Speaker Academy

Sealed Speaker Box Design

A sealed speaker enclosure is one of the simplest loudspeaker cabinet designs. The rear of the driver is enclosed in an airtight chamber, causing the trapped air to act as an additional spring on the loudspeaker cone. The enclosure volume, together with the driver's Thiele-Small parameters, determines the resulting bass response, resonance and damping.

What Is a Sealed Speaker Box?

A sealed speaker box, also called an acoustic-suspension enclosure, is a cabinet with no intentional opening or port connecting the internal air volume to the outside.

             Front of cabinet
        ┌─────────────────────┐
        │                     │
        │       WOOFER        │
        │         ↓           │
        │      ┌─────┐        │
        │      │Cone │        │
        │      └─────┘        │
        │                     │
        │    Trapped air      │
        │                     │
        │                     │
        └─────────────────────┘
              SEALED BOX

The cabinet must be sufficiently airtight for the enclosure to behave as intended.

How a Sealed Box Works

When the speaker cone moves inward, the air inside the enclosure is compressed.

When the cone moves outward, the internal air expands.

This trapped air therefore acts like an additional spring connected to the speaker's mechanical suspension.

Speaker suspension
       +
Enclosure air spring
       =
Combined mechanical system

Changing the enclosure volume changes the strength of this additional air spring.

What Happens When the Box Gets Smaller?

A smaller sealed enclosure contains less air.

The trapped air is therefore compressed more strongly when the cone moves.

This increases the air-spring stiffness acting on the driver.

The result is generally:

Higher system resonance
Higher system Q
Reduced low-frequency extension
Greater restoring force on the cone

What Happens When the Box Gets Larger?

A larger enclosure contains more air, so the cone produces a smaller pressure change inside the cabinet for the same displacement.

The air-spring effect becomes weaker.

The result is generally:

Lower system resonance
Lower system Q
Deeper low-frequency extension
Less acoustic loading from the enclosure

However, making the enclosure larger does not automatically produce a better speaker. The driver's parameters and the desired alignment must be considered.

Thiele-Small Parameters

Sealed-box design commonly uses the driver's Thiele-Small parameters.

Important parameters include:

Fs — free-air resonant frequency
Vas — equivalent compliance volume
Qts — total Q of the driver
Qes — electrical Q
Qms — mechanical Q
Re — voice-coil DC resistance
Sd — effective cone area
Xmax — linear excursion capability

Fs — Free-Air Resonant Frequency

Fs is the driver's resonant frequency when measured without the acoustic loading of a particular enclosure.

It is an important starting point for enclosure design.

A driver with a low Fs is generally more suitable for low-frequency applications than an otherwise similar driver with a very high Fs.

Vas — Equivalent Compliance Volume

Vas describes the compliance of the driver's suspension in terms of an equivalent volume of air.

It is normally specified in litres or cubic metres.

Vas is one of the most important parameters when determining a sealed enclosure volume.

Qts — Total Driver Q

Qts represents the total damping of the driver, combining its mechanical and electrical behaviour.

It is commonly used together with Fs and Vas when evaluating enclosure alignments.

Qtc — System Q in a Sealed Box

When the driver is installed in a sealed enclosure, its Q changes. The resulting system Q is called Qtc.

Qtc is an important parameter for sealed-box design because it describes the damping of the driver and enclosure combination.

The enclosure volume determines the relationship between the driver's free-air characteristics and the final system Q.

Common Sealed-Box Alignments

Different sealed-box alignments can be chosen depending on the desired frequency response.

A commonly referenced alignment is the maximally flat response, often associated with a system Qtc of approximately 0.707.

Other Qtc values produce different response shapes.

Approximate Qtc	General Behaviour
Below 0.707	More damped, gradual low-frequency response
≈ 0.707	Common maximally-flat target
Above 0.707	More pronounced response near system resonance

The best alignment depends on the intended application rather than there being one universally correct Qtc.

Sealed Box Volume Formula

For an idealized sealed enclosure, the relationship between the driver's Vas, Qts, desired Qtc and enclosure volume can be expressed as:

Vb = Vas / ((Qtc / Qts)² - 1)

where:

Vb = net enclosure volume
Vas = driver's equivalent compliance volume
Qtc = desired system Q
Qts = driver's total Q

This formula is a useful starting point for sealed-box design.

