Most of us have heard of many types of car audio subwoofer enclosures. Many enthusiasts have debated the benefits and drawbacks of acoustic suspension (sealed), bass-reflex (vented), bandpass and infinite baffle designs in hopes of choosing the best solution for their application and expectations. In all cases, the enclosure has two specific purposes that are crucial to ensuring that your subwoofer sounds excellent.
Rear Wave Cancellation
If you were to take a subwoofer out of its carton, connect it to an amplifier and play music, it wouldn’t make any bass. As the cone moves forward, the pressurized air in front of the cone is canceled by the rarefied air behind the cone, and vice versa. We call this back-wave cancellation.
In the graph below, you can see two sine waves, one in yellow and another with opposite polarity in blue. When they are added together, they cancel each other out, and you get the green line.
Understanding that the sound coming off the back of the subwoofer (or speaker) cone cancels the sound coming from the front is the first step in comprehending why a subwoofer needs an enclosure. We need to separate those two sound sources from each other for us to hear bass information.
The simplest solution is to create a wall or baffle that keeps these sound sources separate. That’s exactly what happens in an infinite baffle subwoofer installation. Your installer will either cut a hole in the rear deck of your sedan or mount a subwoofer on a board behind the rear seat so that you hear only the music coming from the front of the speaker. The sound coming from the rear is trapped in the trunk. This type of design doesn’t work with SUVs or hatchbacks since they don’t have a trunk to capture the sound from the back of the subwoofer cone. In reality, this is very similar to a sealed enclosure, except that the enclosure is extremely large and doesn’t affect the frequency response of the speaker.
Benefits of an Infinite Baffle Car Audio Subwoofer System
Infinite baffle installations are very popular in factory-installed subwoofers systems in sedans. The subwoofer can be mounted in the rear deck of the vehicle, and the trunk becomes the enclosure. These systems are also relatively easy to construct, as long as the trunk is sealed well. The drawback of this type of installation is that there is nothing to control the motion of the subwoofer cone other than its suspension.
Subwoofer Physical Power Handling
For a midrange driver playing above 300 Hz, cone excursion isn’t a concern because it doesn’t have to move very far at these frequencies. As audio frequencies decrease, cone excursion increases dramatically. As an example, to produce, say, 90 dB of output at 100 Hz, a subwoofer cone might have to move 1 mm forward and rearward. To produce the same output at 50 Hz, the cone needs to move 4 mm, and at 25 Hz, the cone will have to move back and forth a total of 16 mm. Stated scientifically, cone excursion at a given output level increases with the inverse square of the frequency. So, for each drop of one octave, excursion quadruples.
Every speaker has a limit as to how far the cone can move forward or rearward. While there are different methods of determining this limit based on acceptable distortion performance, we’ll use the Xmax standard of the voice coil needing to not move outside of the top plate.
In the case of this Sony XS-GSW101 subwoofer, Xmax is specified as being 6.4 mm in each direction. This definition means the cone can move through a range of 12.8 mm without the strength of the magnetic field imposed on the voice coil by the top plate changing. What happens when the voice coil starts to move out of the top plate? In short, the amount of distortion added to the output skyrockets because the cone movement isn’t linear relative to the drive signal. You don’t want that.
How a Subwoofer Enclosure Improves Power Handling
When we put a subwoofer into an enclosure, the air inside the enclosure acts to limit how far the cone can move. In a tiny enclosure, the total air volume is small, so the cone can’t move much. In a large enclosure, the volume of air is large and is easier to compress and rarify, so the cone can move farther.
The graph above shows how the compliance (resistance to pressurization and rarefaction) of the air inside a sealed enclosure affects the output of the system. The red curve shows the response of the subwoofer in an infinite baffle application. As you can see, there is lots of output down in the 20-40 Hz range. The white curve shows the sub in a 0.6-cubic-foot enclosure. It’s not as loud at lower frequencies. Finally, the blue curve shows the response of the subwoofer in a very small 0.2-cubic-foot enclosure. This is much smaller than you’d ever want to use but serves to show how the cabinet acts to limit cone travel and, consequently, bass output.
In the graph above, the three traces show us how much the subwoofer cone moves relative to the size of the enclosure at a drive level of 350 watts. The red curve is our infinite baffle simulation, the white is 0.6 cubic feet, and the blue is an enclosure with a net internal volume of 0.2 cubic feet. Since we don’t want the subwoofer sound distorted at high output levels, our installer needs to choose an enclosure design that balances bass output with maximum cone excursion capability. In this case, the blue enclosure is the only application where we can provide the full rated 350 watts to the subwoofer without the cone moving too far.
We did some extra simulations and confirmed that an enclosure volume of 0.29 cubic feet allows the full 350 watts of power to be applied without excursion issues.
I Want More Bass Than That!
If we look at the frequency response graph above, it shows us that our subwoofer has a -3 dB output frequency of about 92 Hz. While this doesn’t seem like much fun in terms of making bass, it actually works out OK because our vehicles boost low frequencies. The result would be relatively flat frequency response. What do we do if we want our subwoofer to play louder? The answer is to have the retailer you are working with build a bass reflex (vented) enclosure.
In the image above, you can see how much more bass the same subwoofer produces when installed in a vented enclosure. At 40 Hz, the vented enclosure is 9 dB louder than the sealed enclosure. At 30 Hz, it’s 8.2 dB louder. It seems like a win, doesn’t it? When you combine this increased output with the cabin gain, we find all vehicles benefit; suddenly a single 10-inch subwoofer can really rock!
Is there a drawback to using a bass reflex enclosure? The answer is yes. Below the tuning frequency, the enclosure doesn’t control cone motion well.
The violet curve in the graph above represents the typical cone excursion response from a bass reflect enclosure. The cone moves a little more around 55 Hz, but a lot less at 35 Hz, which is the tuning frequency of the vent. This is because most of the sound produced by the enclosure is coming from that vent. Below the tuning frequency, cone excursion skyrockets. Below 30 Hz, we can run into excursion issues. To prevent this from damaging the subwoofer, it’s best to implement a high-pass filter (often called an infrasonic filter, or incorrectly called a subsonic filter) to limit output at these frequencies.
Choose Your Subwoofer Enclosure Solution Wisely
Your car audio subwoofer needs an enclosure to prevent the sound coming from the rear of the cone from canceling with the sound coming from the front. Likewise, it needs an enclosure to control cone motion to prevent distortion at high drive levels at low frequencies.
When you visit your local specialty mobile enhancement retailer to discuss the prospect of upgrading your vehicle with a subwoofer, it’s important to talk about the style of enclosure that will be used. A design that helps to optimize the efficiency of the overall system can make your car stereo sound great while reducing the power required from the amplifier. When executed properly, even a single 10-inch subwoofer can deliver impressive bass response that will make your music sound amazing.
