When it comes to subwoofers or speakers, they need to be installed in a correctly sized enclosure to deliver accurate performance and useful bass extension. Speakers are complex devices that rely on their suspension components and the enclosure they are installed in to determine how they perform at lower frequencies. When the product specialist you’re working with is designing your sound system, he or she needs to balance many criteria to ensure you get the sound you want. In this article, we’ll look at the importance of installing subwoofers in enclosures with enough internal air volume.

How Enclosure Volume Affects Speaker Performance

Every speaker of every size is affected by the volume of air in the enclosure behind the driver. Even large tweeters are often designed with cavities behind the diaphragm that can change their performance down near the crossover point. For this article, we’ll focus on midbass subwoofers and look at several real-world examples and application comparisons.

Lately, we’ve seen several instances where multiple subwoofers have been crammed into incredibly small enclosures. The most common example of this is in pickup trucks. The space under the rear seat of your truck is limited. In most cases, there is less than 1.6 cubic feet of air space for the woofers to work with. Physically, you can easily install a pair of woofers, and some custom solutions allow for as many as four drivers under the rear seat. Let’s look at how a popular shallow-mount subwoofer like the JL Audio 10TW3-4 behaves in this space.

Let’s start with a single subwoofer in 1.5 cubic feet of space. Our enclosure simulation software predicts a -3 dB frequency of 45.95 Hz and a total system Q of 0.483. This low Q tells us that it is likely too large of an enclosure for this driver. Thanks to the impressive excursion capabilities of the TW3 subwoofers, things will work fine.

Now, let’s say we want more bass. The seemingly logical move would be to add another subwoofer, right? If we ask our technician to install two drivers in this 1.5-cubic-foot enclosure, the -3 dB point raises to 47 Hz, and the Q increases a bit to 0.643. Based on the shape of the graph and those numbers, this combination looks like a great solution.

Thankfully, JL Audio knows how to design a subwoofer to work in a small enclosure. The response of these systems is smooth. Now, let’s double down again and see what happens when we use four woofers in this 1.5-cubic-foot enclosure. Our software says the -3 dB point is now 50 Hz, and the Q is 0.738. Both are great numbers.

With that said, we need to look carefully at where the increase in output happens. The single-driver solution predicts an output of 106.4 dB, the dual driver system predicts 109.4 dB, and the quad-sub solution says 112.4 dB at 60 Hz. As science indicates, that’s a 3 dB increase for every doubling of cone area for a given power rating. But does that mean more usable bass? Let’s look at what happens down at 35 Hz. The single driver enclosure is predicted to produce 102 dB of output. The dual-driver enclosure should produce 104.4 dB. Finally, the quad-driver enclosure should offer 105.9 dB at 35 Hz. We saw a gain of 2.2 dB from the single to dual increase and only 1.5 dB from the dual to the quad system.

Why? Because the volume of air in the enclosure limits cone excursion at lower frequencies. More specifically, the ratio of the available airspace to the driver’s equivalent compliance (this specification is called the Vas) decreases as the effective enclosure volume decreases. In this example, we get more upper bass around 60 Hz with four subwoofers, but beyond two, we don’t get much more deep bass.

Extreme Cases Result in No Bass

As we mentioned, JL Audio did a great job designing its TW-Series subwoofers to work in very small enclosures. Not every subwoofer on the market is designed this way. Balancing the design of a subwoofer trades efficiency for low-frequency extension. Let’s look at an example that doesn’t work. In this case, we have a very shallow 8-inch driver that’s been crammed into a tiny enclosure in the door of a car. The enclosure has a net volume of 0.15 cubic foot. The enclosure fits, but does it work?

As you can see, this is a disaster. There’s a huge peak at 450 Hz and barely any output below 100 Hz. Given that most subwoofer systems operate below 60 Hz, this doesn’t work. Oh, the system Q is 1.79. Target Q-factors are typically in the 0.6 to 0.95 range. This would sound unpleasant in every way.

Does this make this a bad subwoofer? We modeled the driver in a 1-cubic-foot enclosure and it still didn’t make much bass. It’s clearly designed for an infinite baffle application.

A better solution for this ultra-small enclosure would have been a high-quality 6-inch subwoofer. The graph below shows the aforementioned 8-inch sub and a 6.5-inch sub in the same 0.15-cubic-foot enclosure. The 6.5-inch sub is 12 dB louder at 40 Hz.

What About Vented Enclosures?

Let’s circle back to the beginning and look at how a vented enclosure might change our subwoofer system performance. We’ll remind you that we want the subwoofer to be able to produce lots of energy between 30 and 60 Hz for deep bass.

