DIY Speaker Series: Understanding Box Types

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When designing DIY speakers, carefully considering the enclosure’s impact on sound performance is crucial. From the start of our build, enclosure type was one of the first decisions we made.

Enclosures are more than just a structural box; they play a critical role in defining the tonal balance, bass response, and efficiency of the speaker system. Proper design choices will directly affect how well your speaker reproduces low frequencies, manages driver excursion, and handles sound waves from the front and rear of the drivers. For example, the SUB15, being a subwoofer, requires an enclosure that optimally reinforces bass, while the LCR10, as part of the left-center-right array, must balance midrange clarity with controlled low-end output.

When we talk about enclosures or speaker boxes, we’re primarily discussing designs for bass drivers. Tweeters typically come encapsulated and don’t require enclosures, while midrange drivers need their own chambers to isolate them from the bass drivers, ensuring that low-frequency pressure waves don’t interfere with the delicate cone movement responsible for midrange frequencies. Additionally, the volume of the chamber for midrange drivers can affect their overall sound signature, so it’s not something to overlook. However, that is out of the scope of this post, so we will come back to it later to discuss.

A Simple Guide to Speaker Box Types

The fundamental role of a speaker enclosure is to separate the sound waves produced by the front of the driver from those at the rear. These waves are 180° out of phase, meaning they will cancel each other out if they meet, particularly in the lower frequencies, resulting in poor bass response. This phenomenon, known as “acoustic short-circuiting,” makes enclosure design crucial for preventing such phase interference.

Here we’ll explore five primary types of speaker enclosures and the physics behind their designs.

1. Sealed Enclosures

Sealed enclosures, or acoustic suspension designs, are the simplest in terms of construction. The box is airtight, meaning no air can escape when the driver moves. This causes the air inside to act like a spring, providing resistance to the cone’s motion and offering control over its movement.

• Acoustic Damping: The air pressure inside the sealed box acts as a linear spring, helping to dampen the driver’s motion. This provides a predictable, second-order roll-off of -12 dB per octave below the system’s resonance frequency (Fs), resulting in tighter bass response.
• Transient Response: Sealed enclosures tend to offer better transient response compared to ported designs. The lack of ports means no phase shift at low frequencies, which results in cleaner, faster bass transients.
• Less Efficiency: While sealed boxes provide accurate bass, they are less efficient, requiring more power to reach the same output level as other enclosure types, especially in the low-frequency range.
• Room Gain: Sealed enclosures benefit from room gain, a phenomenon where small room dimensions naturally reinforce low-frequency output. In enclosed spaces, the bass roll-off of a sealed enclosure can be compensated by room acoustics, effectively boosting the low-end performance without the need for more aggressive porting or larger boxes. This makes sealed designs ideal for home theater or small room applications where low-frequency reinforcement is needed.

Sealed enclosures are ideal when sound accuracy and transient response are more important than bass volume. They excel in applications where room acoustics or driver quality already emphasize control over deep bass extension. These are the easiest to build if you are just starting your DIY speaker journey.

2. Ported Enclosures

Ported enclosures, or bass-reflex designs, use a tuned port to enhance the system’s low-frequency response. The port functions as a Helmholtz resonator, reinforcing the bass output by harnessing the energy from the rear of the driver and channeling it in phase with the front wave.

• Resonance Tuning: The length and diameter of the port are tuned to the driver’s resonance frequency. At this frequency, the air in the port oscillates in phase with the driver, reinforcing the sound wave and increasing efficiency.
• Phase Issues: A ported design introduces a phase shift near the tuning frequency, which can lead to group delay. This phase shift is important to consider because it can introduce smearing in the bass response if not properly managed.
• Roll-off Characteristics: Ported enclosures exhibit a steeper roll-off below their tuning frequency, typically -24 dB per octave, compared to sealed designs. This can result in uncontrolled driver excursion below the tuning frequency, which may require subsonic filtering.

Ported designs are excellent when you need higher efficiency and deeper bass extension, especially for subwoofer applications or large-format floor-standing speakers. Just be mindful of port tuning and group delay, especially if you’re prioritizing bass clarity.

3. Transmission Line Enclosures

Transmission line (TL) enclosures are among the most complex designs. They use a long, folded internal path, or labyrinth, to guide the rear sound waves from the driver, gradually absorbing higher frequencies while reinforcing low frequencies.

