers, the output increases by 3 dB SPL.

The graph below shows a single X2 subwoofer in 0.664 cubic foot of space in red and a pair of those subwoofers in 1.326 cubic feet in teal. The total power is 200 watts for each simulation.

What happens if we ask our installer to cram both subwoofers into a 0.664-cubic-foot enclosure?

The graph above shows that the subwoofer system produces less bass with two drivers sharing the 0.663-cubic-foot enclosure and 200 watts (total) than with a single driver (in red). Proper subwoofer enclosure design is crucial to maximizing car audio system efficiency. If we doubled the power when adding the second sub, it would be louder, but maybe only by 2 to 2.5 dB.

Ported Subwoofer Enclosure Solutions Add Efficiency

What if you want the most bass output for our investment? What enclosure should you use? The answer depends on how much space you have in the vehicle. Let’s say we have room for two subwoofers in an acoustic suspension enclosure with a net volume of 1.324 cubic feet. This is a large enough enclosure to ensure that the drivers play loudly at low frequencies, right? Sure, but is this the most efficient use of our money? Guess what? No, it isn’t.

If you have the shop you’re working with design and construct a vented enclosure using the 1.324 cubic feet of space and a single subwoofer, the system will produce significantly more bass. Two drivers in an acoustic suspension enclosure with a volume of 1.324 cubic feet, sharing 200 watts, will produce 102.9 dB SPL at 35 hertz. A single driver in a 1.324-cubic-foot bass reflex enclosure would deliver a mind-blowing 107.8 dB of output at the same frequency. That’s 4.9 dB more output. Your sealed enclosure would need 618 watts of power to reach the same output level. Chances are, the subwoofers wouldn’t appreciate receiving that much power.

Does Adding More Subwoofers Make My Car Audio System Play Louder?

So, let’s answer the question, “Does adding more subwoofers make my car audio system play louder?” The answer is yes if your enclosure design has double the air volume every time you double the number of subwoofers. Your system will play 6 dB SPL louder every time you double the number of drivers in this scenario.

Unless the enclosure was grossly oversized, adding more subwoofers to a given volume is unlikely to increase low-frequency output. This is why it’s crucial for the shop you’re dealing with to model the enclosure options so that you get the most bass for your investment. In most cases, especially for an under-seat truck enclosure, a single driver in a bass reflex (vented) enclosure produces significantly more low-frequency energy than two, three or even four drivers in an acoustic suspension design. Drop by a local specialty mobile enhancement retailer today and talk with them about your goals for your subwoofer system upgrade. If they know how to optimize enclosure designs with simulation software, the chances are that you’ll get the best bang for your buck, bass-wise!

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 back, we looked at how adding stuffing to a sealed enclosure affects its performance. It was clear from our measurements that the addition of Dacron lowered the system’s Qtc (Total Q). The original theory was that adding stuffing made the enclosure seem larger and let the driver play louder at low frequencies. Let’s revisit this test and add several acoustic measurements to quantify any changes in efficiency and output.

Results from Our Original Testing

Our original article determined that adding different amounts of stuffing to our 1.358-cubic-foot enclosure lowered the system Qtc. Without stuffing, the Qtc with our audiophile-grade 12-inch subwoofer was 0.9532. That’s a bit high for our liking but offers good efficiency. With 0.25 pound of Dacron added to the enclosure, the Qtc dropped to 0.9148. That’s still on the high side but getting better.

Moving up to a half-pound of stuffing had minimal effect on the driver, and the system stayed roughly the same at 0.919. Cramming another quarter-pound of stuffing into the enclosure made a truly beneficial change. The Qtc was now down at 0.8397. The lower Qtc measurement is better as it results in less resonance and a tighter, more controlled bass perception. In this capacity, stuffing with Dacron does have the same effect as installing the subwoofer in a larger enclosure.

The driver’s resonant frequency in the enclosure barely changed throughout the test. Empty, the system had an F3 of 43.35 hertz. With all the stuffing in place (0.75 pound), the resonant frequency dropped to 41.68 hertz. The difference would be negligible and doesn’t support the claims of stuffed enclosures playing lower.

Round Two of Subwoofer Enclosure Testing

In this test, we’ll use the same enclosure and subwoofer and take several acoustic measurements under strictly controlled conditions. We’ve set the enclosure up in the middle of our lab and placed the Clio Pocket calibrated mic on the floor 50 centimeters in front of the enclosure. This configuration is similar to a typical ground-plane measurement, except the closer proximity to the enclosure will help to reduce the effect the room has on the measurements. A “normal” ground-plane measurement would have the microphone 2 meters from the enclosure. We will continue buying lottery tickets in hopes of financing our own anechoic chamber, but that might take a while!

All measurements are at the same output level. We’ll use 4 volts representing 2 watts of power into the subwoofer’s nominal 4-ohm load. With a drive level any lower than this, the background noise from the HVAC system starts to mess with the very low frequency measurements. Again – anechoic chamber, please!

