BOX DESIGN NOTES
Sealed Boxes VS Ported Boxes:
Sealed Boxes offer the most protection from mechanical damage, plus can offer the most accurate sound quality if not
undersized. Exact size is not too critical and can be adjusted by stuffing (see below). Response decreases gradually at low
frequencies giving a nearly flat frequency response in many vehicles. But, there’s no path for heat to escape from the box (one
reason for mounting “inside out”)
Ported (Vented) Boxes
do offer some heat ventilation. Sound quality is less accurate, but often more powerful. The trade-
off: more powerful sound is often boomier. At frequencies below the port tuning, the enclosure does not restrict cone motion.
Therefore, music with very low frequencies can bottom out the subwoofer, possibly causing damage. Also, the frequency
response rolls off quickly, because the port stops functioning like a port and becomes more like a huge leak-the front and back
waves from the subwoofer start to simply cancel each other. If possible, build the box so you can do test listening before final
assembly. You can then adjust the port length to taste. If it sounds too boomy after final assembly, you can experiment with
stuffing, but not near the port or cone or magnet vents.
Box Stuffing:
Dense stuffing changes the thermal properties of the enclosure, and can make the enclosure act larger than the
measured dimensions. This means sealed boxes can be made somewhat smaller with the same sound. For example, stuffing
with 1 pound (1/2 kilo) of 100% polyester such as pillow stuffing could let you shrink the sealed box 15% but keep basically the
same sound. Ported boxes cannot take advantage of this effect very well, since stuffing the enclosure can negatively affect the
air flow in and out of the port. Coating the inside walls with anti-vibration compounds or sheeting can improve the sound as can
bracing the enclosure.
Ports:
Air in the box resists compression, like a spring. The port basically makes a plug of air, with a certain mass. The system
resonates, kinda like how a car bounces up and down on its springs. A longer port has more mass, lowering the tuning. A
bigger port also has more mass, but the larger area changes how much box stiffness the port “sees” and tuning goes up. To
compensate, the port can be lengthened, but the port needs to be big enough to match the cone excursion. Unfortunately for
auto sound buffs, this all means that a round port big enough to flow all the air often can’t fit in the box! And, ports should NOT
be built with elbows if at all possible, it kills the airflow.
Port Radius:
Rounding the port ends reduces distortion at lower volumes. At high SPL, the air just “jets” in and out and the
radiusing has little effect. (For slot ports, the shape is different, so do radius the inside end if you can).
Crossfire recommends the use of Precision Ports
TM
for all round port applications. Precision
Ports
TM
will increase SPL and reduce airflow turbulence at the entrance and exit of the port
with its unique flare port design.
Sub Dimensions
Sealed Box
Ported Box
RECOMENDED PORTED ENCLOSURES
These recomended enclosures are general enclosure recomendations and can be
varied to suit each customers application. For SPL type applications, enclosure
volumes and tuning frequencies will vary for each application. For custom enclosure
applications please contact Crossfire Technical Service Dept.
Please Note: All enclosure recommendations are based on net enclosure volume.
Please add woofer displacement and port displacement to calculate accurate net
enclosure volume. Example: C7 12D1 has a woofer displacement of .136ft3 and a port
displacement of .29ft3, So your ported enclosure needs to be 2.7ft3 gross volume
before all displacements, giving you a net volume 2.25ft3.
Model:
Internal Volume (Net)
Port Tuning Frequency
Port Area
Port Length
0.75ft
3
38Hz
10 sq inches
22.25”
Model:
Internal Volume (Net)
Port Tuning Frequency
Port Area
Port Length
C7 8D1
C7 12D1
2.25ft
3
35Hz
25 sq inches
20.25”
0.75ft
3
38Hz
10 sq inches
22.25”
C7 8D2
2.25ft
3
35Hz
25 sq inches
20.25”
C7 12D2
C7 15D1
3.25ft
3
34Hz
32 sq inches
18.25”
C7 10D1
1.5ft
3
35Hz
16 sq inches
20”
3.25ft
3
34Hz
32 sq inches
18.25”
C7 15D2
Model:
Internal Volume (Net)
Port Tuning Frequency
Port Area
C7 18D1
5.5ft
3
34Hz
65 sq inches
5.5ft
3
34Hz
65 sq inches
C7 18D2
1.5ft
3
35Hz
16 sq inches
20”
C7 10D2
