Discovering the Different Types of Enclosures

Several different enclosure types are used to house car audio subwoofers, and all have their advantages and disadvantages. The type you choose largely depends on the kind of sound and bass output you want to achieve, how much amplifier power you’ll have available to drive the sub system, and how much space you have in your vehicle. Subwoofers are usually optimized for one type of enclosure or another, although some can work in more than one type of enclosure.

Different Types of Enclosures

Infinite baffle

The most basic kind of subwoofer enclosure is really not an enclosure at all. Called infinite baffle or free-air (see Figure 16-4), this mounting configuration does indeed require an enclosure, but usually it’s simply a car’s trunk. Typically, the subwoofer is installed in a car’s rear deck or on a baffle, usually a piece of wood, behind the rear seat.

The advantages of an infinite-baffle enclosure are the following:

• Simple and inexpensive because there’s no box to build 
• Takes up less space 
• Simple
• dipole

The disadvantages are

• Difficult to isolate the front and back waves of the subwoofer from each other because an infinite baffle isn’t effectively sealed.
• Relatively larger power requirements as compared to other subwoofer enclosures, such as sealed enclosures.
• Less accurate sound because infinite-baffle configurations can sometimes sound a bit tubby (that is, not tight). Tubby is used to describe the sound of bass when it’s not as controlled as it should be or not accurately reproduced — and you know the sound only if you know what the bass is supposed to sound like in the first place.

Seated enclosures

Sealed enclosures (also called air-suspension, shown in Figure 16-5) are the simplest and easiest to construct. As the name implies, this type of enclosure is completely sealed so that the back wave of the subwoofer is isolated from the front wave.

Figure 16-5:

A sealed enclosure.

The advantages of a sealed subwoofer enclosure are the following:

• Simple to build
• More forgiving (enclosure size can be off by as much as 10 percent and the subwoofer will still perform well)
• Higher power handling
• Better transient response, meaning that the subwoofer can respond more quickly to changes in the music
• Smaller enclosure sizes; many of the subs that call for a small box are designed for sealed enclosures
• The disadvantage is that they are less efficient, meaning that they require more power for the equivalent output of an alternative design.

Vented enclosures

A simple vented enclosure, also called bass-reflex, is identical to sealed enclosures except that it employs a port or hole to increase output or the amount of bass (see Figure 16-6). The rear wave of the subwoofer, as tunneled through the port, is used to reinforce output or bass volume. Think of it as using both the front and back of the subwoofer’s cone to help produce bass.

The port is typically a tube made of PVC plastic, and its diameter and length determines the frequency at which the enclosure is tuned. The subwoofer and box combination has a specific frequency at which it operates optimally, and this is referred to as the frequency at which the subwoofer system — the sub¬woofer and enclosure together — is tuned. If either the length or diameter of the port is changed, the tuning frequency will also change. So in a ported enclosure, it’s extremely important to know the proper port dimensions — and use the proper sized port.

Figure 16-6:

A vented or ported box design.

Courtesy of JL Audio

Vented enclosures have the following advantages:

• Above the port-tuning frequency (the frequency at which the subwoofer system performs optimally), a vented enclosure is much more efficient, meaning it takes less power to produce more sound.
• A vented enclosure also has very low distortion around the port-tuning frequency.
• The disadvantages of vented enclosures are
• Below the port-tuning frequency, the subwoofer can become unstable, causing it to distort and even become damaged.
• More complex design and construction.
• Larger enclosure size, which means it will take up more room in your ride. 
• The possibility of port noise caused by air rushing through the port.

Exotic Vented enclosures 

Beyond simple vented enclosures, there are more exotic vented designs called bandpass enclosures. Bandpass enclosures became popular in the late 1980s as a way for sound-off competitors to wring the most sound pressure level (SPL) possible out of a system. Simple sealed enclosures have now replaced these more esoteric designs.

The main benefit of using a bandpass enclosure is that the design features very high efficiency — meaning lots of bass — within a narrow range of frequencies, which is why they were a hit with the SPL crowd. The design is called bandpass because it filters out all frequencies except those in a narrow range, allowing only a certain band of frequencies to pass.

Variations of bandpass designs are called orders. The most basic is a fourth- order or single-reflex design, which is a two-chamber enclosure in which a subwoofer in a sealed chamber fires into a second vented enclosure (see Figure 16-7). In a sixth-order dual-reflex design, both chambers are ported (see Figure 16-8). 

Figure 16-8:

A sixth- order, dual-reflex bandpass subwoofer enclosure.

Courtesy of JL Audio

  

The advantage of bandpass enclosures is that they offer a very high efficiency over a narrow frequency range.

Their disadvantages include the following:

• Narrow frequency response
• Very complex enclosure design
• Large enclosure size 
• Decreased power handling