Example Sealed-Box Calculation

Suppose a woofer has:

Vas = 80 litres
Qts = 0.40

and the desired system alignment is:

Qtc = 0.707

Using:

Vb = Vas / ((Qtc / Qts)² - 1)

gives approximately:

Vb ≈ 36.4 litres

This is the required approximate net internal acoustic volume, not the external cabinet volume.

Net Volume vs Gross Volume

This distinction is extremely important.

If the calculated net enclosure volume is 36 litres, the physical cabinet normally needs to be larger because the driver, bracing and other internal structures occupy space.

Gross internal volume
        │
        ├── Driver displacement
        ├── Bracing
        ├── Terminals
        └── Other internal objects
        │
        ▼
Net acoustic volume

Calculating Gross Cabinet Volume

For a rectangular cabinet:

V = W × H × D

where the dimensions are the internal dimensions.

For example:

40 cm × 40 cm × 25 cm
= 40,000 cm³
= 40 litres

The actual net volume will be smaller after subtracting the volume occupied by the driver and internal structures.

Driver Displacement

The basket and magnet assembly of a large woofer can occupy a significant amount of internal space.

This volume must be considered when calculating the final net enclosure volume.

For accurate designs, use the driver's specified displacement when it is available.

Bracing Displacement

Internal braces improve cabinet rigidity but occupy some volume.

Large braces should therefore be included when calculating the net internal volume.

This is particularly important in compact sealed cabinets where the bracing occupies a relatively large percentage of the total volume.

Sealed Box Frequency Response

The sealed enclosure changes the driver's low-frequency response.

The response is primarily determined by the driver and the selected Qtc alignment.

Unlike a bass-reflex enclosure, there is no port resonance producing additional output below the driver's system resonance.

System Resonant Frequency

The sealed-box system resonance can be approximated by:

Fc = Fs × (Qtc / Qts)

where:

Fc = system resonance
Fs = driver's free-air resonance
Qtc = system Q
Qts = driver's total Q

Example of System Resonance

Using the previous example:

Fs  = 30 Hz
Qts = 0.40
Qtc = 0.707

Then:

Fc = 30 × (0.707 / 0.40)

Fc ≈ 53 Hz

This demonstrates how the enclosure changes the effective low-frequency resonance of the driver.

Sealed Box Low-Frequency Roll-Off

A sealed enclosure has a gradual low-frequency roll-off below its system resonance.

For a typical second-order sealed alignment, the low-frequency attenuation is approximately 12 dB per octave below the system resonance.

The exact response shape depends on Qtc.

Sealed Box vs Bass-Reflex

Characteristic	Sealed	Bass-Reflex
Port	No	Yes
Construction	Simple	More complex
Air leakage	Important	Important around cabinet and port
Low-frequency roll-off	Typically 12 dB/octave	Typically steeper below tuning
Port noise	None	Possible
Driver loading	Air spring	Air spring plus port resonance

Advantages of Sealed Enclosures

Simple construction
No port calculation
No port turbulence
No port tuning required
Good control of the driver's cone
Compact cabinets can be practical
Predictable behaviour
Useful for many hi-fi applications
Easy to combine with electronic equalization

Disadvantages of Sealed Enclosures

Lower efficiency than some ported alignments
Less output at low frequencies for some designs
Requires an airtight cabinet
Small boxes can produce high system Q
Large low-frequency drivers may require substantial cabinet volume

Sealed Boxes and Efficiency

A sealed enclosure does not use a port to add acoustic output.

Consequently, some applications may require more amplifier power to produce a particular low-frequency SPL compared with an appropriately designed bass-reflex enclosure.

However, the trade-off depends on the driver, alignment and intended frequency range.

Sealed Box and Cone Control

The trapped air provides restoring force to the cone.

This can provide useful mechanical control of the driver's movement.

However, cone excursion still needs to be considered when designing a high-output system.

Sealed Box and Amplifier Power

A sealed enclosure can generally tolerate substantial amplifier power within the mechanical and thermal limits of the driver.

Increasing amplifier power does not remove the driver's excursion limit.

At sufficiently low frequencies, the cone can still exceed its allowable excursion.

Sealed Box and Xmax

The driver's Xmax is particularly important for high-output applications.

The enclosure provides additional restoring force, but the cone still has a finite linear excursion range.

If the required SPL causes the driver to exceed Xmax, distortion and possible mechanical damage can occur.

Sealed Box and Subwoofers

Sealed subwoofers are popular because the enclosure can be relatively simple and compact.

