Vented vs. Sealed boxes

Vented/ported boxes are slightly different from sealed ones in that they have a hollow tube (or some other type of port or vent) connecting the internal air volume to the outside air.

At low frequencies, ported loudspeakers have:

• Two masses – the speaker cone and the air mass.
• Two springs – the speaker suspension and the air spring.
• Two damping devices – electromechanical damping of the speaker, and a tube that opposes the velocity of the air flowing to/from the box.

A ported design has several advantages over a sealed design:

• The two resonances can be tuned to provide a lower cut-off frequency than with a sealed design.
• The excursion of the speaker cone at low frequencies can be much smaller for the same output, resulting in lower harmonic distortion and higher maximum output.

However, it also has disadvantages:

• The system is more sensitive to variations in temperature, humidity and other environmental or design factors.
• The system has 4 poles instead of 2. Thus, 'ringing' (boomy bass) is a common side effect due to the relatively high q-factor of one of the pole pairs.
• Sounds at midrange frequencies may leak through the port.
• At high amplitudes the port may produce audible hissing.

Sealed boxes

A basic sealed speaker box usually consists of a hollow box and a large circular hole where the speaker is mounted.

At low frequencies, the air and damping materials form an energy storage device – a mechanical spring – which produces an unavoidable resonance when combined with the spring constant and mass of the speaker cone. This resonance is usually exploited to optimize the low-frequency response of the loudspeaker.

At relatively high frequencies (e.g.: above 500Hz), the wavelength of the sound is short enough to produce various echoes and partial oscillations inside the box. Its purpose then is to absorb as much of that energy as possible. Damping materials such as pillow stuffing are often used, as well as design techniques such as an acoustic labyrinth.

A compromise has to be made between 'tuning' a box volume for good performance at low frequencies, or making it very large in order to reduce the amplitude of midrange resonances. Many designs make the enclosures relatively small because of space constraints. Furthermore, smaller boxes often have practical and structural advantages over relatively large boxes.

A sealed loudspeaker can be modelled as a second-order system at low frequencies. It has:

• One mass – the speaker cone (in parallel with part of air mass inside the box).
• One spring – the spring constant of the air, in parallel with the speaker suspension.
• One damping device – electromechanical damping of the speaker (Qms in parallel with Qes).