The graph below shows our 1.5-cubic-foot truck enclosure’s performance with subwoofers in sealed (acoustic suspension) and vented (bass reflex) designs.

As you can see, between 20 and 50 Hz, the single subwoofer in a vented enclosure is louder than four subs in a sealed design. At 25 Hz, the vented design can produce 106.9 dB, whereas the four drivers are only making 100.7 dB. That’s a HUGE difference! The vented enclosure would cost less, sound better and require less power from your amplifier to recreate the lowest notes in your music. Your installer will also have an easier time blending the system’s output with the midbass speakers in your truck because there’s less of a peak in the response at the crossover point. It’s a complete win-win!

Picking the Right Subwoofer Solution

When it comes time to shop for a subwoofer upgrade for your car or truck, be sure to ask the specialty mobile enhancement retailer you are working with about your options in terms of the enclosure. It’s pretty common to see a single subwoofer in a vented design deliver significantly more output than multiple drivers in a sealed enclosure.

Lead-In Image: Thanks to Jim Skaggs at the team at MTI Acoustics for the lead-in photograph.

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.

Have you ever purchased something, then regretted your decision a few days later? We recently ran into a situation regarding car audio subwoofer amplifiers where the answer to “Which is better?” became a complicated discussion.

We’re going to look at two amplifiers – one is a consumer-grade product that’s extremely popular, the other seems to offer too much bang for your buck but might be well-suited to applications where short play times are acceptable. We’ll be clear: If you’re building an SPL competition system, it’s likely that the majority of what we are going to discuss won’t apply to you. In those instances, power and efficiency are pretty much the only specs worth examining. For those who live with their stereos every day, choosing a reliable amplifier that adds minimal distortion or noise to their music is crucial.

The Contestants

A friend at a local car audio shop called and asked for an opinion about a new line of amplifiers being offered to his store. It’s worth a proper examination in every situation like this, rather than guessing about the product’s performance. Thankfully, we have the BestCarAudio.com Test Drive lab at our disposal. We dropped by the shop to pick up the sample amplifier and brought home the amp that’s usually in the store owner’s car.

For this comparison, the reference amplifier is an ARC Audio XDi 650.1. This amp is rated to produce 650 watts into a 1-ohm load and includes a wired remote level control. Physically, the amp has a footprint of roughly 9 by 6.25 inches and stands only 2.16 inches tall. For the power it produces, the size is compact and convenient.

In this case, the sample amplifier is a high-power subwoofer amplifier that is rated to produce over 2,000 watts into a 4-ohm load and over 3,000 watts into a 2-ohm load. The chassis is just under 9 by 7.5 inches in size, and the amp stands at just under 3 inches tall. That’s a pretty small package for something that claims to make that much power.

Physical Layout Affects Installation Options

OK, let’s talk about the layout of the connections and controls on these amplifiers.

The ARC Audio has power, signal, speaker connections, fuses and remote-level control jacks along the amp’s bottom edge. As such, your installer can place multiple amplifiers side by side in your vehicle to create an elaborate audio system. The crossover adjustments, infrasonic filter, bass boost and sensitivity control are accessible from the top of the amp. The ARC Audio badge in the top-center of the amp can be installed facing in any of four positions to increase installation options while keeping things looking great.

The challenger amplifier has power connections on one side and speaker and signal connections on the other. There’s no way to mount these amplifiers side by side. The signal processing controls and the jack for the optional remote level control are on the right side. Once installed, the tech will need to get very close to the amp to see the controls to make adjustments. There are no fuses on this amp, so an external fuse needs to be added, which will increase the installation cost. At almost 3 inches in height, it’s also unlikely this amp would fit under the seat of most cars on the road today.

Score: ARC Audio 1, Competitor 0

Power Production

These are subwoofer amplifiers. As such, it’s fundamental that they can make generous amounts of power. We set the XDi up on the bench and wired it to our bank of load resistors configured to present 1 ohm of resistance. With 14.2 volts from our power supplies, the little XDi 650.1 cranked out a beefy 720 watts as measured using our D’Amore Engineering AMM-1 meter.

We wired up the competitor amp to see what this little monster could do. When loaded to 2 ohms, the amp produced an astonishing 1,799.8 watts of power. When connected to the 1-ohm load, the amp went into protection mode right at 1,200 watts of power. We realized afterward that this particular amp isn’t rated for 1-ohm loads. Nevertheless, the remote signal had to be removed for the amp to reset. That level of power is downright impressive from an amplifier so small.