However, achieving very high output at extremely low frequencies can require substantial cone excursion and amplifier power.

Drivers specifically designed for sealed subwoofer applications can therefore have long excursion capability.

Sealed Box for Hi-Fi Speakers

Sealed enclosures have historically been used extensively in hi-fi loudspeakers.

Their simple acoustic behaviour and gradual low-frequency roll-off can make them useful in controlled listening systems.

The final design still depends on the driver and intended response.

Sealed Box for PA Speakers

Sealed enclosures are less common for high-output low-frequency PA applications because a ported alignment can provide greater acoustic output from the same driver and cabinet volume in the intended operating range.

Nevertheless, sealed designs can still be appropriate for certain midrange and specialty applications.

Sealed Midrange Enclosures

A sealed chamber is commonly useful for midrange drivers.

It isolates the rear radiation of the midrange driver and prevents interaction with the woofer's internal acoustic volume.

The chamber can be relatively small when the driver does not need to produce deep bass.

Sealed Chamber for a Midrange Driver

┌───────────────────────────────┐
│                               │
│          MIDRANGE             │
│             ↓                 │
│          ┌─────┐              │
│          │     │              │
│          └─────┘              │
│                               │
│       Sealed chamber          │
│                               │
└───────────────────────────────┘

The chamber isolates the rear of the midrange driver from the other sections of the enclosure.

Internal Damping

Damping material can be used inside a sealed enclosure to absorb internal acoustic energy.

Common materials include:

Polyester fibre
Acoustic wool
Suitable acoustic foam
Other purpose-designed damping materials

The amount and distribution of damping should be chosen according to the enclosure and application.

Does Damping Increase the Box Volume?

Damping material can make the enclosure behave acoustically somewhat differently from its simple geometric volume.

The commonly quoted increase in "apparent volume" depends on the type, density and placement of the material.

It is therefore better to treat damping as part of the overall design rather than assuming a fixed percentage increase for every enclosure.

Cabinet Wall Thickness

The enclosure walls must be sufficiently rigid for the intended application.

Typical construction materials include:

MDF
Plywood
Birch plywood
Other suitable rigid sheet materials

The required thickness depends on cabinet size, material and expected acoustic output.

Internal Bracing

Large sealed cabinets should normally include internal bracing.

Bracing reduces panel flexing and helps prevent cabinet vibration from colouring the sound.

┌─────────────────────────┐
│                         │
│     ┌─────────────┐     │
│     │    BRACE    │     │
│     └─────────────┘     │
│                         │
└─────────────────────────┘

Making a Sealed Cabinet Airtight

An airtight cabinet is essential to the intended behaviour of a sealed enclosure.

Potential leakage points include:

Driver gasket
Terminal panel
Cabinet joints
Service openings
Cracks or construction gaps

A small leak can reduce the effective acoustic stiffness of the enclosure.

Driver Mounting

The driver should be mounted securely against a flat, rigid baffle.

A suitable gasket or sealing arrangement can help prevent air leakage around the driver frame.

The mounting screws should be tightened evenly without damaging the driver frame.

Terminal Mounting

The speaker terminals must also be sealed against the cabinet.

A terminal cup with a poor seal can become an unintended air leak.

For DIY cabinets, inspect the terminal area carefully after assembly.

Calculating a Rectangular Sealed Cabinet

Once the required net volume is known, the cabinet dimensions can be selected.

For example, a target net volume of approximately 40 litres could be approached using internal dimensions such as:

40 cm × 40 cm × 25 cm

= 40 litres

The actual cabinet must be larger than this if driver and brace displacement must be added.

Designing the Cabinet Around the Driver

The driver should be selected before the final enclosure dimensions are fixed.

Important physical dimensions include:

Cutout diameter
Overall frame diameter
Mounting depth
Magnet diameter
Magnet depth
Required clearance behind the magnet

A cabinet can have the correct acoustic volume but still be physically unsuitable for the selected driver.

Driver Magnet Clearance

The rear of the driver needs sufficient clearance from the enclosure and internal bracing.

Restricted clearance can interfere with the driver's ventilation and mechanical operation.

Check the manufacturer's recommended mounting requirements when available.

Sealed Box Construction Materials

MDF is a practical material for many DIY sealed boxes.

Plywood is often preferable where lower weight and greater mechanical durability are important.

For small custom enclosures, composite materials can also be used when appropriate construction methods are available.