Sealed Enclosure Stuffing Findings

If you look at the graph below, you’ll see the SPL measurements from the four test conditions. The red trace is the enclosure without any stuffing. The violet trace represents 0.25 pound of stuffing. The black trace represents a half-pound of filling. Finally, the amber trace is 0.75 pound.

As expected, the more stuffing there is, the smaller the bump at the top of the response curve. Why does this happen? Because polyester fiberfill reduces the resonance of the system. With less resonance, the driver returns to rest faster after the signal stops, and less distortion is added to the output.

You’ll notice the difference between no stuffing and the tightly packed enclosure is relatively tiny. Indeed, the maximum difference is a total of 1.4 dB SPL, with the unstuffed enclosure being louder.

I generated a second graph referencing the first three measurements to the fully stuffed measurement. This analysis shows you how much louder the subwoofer is as there is less and less stuffing. While it might be noticeable, the difference is minute.

Sealed Enclosure Stuffing Summary

Unlike what many “old timers” will tell you, adding a large amount of Dacron (or similar) stuffing doesn’t significantly affect output, especially at lower frequencies. It certainly doesn’t cause the same improvement of the low-frequency production that you’d get from a larger enclosure. One consideration, though: If the crossover point for our subwoofer systems is, or should be, around 80 Hz, then a system with a flatter response will seem to be a bit louder at lower frequencies. With that said, we are talking about less than 1.5 dB SPL, so the whole thing regarding output amplitude is effectively irrelevant.

So, is it worth asking the shop building your sealed subwoofer enclosure to add stuffing? Don’t add anything if you’re a bass head and want the system to play as loudly as possible. If you’re into sound quality and want to reduce distortion around the resonant frequency of the subwoofer system a bit, then go for it. It’s not like the cost of some stuffing is significant.

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 should go without saying that a car stereo system designed around multiple amplifier channels and a DSP is the easiest way to get great sound in your vehicle. The technician working on your vehicle has complete control over the output level, sound arrival time and the frequency response of each speaker in the system. The result should be a sound system that is as optimized as possible – assuming the calibration process is executed properly. There are still many misunderstandings about what a DSP can and can’t do. This article will provide a few things to listen for to help you determine whether your DSP has been adjusted properly.

What Is a DSP?

What is a digital signal processor? Though we have covered the topic extensively, we’ll offer a quick “too long; didn’t read” for those new to our magazine. A DSP is a computer chip optimized to perform many calculations quickly and repeatedly on a sequence of digital samples. These samples are typically a digital representation of an analog signal. In the case of our car audio systems, the analog signals are the left and right channels of the audio recording. DSPs are common in other applications, including video processing and radio frequency transmission analysis. Modern recording studios and live venues use DSP-based mixing consoles.

How Is a DSP Used in Car Audio Systems?

In car audio applications, a DSP serves many purposes. A DSP can combine signals from multiple sources, adjust levels, apply equalization and frequency filtering, and add delay to a signal. In short, it helps the technician connect to a factory-installed stereo, optimize the signal to each speaker and compensate for your vehicle’s acoustics.

If your car stereo uses an aftermarket head unit, your installer will likely connect a DSP directly to the radio’s preamp outputs using RCA cables. Suppose you have a vehicle that uses a digital interface between a factory-installed head unit and an amplifier like A2B, MOST, AVB or a SP/DIF connection. In that case, you may have an interface that feeds a digital signal to your DSP over a fiber-optic cable. Finally, many audio system upgrades require recombining signals from multiple factory-installed amplifier channels and removing any processing to create a full-bandwidth signal. A high-quality DSP can help with this.

After the audio signal is in the DSP, the first order of business is to route that signal to the appropriate output channels. You don’t want the left-channel signal from the radio going to the right-side tweeter. Also, you’ll likely want the signals from both the left and right channels going to the subwoofer signal. All reputable DSPs have a signal-routing mixer to perform these tasks.

Next, the technician configuring the system must filter the signals to each speaker. You don’t want bass information going to a tweeter or midrange information to a subwoofer. A properly trained technician knows which high- and low-pass crossovers to apply to the speakers in your vehicle based on their design, directivity characteristics and the speakers that are operating in adjacent frequency ranges.

The last step is for the technician to use a calibrated microphone system and measure each speaker’s acoustic output at the listening position. Using that information, they can adjust the equalizer to smooth out any peaks or dips caused by reflections in the vehicle. Finally, the output level of each speaker is adjusted to ensure that the transition from one driver to another is smooth.

Auditioning Overall Tonal Balance

While a DSP often seems like some mystical black box of audio voodoo, they are quite simple devices. Yet they do have an extensive list of functions and require training and a thorough understanding of the laws of physics to implement properly. A DSP is effectively mandatory if your goal is realism and accuracy from a car audio upgrade. That brings us to the question, how would a consumer know whether the DSP in their car or truck is adjusted properly?

