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Enclosures
Focal offers 4 ready-to-use optimized enclosures. If you decide to 'brew your own', it is safest
to go for a bass-reflex type. This classic simple alignment usually give the best and most
consistent results.
See below regarding our proposals for the various volumes available per subwoofer.
Vented boxes
A bass-reflex system is more efficient than the sealed box since some of the energy from the
rear of the cone is converted via the reflex port to add in phase with the main output. There
is also a reduction of cone travel at the tuning frequency, so this system can also give higher
power handling. However, at very low frequencies bass rolls off faster (24dB/octave). In simple
terms, bass is louder, but not quite as deep.
You can make a vent port from a piece of plastic pipe, or there are many profiled vent custom
designed for loudspeakers that minimize turbulence (prevents what's commonly referred to
as chuffing).
Building an enclosure
The construction should always remain strong and rigid, to eliminate any unwanted
resonances. A good material such as MDF (Medium Density Fibreboard) will effectively ensure
this is possible. Choosing the correct thickness of MDF compared to the surface area decided,
is also critical to ensure the enclosure does not vibrate. For this reason the general thickness
recommended is 19mm MDF. Any loose components such as the internal connections and
cables should always be fixed securely inside. Damping material (foam etc) covering all the
internal walls of the enclosure, will further ensure it is free from unwanted resonances. It is
imperative that your custom designed subwoofer enclosure remains securely mounted at all
times to the vehicle.
Calculating the internal volume
Internal volume calculation: Vb = B x C x {D+(A-D)/2}
1 - External sizes in inches and millimetres with sides of 19 mm thickness
2 - (Ø x L) in cm
Polyglass Subwoofers Manual
Internal
15 L
volume
1
0.53cu.ft.
11"
A
280
11"
B
280
14
3/16
"
C
360
7
1/16
"
Dv
180
27 V1
2
3/4
"x9
13/16
vents
2
7 x 25
33 V1
vents
2
40 V1
vents
2
20 L
30 L
40 L
0.71cu.ft.
1.06cu.ft.
1.41cu.ft.
11
3/4
14
9/16
"
14
300
370
11
3/4
"
14
9/16
"
14
300
370
16
1/8"
14
15/16
"
19
410
380
7
7/16
"
9
5/16
"
9
190
237
"
2
3/4
"x9
13/16
"
2
3/4
"x9
13/16
"
7 x 25
7 x 25
2
3/4
"x9
1/16
"
2
3/4
7 x 23
7 x 23
50 L
60 L
1.77cu.ft.
2.12cu.ft.
9/16
"
17
11/16
"
17
11/16
370
450
450
9/16
"
17
5/16
"
17
5/16
"
370
440
440
1/4
"
17
5/16
"
20
7/8
"
490
440
530
5/16
"
10
1/4
"
10
1/4
"
237
260
260
"x9
1/16
"
2
3/4
"x15
3/4
"
2
3/4
"x15
7 x 40
7 x 40
"
3/4
"
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