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DLS SOUND HANDBOOK - SPEAKER BOXES
WHY DO WE NEED SPEAKER BOXES?
A Hi-Fi speaker for home use is always mounted in a
box to reproduce the best possible sound.
Traditional mounting in cars are in a door side or in
the hat rack, this is a simple baffle mounting. You
can of course achieve a better sound in your car by
using suitable speaker boxes.
In a correct adapted box the sound is improved and
the power handling capacity increases.
If you have a subwoofer in your trunk and a pair of
"open air"- mounted speakers in the rear, the air
pumping from the sub will effect the rear speakers
and make the cones move a little, ruining the sound
from them. This is one of many good reasons to use
boxes in your car as well as at home.
Normally we use boxes for the subwoofer but also
the rest of the speakers sounds better mounted in a
suitable box. We will now describe the different types
of boxes normally used in a car.
SPEAKER BOXES, GENERAL
Build your box in a stable and air-tight material. The
best is MDF-board, 19 mm, or particle board, 22 mm.
Larger boxes must have braces inside to avoid
resonance. The box must be completely air-tight. Use
sealing compound in all joints, also in the conduit
entry. The size of the box is fixed by the speaker
data, but also the type of vehicle and music, have an
influence on the box size. Deep bass demands larger
boxes than disco music.
SEALED (CLOSED) BOXES
Sealed boxes are easy to build. The size is not critical,
but it can´t be too small. The speaker data such as
Fs, Qts, Vas and X-max decides the size of the box.
Large speakers need larger boxes. Two speakers
need a box of the double size etc. The box must be
completely air-tight. Sealed boxes are normally used
for door-panels or kick-panels. Most 4", 5,25" and
6,5" speakers can be used in sealed boxes.
A sealed box should be filled with acoustic wool up
to 75 - 100%. A sealed box has a lower efficiency
than vented boxes but they can handle high power
and are easy to build. A subwoofer in a sealed box
creates a tight bass suitable for the audiophiles list-
ening to classical music, jazz and soft rock.
Advantages:
- High power handling capacity
- Extended low frequency reproduction
- Excellent transient response
- Easy to build
- Not critical with the size
Disadvantage:
- Lower efficiency (needs more power)
If you use a 25-30 Hz subsonic highpass filter on the
line input of your amplifier you will achieve a tight
and well-defined bass in your bass-box. (All DLS
amplifiers have a built-in subsonic filter.)
VENTED BOXES
A speaker in a vented box has higher efficiency (3
dB) than in a sealed box. In a vented box the sound
from the speaker and the port work together creating
a higher sound level. The sound from the port must
come out in the same phase as from the speaker, or
the sound result is bad.
The size of the box is set by the speaker data just as
in the sealed box. Also the car type and music type
have an influence on the box size.
Often the size of the car decides the practical size of
the box. A smaller box has a higher resonant
frequency than the larger one. The size of the box
should not be so large that the speaker plays below
it´s own free air resonance (Fs), then the power hand-
ling capacity drops.
The port in a vented box should be installeded on the
same side of the box as the speaker. But sometimes
this is impossible. The port opening inside the box
must have a free area behind the port, to the wall
behind, of at least the port diameter.
If the port is very long you might have problems
with the install. You can make a trick and "fool"
the speaker. Cut the port approx. 2,5 cm (1") from
the rear wall inside the box. Then the speaker is
fooled to believe the port is longer than it actually
is. This is not a perfect method but it often works
good enough.
There must also be a free area in front of the port.
Don´t cover the port opening with cloth. A large sub
needs a larger port to avoid whistling sounds. Use
ports with conical openings to avoid this. The port
must also be fastened properly to avoid rattle.
3" or 4" PVC tubes are normally used for ports. In a
correct tuned box you should be able to feel the air
pumping out from the port. At high volumes the air
can blow out a burning match, if not the box and port
are mismatched.
The port does not have to be fully inside the box as
long as the area and length are correct. for example
you can mount the port through a hat-rack. In a small
box this can have an effect on the box tuning since
the volume changes.
Suitable port diameters for different speaker sizes:
8":
4 - 8 cm,
12":
8 - 15 cm,
Sometimes you need two or more ports in a box. You
can convert from one to two or more ports as long as
the total port area is the same.
Advantages:
- Less cone movement and lower distorsion at port
tuning.
- Higher sound level at port tuning.
- Improved "bass kick".
Disadvantages
- Less cone control below port tuning.
- Higher frequencies can "leak" through the port.
- The sound from the port can be out of phase
compared with the sound from the speaker cone. Can
give a trailing sub sound.
14
10":
6 - 10 cm
15":
10 - 15 cm
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