• Acoustic Reinforcement: The labyrinth is tuned to quarter-wavelength resonances of the driver’s Fs, which allows low frequencies to emerge in phase with the front wave, reinforcing bass without creating phase issues. These types of enclosures can often get to subsonic frequencies.
• Damping and Absorption: Proper damping inside the transmission line is critical. The goal is to attenuate higher frequencies while allowing low frequencies to pass through and resonate. This often leads to having different amounts of damping material through the length of the port, adding to complexity.
• Phase and Response: Transmission lines offer smooth bass roll-off, lower distortion, and minimal group delay, which makes them ideal for audiophiles who prioritize natural and accurate low-frequency performance. The goal of the transmission line is to get the sound produced by the driver and the port in phase, again this adds complexity as it is hard to get right.

Transmission line enclosures are best for experienced builders aiming for high fidelity and deep bass extension in critical listening environments. The complexity of design and size requirements make these boxes less suitable for beginners, but if done correctly they can provide very high levels of bass.

4. Passive Radiator Enclosures

A passive radiator enclosure is similar in concept to a ported design but uses a passive driver instead of a port to augment the system’s low-frequency output. The passive radiator moves in response to the pressure changes caused by the active driver, extending bass output without the potential drawbacks of port noise.

• Resonant Coupling: The passive radiator is tuned to a similar frequency as the port in a bass-reflex system. The mass of the passive driver and the stiffness of the suspension combine to create a secondary resonant system, reinforcing low frequencies like a port.
• Eliminating Port Noise: Unlike ports, passive radiators eliminate the risk of chuffing, which is caused by air turbulence at high volumes. This allows for a cleaner bass response in compact designs.
• Reduced Group Delay: Passive radiators can exhibit less group delay compared to ports, improving the system’s transient response at lower frequencies.
• Box Size: Passive radiator enclosures can be more compact than their ported counterparts, making them an attractive option for designs where space is a constraint. Since they don’t require the long ports needed in ported systems to tune low frequencies, you can achieve deeper bass in smaller enclosures without sacrificing performance. However, the size of the passive radiator itself still needs to be factored into the overall design.
• Roll-off Characteristics: Passive radiator systems tend to have either a similar or slightly steeper roll-off slope similar to ported designs. The passive radiator will also have a dip in the response at the passive speaker’s resonance, which is considered its cut-off point.

Use passive radiators if you’re looking for the bass extension of a ported system but want to avoid the risk of port noise, especially in smaller enclosures. However, these enclosures tend to be more expensive to build due to the cost of the passive radiator.

5. Infinite Baffle or Free-Air

An infinite baffle enclosure, also known as a free-air system, differs from other designs in that it doesn’t fully enclose the driver. Instead, it isolates the front and rear sound waves by mounting the driver on a large, rigid surface like a wall or ceiling, where the rear sound waves are dissipated into an adjacent space (such as an attic or another room). This effectively simulates an infinitely large enclosure.

• Acoustic Isolation: The goal of an infinite baffle is to provide enough isolation between the front and rear waves so that they don’t interact, preventing phase cancellation and maintaining bass clarity.
• Driver Requirements: This design places less mechanical resistance on the driver, meaning that only specific drivers designed for free-air operation should be used. These drivers generally have high compliance, allowing for extended low-frequency output without relying on an enclosed air volume for damping.

Infinite baffle designs are often used for subwoofer installations in home theaters or automotive systems, where space is at a premium and the builder wants to avoid complex enclosure builds. However, this design requires careful attention to driver specs and installation location to ensure phase integrity and sound isolation.

To Summarize

Enclosure design is not just a box-building exercise—it’s about leveraging the physics of acoustics to maximize performance. Whether you’re a beginner or seasoned builder, understanding these fundamental enclosure types will set the foundation for achieving the best possible sound in your DIY speakers.

While there are numerous types of enclosures, this post focuses on the most popular designs to help you make an informed decision. For those just starting out, the choice often comes down to sealed versus ported. If you’re looking for simplicity and a compact design, the sealed enclosure is ideal. However, if you want more output and deeper bass, a ported (bass-reflex) design is the better option. More advanced enclosures like transmission lines and passive radiators offer great potential but require deeper knowledge and precise calculations.

Ultimately, by focusing on the core principles of speaker box design, you can confidently choose the enclosure that best suits your needs and start building speakers that deliver high-quality sound.