Sealed box: the simplest and the most accurate

A sealed box is a box with no opening. The driver plays forward, and the sound from the back of the cone stays shut inside, where the damping material absorbs it. It is the oldest and simplest type of loudspeaker cabinet, and many designers still consider it the most accurate.

Section through a sealed box. The driver on the left, behind it the closed space with damping material. The air inside is compressed and expanded like a spring as the cone moves.

How it works

The air in the box acts as a spring that adds to the stiffness of the driver's suspension. The cone is held more stiffly, so its resonance rises. Below this resonance the bass falls at 12 dB per octave, which is slow and smooth.

The character of the bass is set by the number Qtc. At a Qtc of 0.5 the bass is heavily damped and rolls off early. At 0.707 the response is the flattest, with no hump. At 1 and above the bass has a hump above the resonance and sounds bigger, but less accurate. A smaller box gives a higher Qtc and a higher resonance.

What the calculation shows

All charts in this series on enclosure types are calculated by Loudsmith for the same 6.5" woofer, the SB Acoustics SB17NRX2C35-8 (Fs 36.5 Hz, Qts 0.43, Vas 27 l), so they can be compared. The 0 dB level is the driver's level in the midrange. For this woofer Loudsmith designed a sealed box with a net volume of 13.6 l and a Qtc of 0.71.

Total

Sealed box, 13.6 l. The −3 dB point is at 62 Hz, at 40 Hz the bass is 8 dB down and at 30 Hz 13 dB down.

The bass rolls off slowly from 62 Hz. In a room it is better than the chart suggests: the wall and floor add several decibels in the lowest bass, and the slow roll-off of a sealed box works well with that.

The impedance of a sealed box has a single peak, here 52 Ω at 61 Hz. Its position is the resonance of the driver in the box.

Pros and cons

Pros: the smallest box for a given driver, accurate bass with a short decay, no port to make noise. It forgives an inexact volume and does not mind standing near a wall.
Cons: for the same driver it gives the least deep bass. Below the resonance the cone excursion grows, so loud music needs a woofer with a lot of excursion.

Which driver and which room

It suits woofers with a Qts of about 0.3 to 0.5 and a lot of excursion. It is ideal for small rooms, near a wall, and anywhere the bass is filled in by a subwoofer or by equalisation. Many subwoofers that are corrected with DSP are sealed as well.

Well-known designs

THOR by Siegfried Linkwitz: a 12" Peerless in a sealed box of about 47 l with no damping material, equalised down to the lowest bass. Linkwitz published both the calculation and the measurement.
ZD5 by Zaph Audio: a 5" Scan-Speak in 14 l, F3 about 65 Hz. The author writes that the measured impedances matched the model exactly.
Dayton Audio lists recommended sealed boxes for its Ultimax subwoofers, for example 28 l for the 12" with a Qtc of 0.83 and an F3 of 46 Hz.

In Loudsmith

Loudsmith designs a sealed box for every woofer. The Build tab shows the Qtc, and in Dimensions you set the volume and the box damping. Damping material increases the acoustic volume of the box, so the Qtc drops slightly. We compared the result with fourteen boxes published by Dayton Audio and with Linkwitz's THOR measurement.

Enclosure types series: sealed · bass reflex · passive radiator · aperiodic · transmission line · TQWT · ML-TL · open baffle · back-loaded horn. You will find the formulas in the article Speaker box calculations step by step.