Some obversations on
Sealed vs. Vented
This will not be an exhaustive article on the Helmholtz effects. This has been well covered in many other
articles. Instead this will be a look into certain generalizations of the vented design, and demonstrate a
non-traditional alignment for consideration.
There has always been some confusion in understanding what exactly happens when we put a hole in
a sealed box, and how the effects of different alignments relate to the various parameters. One
misconception is the resultant increased efficiency, which often is taken to mean increased
The output of a given driver at frequencies above fb will be exactly the same for either sealed or vented
design. The additional low frequency output provided by the vent is the result of this increased
efficiency. This is demonstrated on the plot below: The modeled driver is an RS225-8, and the identical
source voltage drives both models. The yellow plot is the sealed alignment with a Qtc of .707, while the
green one is a vented alignment
Another misconception would be that sealed boxes always play louder than vented ones. This
generalization is certainly true in some instances, but it is not always correct. The plot below shows the
excursion of the vented driver to be equal or less than the sealed driver until some frequency far below
20 Hz. At that frequency, the cone unloading would be outside the passband of normal music content,
and would not be excited. If the driver selected required higher vent tunings cone unloading would of
course, be of more concern, but could be ameliorated by an active or passive high pass filter. Note that
while the vented excursion is within 115% of its Xmax, the source voltage would have to be reduced to
keep the sealed excursion within its Xmax parameters. Consequently, the sealed speaker would have
about 2 dB less max SPL than the vented alignment. .
Vented designs roll off 4th order, or 24 dB/octave. -Twice that of a sealed design:
Well of course this is ultimately true, but often the alignment can be manipulated to emulate much lower
transfer functions for a significant portion of the frequency band.
Note in the response pic above the vented (green) response closely follows the sealed response to well
below f10. In this instance a larger enclosure with the same tuning would result in an even shallower roll
off and still lower f3/f10 frequencies.
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