The first thing to listen for is a smooth frequency response. There shouldn’t be emphasis or deficiencies in any frequency range. For example, if the letters S and T seem overly prominent, the equalizer bands around 3 and 5 kHz might need adjustment. If voices are boomy or chesty, there might be too much output around 200 Hz. The bottom line is that if every genre of music doesn’t sound right, then the DSP needs more adjustment.

An idea offered by long-time car audio competitor Harry Kimura is to listen to a well-engineered piano recording. The lowest note on a piano has a fundamental frequency of 27.5 hertz. That’s well below what an audio system without a subwoofer can reproduce with any authority. The highest note has a fundamental frequency of 4.186 kHz. It’s crucial to remember that each note includes several octaves’ worth of harmonics to give the instrument its “sound.” There’s still important audio information beyond 12 kHz from this 4.186 kHz note. If someone plays a scale from the highest to the lowest notes, each should be reproduced by your car audio system with the same volume or intensity. If something is too loud or quiet, the DSP’s equalizer needs adjustment.

What about the Bass?

We can confidently tell you that a car audio system that plays the bottom two octaves of a piano at the same level as middle C won’t be much fun on the road. It might be super-accurate, but the bass will be drowned out when competing with wind, road and exhaust noise. The subwoofer in your car audio system should be 8 to 12 dB louder than the midrange for the system to be enjoyable while in motion. If you’re a basshead, fill your boots!

The Source of Sound

The second criterion to listen for is staging and imaging. Imaging refers to the ability of an audio system to render the sound of specific instruments accurately on a virtual soundstage. Think of yourself listening to a live acoustic music performance. A four-piece jazz band with a drummer, pianist, upright bass player and lead singer would be a perfect example. No matter where you sit in the audience, the sound source from their instruments is easy to detect. You’d know if the lead singer walked across the stage while performing, even if you didn’t see them.

In your car audio system, you should be able to pick out the specific instruments in a well-recorded track. If the singer and drummer were in the center of the stage, they should sound like they are in the center of the dash or windshield. If the bassist is on the left, then the sound should come from in front of the steering wheel. If the piano was on the right side of the stage, it should sound like it’s coming from the airbag on the right side of the dash.

Here’s a good example of what you should hear if the system has a solid soundstage and good imaging. “Listen to Money for Nothing” by Dire Straits. At 1:12 into the track, Pick Withers’ drums pan from the far right to the center. Each drum appears to have a dedicated microphone, and their signals are panned to fill the soundstage. As he hits different drums, the sound source should move. The system isn’t configured correctly if the drums are a big blurry mess.

How Do You Want Your System to Sound?

The above description assumes you wanted your car audio system calibrated as though you were in the audience. The other option is to have the system configured as though you were on stage with the performance. In this scenario, the sound in your car would be more like what you’d hear when wearing headphones. The vocals and drums might be in the middle of your head. The bassist would be to your left, and the piano to the right. Some call this a “club” sound, where music comes from around you. The product specialist you’re working with to design your mobile audio system should ask you about your listening preferences during the client qualification process.

A car audio system with high-quality amplifiers and an excellent DSP will offer better focus for each instrument. We call this better imaging. We’ve heard many factory-installed audio systems where the center-stage vocals came from a space the size of a large pizza. The best aftermarket systems we’ve listened to reproduced that same track from a point in space the size of a tennis ball. It’s not just equipment that achieves this goal. The technician adjusting the DSP needs to know what to look for and what to ignore in the acoustic measurements to get this right.

What Can’t a DSP Fix?

There are some product and installation issues that a DSP can’t resolve. If you’ve chosen speakers with resonance and distortion issues, the DSP can’t remove that harmonic information added to your music. If you find the high-frequency response harsh or fatiguing, you are probably hearing harmonic distortion from your speakers. The only solution is to switch to better-designed speakers that include distortion-reducing designs and technologies.

The same goes for sloppy midbass. If a rim hit on a tom drum sounds like a thud or bump rather than a sharp crack, you may have a speaker or amplifier distortion problem. Audio components (primarily amplifiers and speakers) with high levels of intermodulation distortion typically cause muddiness or unwanted warmth in the lower midrange frequency range. No amount of DSP adjustment can fix this.

Learning About High-Quality Audio Systems and Accurate DSP Adjustment

This article kicks off a series on how to listen to audio systems and components from a technical perspective. We’ve noticed that many consumers think certain products “sound really good” when they are, at best, mediocre. We hope that educating everyone about what to listen for when auditioning speakers and listening to demo vehicles will help people purchase solutions that offer the best performance possible for their investment. As you learn what high-quality car audio upgrades sound like, use that information when working with a local specialty mobile enhancement retailer to pick the best upgrades possible.
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.