Saturday, May 26, 2007

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.

3 comments:

Gio said...
This comment has been removed by the author.
Gio said...

The excursion is only smaller near the box resonance frequency. However, there is high excursion potential at frequencies substantially below resonance.

Cheers,
Gio.
DIY Audio Projects

Lech said...

Hi Gio,
Thanks for clarifying that, and yes I agree. For ported boxes the cone excursion dips at resonance, which is quite useful to have near its low frequency limit of operation.