Closed box vs ported enclosure — the fundamental trade-offs

Why an enclosure is needed

A bare driver mounted without an enclosure radiates from both the front and rear of the cone. At low frequencies, where the wavelength is much larger than the driver, the front and rear outputs are nearly equal in amplitude and opposite in phase — they cancel almost completely. This acoustic short circuit causes the driver's output to fall at 12 dB/octave below a frequency determined by the driver and baffle dimensions.

An enclosure isolates the rear radiation, eliminating the cancellation and allowing the driver to develop useful output at low frequencies. The type of enclosure determines the shape of the low-frequency rolloff and how far down in frequency useful output extends.

The closed box

A closed box (also called a sealed enclosure) encloses the rear of the driver in an airtight volume. The trapped air acts as an additional spring in parallel with the driver's own suspension compliance, raising the effective resonant frequency of the driver-enclosure system (fsb) above the free-air resonant frequency (fs):

fsb = fs × √(1 + Vas/Vb)

where Vas is the equivalent compliance volume of the driver and Vb is the enclosure volume. A larger box approaches the free-air behaviour (fsb → fs as Vb → ∞); a smaller box raises fsb further.

The system behaves as a second-order high-pass filter, rolling off at 12 dB/octave below fsb. The shape of the rolloff in the transition region is controlled by the system Q factor (Qtc), which is determined by the driver's mechanical and electrical parameters and the enclosure volume. Three alignments are commonly used:

• Qtc = 0.5 (Bessel/Linkwitz): Maximally flat group delay, no overhang or bump in the response, very gentle rolloff onset. Requires a large box. Preferred for time-domain accuracy.
• Qtc = 0.707 (Butterworth): Maximally flat magnitude response, -3 dB at fsb. A practical all-around alignment.
• Qtc = 1.0 (Chebyshev-like): A 3 dB peak in the response near fsb, then steep rolloff. Extends apparent bass at the cost of a coloured response and poor transient behaviour.

Values below 0.5 are overdamped and rarely used. Values above 1.0 produce progressively worse audible effects — the enclosure "honks" at the resonant frequency.

Key properties of the closed box:

• 12 dB/octave rolloff — below the -3 dB point, output falls relatively gently
• Cone is always loaded by the enclosure air spring — even below resonance, the compliance limits cone excursion
• Robust to low-frequency programme: the enclosure loading continues to control the driver below the system resonance
• Smaller maximum output at low frequencies compared to a ported design of the same size

The ported enclosure (bass reflex)

A ported (or bass reflex) enclosure adds a tuned port — a tube opening in the enclosure — which acts as a Helmholtz resonator. The port's resonant frequency (fb) is determined by the port dimensions and the enclosure volume:

fb ≈ (c/2π) × √(Sp / (Vb × Leff))

where Sp is the port cross-sectional area and Leff is the effective port length (physical length plus end corrections). At fb, the port radiates the majority of the acoustic output, while the cone motion is minimal — the enclosure and port together absorb the driver's excursion, unloading the cone.

The system behaves as a fourth-order high-pass filter, rolling off at 24 dB/octave below the lower -3 dB frequency. Properly aligned, the ported system can extend the -3 dB frequency lower than a closed box of the same volume, or achieve the same -3 dB frequency in a smaller volume, with higher maximum output.

Common alignments:

• QB3 (quasi-Butterworth 3rd order): Slight overhang, extended bass, commonly used in commercial designs.
• B4 (Butterworth 4th order): Maximally flat response, clean rolloff.
• SBB4 (super Butterworth): Maximally flat group delay below fb, extended response, large box required.

Key properties of the ported enclosure:

• 24 dB/octave rolloff — below fb, output falls very steeply
• Cone is unloaded at and around fb — excursion is low at fb but rises rapidly below it
• Critical vulnerability: below fb, the port no longer contributes and the cone is unloaded. Large cone excursions at subsonic frequencies (record warp, room pressurisation events, very low bass content) can cause damage. A high-pass filter at or slightly below fb is essential in any serious ported design.
• Higher maximum output at low frequencies compared to an equivalent closed box, at the cost of steeper below-tuning rolloff

Group delay

The ported enclosure introduces more group delay — frequency-dependent delay of the signal — than a closed box. At frequencies near the port tuning frequency, group delay can be 20–40 ms in typical designs, compared to 5–15 ms for a comparable closed box. The audibility of this difference is debated, but it is measurable and becomes more audible as the tuning frequency rises into the midrange.

Choosing between them

Use a closed box when:

• Time-domain accuracy is the priority (recording studio subwoofer, accurate monitoring)
• The driver has a low fs and sufficient Qts for a sensible Qtc in the target volume
• The low-frequency programme is uncontrolled or unpredictable (no subsonic filter available)
• Compact dimensions are required and modest bass extension is acceptable

Use a ported enclosure when:

• Maximum bass extension or output is the priority
• A subsonic filter is in the signal chain
• A driver with low Qts is available (ported alignments favour low Qts drivers; closed boxes favour higher Qts)

Many practical designs use sealed alignments for subwoofers in systems where DSP provides the subsonic filter and group delay correction, and ported alignments for full-range woofers where extension and output are the primary goals.