Enclosure Shape and Internal Volume

The exact external shape is flexible as long as the required internal acoustic volume and structural requirements are maintained.

A rectangular enclosure is generally easiest to calculate and build.

Non-rectangular cabinets require more careful volume calculations.

Panel Resonance

Large unsupported panels can vibrate when exposed to high sound pressure.

This vibration can produce unwanted sound from the cabinet itself.

Bracing, thicker panels and suitable construction can reduce this problem.

Testing a Sealed Cabinet

After construction, inspect the cabinet for:

Air leaks
Loose panels
Loose bracing
Driver movement
Internal rattles
Terminal leakage

Then test the speaker at low level before applying high power.

Checking for Air Leaks

Air leakage can often be identified by inspecting all cabinet joints and listening for unusual air movement during low-frequency operation.

The driver mounting gasket, terminal panel and cabinet joints deserve particular attention.

Measuring the Finished Speaker

The finished enclosure should ideally be measured rather than evaluated only from theoretical calculations.

Useful measurements include:

Frequency response
Impedance response
Resonant frequency
Distortion

These measurements can reveal whether the finished system behaves as expected.

Impedance Measurement of a Sealed Box

The impedance curve of a driver changes when it is installed in a sealed enclosure.

The resulting resonance peak can be used to determine the system's resonant behaviour.

Impedance measurement can therefore be a useful diagnostic tool when building DIY sealed cabinets.

Common Sealed Box Problems

Problem	Possible Cause
Weak bass	Box too small, leakage, incorrect driver or insufficient excursion
Excessive bass peak	Qtc too high or enclosure alignment unsuitable
Cabinet rattle	Loose panel, brace or component
Reduced output	Driver limitation, amplifier limitation or incorrect enclosure
Unexpected resonance	Internal structure, panel resonance or driver behaviour

Common Sealed Box Design Mistakes

Using the external cabinet dimensions as the acoustic volume.
Ignoring driver displacement.
Ignoring bracing displacement.
Building a cabinet that is not airtight.
Choosing enclosure volume without considering Qts.
Ignoring the driver's Fs and Vas.
Assuming a larger cabinet is always better.
Using insufficient cabinet bracing.
Failing to provide sufficient magnet clearance.
Applying excessive amplifier power below the driver's safe excursion range.
Adding damping without considering the actual enclosure design.
Never measuring the completed cabinet.

Practical Sealed Box Design Workflow

Select the woofer.
Obtain its Thiele-Small parameters.
Record Fs, Vas and Qts.
Choose the desired Qtc alignment.
Calculate the required net enclosure volume.
Choose practical internal dimensions.
Calculate driver and brace displacement.
Increase the gross cabinet volume accordingly.
Select suitable construction material.
Design internal bracing.
Plan the driver mounting.
Plan the terminal position.
Add appropriate damping.
Construct the cabinet.
Seal all joints and openings.
Install the driver.
Test the enclosure.
Measure the finished speaker.
Compare the measured response with the design target.

Sealed Box Design Checklist

Driver selected
Fs known
Vas known
Qts known
Target Qtc selected
Net enclosure volume calculated
Driver displacement included
Brace displacement included
Cabinet dimensions calculated
Wall thickness selected
Bracing designed
Damping planned
Driver mounting sealed
Terminal mounting sealed
Cabinet joints sealed
Magnet clearance checked
Low-frequency testing performed
Final enclosure measured

Key Takeaways

A sealed enclosure is an airtight cabinet with no intentional acoustic port.
The trapped air acts as an additional spring for the driver.
Smaller boxes generally produce stronger air-spring loading.
Larger boxes generally produce weaker air-spring loading.
Fs, Vas and Qts are important parameters for sealed-box design.
Qtc describes the damping of the complete driver and enclosure system.
A Qtc around 0.707 is commonly associated with a maximally-flat sealed alignment.
The required net volume can be calculated from Vas, Qts and the desired Qtc.
Net volume is different from gross internal cabinet volume.
Driver and brace displacement must be subtracted from gross volume.
A sealed enclosure typically has a second-order low-frequency roll-off.
Sealed cabinets do not suffer from port turbulence.
Sealed cabinets must be airtight for the intended acoustic behaviour.
Internal bracing helps reduce cabinet-panel vibration.
Damping can be used to control internal acoustic energy.
The driver must have adequate excursion capability for the required low-frequency output.
The final cabinet should be tested and, ideally, measured.