Enclosure Types Comparison

Understanding the trade-offs between sealed, vented, and passive radiator enclosures helps you choose the right design for your application.

Overview

Each enclosure type has distinct characteristics that make it better suited for different applications. There's no universally "best" type, only trade-offs between size, extension, efficiency, and transient response.

Sealed (Closed Box)

How It Works

The driver is mounted in a sealed, airtight box. The air inside the enclosure acts as a spring (acoustic compliance) that works against the driver's suspension. The enclosed air makes the driver's suspension effectively stiffer, raising the system resonance frequency and damping cone motion.

Key Characteristics

• Roll-off: 12 dB/octave below F3
• Response shape: Controlled by Qtc (system Q)
• Box size: Can be quite small relative to vented for same driver
• Group delay: Lowest of all types, good transient response

System Q (Qtc)

Qtc determines the response shape:

• Qtc = 0.5: Critically damped, no peak, earliest roll-off
• Qtc = 0.707: Butterworth (maximally flat), standard target
• Qtc = 1.0: 2-3dB peak before roll-off, extended response
• Qtc > 1.0: Underdamped, "boomy" peak, poor transient response

Qtc is determined by driver Qts and box size (Vb/Vas ratio). Smaller box raises Qtc.

Advantages

• Simple to design and build
• Excellent transient response and timing
• Predictable, well-behaved response
• No port noise or tuning issues
• Smallest possible box for some applications
• Tolerant of parameter variations

Disadvantages

• Lower efficiency than vented (need more power for same output)
• Higher excursion at low frequencies (more strain on driver)
• Earlier low-frequency roll-off than vented
• Less maximum output capability

Best For

• Home theater where accuracy and transient response matter
• Sealed room with boundary reinforcement
• Applications where box size must be minimal
• High-Qts drivers (0.6-0.8)
• Musical applications where "tight" bass is valued

Design Guidelines

• Target Qtc of 0.6-0.7 for flat response
• Higher Qtc (0.8-1.0) for extended response with slight peak
• Typical box size: Vb = 0.5 to 2 × Vas
• Use high-pass filter to protect from over-excursion

Vented (Ported/Bass Reflex)

How It Works

The enclosure has a port (tube or slot) that's tuned to a specific frequency (Fb). At the tuning frequency, the port and the air inside it resonate, creating a Helmholtz resonator. The port's output adds to the driver's output, increasing efficiency and reducing driver excursion near Fb.

Below the tuning frequency, the port goes out of phase with the driver and the response drops off steeply (24 dB/octave). The driver is also less controlled below Fb, leading to increased excursion.

Key Characteristics

• Roll-off: 24 dB/octave below Fb
• Response shape: Depends on alignment (QB3, BB4, C4, etc.)
• Box size: Typically larger than sealed for same driver and extension
• Efficiency: Higher than sealed, especially near Fb

Common Alignments

QB3 (Quasi-Butterworth, 3rd order)

• Fb ≈ 0.8 × Fs
• Flattest response
• Earliest roll-off of vented types
• Best transient response for vented

BB4 (Butterworth, 4th order)

• Fb ≈ Fs
• Slight peak (1-2 dB) before roll-off
• Extended low-frequency response
• Common general-purpose alignment

C4 (Chebyshev, 4th order)

• Fb > Fs
• Larger peak (2-3 dB) before roll-off
• Maximum extension
• Slightly compromised transient response

Advantages

• Higher efficiency, more output with same power
• Lower excursion near tuning frequency
• Extended low-frequency response (lower F3 than sealed)
• Higher maximum output capability
• Works well with low-Qts drivers (0.2-0.4)

Disadvantages

• Larger box required for same driver and similar extension
• More complex to design and build
• Port noise (chuffing) possible at high power
• Steep roll-off below tuning can be problematic
• Poorer transient response than sealed
• Sensitive to parameter variations and tuning accuracy

Best For

• PA and sound reinforcement (maximum output priority)
• Car audio (cabin gain extends response below Fb)
• Large home subwoofers where box size isn't constrained
• Low-Qts drivers (0.2-0.4)
• Applications where efficiency matters (limited amplifier power)

Design Guidelines

• Start with recommended alignment (QB3, BB4, C4)
• Typical box size: Vb = 1 to 3 × Vas
• Port area should be 1/6 to 1/10 of Sd (driver area)
• Keep port velocity under 17 m/s to avoid noise
• Use high-pass filter at or slightly below Fb to protect driver

Port Considerations

Port diameter/dimensions:

• Larger diameter = lower velocity = less noise
• But larger diameter = longer port for same tuning
• Multiple smaller ports can be used instead of one large port

Port length:

• Determined by box volume, port area, and tuning frequency
• Simulator calculates this automatically
• Can be adjusted with flared ends (reduces effective length)

Passive Radiator

How It Works

Similar to a vented enclosure, but instead of a port, a second cone (passive radiator) is used. The passive radiator has no motor (no magnet or voice coil) and is driven by pressure changes inside the box.

Tuning is controlled by adjusting the mass of the passive radiator. Adding weight lowers the tuning frequency. The passive radiator's mass and compliance form a resonant system with the box volume, similar to how port mass and box volume resonate in a vented design.

Key Characteristics

• Response: Similar to vented (depends on alignment)
• Roll-off: 24 dB/octave below tuning, like vented
• Box size: Can be smaller than vented for same tuning
• Tuning: Adjustable by changing passive radiator mass

Advantages

• No port noise (no air velocity through a tube)
• Smaller boxes possible (no port taking up space)
• Tuning is adjustable (add/remove mass)
• Cleaner appearance (no visible port)
• Works well in compact designs

Disadvantages

• More expensive (passive radiator costs more than a port)
• Passive radiator has excursion limits too (must stay within Xmax)
• Harder to source passive radiators (fewer options than drivers)
• Added mass must be carefully calculated and applied
• Slightly more complex to design than sealed

Best For

• Compact sealed-like boxes that need more output
• Applications where port noise is unacceptable
• Designs where adjustable tuning is valuable
• Aesthetic considerations (clean appearance)

Design Guidelines

• Passive radiator Sd should be similar to or larger than driver Sd
• Typical box size: Similar to vented or slightly smaller
• Added mass determines tuning (more mass = lower tuning)
• Check passive radiator excursion, especially at high power
• Use high-pass filter at or slightly below tuning frequency

Side-by-Side Comparison

Choosing the Right Type

Choose Sealed If:

• You value accuracy and transient response
• Box size must be minimal
• You have a high-Qts driver
• You want the simplest possible design
• You have adequate amplifier power

Choose Vented If:

• Maximum output is the priority
• You have a low-Qts driver
• Box size isn't severely constrained
• Efficiency matters (limited power)
• You need the deepest possible extension

Choose Passive Radiator If:

• You want vented-like output in a smaller box
• Port noise is unacceptable
• You want adjustable tuning
• You value clean aesthetics
• You can source an appropriate passive radiator

Further Reading

• Thiele-Small Parameters Reference
• User Guide
• Glossary