Score: ARC Audio 1, Competitor 1

Efficiency Calculations

It isn’t easy to monitor input and output power simultaneously while trying to measure the maximum output capabilities of an amplifier. To simplify the task, we set the XDi up again and measured efficiency at its rated output level of 650 watts. The ARC was drawing 47 amps of current, and there were 14.2 volts at the terminals. That works out to an efficiency of 96.7% at a specific power output level of 645.3 watts, according to the AMM-1. For the competitor, it turns out it never made its rated 2200+ or 3300+ levels. So, to keep things stable, we measured efficiency at 1,100 watts into 2 ohms – an output level that will keep our power supplies and load resistors happy. We calculated the efficiency to be 85.1%, which is very, very good. This efficiency might have increased a percent or two at full power. The ARC measurement, however, is astonishing.

Score: ARC Audio 2, Competitor 1

Amplifier Frequency Response

Sadly, many consumers think that subwoofer system frequency response isn’t a big deal as it’s “just bass.” For anyone who’s had an audio system upgraded with a digital signal processor, they’ll know that a statement like that couldn’t be further from the truth. Choosing source units, amplifiers, speakers and subwoofers that deliver flat frequency response is the first step in creating a high-quality audio system. We connected both amps to our digital interface and took frequency response measurements with the infrasonic filter set to their lowest frequency, the crossover at the highest frequency and the bass boost controls turned down.

In terms of the XDi amp, the overall bandwidth is on the narrow side. The -3 dB point is 25 Hz on the bottom and about 210 Hz on the top. I’d like to see the bottom end extended a little. Still, for most consumers using vented enclosures, this design will protect the woofers from potential damage due to over-excursion caused by the driver unloading below the tuning frequency. This would have been a conscious decision by the ARC Audio design team.

For our competitor, we see a lot of bandwidth. On the bottom, the -3 dB point is at 6.8 hertz, and we saw 500 Hz on the top. With that said, the dip at 60 Hz and a peak at 45 Hz are troubling. Yes, the amplitude varies by only a half-decibel, but amplifiers should never apply any equalization to the signal unless specifically requested.

We’ll call this a draw and give both amps one point. Score: ARC Audio 3, Competitor 2

Harmonic Distortion Test

Here’s where we step away from the typical measurements and get into the nitty-gritty of what makes one amplifier sound better than another when reproducing music. We set the amps up on the bench and measured the frequency output of each when a 1-volt, 60 Hz test tone was produced. The differences are dramatic!

Looking at the graph generated for the XDi, we see our test signal labeled with marker M0 at a level of 0.74 dB, which is 1.088 volts. The first harmonic at 120 Hz, marked with M1, is at a relative level of -70.33dB for a distortion rating of 0.0318%. That’s good for an efficient subwoofer amplifier.

For our competitor, the results weren’t as good. In fact, they were bad. Very bad. We can see the test stimulus signal at -0.07 dB (0.992 volts) labeled with M0 and the second harmonic (M1) of roughly 180 Hz at a level that’s only -40 dB quieter. That’s a THD specification of 0.995%. This is one of the worst distortion measurements we’ve ever seen on our bench.

Score: ARC Audio 4, Competitor 2

Signal-to-Noise Ratio

An amplifier’s signal-to-noise performance describes the amount of unwanted noise or hiss added to the output signal. In most cases, amplifiers that add noise at a level of -80 dB are considered pretty good. The great amps get up into the -90 dB range. For the XDi amp, the QuantAsylum software provides a measured SNR spec of -84.6 dB. You can see the hash (squiggly yellow lines) down at the bottom, sloping downward from about -100 dB to around -105 by 100 Hz. This is good performance.

Next, we have our competitor. Wow. Just wow. The software specs SNR at 30 dB, and you can see the noise floor sitting at just below -60 dB from 10 to 100 Hz. Once again, this is the worst we’ve ever seen on the bench.

While it’s hard to hear hiss through a subwoofer compared to a tweeter, this is frustratingly bad performance. Chances are, you can feel a subwoofer moving with that much noise. Can we take marks away? I suppose not.

ARC Audio 5, Competitor 2

Amplifier Controls and Adjustments

As we mentioned earlier, the XDi has all the controls on the top panel of the amp. This location makes it easy to adjust once the amp is installed in the vehicle. In contrast, the competitor has the controls on the side. Once installed, the tech would have to either feel around for the adjustment knob or physically get close to the amp to read the labels. We do like that the controls are raised so that they can be adjusted with bare fingers, though; that’s a nice touch.

We noticed with the competitor amp that with the sensitivity control at its lowest setting, the amplifier produced no output – no matter how much signal was applied. For installers who aren’t accustomed to this brand, this design flaw could easily waste a lot of time during the level-setting process.

