Overly large vent areas (or diameters) may
be divided into smaller ports as long as the overall vent area and length
remain unchanged. An example of this is the case where one 8" diameter tube
is replaced by four 4" diameter tubes of the same length. The four smaller
tubes have the same area, but also have twice the wall surface area. In
cases such as this it is often necessary to trim the tube length by perhaps
10% to 20% to maintain the same helmholtz frequency that the single tube
would have provided, since multiple tubes (more wall area) tend to tune a
bit lower because of the added air friction on the greater wall surface
area.
You can determine actual tuned frequency under large-signal conditions by
measuring the A.C. current maximum near the target helmholtz frequency with
an A.C. ammeter hooked in series between the power amplifier and the
loudspeaker; the maximum current reading will coincide with tuning. Once
actual helmholtz frequency is known, you can trim excess length (equally
from each tube) to achieve the desired frequency increase.
Port Placement on the Enclosure
It is generally not critical where ports are placed on an enclosure as far
as low frequency operation is concerned, except that the port and woofer
form a "system" which should not be disturbed or limited by near- by
obstacles. It is perfectly acceptable to put a port on the back of an
enclosure as long as the enclosure will not then be located too close to a
wall so that the air flow at the vent opening is restricted. The same rule
applies to the outside of the vent as to the inside-the-box end: the end of
the vent should be kept away from obstacles if possible, as a rule by about
twice the port diameter. The fiberglass or other insulating material inside
the box should be fastened so as not to be drawn toward the port by the air
flow. If necessary, insulation should be forgone in the immediate vicinity
of the port end. Port material can be anything rigid, e.g. cardboard carpet
tube. More expensive PVC or plastic tube is not necessary unless some odd
size is called for where cardboard is not available. Port ducting can be
square or rectangular using wooden sides as long as extreme length-to-width
ratios are avoided. For example, a 9" tube and an 8" X 8" square duct have
approximately the same area. One or two sides of the box may be used as
sides of such a port, but this will cause an alteration in the expected
tuning. Common wall ducts should therefore be designed to allow for some
length adjustment after the box is completed.
