🔰 BEGINNER LEVEL: Getting Sealed Right

Why Sealed Sounds Different

Sealed enclosures have a reputation: accurate, punchy, controlled — but not as loud as ported. Understanding why helps you choose correctly and set realistic expectations.

The air spring effect:

The air trapped inside a sealed box acts as a spring working against the driver's suspension. When the cone moves forward, it compresses the air inside, which pushes back. When the cone moves backward, it creates a partial vacuum, which pulls back. This additional restoring force stiffens the total suspension system.

What this does to performance:

Raises the effective resonant frequency (Fc > Fs)
Increases effective Q factor (Qtc > Qts)
Creates a 12 dB/octave rolloff below Fc — gentle, predictable, safe
Protects the driver from over-excursion at very low frequencies
Does NOT help efficiency — sealed boxes are typically 3–6 dB less efficient than ported at the same frequency

Volume determines character:

A very small sealed box creates high Qtc (>1.0) — the response peaks before rolling off, creating that "boom box" one-note bass many people actually like. A large box creates Qtc near 0.5 — extended, flat, dry, audiophile.

For most music-oriented builds, target Qtc between 0.7 and 0.85. This gives a slight warmth (gentle lift above the rolloff) without excessive bloom.

Polyfill: Real Effect or Myth?

Polyfill (polyester fiber stuffing, the same material used in pillows) inside a sealed enclosure is real and beneficial — when used correctly.

What it does:

Acoustic wave energy hitting the polyfill is absorbed rather than reflecting back through the cone. This reduces standing waves and coloration inside the box. More importantly, it slows the effective speed of sound inside the enclosure, making the trapped air behave as if it has a higher compliance (softer spring). The box appears acoustically larger than it physically is.

How much to use:

0.5 lb per cubic foot is a good starting point. Loosely filled — not tightly packed. Packing it tight causes excessive damping of the driver's motion, which worsens performance.

Maximum effective increase: About 15–20% of box volume. A 1.0 ft³ sealed box with proper polyfill behaves like roughly 1.15–1.20 ft³. Useful for fine-tuning Qtc when your box came out slightly too small.

Dimensioned blueprint for a practical 12 inch sealed subwoofer enclosure showing front, side, and top views, panel thickness, and target net internal volume. — This reference blueprint shows how a simple 12-inch sealed enclosure turns target volume into real exterior dimensions. Use it as a starting template, then adjust for the actual driver, brace plan, and displacement losses before cutting material.