ARC Audio 6, Competitor 2

Preamp Inputs

The XDi has four RCA jacks along the lower edge panel. Two jacks are inputs, and the others are input and output for use when using the amp in a master or slave configuration. The competitor amp has only one RCA input jack, despite there being two terminals on the side of the amp with the standard red and white labeling. The lower terminal is an output to drive another amp. It’s not hard to imagine someone connecting a second input to the lower terminal. We checked to ensure that both amplifiers used a differential input design to keep noise out of the system. The XDi does; the competitor doesn’t. This isn’t 1988. Speechless.

ARC Audio 7, Competitor 2

Power Connections

Both amplifiers have good quality terminal blocks for power and ground connections; both will accept four-AWG cables. For the XDi amp, it draws about 60 amps of current when pushed hard. That’s a reasonable amount of current for four-AWG cable. The competitor, on the other hand, can draw well over 150 amps of current. That’s a lot more than is recommended for a four-AWG cable. The amp really should have 0-AWG terminals, or you should invest in those little billet adapters that let your installer connect 0-AWG to four-AWG terminals. One hundred fifty amps of current, drawn through 30 feet of high-quality, all-copper four-AWG power cable, will result in a voltage drop of about 1.3 volts. The XDi would only lose about 0.5 volts over the same run in a cable of that size.

ARC Audio 8, Competitor 2

Cooling Design Determines Play Time

Last and certainly not least, we examined the physical design of the amplifiers to evaluate their cooling capabilities. When you drive a subwoofer at high output levels, an amplifier, even as efficient as these, needs to dissipate the heat it generates. The ARC amp uses a small cooling fan that is mounted on the back edge of the amplifier’s vertical heatsink. The fan draws air in from the sides of the amp, then pushes it out the back. There’s a plate behind the finned heatsink that causes the air to flow along the heatsink and out of the amp’s ends to maximize the moving air’s cooling effect.

The competitor amp also has a fan on the side of the amp. It draws air into the case, and it exits out the other side. The airflow path is near the output switching devices but isn’t routed through any part of the heatsink.

We let each amp play at its maximum output level for a few minutes. The ARC reached 140 degrees Fahrenheit (60 degrees Celsius), and the competitor was a mind-blowing 204.8 degrees Fahrenheit (96 degrees Celsius) in less than three minutes. The chances of this high-power amp overheating due to the heatsink’s small size is a serious concern.

A look inside the XDi amp showed temperature sensors near the output switching devices and the power supply transformer. We couldn’t find any temperature sensing in the competitor amplifier.

A microprocessor controls the fan in the ARC Audio amp. The fan goes to full speed when the amp gets hot and continues to run until it is cool. On the competitor amp, the fan speed seems related to the power being produced by the amp. If you get the amp hot then turn the signal down, the fan slows down. That doesn’t make any sense.

ARC Audio 9, Competitor 2

Factory Stereo Integration Features

If you’re hoping to add one of these amplifiers to the factory-installed radio in your car or truck, then the ARC will be the best choice. The XDi has automatic turn-on detection so that the amp will turn itself on when you turn on the radio. Likewise, it includes a pair of RCA adapter pigtails that can be connected to the speaker wires from the radio or amp to provide a signal. The competitor amp will need an external line-level converter that includes a remote turn-on output.

ARC Audio 10, Competitor 2

Remote Level Control

The XDi 650.1 includes a remote level control. The other amp doesn’t, and we didn’t see a part number listed in the owner’s manual or on the product page on their website.

ARC Audio 11, Competitor 2

Upgrade Your Stereo with a Quality Subwoofer

As we said at the beginning, the right choice of amplifiers for you depends on your application. If all that matters is a three-second burp at an SPL contest, then the choice is crystal clear. If you’re shopping for an amplifier that will sound good and continue to play for hours and hours, the information provided above should make your choice obvious. In terms of power, no, these weren’t equal. With that said, ARC has two larger amplifiers in the X2 family (which replaced XDi). There’s the 1,100-watt X2 1100.1 and the monster X2 2500.1 2,500-watt unit. Both of those amps share the same basic design as the XDi family but add a few more features and updated circuitry.

If your local specialty mobile enhancement retailer isn’t an ARC Audio dealer, similar amplifier solutions such as the JL Audio XD600/1v2 or XD1000/1v2, the Rockford Fosgate P500X1bd or P1000X1bd, the Audison AP F1D, the Hertz ML Power 1, the AudioControl LC-1.800 or the Helix M ONE X would be great options in the same class of amplification. If the power ratings on an amplifier you’re considering seem too good to be true, if the heatsink is tiny or the price seems like too much of a bargain, chances are you’re going to be missing out on some critical features or performance factors. The best amp isn’t always the one that makes the most power.

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.