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Car Audio: Subwoofer Enclosures/Fourth Order
A fourth order enclosure (vented box) is a sealed enclosure with the additon
of a port tuned to a specific frequency. The port extends the low frequency response
of the basic sealed enclosure usually providing a lower cut-off frequency. The
enclosure offers a good transient response (second to that of a sealed enclosure),
good power handling within it's bandpass region, and excellent efficiency making it a
popular choice for many competitors and everday users, but
below the tuned frequency power handling is poor and damage to the woofer is likely
to occur if over driven. Lowering the port frequency below that of the manufacturer's recommended
frequency will only worsen matters and cause the sound to become "muddy"....bleah!
With any ported design enclosure, the largest port diameter or area should be used to minimize
port noises (or whistling). Flaring the edges of the port is also recommended for the same reasons.
Ports may be any shape, but a cyclinder is usually the easiest to incorporate into your design.
These enclosures are obviously more complex to build, less forgiving of design errors, and usually larger
than sealed enclosures, but if done properly, they can provide that additional output a competitor desires.
Fourth Order/Bass Reflex and Bandpass Enclosures:
• Fourth Order / Bass Reflex
• Isobaric - Bass Reflex
• Isobaric - Bass Reflex (Compound Loading)
• Bass Reflex - Passive Radiator
• Isobaric Bass Reflex - Passive Radiator
• Single Reflex - Band Pass
• Isobaric Single Reflex Bandpass
• Three Chamber Single Reflex Bandpass
• Isobaric Three Chamber Single Reflex Bandpass
Fourth Order/Bass Reflex |
Most often used in hatchbacks, vans, SUVs, and other vehciles where the enclosure can be
mounted inside of the vehicle, the bass reflex enclosure is a poplular choice for competitors and the easiest of the fourth order enclosures
to build. The shape of the box may vary, often seen with the facing or rear wall angled, but may be a simple box shape as shown.
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Isobaric - Bass Reflex |
If space is at a premium or just a smaller enclosure is desired, an isobaric
design may be used. The volume of the enclosure wil be approximately
half the space of the above enclosure with the same results.
The downside is you will need two woofers vs. one and the output will be the same.
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Isobaric - Bass Reflex (Compound Loading) |
This is a variation of the above enclosure. As long as the woofers are
mechanically in phase, it doesn't matter which way they face each other, however this will require a
larger enclosure to accomodate the magnet and frame assembly of the outer woofer and is more complex to build.
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Bass Reflex - Passive Radiator Enclosures |
Passive radiator systems are very similar to ported systems.
Instead of a tuned port, a "drone cone" is used to extend the low frequency response.
Most look like a flat diaphram, some may look like a woofer, but will have no magnet
structure.
The response is similar to a ported system, but the cut-off frequency is slightly higher,
and the cut-off slope is slightly steeper. Many of you may have seen the very small
enclosures made by Stillwater Designs®
that used 4'' drivers with a passive radiator (not sure if they still make these).
These worked extremely well (for what they were) in vehicles that literally had
next to no room at all to mount a conventional enclosure inside of the vehicle
(i.e.: Jaguars XJS, mounted under the seats).
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Single Reflex - Band Pass Enclosures |
The single-reflex band pass enclosure is basically a combination of a
sealed and vented enclosure. These can be fairly small enclosures, though larger than a sealed enclosure.
They offer great power handling, good low frequency response, but are not as efficient as vented enclosures
(top of this page), and are more complex to build. However, when built correctly, these make excellent enclosures
and are often used by competitors. This is often referred to as a fifth order enclosure, but it's not.
The addition of a crossover makes it a fifth order system.
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